Citation
Effect of bed shear stress on the erosional characteristics of kaolinite

Material Information

Title:
Effect of bed shear stress on the erosional characteristics of kaolinite
Series Title:
UFLCOEL
Creator:
Parchure, T. M. ( Trimbak Mukund ), 1943- ( Dissertant )
University of Florida -- Coastal and Oceanographic Engineering Laboratory
Mehta, Ashish J. ( Thesis advisor )
Eades, J. L. ( Reviewer )
Benedict, B. A. ( Reviewer )
Place of Publication:
Gainesville, Fla.
Publisher:
University of Florida
Publication Date:
Copyright Date:
1980
Language:
English
Physical Description:
xiii, 174 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Clays ( jstor )
Eroded soils ( jstor )
Moisture content ( jstor )
Sediments ( jstor )
Shear strength ( jstor )
Shear stress ( jstor )
Soils ( jstor )
Stress functions ( jstor )
Stress tests ( jstor )
Water erosion ( jstor )
Coastal and Oceanographic Engineering -- Dissertations, Academic -- UF
Coastal and Oceanographic Engineering thesis M.S
Erosion ( lcsh )
Kaolinite ( lcsh )
Shear strength of soils ( lcsh )
City of Gainesville ( local )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Abstract:
The degree of resistance to the erosion of a cohesive sediment bed under an applied shear stress is controlled by the physico-chemical properties of the sediment and the fluid, as well as by they inter-particle bonds. Previous attempts to correlate the erodibility of cohesive sediment beds with the shear strength determined by such devices as a penetrometer or a viscometer, or with soil indices, have not been successful. the objective of this study was to evolve a test procedure for conduction laboratory experiments to obtain the "layer by layer" shear strength of deposited beds of various density structures. Experiments were conducted using kaolinite with salt water of 35 ppt concentration as the eroding fluid. All the tests were carried out in an annular rotating channel apparatus. New techniques developed for measurement of the density structure of deposited beds have been described. An illustrative example outlining the procedure for the determination of the depth-variation of the bed shear strength is given.
Thesis:
Thesis (M.S.)--University of Florida, 1980.
Bibliography:
Includes bibliographical references (leaves 169-173).
General Note:
Typescript.
General Note:
Vita.
Funding:
This publication is being made available as part of the report series written by the faculty, staff, and students of the Coastal and Oceanographic Program of the Department of Civil and Coastal Engineering.
Statement of Responsibility:
by T.M. Parchure.

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
The University of Florida George A. Smathers Libraries respect the intellectual property rights of others and do not claim any copyright interest in this item. This item may be protected by copyright but is made available here under a claim of fair use (17 U.S.C. §107) for non-profit research and educational purposes. Users of this work have responsibility for determining copyright status prior to reusing, publishing or reproducing this item for purposes other than what is allowed by fair use or other copyright exemptions. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder. The Smathers Libraries would like to learn more about this item and invite individuals or organizations to contact Digital Services (UFDC@uflib.ufl.edu) with any additional information they can provide.
Resource Identifier:
07367591 ( OCLC )

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Full Text

O
^ \
I















EFFECT OF BED SHEAR STRESS ON THE EROSIONAL
CHARACTERISTICS OF KAOLINITE





By

T.M. PARCHURE


A THESIS PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF MASTER OF SCIENCE















UNIVERSITY OF FLORIDA


1980











ACKNOWLEDGEMENTS


My deepest gratitude goes to Dr. Ashish J. Mehta, Associate Professor,

Department of Coastal and Oceanographic Engineering, who has been my

advisor and chairman of the committee, for his valuable guidance,

encouragement, and support during the course of studies. I thank

Dr. J. L. Eades, Professor, Department of Geology, and Dr. B. A. Benedict,

Professor, Department of Civil Engineering, for serving on my supervisory

committee. I have to thank Dr. E. Partheniades, Professor, Department of

Engineering Sciences, for his advice regarding measurement of bed density.

I am grateful to the personnel at the Coastal Engineering Laboratory,

Mr. George Jones in particular, for their excellent cooperation and for

extending every possible help for a successful completion of the experi-

mental work. I am very thankful to Adele Koehler for careful typing of

the manuscript and to Lillean Pieter for the excellent drafting work.

I would like to express gratitude to my loving wife Aparna and my

mother Indira for their continued encouragement in my endeavor and for

accepting to bear the hardships caused during my long stay away from them.

The present study was conducted as a part of the research project

entitled "Deposition of Fine Sediments in Turbulent Flows" supported by

the National Science Foundation under Grant Number GK-31259. Support

was also received partially from the Environmental Protection Agency

under Grant Number R806684010. This support from both the agencies is

sincerely acknowledged.











TABLE OF CONTENTS


ACKNOWLEDGEMENTS

LIST OF TABLES

LIST OF FIGURES

NOMENCLATURE .

ABSTRACT .

CHAPTER


INTRODUCTION . .

FINE SEDIMENTS . .

2.1 General Description .
2.2 Properties of Fine Sediments .
2.3 Parameters Influencing the Properties of Cohesive
Sediments . .
2.4 Processes for Deposited Beds .
2.5 Clay-Water System .

PREVIOUS LABORATORY STUDIES .

3.1 General Review .
3.2 Review of Literature on Erosion ...
3.3 Review of Literature Pertinent to the Present
Study . .
3.4 Shear Strength of Clay .
3.5 Shear Strength and Bed Density of Clay .

PRESENT INVESTIGATION .

4.1 Objective . .
4.2 Material . .
4.3 Apparatus for Erosion Tests .
4.4 Experimental Procedure .


4.5

V TEST

5.1
5.2
5.3
5.4
5.5


Apparatus for Measurements of Bed Density .

RESULTS AND ANALYSIS .

Effects of Parameters in Steps I, II and III
Multiple Steps of T .
Discretized Sinusoidal Velocity Variation
Correlation Plots of C30 and C60 Values .
Analysis of Data .


I

II


III







IV


74


Page

ii

v

vi

xi

xiii



1

5

5
7

18
26
31

37

37
39

45
54
62

68

68
71









CHAPTER Page

VI DISCUSSION AND PROPOSED TEST PROCEDURE ........ 132

6.1 Bed Density and Other Soil Parameters Correlated
to the Shear Strength .... 132
6.2 Critical Shear Stress .... 139
6.3 Proposed Test Procedure .... 143
6.4 Illustrative Example .... 151

VII SUMMARY AND CONCLUSIONS .... 164

7.1 Summary of Literature Review ........... 164
7.2 Conclusions of the Present Study ... 165

REFERENCES . ... ......... 169

BIOGRAPHICAL SKETCH . ... ..... 174











LIST OF TABLES


Specific surface area and liquid limit for typical
clays

Summary of selected studies on cohesive soil erosion

Experimental conditions for tests conducted with a
multiple steps of Te


V


Table

1


2

3


Page











LIST OF FIGURES


Figure Page

1 Inter-particle forces on clay minerals and the clay
micelle 14

2 Rheological models 33

3 Concentration versus time plot obtained in erosion of
placed bed: Partheniades (1962), Expt. Series-I 48

4 Concentration versus time plot obtained in erosion of
remolded bed: Partheniades (1962), Expt. Series-II 49

5 Concentration versus time plot obtained in erosion of
deposited bed: Partheniades (1962), Expt. Series-III 50

6 Relationship between rate of erosion and average bed
shear stress: Partheniades (1962), 50

7 Concentration versus time plot: Krone (1962) 52

8 Concentration as a function of bed shear stress:
Krone (1962) 52

9 Results of a 500 hour long erosion test: Krone (1962) 53

10 Concentration versus time plots for Series I obtained
by Lee (1979) 55

11 Concentration versus time plots for Series II obtained
by Lee (1979) 55

12 Lee's (1979) data for Series I re-plotted to indicate
variation of suspension concentration as a function of
time and bed shear stress 56

13 Lee's (1979) data for Series II re-plotted to indicate
variation of suspension concentration as a function of
time and bed shear stress 57

14 Concentration versus time data obtained by Yeh (1979)
for erosion of Kaolinite 58

15 Concentration as a function of bed shear stress obtained
by Yeh 59








Figure

16 Schematic diagrams showing shear strength of cohesive
soil related to other parameters

17 Relationship between shear strength and bed density
observed by Owen (1970)

18 Bed density profiles: Owen (1970)

19 Definition sketch for notations used to describe experi-
mental conditions

20 Size gradation of Kaolinite used for the experiments


21 Photograph:

22 Photograph:
the ring

23 Photograph:

24 Photograph:
the ring

25 Photograph:

26 Photograph:

27 Photograph:
of sediment

28 Photograph:

29 Operational
and channel

30 Correlation
channel and


The rotating channel facility

Close view of the annular channel and


The motor controllers

The electric motors for the channel and


Millipore filter apparatus assembly

Device for measurement of bed density

Equipment for determining the concentration
suspensions

Sampling bottles

speeds and controller meter readings for ring
at different bed shear stresses

between r.p.m. and meter reading for the
the ring


31 Schematic drawings of apparatus developed for measurement
of bed density

32 Effect of parameters in Step I on suspension concentration

33 Suspension concentration versus time for Expt. 3

34 Suspension concentration versus time for Expt. 4

35 Suspension concentration versus time for Expt. 5

36 Effect of Step II parameters on suspension concentration


Page








Figure

37 Effect of Step III parameters on suspension concentration

38 Representation of a linearly varying bed shear stress by
two different discretized time step functions

39 Suspension concentration versus time for Expt. 7


Suspension concentration versus time for

Suspension concentration versus time for

Suspension concentration versus time for

Suspension concentration versus time for

Suspension concentration versus time for

Suspension concentration versus time for

Suspension concentration versus time for

Time-step function for bed shear stress

Suspension concentration versus time for

Suspension concentration versus time for


Expt.

Expt.

Expt.

Expt.

Expt.

Expt.

Expt.



Expt.

Expt.


Page

93


97

98

99

100

101

102

103

104

105

106

107

108


50 Variation
stress as

51 Variation
stress as

52 Variation
stress as

53 Variation
stress as

54 Variation
stress as

55 Variation
stress as

56 Variation
stress as

57 Variation
stress as


of suspension
a function of

of suspension
a function of

of suspension
a function of

of suspension
a function of

of suspension
a function of

of suspension
a function of

of suspension
a function of

of suspension
a function of


concentration with bed shear


time for Expt. 8

concentration with
time for Expt. 9

concentration with
time for Expt. 10

concentration with
time for Expt. 11

concentration with
time for Expt. 12

concentration with
time for Expt. 13

concentration with
time for Expt. 14

concentration with
time for Expt. 15


bed shear


bed shear


bed shear


bed shear


bed shear


bed shear


bed shear


58 Effect of shear stress variation on suspension
concentration


viii


110








Figure Page

59 Comparison of suspension concentration obtained under
two different discretized time step functions 119

60 Variation of suspension concentration with bed shear
stress, all data for Td = 24 hours 120

61 Variation of suspension concentration with bed shear
stress, a-i data for Td = 40 hours 121

62 Suspension concentration versus bed shear stress
(Expt. 7) 124

63 Variation of suspension concentration as a function of
bed shear stress (Expt. 9) 125

64 Variation of suspension concentration as a function of
bed shear stress (Expt. 11) 126

65 Variation of suspension concentration with bed shear
stress for two different discretized time step functions 127

66 Cr versus Tr at 10 minutes 128

67 Cr versus tr at 20 minutes 129

68 Comparison of Cr versus T for two different bed density
structures

69 (AC)ex plotted against the corresponding values of (AT)ex 131

70 Example of erosion test result and critical shear stress
for erosion as a function of dry density of mud surface
given by Thron and Parsons (1980) 135

71 Parameters influencing bed density and plasticity of soil 138

72 A typical test result reported by Espey (1963) 140

73 Notations for critical shear stress 142

74 Erosion rate versus time for different values of bed
shear stress 144

75 Definition sketch for various parameters 145

76 Explanatory sketch for EQ type profile of c-t curve 147

77 Explanatory sketch for ER type profile of c-t curve 148

78 Suspension concentration during deposition under the
bed shear stress of 0.05 N/m 153








Figure Paoe

79 Suspension concentration during deposition under the
bed shear stress of 0.015 N/m2 154

80 Suspension concentration during deposition under zero
bed shear stress 155

81 Variation of bed density with depth for three different
conditions of flow deposited beds 156

82 Suspension concentration versus time for Expt. 17 157

83 Suspension concentration versus time for Expt. 18 158

84 Suspension concentration versus time for Expt. 19 159

85 Variation of suspension concentration with bed shear for
different flow deposited beds 160

86 Suspension concentration versus bed shear stress for
different flow deposited beds 161

87 Variation of erosion rate as a function of bed shear
stress for different flow deposited beds 162

88 Shear strength of bed as a function of depth 163













NOMENCLATURE


C = Ratio of the consecutive suspension concentrations, e.g.
C2 C
S2 etc.
C C2

AC = Excess (suspension) concentration, e.g. C2 C1.

C2 C1
(AC)ex = Normalized excess concentration, e.g. C
C1

Co = Suspension concentration at the end of initial mixing.


C30 = Suspension concentration at the end of 30 minutes after change

of bed shear stress.


p = Density of bed.


Tm = Bed shear stress for initial mixing.


Tm = Duration of initial mixing.


Td = Bed shear stress for deposition.

Td = Duration of deposition plus consolidation.


Te = Bed shear stress for erosion (varied as a function of time).


Ts = Time step for Te, i.e. duration over which different magni-

tudes of Te prevailed.


r = Ratio of the consecutive values of bed shear stress, e.g.
r T T
e2 e3
etc.
Te Te
el e2








AT = Excess shear stress, e.g. T Te
22 -
(AT)ex = Normalized excess shear stress, e.g.
e1








Abstract of Thesis Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the Requirements
for the Degree of Master of Science


EFFECT OF BED SHEAR STRESS ON THE EROSIONAL
CHARACTERISTICS OF KAOLINITE

By

T.M. Parchure

December 1980

Chairman: Dr. Ashish J. Mehta
Major Department: Coastal and Oceanographic Engineering

The degree of resistance to the erosion of a cohesive sediment bed

under an applied shear stress is controlled by the physico-chemical

properties of the sediment and the fluid, as well as by the depth-

variation of the bed shear strength characterized by the inter-particle

bonds. Previous attempts to correlate the erodibility of cohesive

sediment beds with the shear strength determined by such devices as a

penetrometer or a viscometer, or with soil indices, have not been suc-

cessful. The objective of this study was to evolve a test procedure

for conducting laboratory experiments to obtain the "layer by layer"

shear strength of deposited beds of various density structures.

Experiments were conducted using kaolinite with salt water of 35 ppt

concentration as the eroding fluid. All the tests were carried out in

an annular rotating channel apparatus. New techniques developed for

measurement of the density structure of deposited beds have been de-

scribed. An illustrative example outlining the procedure for the

determination of the depth-variation of the bed shear strength is

given.


Chairman


xiii













CHAPTER I

INTRODUCTION


Study of the properties of transportation and deposition of sedi-

ments has engaged the attention of several research workers. The

motive behind these studies has primarily been to assist in the design

of dams and irrigation canals, water treatment and sewage disposal

works, navigation channels, docks and harbors, etc. Special phenomena

such as mud-banks along the shorelines, density flows at docks, floc-

culation and deposition of sediments due to mixing of salt water and

fresh water in estuaries are associated with the fine sediments. Among

various ways of classifying the sediments based on their geological

origin, chemical properties, physical properties, etc., one of the

classifications has been to treat the fine sediments of various composi-

tions as a class in itself due to their special properties which differ

considerably from the other sediments, and hence they need to be studied

separately.

The aspects related to fine sediments in some of the engineering

projects are illustrated by case studies. In the Mersey Estuary,

England, because of the abundance of fine cohesive sediment, attempts

to increase the depths in one channel by removing 2.3 million m3 of

material each year failed utterly. The channel in fact became shallower

after five years of intensive dredging than it was before. Another

example is Savannah harbor in the U.S.A., where in spite of continuous








dredging over thirty years, the siltation rate has almost doubled and

displaced the major deposition zone 20 km upstream along the river to

an inconvenient location. Construction of a dam across a river trans-

porting fine sediments in suspension results in the deposition of

sediments immediately upstream of the structure and may, as in the case

of the Aswan Dam, Egypt, deprive farmers downstream of their annual

replenishment of fertile sediment during the flood season. Density

currents created by the presence of fine sediments in sea water cause

excessive siltation of navigation basins as experienced at the Tilbury

tidal basin on the Thames River, England.

Cohesive sediments also form an important consideration in the

nuclear power generation planning and disposal of radioactive waste.

Research has shown that the transuranic elements such as plutonium and

americium are reconcentrated strongly by marine sediments. Their

presence in sediment has focused attention on routes by which contaminated

sediment might present a route of exposure to man especially in the

longer term in the form of airborne dust in a respirable form or uptake

through crops grown in soils reclaimed from areas of contaminated

sediments.

In the more recent years the study of fine sediments has assumed

an important place in the context of pollution control. The transport

and ultimate fate of contaminants is a complex process involving

physical, chemical, and biological aspects, all of which play an

important role. Sediments in a way are pollutants themselves since

they increase turbidity. Their deposition may pose serious engineering

and environmental problems. However, the more important aspect is their

property to adsorb other pollutants very effectively and transport them








along. The content of heavy metals adsorbed to the sediments is found

to depend on the grain size of the sediment. The smaller the grain

size, the higher is the contamination with heavy metals. The sedi-

ments less than 16 microns in size are likely to have very high con-

taminants. The bulk of the pollutants may be carried on the sediments

rather than the water. Hence, the dispersal of pollutants cannot be

dissociated from the dispersal of sediments. Study of dispersal of

sediments alone, however, cannot be expected to provide the required

information related to pollutants since the chemical, biological, and

physical aspects involving oxidation, reduction, exchange of elements,

decay, etc. considerably change the properties of pollutants.

An understanding of the depositional and resuspension character-

istics of the fine sediments would therefore be beneficial in obtaining

better solutions to engineering problems and in exercising a more

effective pollution control. Over the last about twenty years, studies

have been carried out to investigate the erosional and depositional

properties of fine sediments. The influence of different parameters

connected with the sediment and the eroding fluid on the character-

istics of erosion and deposition has also been studied. These have

been described in Chapter III. The experimental work reported in

this thesis was carried out to study the resuspension of flow deposited

bed of Kaolinite under varying shear stress and to study the effect of

bed structure on the process of resuspension. Saline water with 35

parts per thousand concentration was used as the eroding fluid.

An understanding of the physical processes associated with the

movement of fine, cohesive sediments is clearly essential for obtaining

improved engineering solutions to estuarine problems. The phenomena of





-4-


fine sediment transport, deposition, bed formation and consolidation,

and bed resuspension are rather complex, and in the estuarine environ-

ment they are inter-linked in a cyclical manner within time-scales

imposed typically by the astronomical tides. Investigations of these

phenomena under laboratory scales is an important first step towards

an elucidation of the mechanics of the transport processes in the

prototype, since it is possible to isolate and control the important

governing parameters in laboratory tests. In that context, this in-

vestigation is concerned with studying the characteristics of resus-

pension of flocculated cohesive sediment beds. Under an applied bed

shear stress, the surficial erodibility of such a bed is contingent

upon the structure of the bed, as defined by the inter-particle bond

strength of the floc network. Inasmuch as this network is formed

under a given set of conditions specified by floc deposition and con-

solidation of the settling suspension at the bed, the magnitude and

the duration of the applied shear stress during bed formation are

important governing parameters for the subsequent process of resus-

pension. Hence the investigation of the erosion of beds formed under

a variable shear stress is emphasized in this study. The overall

objective of this study was an attempt to establish a laboratory test

procedure for specifying the "layer by layer" erodibility of a deposited

bed in terms of parameters) involving the critical shear stress for

the erosion of a particular layer. The observed variation of the

erodibility of a given bed with depth has been discussed with reference

to the depth-variation of the floc shear strength and the bulk

density.














CHAPTER II

FINE SEDIMENTS


2.1 General Description


Fine sediments, commonly called clays or muds,are a product of

weathering or hydro-thermal action on the rock and other soil on

earth's surface. Although the maximum size of particles in the clay

grade is defined somewhat differently in different disciplines, the

general tendency has been to classify sediments finer than two microns

as clays. Mixtures of clays and silt are usually called muds. The

classification of fine-grained soil as either a silt or a clay is not

merely on the basis of particle size but rather on the plasticity or

non-plasticity of the material. Clay soil is plastic over a range of

water content; that is, the soil can be remolded or deformed without

causing cracking, breaking, or change of volume, and will retain the

remolded shape. The clays are frequently cohesive. When dried, a clay

soil possesses very high resistance to crushing. A silt soil possesses

little or no plasticity and when dried has little or no strength.

The basic differences between the elementary particles of non-

interacting coarse minerals and the interacting fine minerals such as

Kaoline could be briefly described as follows:

1. The non-interacting particles have no electric charge. Hence, they

interact only hydrodynamically without any inter-particle attraction.




-5-








The fine sediments are interacting particles which attract or repel

each other due to the presence of electric charge. Hence, the non-

interacting particles remain separate from each other, whereas the

interacting particles can form flocs under suitable environment.

2. If a small sample of moist silt is shaken easily but rapidly, water

will appear on the surface but disappear when shaking stops. This

phenomenon is referred to as dilatancy. The non-interacting par-

ticles show dilatancy with high concentration of sediment, whereas

a sample of moist clay when shaken similarly does not show wetting

of the surface.

3. The interacting particles exhibit elastic or plastic properties.

4. The non-interacting particles are usually unsaturated, whereas the

flocs of interacting particles are saturated with water molecules.

5. With a low concentration of sediment, the suspension of non-

interacting sediment is close to a Newtonian fluid, where the

deformation is linearly proportional to shear stress. The sus-

pension of interacting particles is non-Newtonian in behavior.

6. The erosional, transport, and depositional characteristics of

non-interacting sediments are based mainly on the physical proper-

ties such as size, specific gravity, compaction, etc. The proper-

ties of the fluid such as salinity, pH, temperature have no sub-

stantial effect. On the other hand, the fluid properties have a

substantial effect on the formation of flocs and hence on the

erosional and depositional properties of interacting particles.

7. Fine sediments have a relatively much higher compressibility than

the coarser non-interacting sediments.





-7-


8. Surface forces are dominant in respect to fine sediments, whereas

gravitational forces predominate in the case of non-interacting

particles. Due to the tendency of fine sediments to attach to each

other due to surface forces, fine sediments are also sometimes

referred to as cohesive sediments and the other sediments as non-

cohesive sediments.

9. Fine sediments are transported in the form of the suspended load or

wash load, whereas the coarser sediments are predominantly trans-

planted as bed load.



2.2 Properties of Fine Sediments


Several parameters affect the properties of clay materials, par-

ticularly the following:

1. Clay mineral composition.

2. Non-clay mineral composition.

3. Organic matter.

4. Exchangeable ions and soluble salts.

5. Texture, i.e. the particle size distribution of the constituent

particles, the shape of the particles, their orientation in space

relative to each other, and the forces tending to bind the par-

ticles together.

In the context of clays, it is necessary to distinguish between

material structure and property anisotropy. In general, "anisotropy"

refers to the material structure and/or properties which do not exhibit

the same characteristics and/or properties in every direction. The

material structure anisotropy relates primarily to the anisotropy of








fabric which would influence development of interparticle force rela-

tionships. Property anisotropy refers to strength, compressibility,

permeability, conductivity, and other mechanical properties which are

not equal in all directions, i.e. the material property demonstrated

is a function of the sample tested. The external constraint anisotropy

refers particularly to the applied stresses and boundary constraints.

While considering properties of fine sediments, the anisotropy needs

to be taken into account.



2.2.1 Size, Range, and Definition


The maximum size of particles in the clay size grade is defined

differently in different disciplines. In geology, the tendency has been

to follow the Wentworth Scale to define the clay grade as materials

finer than about 4 microns. In soil investigations, the tendency is

to use 2 microns as the upper limit of the clay size grade. Although

there is no sharp universal boundary between the particle size of clay

minerals and non-clay minerals, in argillaceous materials, a large

number of analyses have shown that there is a general tendency for the

clay minerals to be concentrated in a size less than 2 microns (Grim,

1968).

Clays contain varying percentages of clay-grade material and

therefore varying relative amounts of non-clay-mineral and clay-mineral

constituents. Clays almost always contain some non-clay mineral

material coarser than the clay grade, although the amount may be very

small. In many materials called clays the clay grade and the clay-

mineral constituents make up considerably less than half the total.








In such materials the non-clay is frequently not much coarser than the

maximum for the clay grade, and the clay mineral fraction may be par-

ticularly potent in causing plasticity. In general, fine grained

materials have been called clays so long as they have distinct plasti-

city and insufficient amounts of coarser material to warrant the

appellations "silt" or "sand." If particle size anslyses are made, the

term clay would be reserved for a material in which the clay grade

dominates. However, names have been and are applied most frequently

on the basis of appearance and bulk properties of the material.

The expression clay material is used for any fine-grained, earthy,

argillaceous, natural material. Clay material includes clays, shales,

and argillites. It would also include soils, if such materials were

argillaceous and had appreciable contents of clay-size-grade material.

Clay particles are usually within a range of diameter smaller than

0.002 mm, but larger than the molecular size (10-6 mm).

For the civil engineer, the fine sediments eroded from the earth's

crust are of interest and the term clay is primarily a particle size

term. For the chemical engineer the interest includes synthetic and

other materials in their fine form, rather than the natural clay

minerals.

There are three levels of first order fabric recognition of clay

structure. These are categorized on the basis of the degree of magni-

fication required for a proper observation of the fabric pattern.

1. Macroscopic: The fabric units are distinguishable by the naked

eye. They consist of an aggregation of clay particles called peds.

2. Microscopic: The fabric units are visually observed under the light

microscope. For clays, single particles are not distinguishable





-10-


at this level of viewing. The fabric units identified in the

microscopic range consist of several particles or groups of par-

ticles defined as clusters, also sometimes called flocs.

3. Ultra-microscopic: The fabric units are visually observed in the

ultra-microscopic level using electron microscopy (either trans-

mission or scanning electron microscopy). Single or individual

clay particles can be distinguished at this level.



2.2.2 Surface Area


Colloidal properties such as plasticity and adsorption of molecules

arise from the large surface area associated with a small mass. The

surface forces are dominant in respect to fine sediments and the in-

fluence of gravitational forces is small. The clay minerals are plate-

shaped or tabular because the layer-lattice structure results in strong

bonding along two axes but weak bonding between layers. The clay par-

ticle thickness depends upon the magnitude of forces of attraction

between the layers. The variation in specific surface area of different

minerals is primarily due to different thicknesses of the plate-shaped

particles. Variation in the other two dimensions of clay particles is

related to the degree of crystallinity of the clay minerals. A well

crystallized Kaolinite has large particles. If it is poorly crystalline,

the Kaolinite particles may not be larger than those of montmorillonite.

The true meaning of the particle size becomes more evident in terms

of its specific area. For example, a single sphere 1 cm in diameter

has a surface area of 3.14 cm2. The same volume in terms of one micron (1 u)

diameter spheres has a surface area of 10,000 times greater and for one





-11-


millimicron diameter spheres the surface area is 10 million times as

great.

The specific surface area of different clay minerals is as

follows:

Montmorillonite: 800 m2/g Chlorite: 80 m2/g

Clay Mica: 80 m2/g Kaolinite: 15 m2/g

Kaolinites show the most uniform crystals, often hexagonal plates

with a typical diameter of 0.3 to 0.5 p and a thickness of 0.05 to 2 u.

Montmorillonite particles are thin plates typically around 30 A thick

and 0.1 to 1 p in diameter. Illite particles are plates with a typical

thickness of 300 A.

Surface area is one of the most important properties of fine sedi-

ments. Most of the differences between clay minerals in properties such

as water retention, plasticity, or cohesion can be explained by the

differences in the surface areas of particles. This explains the high

swelling and high liquid limit of montmorillonite. Liquid limit being

closely related to the surface area, it is considered as a measure

of the nature of the surface as well as the area.




2.2.3 Shape


The shape of a particle is also an important factor in determining

the specific area. A spherical particle has the least surface area,

and a disc or plate shaped particle has the greatest surface area. A

sphere of 1 cm3 has an area of 4.836 cm2, while an equi-volume disc

one micron thick has 20,000 cm2 surface area. Typical plate shaped





-12-


particles may have surface areas as much as ten times as great as the

spherical particles of equal volumes.



2.2.4 Electric Charge


Substitution of one ion for another in the clay crystal lattice

and imperfections at the surface, especially at the edges, lead to

negative charges on clay particles. Cations from the pore water are

attracted to the particles (and anions repelled) to maintain electro-

neutrality. These are the exchangeable cations and their number is

the cation-exchange capacity (i.e. the amount of negative charge per

unit weight or per unit surface area) of the clay. This is usually

expressed as milliequivalents per gram (me/g) or per 100 g (me/100 g).

The force with which ions are held at the surface varies with the nature

of the charge. The amount of charge for different clay minerals is

given below:

Kaolinite: 5 to 15 me/100 g

Clay Mica and Chlorite: 20 to 40 me/100 g

Montmorillonite: 80 to 100 me/100 g

Vermiculite: 100 to 150 me/100 g

The kind and number of exchangeable cations have an important in-

fluence on the behavior of soils, e.g. monovalent cations such as

sodium increase the activity of the clay, its swelling, etc.



2.2.5 Inter-Particle Forces


The behavior of clay particles is controlled more by the surface

forces than by the gravity forces. Thus it can be shown that the





-13-


average electrochemical force exerted on one clay particle is of the

order of one million times greater than the average weight of the

particle.

The inter-particle forces are both repulsive and attractive in

nature. The most important repulsive forces generated by the electrical

charges are

i) Repulsion caused by the negatively charged particle faces.

ii) Repulsion of adsorbed positively charged cations.

iii) Osmotic pressure resulting from the high concentration of cations

near the surface of the particles in the pore water.

The attractive forces binding the particles together in clay

minerals are the following:

i) Forces due to the attraction of the mass of one clay mineral

particle and the mass of another.

ii) Inter-molecular forces resulting from the nearness of one par-

ticle to another with the overlap of fields of force of molecules

in the surface layers of adjacent particles.

iii) Electrostatic forces due to changes in the lattice resulting

from unbalanced substitution within the lattice, broken bonds

on the edges of the lattice, and the attractive force of cation

ions adsorbed on the clay-mineral surfaces.

iv) Chemical cementation between particles by various compounds.

v) Cation bonds: Forces exerted by cations attracting and trying

to neutralize negatively charged particles (Fig. 1.A).

vi) Water dipole linkage is the bonding action of adsorbed polar

molecules (Fig. 1.B). Oriented water molecules between two clay-

mineral surfaces may form a bridge of considerable strength if




-14-


+ And Indicate Electric Charge


(+ () (W+)


.--- Clay Particle


(A) Cation Bond


E:]
t+-


---- Water Dipole
E:l Clay F


Particle


(B) Water Dipole Linkage


S---o Clay Particle


-- Water Dipole
-- Cation
1?


(C) Dipole Cation Dipole


@0


a @



E G


H-


_-b Diffused
Double Layer


(D) The Clay Micelle


Fig. 1 Inter-particle forces on clay minerals and clay micelle


I- -


_1


I- -


i_ -- 1-


I -I


G a





-15-


only a few molecules thick, and of practically no strength if

more than a few molecules thick. Similarly, adsorbed polar

organic molecules could serve as a bond between clay-mineral

particles.

vii) Dipole-cation-dipole linkage (Fig. 1.C).

viii) Hydrogen bond occurs when an atom of hydrogen is strongly attracted

by two other atoms.

ix) van der Waals forces are secondary valance forces of an electro-

chemical nature. They are generated by the mutual influence of

the motion of electrons of the atoms and they are always attrac-

tive. These forces acting between all units are property of

the matter and are independent of the chemical characteristics

of water. Although other attractive forces of electrical nature

may exist, the van der Waals forces are the main cause for bond-

ing together of clay particles. An important characteristic of

these forces is that they decay very rapidly with distance and

hence the particles must come very close to each other so that

the forces can be effective.


2.2.6 Flocculation


The bonding of clay particles when they are brought together is

known as flocculation. Both collision and cohesion are essential to

flocculation. Cohesion is believed to result from the predominance of

attractive forces on the surface of clay particles. Collision of par-

ticles may be caused by the Brownian motion of the suspended particles,

by internal shear of water, and by the differential settling velocities





-16-


of the flocs. Brownian motion is the erratic movement of small sus-

pended particles caused by the thermal agitation of the suspending

medium, which enables particles to come in contact with each other.

The inter-particle forces have been described in the previous

paragraph. Whatever the origin of the surface electric charges, any

such charged particle in an ion containing water will attract ions of

opposite charges, called "counter-ions,' to compensate its own electric

charges. At the same time, the ions tend to diffuse away from the sur-

face because of their thermal activity since such a diffusion takes

place from a zone of high concentration to a zone of lower concentration

in a way analogous to the diffusion of the molecules of the air in the

atmosphere. Thus a clay particle idealized by a thin rectangular plate

will be surrounded on either side by a diffused layer of counter-ions

whose positions will be determined by the balance of the electrostatic

attraction and their thermal activity. This layer is known as a

"double layer" and it plays a dominant role in the mechanical properties

of suspended clays and clay deposits. The system of clay particle and

double layer is electrically neutral and is known as "clay micelle."

Figurel.D shows a simplified, schematic diagram of a clay micelle.

Any form of agitation, including Brownian motion,will eventually

cause two particles to approach each other sufficiently close for their

double layers to interact. This interaction causes changes in the

distribution of the cations in the double layer of both particles. The

result will be determined by the potentials of the van der Waals force

and the electric forces. Sufficiently far from the particle, the

repulsive forces may dominate, whereas closer to the surface the net

effect would be attractive which causes flocculation of particles.





-17-


It has long been observed that gentle stirring promotes floccula-

tion. This is due to the velocity gradients which are induced in the

liquid causing relative motion of the particles. Such velocity

gradient-controlled flocculation is called orthokinetic flocculation.

A simple theory of flocculation kinetics can be derived for a uniform

liquid shear field, giving a constant velocity gradient. Such constant

velocity gradients are difficult to achieve in practice; the closest

experimental form has been in the annular gap between coaxial rotating

cylinders, also known as Couette apparatus. Consequently, the theory

has to be extended to velocity gradients created in turbulent flow con-

ditions. The orthokinetic rate of flocculation has a high dependence

on initial particle size, is linearly dependent on velocity gradient,

and it is independent of temperature.

While considering Brownian motion, it is useful to consider one

particle (the collector) as stationary, and to calculate the diffusion

rate caused by Brownian movement of other particles to this collector.

Because particles become attached to the collector, and are therefore

removed from the suspension, a concentration gradient is formed radially

outwards from the collector. This diffusion-controlled flocculation is

called perikinetic flocculation. Temperature and viscosity effects are

significant under perikinetic flocculation, and the rate of flocculation

is independent of particle size. In the benthic boundary layer, ortho-

kinetic (i.e. velocity gradient-controlled) flocculation has a much

greater influence on the frequency of collision between particles than

perikinetic (i.e. diffusion-controlled) flocculation (Williams, 1980).

The effect of flocculation is formation of aggregates. Repeated

inter-particle collisions in a turbulent flow field are predominantly





-18-


due to the internal shearing of the suspending water. Aggregates formed

by fluid shearing are denser and stronger than those formed by the

Brownian motion or by differential settling velocities. The aggregates

can be dispersed by local high shearing rates and may re-aggregate later.

The settling velocities of aggregates and the probability of their

sticking to the bed are determined by the size, density, and shear

strength. Mineral particles cohering in a cluster with uniform porosity

are called primary aggregates or flocs. At lower shearing rates,

primary aggregates would collide with each other and bond to form

first-order aggregates. At still lower shearing rates, the first order

aggregates would collide and bond with each other to form weaker, less-

dense second-order aggregates, and so on. Each higher-order aggregate

would include water in the new pore volume formed, and because shear

stress can be transmitted only through inter-aggregate contacts, the

higher order aggregates are weaker. A procedure for determining the

order of aggregation has been given by Krone (1976).



2.3 Parameters Influencing the Properties of Cohesive Sediments


An attempt wasmade by the Committee on Tidal Hydraulics (1960) to

identify the soil parameters which affect the process of shoaling in

estuaries. However, the literature review contained in the report is

limited. Based on the findings of subsequent research, Paaswell (1973)

reviewed the causes and mechanism of cohesive soil erosion and identified the

following parameters used in evaluating the erosion of cohesive beds:





-19-


Character Parameter

Physical Soil type (clay mineral)
Percentage of clay ,
Liquid and plastic limits, and activity
Specific gravity
Physico-chemical Base exchange capacity
Sodium absorption ratio
Pore fluid quality
Pore fluid environment
Mechanical properties Shear strength (surface and body)
Cohesion
Thixotropy
Swelling and shrinkage properties
Conditions of environment Weathering (wet-dry)
Freezing and thawing
Prestress history
In addition to the above, Alizadeh (1974) has included the parame-

ters bulk density, water content, effective stress, time (aging),

testing methods, and soil preparation methods.

Kandiah (1974) mentioned that the erodibility of cohesive soil is

controlled by the mineralogical, chemical, and environmental factors.

The mineralogical factors include the type and amount of clay mineral

present in the soil while the chemical factors include the total salt

concentration, sodium absorption ratio, and pH of the pore fluid. The

influence of these two factors is mutually independent and they are the

"key parameters" of erosion since their effect is far more pronounced

than other factors.

Attempts have been made to correlate the critical shear stress to

various parameters affecting soil erosion. For example, Smerdon and

Beasly (1959) presented the following equation between critical shear

stress ( c) and plasticity index (PI):


Tc = 0.0017 (PI)0.84 (2.3.1)





-20-


Dunn (1959) found that not only the plasticity index but vane

shear strength (S ) needs to be included in the expression as follows:


Tc = 0.0098 + 0.00049 (Su + 180) tan (30 + 1.73 PI) (2.3.2)


Carlson and Enger (1963) suggested the following relationship:


Tc = -0.017 + 0.000181(PI) + 0.000186(v)

+ 0.00268(K) + 0.000465(LL) (2.3.3)

where

v = sample density

K = phi-skewness of the grain size distribution

LL = liquid limit

Sargunam (1973) presented the following expression related to the

composition of pore fluid C, and sodium adsorption ratio (SAR)


Tc = C1 + (C2 n log SAR) log C (2.3.4)

where

C = pore fluid concentration

C1, C2, and n = constants which vary with the type and amount

of clay minerals
For describing the influence of various parameters the following

classification appears more appropriate:

a) Hydrodynamic factors.

b) Properties of sediment.

c) Properties of bed.

d) Properties of pore fluid and eroding fluid.





-21-


a) Hydrodynamic factors (bed shear stress). These are principally

embodied in the instantaneous bed shear stress and its frequency dis-

tribution, as specified by the flow characteristics, including the

surface roughness of the bed-fluid interface. The studies reported

later in this thesis have indicated that the concentration of suspen-

sion resulting from the erosion of bed is not only a function of the

applied shear stress but also the shear stress previously acting on

the bed.

b) Properties of sediment (composition, shape, size gradation,

organic matter, cation exchange capacity, moisture content). Fine

sediments include interacting particles such as clays, as well as non-

interacting fine particles such as silt. The differences in the

physical properties of these have already been described. The clays may

be composed of different clay minerals each of which has its own physi-

cal properties such as shape, size, surface area, liquid limit, etc.

Since these properties in turn influence the erosional properties, the

basic composition of fine sediments in terms of individual clay

minerals or their mixtures, and the clays alone or mixture of clay and

silt has a considerable influence on the erosional properties.

Sediment composition is specified by the clay mineral, its weight

fraction, and the amount and type of organic matter. Recent attempts

at the University of California, Davis, have been directed toward

characterizing the composition of clays through measurements of the

dielectric constant at selected frequencies. A dielectricc dispersion

parameter" is derived from these measurements. Each clay tested seemed

to have a characteristic value of this parameter (Alizadeh, 1974;

Arulanandan et al., 1973). The dielectric constant is a measure of





-22-


the ability of clay to store electrical potential energy under the in-

fluence of an electric field. The dielectric constant for a soil

sample is defined as

Scd (2.3.5)
VA
where

c = capacitance

d = length of specimen

A = cross-sectional area

ev = dielectric constant of vacuum

= 8.85 x 10-14 farad/cm

The dielectric constant of a dry silicate mineral is 4, and of water

about 80.

Alizadeh (1974) has defined the magnitude of dielectric dispersion

(AE') as the total amount of decrease in the measured dielectric con-

stant. The dielectric dispersion depends mainly on the type and amount

of clay; the other factors such as pore fluid composition, water con-

tent, particle orientation, etc. have a secondary effect. It has been

used as a quantitative index for soil characterization. Measurements

have shown that for 10 percent Kaolinite and 21.2 percent water content,

AE0 has a magnitude 7.5, whereas for 60 percent Kaolinite and 30.4

percent water content, AE' is 18.

The shape and size of individual clay particles have an influence

on their surface area. Since the surface forces predominate in respect

to cohesive sediments, the colloidal properties such as plasticity and

adsorption of molecules are governed by the surface area. Natural

sediments usually have a wide range of particle sizes. The effect of





-23-


such heterodisperse sediments is to increase the probability of colli-

sion of primary particles. Also, natural sediments typically are not

uniform in their shape. Since Brownian motion is rotary in character,

the largest dimension is appropriate for the collision diameter,

whereas the mean diameter is applicable to the diffusion constant.

Consequently, anisodimensional particles have a relatively large colli-

sion diameter combined with a relatively large diffusion constant, thus

enhancing the collision rate. After flocculation has progressed, how-

ever, the flocs tend to be more nearly spherical, and the effect of

anisotropy on collision rate is small.

Organic matter has an effect on the properties of flocculation of

sediment and hence it affects the erosional properties of sediment.

Kandiah (1974) found that organic matter strengthens the soil aggre-

gates against slaking. Studies on the erodibility of 30 percent

illitic soil showed that the critical shear stress for erosion increased

from 1.7 N/m2 to 4.0 N/m2 when the organic content was increased from

0 to about 4 percent.

It has been mentioned earlier that the type of clay is one of the

factors determining the erosional characteristics. The Cation Exchange

Capacity is a property of the soil which can be used to indicate the

type while associating it with the corresponding critical shear stress

for erosion. Cations from the pore fluid are attracted by the nega-

tive charge on clay particles and the anions are repelled in order to

maintain electro-neutrality. The number of these exchangeable cations

is known as the cation exchange capacity, which is usually expressed

as milliequivalent per gram (me/g). The kind and number of exchangeable

cations have an important influence on the behavior of soils. For





-24-


instance, monovalent cations such as sodium increase the activity

of clay and its swelling. The predominant exchangeable ions in a soil

are usually Na Ca+, and Mg The rate of cation exchange reaction

varies with the clay mineral, the concentration of the cations, and with

the nature and concentration of the anions. In general, the reaction

for Kaolinite is most rapid, slower for Illite, and still slower for

Montmorillonite. The CEC is independent of the ionic strength of the

solution with which the clay is.in contact or the physical structure

of clay. However, it is a function of pH of the fluid in contact.

Kandiah (1974) has shown that the critical shear stress of cohesive

soil increased from 1.3 N/m2 to about 2.7 N/m2 with a change in CEC

from 2 me/100 g to 34 me/100 g. These results were with an average

sodium absorption ratio of 2.5. A decrease in critical shear stress

was observed for the same range of CEC values when the average sodium

absorption ratio was 48.0 (Fig. 4).

c) Properties of the bed (moisture content, density). Attempts

have been made to study the effect of moisture content in the bed

which can be compared with standard soil indices such as plastic limit

and liquid limit. Lutz (1934) noted that soils with high plasticity

and low plasticity had different resistance to erosion. The low plas-

ticity soil was more erosive than the high plasticity soil. Fukuda

(1978) has shown that an increase in suspension concentration can be

expected as the water content of the sediment increases, with the stress

being held constant. A very small change in the water content of the

sediment may give a large increase in the concentration of suspension. For

instance, for a 12 percent increase in water content in his tests, the

concentration of suspended sediment increased from 50 to 3000 mg/l.





-25-


Owen (1970) reported that the shear strength variation within

the bed could be correlated satisfactorily with the variation of den-

sity. The shear strength was observed to increase rapidly with density.

The method of formation of bed namely, remolded, deposited, or com-

pacted with external force, have an effect on the density structure

of the bed and hence on the erosional property of the soil.

d) Properties of the pore fluid and the eroding fluid (salt

concentration, sodium adsorption ratio, pH, temperature). In the case

of a fully saturated soil which has been equilibrated with the eroding

fluid over a sufficiently long time, the pore fluid and the eroding

fluid are expected to have the same properties. However, in case they

have different properties, osmotic pressure gradient is formed which

changes the properties of the pore fluid and this in turn may change

the erosional properties of the bed. The soil having an aggregated

structure resists erosion more effectively than a soil having a dis-

persed structure. This is because the net force between the particles

in an aggregated structure is attractive while in the latter it is

repulsive. Since the pore fluid has a substantial influence on the

structure, it influences erosion. An increase in the salt concentra-

tions of the pore fluid usually increases resistance to erosion.

Sodium Adsorption Ratio (SAR) is defined as


SAR = Na (2.3.6)
[- (Ca+ + Mg )]'1

The concentrations of individual ions are in milliequivalents per

liter.

The SAR is used as an index to characterize the pore fluid and

eroding fluid in terms of the relative strength of the Na, Ca, and





-26-


Mg ions. Fluids with the same salt concentration can have different

SAR. An increase in SAR decreases cohesion, lowers the critical shear

stress, and hence increases the erosion rate (Kandiah, 1974). Alizadeh

(1974) has presented experimental data to this effect.

Kandiah (1974) also studied the effect of pH and concluded that

"pH influences the interparticle cohesion which strongly affects

aggregation and dispersion of clay soil properties" (p. 118).

The effect of water temperature on erosion rate has been studied

by Grissinger (1966), Christensen and Das (1973), Randkivi and Hutchison

(1974), and Gularte (1978). It is observed that the temperature had a

significant effect on the strength of inter-particle cohesion and the

rate of erosion increased significantly with increasing temperature.

Kandiah (1974) showed from particle-by-particle surface erosion

of a remolded illite soil that the critical shear stress for erosion

varies as


Tc = 1.8 x 10-5 exp[4100/T] (2.3.7)


where T is the absolute temperature in the Kelvin scale. The critical

shear stress dropped from 3.6 to 0.8 N/m2 over the temperature change

from 9.50C to 420C.


2.4 Processes for Deposited Beds


The deposited beds are distinguished from the placed or remolded

beds because of the fact that they are formed by the process of settling

of the sediment in suspension. The settling may take place under

quiescent conditions or under a low shear stress which permits





-27-


deposition of the suspended material. The settled material may undergo

consolidation and form a bed. The aspects related to the settling

and consolidation which are the primary processes in formation of

deposited beds are briefly described below.


2.4.1 Settling


The effect of Brownian motion on a suspension of sediment under ap-

parent quiescent conditions is to induce inter-particle contacts. This

may result in bonding of particles through the process of flocculation

(Einstein and Krone, 1962). The mechanics and importance of floccula-

tion have already been described. Hence only the parameters which

influence settling velocity of the flocs are briefly described here.

Owen (1970) conducted detailed study of the settling velocities of

an estuary mud and noted the following conclusions:

i) Suspended sediment concentration: The median settling velocity

increases with concentration up to a value between 4 and 20 grams

per liter, depending on the salinity, at which hindered settling

begins. For a concentration higher than this, the settling

velocity reduces.

ii) Salinity: The median settling velocity increases with salinity

(except during hindered settling) up to a value between 28 and

43 grams per liter depending on the concentration. For higher

values of salinity, the settling velocity decreases. It may

however be noted here that certain clays such as Kaolinite

flocculate even in distilled water.

iii) Depth: The effect of depth of settling is fairly complex, the

settling velocity reducing to a minimum at a depth of 1 meter





-28-


and then increasing with depth to reach its terminal value at

about 2 meters. For a fixed depth of settling, there is an

absolute maximum value of settling velocity, which is attained at

a fixed salinity and concentration.

iv) Temperature: The effect of temperature is not very clear. It

is largely limited to the effect temperature changes have on the

viscosity of suspension. There appears to be a slight tendency

to increased flocculation at higher temperatures, generally

accompanied by slightly greater median settling velocities.

However, at temperatures above 150C, the settling velocities of

flocs formed in low salinity suspensions decreases.

Effect of other parameters is as follows:

v) pH: High pH contributes to dispersion, whereas low pH enhances

flocculation.

vi) Organic matter: Usually flocculation is promoted by the

organic matter.

vii) Dissolved chemicals: Only those chemicals which enter in some

way into the physico-chemical reaction with soil can probably

have an effect on flocculation and settling.


2.4.2 Consolidation


Consolidation is the term used to refer to that portion of the

compressibility of a soil that is essentially inelastic, i.e. its volume

changes under load. Since the pore water and the soil grains in a

saturated system are relatively incompressible, the volume change ob-

served under load is the result of the expulsion of water from the

interstices between soil grains.





-29-


Most soils regain only a fraction of the volume lost during con-

solidation. This results from the fact that in order to undergo volume

change, the soil particles are displaced relative to one another to

assume a more closely packed condition, and consequently greater

density. In sands these movements are not reversible, and in most

clayey soils, they are only slightly reversible. For sand, the volume

decrease is proportional to the logarithm of the pressure. For clay,

the relation is not linear although at higher pressures, it is approxi-

mately so.

In a process of continuous sedimentation in water, the soil at any

depth is being consolidated under the influence of the weight of the

soil above it. Since new material is continually being added, a gradient

tending to cause the pore water to flow upward out of the system exists

at all points within the stratum. If the material being deposited is

sand, consolidation takes place at a rapid rate since the spaces between

particles are relatively large and the water can escape readily. Fur-

thermore, the sand particles are essentially inert and are not greatly

influenced by adjacent particles, and the initial position of each grain

within the mass is relatively stable. Very little volume change can

take place in a sand deposit except in certain circumstances as a

result of outside influences such as mechanical vibration or shock.

Therefore, sands can be considered to be virtually completely consoli-

dated at all times even when the accumulation of sediment is rapid.

Consolidation of clayey deposit proceeds at a comparatively much

slower rate. The total pore space in a clay mass is large but is com-

posed of a multitude of small channels between the individual particles.

The flow of water in the channels is restricted by their small size





-30-


and also by the affinity of the particle surfaces for water, which in

effect reduces even further the dimensions of the available flow

channels. Also, unlike sand grains, clay particles, due to their

shape and the interactions of their force systems, do not tend to fall

into stable positions. As a result, the upper portion of a clay de-

posit is very porous and contains a large percentage of water. The

actual porosity at the clay surface can vary considerably, depending

upon the amount and type of clay mineral present, and on the factors

that affect the interparticle forces, but it is always large in com-

parison with that of sand.

Soil concentrations of the order of 10 to 20 percent by weight may

be expected in newly formed clayey deposits. It is likely that at about

this concentration, a continuous, interdependent network of soil par-

ticles is formed. This condition has been referred to as the "hindered

settling." It might as well serve as a useful dividing line between

the processes of sedimentation and consolidation. At lesser concen-

trations, settling occurs as the individual particles of flocs inde-

pendently move downward through water. At higher concentrations, where

the units can no longer move independently, the downward movement is

accomplished by water moving up through the soil voids.

In continually accumulating deposits, hydraulic gradients indica-

tive of incomplete consolidation are present throughout the clay layer.

If the accumulation is relatively rapid, the degree of consolidation

at any depth is slight and it will be found that the density of the

clay deposit will be virtually independent of the depth. When sedi-

mentation ceases, the thickness of the layer will continue to decrease

for some time probably for many years, until the water pressures





-31-


induced by the weight of the sediment have been dissipated (Committee

on Tidal Hydraulics, 1960).

The properties of clays in respect to settling and consolidation

are very important in connection with the studies for their depositional

and erosional properties.


2.5 Clay-Water System


Water which can be held by the clay system is grouped into two

categories, namely low-temperature water, which can be driven off by

heating to about 100C to 150C, and the OH lattice water which is lost

at temperatures above about 3000C. The nature of low-temperature water

and the factors that control its characteristics are of great impor-

tance, since they largely determine the plastic, bonding, compaction,

suspension, and other properties of clay materials, which in turn con-

trol their behavior under the given flow field.

Water, though neutral, has its oxygen and hydrogen atoms spaced

in such a manner that the center of gravity of the positive and nega-

tive electrical charges do not coincide. The resulting molecule has a

positive charge acting at one end and a negative charge acting at the

opposite end. Water molecules are thus considered polar molecules.

Because of the net positive charge of the cations, they attract negative

charges. The negative tips of water molecules are attracted and held

to the cation, which in turn is held by the clay particle due to the

negative charge on its surface. The resulting effect is that water

becomes bonded to the clay. Additional water molecules are also at-

tracted to the clay particle because of a chain-like arrangement of

negative ends to positive ends of molecules.




-32-


The term diffused double layer has already been described. With

the water and clay molecules in contact with each other, it is believed

that immediately surrounding the clay particle, there'is a thin, very

tightly held layer of water, perhaps 1 x 10-6 mm (10 A) thick, and a

second, more mobile, diffused layer extends beyond the first layer to

the limit of attraction. The molecular movement occurs continually

in both the layers. The water which is held in the diffused double

layer is frequently termed adsorbed water or oriented water to dif-

ferentiate it from normal pore water which is not oriented.

The plasticity possessed by clay soils is attributed to the water

which is attracted and held by the clay particles. Experiments per-

formed with clay using non-polar liquid in place of water do not in-

dicate plasticity and the particles act similar to those of a coarse

grained sandy soil (McCarthy, 1977).

A dispersion of particles subject only to hydrodynamic interaction

will exhibit Newtonian flow characteristics, i.e. the shear stress and

shear strain have a linear relationship no matter how small the mag-

nitude. A clay-water suspension of high concentration on the other

hand shows properties of a non-Newtonian fluid as shown in Fig. 2 which

is a plot of equilibrium shear stress (T) versus shear rate (v). Curve

A represents Newtonian behavior. Very dilute cohesive suspensions may

exhibit this flow behavior, particularly if they are weakly flocculated.

Curve D describes Bingham plastic behavior. A Bingham fluid will not

flow at all until the yield stress is exceeded. This implies that the

soil structure fails at some critical stress Ty and for stresses in

excess of this the dispersion flows in a quasi-Newtonian manner.

Curves B and C show more realistic rheograms where there is a gradual







(D) Bingham Plastic
(C) Plastic

(B) Pseudo-Plastic

'(A) Newtonian


SHEARING RATE


Fig. 2 Rheological models


(V)





-34-


reduction in the contribution of structure to apparent viscosity as the

shear rate is increased. These indicate pseudo-plastic and plastic

behavior, respectively, also known as shear thinning.' The intercept

T shown in Fig. 2 is the lower yield stress, whereas TB is known as

the upper Bingham yield stress which is a measure of the work done in

disrupting the floc structure in cohesive suspensions.

The content of water in a clay soil determines the consistency in

the remolded state. The criteria to determine the various states of

consistency are known as Atterberg Limits. At high water content, the

soil-water mixture possesses the properties of a liquid; at lesser water

content the volume of the mixture is decreased and the material exhibits

the properties of a plastic; at still lesser water content, the mixture

behaves as a semi-solid and finally as a solid. The water content

indicating the division between the liquid and plastic state is called

the liquid limit. The division between the plastic and semi-solid

state is the plastic limit. The water content at the division between

the semi-solid and the solid state is the shrinkage limit. All these

three limits are expressed as percentage of water with respect to the

weight of solids. Below the shrinkage limit, there is little or no

change in volume as water content varies. However, above the shrinkage

limit, the change in total volume of soil-water mixture is related to

the change in water content. The plasticity index is the numerical

difference between the liquid limit and the plastic limit, and indicates

the range of water content through which the soil remains plastic. It

is necessary to know both the liquid limit and the plasticity index for

a proper evaluation of the plasticity properties of soil.





-35-


The liquid limit expresses the overall effect of the inter-particle

forces within the clay mass and this soil index varies with clay

mineralogy and with the associated cation. The following data given

in Table 1 were presented by the Committee on Tidal Hydraulics (1960).



Table 1. Specific surface area and liquid limit for typical clays.


Primary Specific
Clay Associated Surface Liquid
Mineral Cation Area Limit

Montmorillonite Na 847 710

Montmorillonite H 768 490

Illite (< 2 p) H 79.8 100

Kaolinite H 13.1 53

Kaolinite (< 5 v) H 26.1 110


Many clay soils exhibit the property of rheotropy at water con-

tents above the liquid limit, and also to a lesser degree at water

contents in the plastic range. Rheotropy is the change to a more

fluid consistency on stirring or disturbance. When the disturbance

has ceased, the system reverts to its less fluid or more rigid condi-

tion. This is often called thixotropy, although the strict definition

of thixotropy is a reversible, isothermal sol-gel transformation. A

sol, by definition, has no yield value, while a gel has rigidity. The

change in clay-water systems is generally from a system with higher

yield value to one with a lower yield value. A sol may be considered

as a colloidal dispersion. This restricts sols to liquid-like behavior.





-36-


When hardening of the sol occurs, a gel is formed. This requires a

change of state from a semi-liquid substance (sol) to a semi-solid

(gel).

Rheotropy of clay soils can be measured by a vane shear or at

higher water contents by means of a viscometer.

The property of thixotropy (or rest-hardening) has been explained

either by changes in particle rearrangement and inter-particle forces,

or by changes in adsorbed water. On stirring, the particles and

fabric units are rearranged and the bonds between particles and units

are broken. Also, the structure of the adsorbed water is broken up

and the clay mass will be more susceptible to deformation under self-

weight. After deformation, the clay fabric will seek a status of

minimum energy with maximum attraction between particles and fabric

units. The adsorbed water also regains its quasi-crystalline form to

give the system sufficient rigidity to have a yield value. There are

several factors which contribute to the regaining of part or all of the

strength. These are original structure, activity of the clay minerals,

and the degree of disturbance. The activity is a characteristic

parameter of the electrochemical action of the colloids and is defined

as the ratio of plasticity index and clay fraction less than two

microns.













CHAPTER III

PREVIOUS LABORATORY STUDIES


3.1 General Review


Over the past two decades, considerable laboratory work has been

carried out on cohesive sediments. In order to get an idea about the

variety of different ways in which the research work has been carried

out, some of the topics under which the literature could be classified

are given below along with a typical reference on the same as an

illustration.

a) Sediment used:

Clay mineral alone: Kaolinite: Christensen and Das (1973)

Mixture of clay minerals: Yolo Loam: Arulanandan et al. (1975)

Mixture of clay and silt: Grundite: Gularte (1978)

Natural sediments: Brisbane Mud: Thorn and Parsons (1980)

Fernandina Mud: Yeh (1979)

San Francisco Bay Mud: Partheniades (1962)

b) Fluid used:

Salt water: Partheniades (1962)

Distilled water: Mehta and Partheniades (1979)

Fresh water: Fukuda (1978)

c) Type of bed:

Remolded: Gularte (1977)


-37-




-38-


Deposited from suspension: Yeh (1979)

Compacted: Christensen and Das (1973)

d) Characterizing indices:

Dielectric dispersion: Alizadeh (1974)

Sodium adsorption ratio: Kandiah (1974)

Cation exchange capacity: Kandiah (1974)

Chemical and electrical parameters: Arulanandan et al. (1973)

e) Basic parameters:

Bed density and salinity: Owen (1977)

Temperature: Gularte (1978)

pH: Kandiah (1974)

Pore fluid and eroding fluid: Arulanandan et al. (1975)

Water quality (pH, conductivity): Migniot (1968)

f) Microstructure studies:

Kaolin: McConnachie (1974)

Marine sediments: Bowels (1969)

g) Other studies:

Hydrodynamic aspects: Turbulent drag reduction: Gust (1976)

Colloidal dispersion: Zeichner and Schowalter

(1977)

Attempts have been made from time to time in the past to review

the information available in respect to cohesive sediments. These are

listed below in chronological order:

1960: The Committee on Tidal Hydraulics, U.S. Army Corps of Engineers

conducted literature review to study soil as a factor in shoaling

processes (Committee on Tidal Hydraulics, 1960).





-39-


1964: The present knowledge on the behavior of fine sediments in

estuaries was summarized by Partheniades (1964).

1968: Task Committee of ASCE on Erosion of Cohesive Materials prepared

a report on literature review (Task Committee, 1968).

1973: State of the art paper on causes and mechanisms of cohesive soil

erosion was presented by Paaswell (Paaswell, 1973).


3.2 Review of Literature on Erosion


Since the scope of the present study deals with erosion of Kaolinite,

a brief account of the work carried out in the past on erosion of co-

hesive sediments is given here. Effect of various parameters and pro-

cesses influencing the erosional properties of cohesive sediments has

been studied by several research workers. Important findings of these

studies have already been given in Chapter II.

The earlier experiments conducted to study the erosion of cohesive

sediments were oriented to obtain solution to a specific engineering

problem such as the model studies performed by the Tennessee Valley

Authority for the Fontana Project (1953) and the Fort Patrick Henry

Project (1960).

Most of the work to understand the basic physical processes related

to cohesive sediments has been carried out during the past two decades.

Smerdon and Beasley (1959) applied the tractive force theory to the

stability of open channels in cohesive soils. Dunn (1959) used a sub-

merged jet to determine the tractive resistance of cohesive soils and

correlated it with plasticity index. Masch et al. (1965) conducted

studies on remolded cohesive sediments using a rotating cylinder





-40-


apparatus and found that the critical shear stress was related to water

content and vane shear strength. Flume study of natural soils conducted

by Lyle and Smerdon (1965) showed that the critical tractive force

correlated to void ratio, cation exchange capacity, and plasticity

index. Grissinger (1966) found that the erosion rate of soil decreases

with increasing clay content and decreasing void ratio and temperature.

Findings of various research workers related to determining the

effect of various parameters and characterization indices on erosion

have already been described in Chapter II. Paaswell (1973) summarized

selected studies on cohesive soil erosion. The same are reproduced

from his paper along with addition of subsequent investigations. It

may be noted that the mode of formation of bed in the laboratory equip-

ment is an important factor. This can be done in the following 3 dif-

ferent ways.

a) Placed or remolded or uniform bed: Formed by mixing the sediment

thoroughly with required water content and placed evenly in the

apparatus so as to have uniform density without any external com-

paction.

b) Flow deposited or flocculated or stratified bed: Formed by allowing

a sediment suspension with high concentration under a low flow

velocity which would permit most of the material to slowly deposit

on the bed. When the flow velocity is zero, the term deposited bed

is used. Such a bed is flocculated and has density stratification

over depth.

c) Compacted bed: Sediment with low moisture content is compacted

with external pressure.







Table 2. Summary of selected studies on cohesive soil erosion.


Investigator Mode of Placement of Sample Mode of Measurement of Erodibility


Lutz (1934)


Peele (1937)

Anderson (1951)


Dunn (1959)


Smerdon and Beasley (1959)


Laflen and Beasley (1960)


Flaxman (1962)




Moore and Masch (1962)


Abdel-Rahman (1964)


Comparison of physical tests with
erosive properties of natural soils

In-place topsoils

In-place topsoils


Remolded, subjected to jet


Slightly recompacted natural soil,
top leveled

Remolded at unspecified percentage
of water, then saturated

Natural soils


Remolded and natural (trimmed) jet


Remolded in duct


Use of qualitative physicochemical
analyses

Soil loss and runoff tables

Correlation of erodibility with
shear measurements

Jet to produce erosion; visual
measures

Visual observation of bed movement


Visual correlation or erosion with
calculated inactive stress

Correlation of permeability and
unconfined compressive strength
with natural erosion (channel
measures)

Measurement of scour depth and
weight loss

Visual; measurement of erosion
depth










Table 2. Continued


Investigator Mode of Placement of Sample Mode of Measurement of Erodibility


Partheniades (1965)


Grissinger (1966)

Masch, Espey, and Moore
(1965)

Mirtskhulava (1966)

Liou (1967)


Liou (1970)

Arulanandan et al. (1973)

Christensen and Das
(1973)

Grissinger (1973)

Sargunam et al. (1973)


Alizadeh (1974)


Remolded natural deposited (salt
water) in duct

Remolded in channel

Unspecified but trimmed as hollow
cylinder

Remolded in flume

Remolded in flume


Remolded in flume

Molded in ring

Remolded in tube


Natural samples remolded in channel

Remolded (compacted) in rotating
cylinder test apparatus

Remolded (compacted) in rotating
cylinder test apparatus


Measurement of suspended sediment
concentrating with time

Rate of erosion by weighing

Weight loss versus rotating shear;
visual correlated with shear

Weight of floc loss

Point-gauge measurement of erosion
depth


Weight comparison

Weight comparison


Rate of erosion by weighing

Weight loss of sample


Weight loss of sample







Table 2. Continued


Investigator Mode of Placement of Sample Mode of Measurement of Erodibility


Kandiah (1974)


Raudkivi and Hutchison
(1974)

Thorn and Parsons (July 1977)




Owen (Nov. 1977)


Gularte et al. (1977)


Gularte (1978)


Remolded (compacted) in a rotating
cylinder test apparatus

Remolded in a recirculating refri-
gerated water tunnel

Deposited in a flume




Deposited in a flume


Remolded in a
gerated water


Remolded in a
gerated water


recirculating refri-
tunnel


recirculating refri-
tunnel


Weight loss of sample


Weight loss of sample before and
after test

Withdrawal of samples, filtering
and weighing or use of photo-
absorptiometer to determine sus-
pension concentration

Withdrawal of samples, filtering
and weighing or use of photo-
absorptiometer to determine sus-
pension concentration

Measurement of suspension con-
centration by using laser-photocell
system

Measurement of suspension con-
centration by using laser-photocell
system


Fukuda (1978)


Deposited Measurement of suspension con-
centration by using laser-photocell
system











Table 2. Continued


Investigator Mode of Placement of Sample Mode of Measurement of Erodibility


Lee (1979) Deposited Measurement of suspension concen-
tration by filtering and weighing

Mehta and Partheniades Deposited Measurement of suspension concen-
(1979) tration by filtering and weighing

Yeh (1979) Deposited Measurement of suspension concen-
tration by filtering and weighing

Thorn and Parsons (1980) Deposited Measurement of suspension concen-
tration by using photo-absorptio-
meter





-45-


Types (a) and (b) mentioned above are usually used in flumes,

whereas type (c) is adopted in rotating cylinder type apparatus.


3.3 Review of Literature Pertinent to the Present Study


It may be noticed from 3.2 above that the literature even on the

erosion aspects of fine sediments is quite extensive. Comparative

study of the results obtained by various research workers may pose

some problem due to the experimental technique involved in these studies.

Details of some of these are given below.

a) Type and size of apparatus used:

Christensen and Das (1973): Rotating circular cylinder: 2.5 cm

dia, 10 cm long

Gularte (1978): Water tunnel: 5.5 m long, 2.0 m wide, 1.5 m high

Yeh (1979): Circular flume: 20 cm wide, 1.5 m centerline dia

Krone (1962): Straight steel flume: 0.9 m wide, 30.5 m long

Partheniades (1962): Straight steel flume: 0.3 m wide, 0.45 m

deep, 18 m long

Owen (1977): Straight flume: 0.3 m wide, 0.2 m deep, 17.6 m long

Thorn and Parsons (1980): Straight flume: 0.3 m wide, 0.2 m

deep, 17.6 m long

b) Method of reproducing shear stress:

Rotating the apparatus in the case of circular cylinders.

Flow of fluid in the case of straight flumes and water tunnels.

Rotation of ring alone or both ring and channel in the case of

circular flumes.




-46-


c) Measurement of erosion:

By photo-electric cell, filtration, and weighing weight loss of

sample before and after test.

Although the importance of several parameters in influencing the

erosion of cohesive sediments has now been established, data in respect

to these are not available for each study. The variation in respect

to fluid used, sediment used, and type of bed have already been men-

tioned under paragraph 3.1. Also there exists considerable variation

in duration of test from 1 minute (Espey, 1963) to 500 hours (Krone,

1962). Sampling time for measurement of concentration is substantially

different from one test to another. Hence, some of the observations

made at small intervals of concentration-time history are not avail-

able in respect to studies where observation of concentration was made

only at long time intervals. Visual observations of erosion permitted

by the transparency of apparatus help in a more realistic interpreta-

tion of data than in the case of an opaque apparatus where erosional

characteristics are indirectly inferred from the observations available.

It is therefore necessary to exercise caution while comparing the

results of various studies.

In view of the implications mentioned above, results of only those

studies where the size of the apparatus was comparable in order of

magnitude, the bed was of a deposited type, and the emphasis was on

the bed shear stress and the bed density are primarily considered here.

Partheniades (1962) conducted erosion tests on San Fransisco Bay

mud using a straight flume. He tested two types of beds, viz. i) placed

bed at natural density and water content and ii) flow deposited bed.

The shear strength of the flow deposited bed was 1/136 to 1/14th of the





-47-


strength of the placed bed. The entire experimental work consisting

of 32 runs was divided into three series.

Series I: Tests on placed bed by changing the flow'velocity by small

positive and negative increment. Results are given in

Fig. 3 (runs 1 to 13 only).

Series II: The same bed was used as for series I except that it was

remolded after the upper surfacing was removed. Results

are given in Fig. 4.

Series III: Tests on flow deposited bed. Results of these tests are

given in Fig. 5.

Comparison of the test results of series I and series III are given

in Fig. 6 in terms of rate of erosion.

Important conclusions drawn by Partheniades (1962) from his erosion

tests were as follows:

i) The rates of erosion were independent of concentration.

ii) The erosion rates for the flocculated bed changed abruptly several

times. This change was proven to be caused by changes of the

bed properties.

iii) The eroded surface did not cause any measurable increase of the

frictional resistance of the bed.

iv) The minimum shear stress to start erosion was about 0.05 N/m2

for both the placed and the flocculated bed, although they had

different densities.

v) Erosion rates for both the beds were of the same order of

magnitude.

vi) The overall resistance to erosion of a cohesive bed is inde-

pendent of the macroscopic shear strength of the bed.
















E 12-

z
0 -
I- 10-5 -
I-

u 8-4 O
z
o w
o >

z 6-3 o
u .J CONCENTRATION
S uJ ------ AVERAGE FLOW VELOCITY--
o w .

o w




TIME AFTER START OF SERIES I hrs
Fig. 3 Con tr n er run ruin in run of rup n bed: Parthen run(1
Expt. Series I 13
200 400 600 800 1000 1200 1400
TIME AFTER START OF SERIES I hrs




Fig. 3 Concentration versus time plot obtained in erosion of placed bed: Partheniades (1962),
Expt. Series I





-49-


E -
10
z
o uw
u



0 < -
r- 4
8 6- 0
w






S2- rn run run CONCENTRATION
w _
S------ VELOCITY

00
0 200 400 600 800
TIME AFTER STARTOF SERIES -I,hrs







E



8 -
w*
6-



z
( ) 4
c?.



C )







run 22

0 20 40 60 80
TIME AFTER START OF RUN, hrs
Fig. 4 Concentration versus time plot obtained in erosion of remolded
bed: Partheniades (1962), Expt. Series II




-50-


Concentration versus
Partheniades (1962),.


200 400 600 800
TIME AFTER START OF SERIES-M, hrs
time plot obtained in erosion of deposited bed:
Expt. Series III


O 002 0.04 006
AVERAGE BOTTOM SHEAR STRESS (bs/fl2)
Fig. 6 Relationship between rate of erosion and average bed shear stress:
Partheniades (1962)


Fig. 5





-51-


vii) The erosion rates strongly depend on the average shear stress.

For both dense beds, the erosion rates increase very rapidly for

shear stress greater than 0.478 N/m2 for series I and 1.34 N/m2

for series II.

viii) The observed independence of erosion rates from the macroscopic

shear strength of the clay and the fact that clay gets eroded

at shear stresses which are infinitesimal compared to its

strength suggest that the mechanism of failure of clay particles

by surface erosion is basically different than the mechanism of

failure of clay particles in the interior of the clay mass, when

subjected to shear stresses.

Krone (1962) conducted studies on San Fransisco Bay mud in order

to relate transport and deposition processes to properties of the

sediment. Erosion tests were carried out on flow deposited beds. The

concentration-time data of erosion tests plotted on log-log coordinates

are given in Fig. 7. Concentration as a function of bed shear stress is

given in Fig. 8. Results of a 500 hour long erosion test are given

in Fig. 9. An arithmetic coordinate plot of these data would pro-

duce a curve with a steadily decreasing slope, suggesting an approach

to an equilibrium or steady state concentration. However, the con-

tinuing straight line plot on log-log coordinates as shown in the

figure discourages this suggestion. Krone (1962) found that "the

log-log erosion curves and their slopes were difficult to explain." He

presented a qualitative hypothesis based on interchange between sus-

pended flocs and the bed and on the dependence of erosion on time and

weakly, if at all, on shear (p. 86).





-52-


I10,". I 1 1I .1 I I I I I I I ---- -- I /


..l



~.\,


oo0
TIME AFTER VELOCITY INCREASE, rin


loo0


10000


Fig. 7 Concentration versus time plot: Krone (1962)


SHEAR ON SEDIMENT BED Ir,), dynes/sq nc


Fig. 8 Concentration as a function of bed shear stress: Krone (1962)


BED NO. 7


S, 20 0
-. 0


D07


0 0 v 0.34


*


' '


" '"


L






O L ~ .. ...." .I I. | ''1.I ..'---- -- ,.



t '

Z
W



S10
I-
0

0



SC 316 MARE ISLAND STRAIT SEDIMENT U-
St VELOCITY, 1.14 ft/sec
A)
j I
o : OPTICAL DENSITY

0 SUSPENDED SOLIDS
w


a1.



0.1
0.01 0.1 1.0 10 100 1000
(
/ TIME AFTER VELOCITY CHANGE, hr


Fig. 9 Results of 1 500 hour long erosion test: Krone (1962)






I.-- ----- -- '' ,- tJI-- .





-54-


Lee (1979) conducted studies on resuspension and deposition of

Lake Erie sediments, using a circular channel apparatus with an

annular ring similar to the apparatus used for the present studies.

However, in the case of apparatus used by Lee, the shear stress was

produced by rotating the ring alone. The results of concentration-

time curves for series I and II of tests as presented by him are given

in Figs. 10 and 11. The same data have been re-plotted in the form of

concentration under a time-variant bed shear stress (Figs. 12 and 13).

Lee found that the entrainment rate was a strong function of shear stress,

water content, and mineralogy.

Concentration-time data obtained by Yeh (1979) for different

values of bed shear stress are given in Fig. 14, and concentration as a

function of bed shear stress given in Fig. 15.


3.4 Shear Strength of Clay


The classical Coulomb's equation for shear strength of soils is

s = c + p tan ( (3.4.1)

where

s = shear strength

c = cohesion

: = angle of internal friction

p = pressure normal to the failure plane

Since c and are found to depend on the loading rate and drainage

condition, the modified Coulomb's equation is given as follows:

s = ce + o tan e (3.4.2)


where






-55-


-s
SI
00JO
U



100






Fig. 10 Concentration
Lee (1979)


scole
90



70



50-

6 owl


run 3 Tw=6.0




-*- --- .--- run 2 T,=4.3




SERIES I







- --o run I Tw=3.2 dyne/cm2


versus time plots for Series I obtained by


100 200
/ (mi;n


Fig. 11 Concentration versus time plot for Series II obtained by
Lee (1979)









1.0 I I

Reference Lee (1979) Series I 0.60
E

z
0
Bed Shear Stress in N/m2 0.43


z
U
z
S0.5- T AT (AT)ex
0.32
z 0.11 0.34
o 0.43
(n 0.17 0.39
z -0.60

D 0.32




0 I I I i I I -I I
0 2 4 6 8
TIME (hrs)

Fig. 12 Lee's (1979) data re-plotted to indicate variation of suspension concentration as a
function of time and bed shear stress




































2 4 6 8 10 12 14
TIME (hrs)


Fig. 13 Lee's data re-plotted to indicate variation of suspension concentration as a
time and bed shear stress


function of


I, I;. r


0 U
0




-58-


- A LV.J I 11/ItI

C'A
z 8
0




S6
z
0



4





2-





0I- IL
0 40 80 120 160 200

TIME (Hours)

Fig. 14 Concentration versus time data obtained by Yeh (1979) for
erosion of kaolinite





-59-


KD = Kaolinite in Distilled Water




z
0
S20-
cr
z
LuJ
C-)
Z
U
O
0





10

00










OO 0.2 0.4

Tb BED SHEAR STRESS (N/m2)

Fig. 15 Concentration as a function of bed shear stress obtained by
Yeh (1979)





-60-


ce = true cohesion, being only a function of the void ratio

of the material (Fig. 16(a))

(e = true angle of internal friction, practically independent
of the void ratio

a = the effective pressure normal to shear plane

For normally consolidated clays, the following expression is

used:

s = Pc tan d (3.4.3)

where

P = consolidation pressure for 100 percent consolidation

6d = angle of drained shear resistance
The magnitude of Pd can be much greater than
the increase of cohesion with increase of consolidation pressure (Fig.

16(a)).

The shearing strength that a clay deposit possesses is related to

the type of clay mineral and the water content but the more important

factor is the stress history, that is, the effective stress or con-

solidation pressure to which the soil has been subjected previously

(McCarthy, 1977). The pore water drainage occurring due to shearing

deformation results in change of shear strength of clay. Three condi-

tions could be considered to be related to the shear strength of soil,

viz. i) unconsolidated-undrained (U-U). (Both time and drainage

which are necessary for consolidation are not permitted.)

ii) consolidated-undrained (C-U). (Consolidation permitted, but

no drainage or volume change permitted during shearing.)

iii) consolidated-drained (C-D). (Drainage and volume change are

permitted.)





-61-


Pe =True Angle of Internal Friction
IPd = Angle of Drained Shear


Resist


(a)


ance

RPfiro nr'. :


Partheniades (1962)


EFFECTIVE NORMAL STRESS


(b)


Reference:
McCarthy (1977)


NORMAL STRESS 0-


C- D = Consolidated, Drained Soil
C-U = Consolidated, Undrained Soil
U-U = Unconsolidated, Undrained Soil


(c)


Reference:
McCarthy (1977)


% WATER CONTENT
-J




Fig. 16 Schematic diagrams showing shear strength of cohesive soil
related to other parameters


~1-

Ce

T


m m I


1- 01


\3'





-62-


A qualitative comparison of shear strength results for these conditions

is shown in Fig. 16(b). Shearing of U-U type is completed relatively

quickly because the prevention of volume change results in development

of excess pore pressures and consequent reduction of the shear strength.

Shearing of C-D type takes place very slowly since both drainage and

the volume change are permitted during shearing.

The shear strength of clay is essentially composed of two com-

ponents: i) physical component due to frictional resistance and inter-

locking between particles, and ii) physico-chemical components due to

the inter-particle attractive and repulsive forces.

The shear strength of clay soil improves with consolidation, pro-

vided that time is available for permitting the necessary pore water

drainage to take place. In effect, consolidation results in decreasing

the water content of the clay with a subsequent increase in shear

strength (Fig. 16(c);McCarthy, 1977, p. 234).


3.5 Shear Strength and Bed Density of Clay


Attempts have been made by several research workers to measure

the bed density and shear strength of clays and to establish correla-

tions between the two parameters.

Ariathurai and Kandiah (1979) have developed an electrical method

to measure in situ sediment densities. Dayal et al. (1980) have developed

a method for obtaining in situ soil strength by use of low velocity

projectile penetration technique. Gularte (1978) measured shear

strength with a fall cone device. He also used a modified viscometer

for this purpose.





-63-


Krone (1962) measured shear strength of the bed by using a screen

penetrometer for consolidation time of 8, 24, 46, 72, 97, 120, 146, 168,

240, and 312 hours, and also measured densities. He'concluded that the

ultimate density appears to be independent of total depth, i.e. con-

solidation occurs independently of the weight of material above a

consolidating layer.

Partheniades (1962) used a conventional vane shear test apparatus

as well as a simple penetration test device for measurement of the shear

strength of soil in connection with the erosion tests conducted on a

flocculated bed and arrived at the following important conclusion

(p. 108): "The observed independence of erosion rates from the macro-

scopic shear strength of the clay and the fact that clay gets eroded

at shear stresses which are infinitesimal compared to its strength

suggests that the mechanism of failure of clay particles by surface

erosion is basically different than the mechanism of failure of clay

particles in the interior of the clay mass, when subjected to shear

stresses." He also observed that the erosion resistance of the floc-

culated bed seemed to increase with depth, and attributed this to the

heterogeneity of the bed which was deposited from a suspension of high

initial concentration containing a wide range of particles from clay

size to fine sand.

Owen (1970) measured the shear strength of the surface layers of

the bed by using a Brookfield viscometer. After measuring the shear

strength of the top layer, that portion of the bed was allowed to spill

slowly. The shear strength of the next layer thus exposed was again

determined by using the viscometer. Samples of the bed were taken

simultaneously and a correlation of shear strength with bed density was





-64-


established (Fig. 17). The tests were carried out in a perspex settling

column 10 meters high and 99 mm internal diameter. The variation of

the density at various depths for different consolidation times is

given in Fig. 18. He concluded that the shear strength variation with-

in the bed could be correlated satisfactorily with the variation of

density and that the shear strength increased rapidly with density.

It has been mentioned earlier that consolidation results in de-

creasing the water content of the clay with a subsequent increase in

shear strength (McCarthy, 1977, p. 234). However, the following

interesting observations have been made regarding the erosion rate as

a function of moisture content:

i) Partheniades (1962, p. 54) noted that "in spite of the lower

overall strength of the bed and its higher water content, the

erosion rates of series II were lower than the corresponding

rates of series I."

ii) Christensen and Das (1973, p. 13) noted the following: "It is

generally assumed that under similar conditions, the rate of

erosion will decrease with increasing density. However the evi-

dence in previous studies has not been conclusive. For this

phase of the laboratory investigation, saturated soil samples

were prepared at varying densities and moisture content and sub-

jected to a constant hydraulic tractive stress. Because the soil

samples were saturated, the density decreases with increasing

moisture content. The duration of the test and the temperature

of the water were kept constant for each type of soil. The

laboratory test results exhibit a sharp decrease in erosion with

increasing moisture content."





-65-


CONCEN-
10 0 ___TRATION SALINfIT7Y fEr
17 mg/1 g/1 m
90 0 16 290 32 9 10 06
15 520 17 0 9-39
SO 17 475 17 8 6 96
i0a 17280 16-7 4 64
7.0 _____ 6 705 2 7 9 73
7-0
SI 7 2d 4 6 9 74
0 0 4 392 a 8 9-76
6.0 10 272 16 8 0 02
a 6 666 33 3 9 72
+_ 6 10 6 974
5-0


4.0 _



3.0 x -




2 -5
SHEAR 2-0
STRENGTH x
N/m2







1.0
09 -





06


0-5 -


0. I 4
60 90 100 150 200 250 300 350 400
DENSITY g/1


Fig. 17 Relationship between shear strength and bed density
observed by Owen (1970)













(A) AFTER 250 MINUTES (B) AFTER 500 MINUTES
10 -I- 1-0-
-0 \
09 09-... -- 09 ..
08 -- ----- ---- ----- 08- -
07 ---- -- ---- --- 0 -- --

06 --- -- 06 .-
0 5 _- -- --- -- -F g.'l


03 ------- ----- 3 SUS PEND 0
2 -- -- -- -- 02 CONCENIO
01 --- --- 01 SLINIIY:
S 173 g/1
o 0 020 06 08 1.0 I I 14 16 1820 22 24 26 2.8 30 0 0-2 04 06 08 10 12 I1 16 18 20 22 2i 26 28 30

100



08 I -- 7


06 ---- ----- 0 1--06
07 -- -~- -- 07-----------------------6 4
o 2 03 --



04 --- -- 0 4- -



0 -- ---- -- ------ ----- o0 ------_-_-_-
0.1 _IF 10
0 0
0 02 04 0608 10 12 1 1 1.6 1 8 20 22 24 26 2-8 30 0 02 04 06 08 10 12 1 16 18 20 22 2, 26 28 30
Wp DENSITY/ EAN DENSITY
CALCULATED DENSITY PROFILES FOR VARIOUS BED THICKNESSES


Fig. 18 Bed density profiles: Owen (1970)





-67-


iii) Owen (Nov. 1977, p. 11) conducted studies on erosion of Avonmouth

mud and concluded the following: "In terms of mean shear stress

the onset of continuous erosion is almost simultaneous for mud

beds of different density, but the rate of erosion is greater for

mud beds of lower density."

iv) Thorn and Parsons (July 1977, p. 8) studied properties of Grange-

mouth mud and made the following observation: "There does not

seem to be any strong relationship between bed shear stress and

surface density, although further tests would be needed to estab-

lish this with confidence. This result is rather surprising as

it would seem likely that the thicker or denser the mud the more

resistant it should be to erosion. The surface density at

equilibrium was divided by mean bed density to give a relative

density but this did not give any stronger relationship with bed

shear stress. This is an interesting result because an earlier

investigation of Avonmouth mud showed that both equilibrium sur-

face density and relative equilibrium surface density were

linearly related to bed shear stress."

v) Arulanandan et al. (1980) studied the effect of changing the

density structure of bed by remolding the soil and found that

remolding generally decreased both the critical shear stress and

the rate of change of erosion rate. They also found that the

salt concentration of eroding fluid influenced the erosion of

remolded soil samples. A decrease in salt concentration of eroding

fluid decreased the critical shear stress and increased the rate

of change of erosion rate.













CHAPTER IV

PRESENT INVESTIGATION


4.1 Objective


The parameters and processes influencing the behavior of fine sedi-

ments in contact with water have been described in Chapter II. Also,

the results of important investigations carried out to study the ero-

sional properties of fine sediments have been presented in Chapter III.

It is clear from the presented information that a range of physical and

chemical parameters are necessary for characterizing the properties of

the sediment bed as well as the properties of the eroding and the pore

fluids. When a given sediment is equilibrated with the eroding fluid

over a sufficiently long time, the pore fluid and the eroding fluid

have the same properties. The erosion process is then predominantly

governed by the following parameters:

i) The structure of the bed in terms of its floc shear strength varia-

tion over the depth, which is a function of the type of bed, viz.

placed, deposited, or compacted.

ii) The bed shear stress Te which causes erosion when it has a magni-

tude greater than the critical shear stress for erosion.

The moisture content is an important parameter in the case of the

placed bed and the compacted bed. In the case of the deposited bed,

which is of interest in this study, the process of bed formation is

important, involving the following parameters (Fig. 19):


-68-










Deposition
ITm


H--Step I
Mixing









r- Tm --


Step I and Step I1
=Pre-erosion stress history
STm= Bed Shear Stress for Initial Mixing

Td = Bed Shear Stress for Deposition and
Consolidation of Sediment

Tel Te2, = Bed Shear Stress for Erosion


Step U
and Consolidation


-Step III ----
Erosion


+Ts _+TS H_


etc.


Tm = Duration of Initial Mixing
Td = Duration of Deposition and
Consolidation
Ts = Duration of Time Step


Fig. 19 Definition sketch for notations used to describe experimental conditions


Te3

Te I
Td I




-70-


a) The process of bed formation in the case of deposited bed starts

with an initial concentration of suspension, Co. Under the labora-

tory conditions, the sediment and the eroding fluid are mixed under

a comparatively high shear stress in order to obtain a suspension

with C as its uniform concentration throughout the depth of fluid.

b) The shear stress Tm under which the initial mixing of the sediment

and the eroding fluid are carried out.

c) The duration of mixing, T With a sufficiently long duration of

Tm, the maximum size of the flocs in suspension is controlled by

the balance between the local shear stress and the floc shear

strength.

d) The bed shear stress Td which is sufficiently small in its magnitude

so as to permit deposition of most of the material in suspension.

e) Duration of the total time for deposition plus bed consolidation,

Td, which influences the density of the bed.

If Co is kept constant, the erosion of bed will depend upon the

following two important processes.

Formation of bed: influenced by Tm, Tm, Td, and Td'

Erosion of bed: influenced by Te which may vary in its magnitude and

duration.

Attempts made by previous research workers to directly measure the

shear strength of the bed or to correlate it to the bed density have

not been satisfactory. The overall objective of the present study was

to develop a laboratory test procedure which would enable the deter-

mination of the variation of the shear strength of a deposited bed

over the depth. This was accomplished by increasing the applied bed

shear stress in small increments of selected short time periods.





-71-


Different types of bed structures were formed by using different com-

binations of Tm, Tm, Td, and Td. Concentration of suspended sediment

resulting from the different values of Te was measured as a function of

time.

The term resuspensionn" is usually used in the case of erosion of

a flow deposited bed. Although all the experiments reported under the

present study were for the deposited beds, the terms erosion and resus-

pension are considered to be synonymous.


4.2 Material


Commercially available Kaolinite was used as sediment in the studies.

Size gradation curve for the material is given in Fig. 20. The median

diameter was 1.4 microns. Ninety-five percent of the material was within

the size range of 1 to 7 microns. Seventy-four percent of the material

was finer than 2 microns. The maximum size was 15 microns. This size

distribution was obtained by using "Sedigraph" Particle Size Analyzer.

Before using Kaolinite for conducting tests, it was kept submerged

under the eroding fluid for a period of three months for the purpose of

equilibration.

The pore fluid and the eroding fluid was identical in these studies.

The fluid was prepared by dissolving commercial salt in tap water and

was adjusted to have a concentration of 35 parts per 1000 by weight.

The pH of the eroding fluid was 7.6.












100






z
w

w
<0 50




w
i-"




50














100 50 10 5 I 0.4
EQUIVALENT SPHERICAL DIAMETER, LLm
Fiq. 20 Si7p qradation of Kaolinite tisd for thp experiments
EQUIVALENT SPHERICAL DIAMETER, JLnm
Fiq,. 2(? Si7p qrarlation of Kaolinite nlspd for the experiments





-73-


4.3 Apparatus for Erosion Tests


A system of rotating circular channel and ring was used for

conducting experiments. The annular channel was 20 cm wide, 46 cm deep,

and had a mean radius of 76 cm. The channel was made of 9.5 mm thick

fiberglass. Four windows with a transparent plexiglass were provided

on the channel to permit visual observations. The channel was supported

on a rigid steel frame. An annular ring made of 6 mm thick plexiglass,

having the same mean radius as the channel, was provided within the

channel. The width of ring was smaller by 6 mm than the width of

channel. The ring could be positioned at any required height within

the channel and it could be freely rotated while in contact with the

water surface. Taps were provided on the vertical outer wall of the

channel for obtaining samples of suspension from the channel. Details

of the apparatus assembly are given by Mehta (1973).

Accessary equipment consisted of Millipore Filtering Apparatus,

an oven, and a Mettler balance having 0.05 mg precision. For deter-

mining the concentration of sediment in suspension, the following

procedure was adopted:

i) Obtain a sample of the eroding fluid having sediment in suspension

through the tap provided on the channel and collect it in a

sampling bottle.

ii) Measure volume of the sample.

iii) Filter the sample through pre-weighed Millipore filter paper

discs with 0.45 micron pore diameter.

iv) Wash the salt using distilled water.

v) Dry the filter paper discs in the oven at 600C.





-74-


vi) Weigh the filter papers containing sediment.

vii) Obtain concentration by calculating the weight of sediment from

the difference in the weight of the filter paper with and without

the sediment, and dividing it by the volume of the sample.

A mercury thermometer was used to measure water temperature.

Photographs of the apparatus and accessory equipment are given

in Figs. 21 to 28.

The rotating channel facility was previously calibrated for measure-

ment of bed shear stress, details of which are given by Mehta (1973).

The required bed shear stress could be attained by adjusting the speeds

of rotation of the ring and the channel. Calibration curves used for

this purpose are given in Figs. 29 and 30. The fing and the channel are

rotated in directions opposite to each other in order to minimize the

effects of the radial secondary currents (Mehta, 1973).



4.4 Experimental Procedure


The experimental procedure consisted of the following three

parts:

i) Formation of bed in the rotating channel: Kaolinite equili-

brated with the eroding fluid for a period of 3 months was put in the

channel. The quantities of Kaolinite and the eroding fluid (which was

saline water with 35 parts per 1000 concentration) were adjusted in

order to have a sediment suspension of the order of 40 parts per 1000

concentration by weight when fully mixed (C ). The bed was formed in

the channel by initial mixing and allowing the sediment to deposit on

the channel bottom. Figure 19 schematically indicates the procedure




-75-


Fig. 21 The rotating channel facility


Fig. 22 Close view of the annular channel and the ring
Fig. 22 Close view of the annular channel and the ring




-76-


Fig. 23 The motor controllers
Fig. 23 The motor controllers


,kI



*-- .i- ,a $ '
..o'-. o?


Fig. 24 The electric


motors for the channel and the ring




-77-


Fig. 25 Millipore filter apparatus assembly


Fig. 26 Device for measurement of bed density





-78-


c ~-I


MIU











Fig. 27 Equipment for determining concentration


of sediment suspensions


Fig. 28 Sampling bottles







RING CONTROLLER


15 20


30


35


40


RING SPEED (RPM)


Fig. 29 Operational speeds and controller meter readings for
bed shear stresses


ring and channel at different


30 2

2


2
m
r

5
0
o

r
r
m
15

m
-3r




5 0
z
oZ


METER READING



















30- Ring
Sco



- 20 -




10-
I0




5 10 15
REVOLUTIONS PER MINUTE (rpm)

Fiq. 30 Correlation hbtween r.p.m. and mnter reading for the channel and the ring





-81-


followed and the notation used. Initial mixing was carried out at a

shear stress Tm over a mixing duration of Tm. The bed shear stress was

then reduced to Td to permit settlement of the suspended sediment and

form a bed. The time for settling plus consolidation is indicated by

Td in the sketch.

All the tests reported in this study were conducted with a total

depth of 30.5 cm in the channel which was kept constant. The quantity

of individual sample withdrawn from the rotating channel was of the

order 20 c.c. out of a total volume of 300 liters in the channel. With

a suspension concentration of say 2 percent, the amount of sediment

withdrawn at each sample was about 0.4 grams out of a total of 12 kg

of sediment in the channel. The quantity of water and sediment with-

drawn from the channel were small enough not to have any measurable

effect on the fundamental processes taking place during the experiment.

A large quantity of the eroding fluid having properties identical to

that in the channel was kept in stock. This was used to replenish the

eroding fluid by adding small quantities from time to time over the

duration of experiment in order to keep a constant depth. Sediment was

replenished at the mixing stage of the next experiment. The sediment

and the eroding fluid was the same throughout the study. All the tests

were conducted on the deposited bed. No remolding or compaction with

external force was carried out.

ii) Erosion of bed: The bed shear stress (re) varying in its

duration and magnitude was applied by rotating the ring and the channel

in accordance with the calibration curves. The time-step function of

bed shear stress is shown schematically in Fig. 19. For any one experi-

ment, the duration of time step (Ts) was kept constant (such as 30 min,





-82-


60 min, 90 min) and only the magnitude of shear stress was varied.

The values of re were obtained by discretization procedure of a linearly

increasing shear stress or the one equivalent to a sihusoidal velocity

variation, etc. If T and T are the two consecutive magnitudes of
e e
bed shear stress, then excess shear stress is given by


AT = T (4.4.1)
e2 e

The normalized excess shear stress was defined as

T -T
e e
ex T


For example, (Ar)ex = 0.2 represents magnitude of T which is 20 percent
ex e2
greater than the magnitude of T and so forth. Different values of
e1
AT as well as (AT)ex were selected for variation of bed shear stress.

iii) Data analysis: Data collection consisted of obtaining samples

of suspension at pre-determined time intervals after every change of

the bed shear stress. The sampling time used was 1, 2, 3, 5, 10, 15,

20, and 30 minutes in the case of Ts = 30 min. For Ts of longer dura-

tion such as 60 and 90 minutes, additional samples were taken at every

10 minutes after the first 8 samples were collected in 30 minutes.

In order to study the variation of suspension concentration over the

water depth, samples were taken from two locations, viz. Tap A located

125 mm above the channel bottom and Tap B located 225 mm above the

channel bottom.

The concentration of sediment in suspension for each sample was

determined by following the procedure described earlier under section

4.3. The basic data consisted of plotting of a concentration versus





-83-


time graph for each experiment. Further analysis of these data was

used to study the variation in suspension concentration as a function

of bed shear stress and for computation of erosion rates.

Although the room housing the rotating channel assembly was air-

conditioned, facilities to maintain a constant temperature of water in

the channel were not available. Hence, typically a change in the water

temperature of the order of 2 to 30C took place over the duration of

the experiment.


4.5 Apparatus for Measurements of Bed Density

During the course of the present study, two different apparatus

were developed for the following measurements:

(a) Measurement of bed density for sediment deposited under

quiescent conditions (Td = 0). The apparatus developed for this pur-

pose consisted of a 30 cm high, 15 cm dia. polyvinyl chloride cylinder

provided with a bottom plate. Ten plastic tubes of various heights

ranging from 0.95 to 6.35 cm, all having a 0.95 cm inner diameter,

were glued to the bottom plate (Fig. 31). The cylinder was made in

two pieces, the bottom cylinder being 7.5 cm high (photograph in Fig.

26). After placing the 22.5 cm tall piece of cylinder on the bottom

cylinder, the circumferencial joint was sealed with a tape to make it

water-tight. The cylinder was then filled with a sediment suspension

of known concentration. The sediment was allowed to deposit under

quiescent conditions (rd = 0) for the required consolidation time

(Td = 24 hrs, 40 hrs, etc.). The supernatant water was siphoned out,

and the top cylinder was removed after peeling the tape off. All the




-84-


I--Top Cylinder 15cm dia.2 2 I



Plastic Tubes of various heights,
0.95 cm dia. glued to the
bottom plate
Bottom
Cylinder
15cmdia.-
In fl II 1


Bottom Plate


SKETCH OF APPARATUS I

-2 cm dia plastic tube

-15 cm dia. plexiglass cylinder
2.5 cm dia metal tube

-- Annular space for mixture of alcohol
and dry ice


Porcelein
Dish -


Piston with Screw Rod


SKETCH OF APPARATUS TI


Fig. 31 Apparatus developed for measurement of density as a function
of depth for deposited beds


5 cm
225 cm


1


T
15cm

_L





-85-


sediment outside of the tubes was removed. Entire sediment from each

tube was taken out in porcelain dishes by using a hypodermic syringe

with repeated washing by small quantities of distilled water. The weight

of the sediment was determined after evaporating water in an oven at

500C temperature. From the height and diameter, the volume of sedi-

ment in each tube was calculated and by knowing the weight, the density

of sediment in each tube was calculated. Further calculations were

made as follows:

Let L1 and L2 be the heights of two adjacent tubes 1 and 2 with a small

change in heights (of the order of 0.3 cm).

Let p, and P2 be the densities of sediment in each tube, calculated as

above.

Let V1 and V2 be the volumes of sediment in each tube.

Let L2 < L1 and hence V2 < V1.

It was assumed that the bottom sediment of height L2 in tube L1 had the

same density as that of tube 1, viz. pI. The reason p2 is not equal to

p1 is the fact that the sediment contained in the upper portion of tube

L1, viz. in the incremental height (L1 L2), has a different density

(Ap)1-2 which was calculated as follows:


A "pV1 P2V 2 (4.5.1)
plV1 P2V2
(Ap)1-2 V- V (4.5.1)


(b) Measurement of bed density for sediment deposited in the
rotating channel under a low bed shear stress (td = 0.015 N/m2,

0.05 N/m2, etc.). The apparatus consisted of a 2.5 cm dia. metal tube

15 cm high placed concentric in a 15 cm dia., 15 cm high circular

plexiglass cylinder having a sealed bottom. At the center of bottom





-86-


plate 2.5 cm dia. hole was provided in the plate to match with the con-

centric metal pipe, thus leaving the bottom only for the annular space

between the metal tube and the plexiglass canister. After the bed was

formed in the rotating channel under the required conditions of Td and

Td, a transparent plastic pipe of about 2 cm dia. was placed vertically

through the sediment bed over the bottom of rotating channel. The

plexiglass canister was then lowered vertically so as to insert the

plastic tube through the metal tube of the apparatus. The annular

space around the metal tube was filled with commercial grade denatured

alcohol and dry ice was added in pieces to the alcohol. In less than

about 30 minutes this resulted in freezing of the suspension inside

the plastic tube which was then removed and placed horizontally covered

with ice cubes in order to keep it frozen. A piston which could be

activated by a threaded rod was used to push about 4 mm length of

frozen sediment projecting outside the plastic tube. A metal plate

held vertically in contact with the projected portion quickly melted

the frozen sediment which was collected in a porcelain dish. Next, a

5 mm portion of the sediment was then pushed out and the process was

repeated. The density of each 5 mm thick layer could be determined

by knowing the volume and the weight. The freezing resulted in swelling

of the sample and thus increasing the height of the sediment bed in

the tube. The total thickness of frozen sediment was therefore divided

into ten equal parts and the density of each layer was measured which

was taken to be corresponding to the ten parts of the thickness of the

original depth of the bed.




Full Text

PAGE 1

I I I I '10.70 -Tm = 0.9 N/m2 25 25 5 0.6T = 24 hrs Td =0N/m2 STd = 40 hrs z F-~ S/ w 030.4 Bed Shear Stress zo15 -in N/m2 0.35/ o H 0 / n 0 -.I z 0.23 0 SI I0.14 5 0.0 10 -,//C(3o)Values oL -----I___o.-----o---'------10 1.0 2.0 3.0 4.0 TIME (hrs) Fig. 51 Variation of suspension concentration with bed shear stress as a function of time (Expt.9 )



PAGE 1

EFFECT OF BED SHEAR STRESS ON THE EROSIONAL CHARACTERISTICS OF KAOLINITE By T.M. PARCHURE A THESIS PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 1980



PAGE 1

-78. Surface forces are dominant in respect to fine sediments, whereas gravitational forces predominate in the case of non-interacting particles. Due to the tendency of fine sediments to attach to each other due to surface forces, fine sediments are also sometimes referred to as cohesive sediments and the other sediments as noncohesive sediments. 9. Fine sediments are transported in the form of the suspended load or wash load, whereas the coarser sediments are predominantly transplanted as bed load. 2.2 Properties of Fine Sediments Several parameters affect the properties of clay materials, particularly the following: 1. Clay mineral composition. 2. Non-clay mineral composition. 3. Organic matter. 4. Exchangeable ions and soluble salts. 5. Texture, i.e. the particle size distribution of the constituent particles, the shape of the particles, their orientation in space relative to each other, and the forces tending to bind the particles together. In the context of clays, it is necessary to distinguish between material structure and property anisotropy. In general, "anisotropy" refers to the material structure and/or properties which do not exhibit the same characteristics and/or properties in every direction. The material structure anisotropy relates primarily to the anisotropy of



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-133(b) A significant decrease in the erosion rate was noticed with increasing moisture content (Christensen and Das, 1973). These tests were performed on soils containing kaolinite and grundite as basic clay minerals. The studies were conducted by providing a lining of compacted clay inside a small brass tube (25 mm diameter x 100 mm length). A steady flow of water was maintained through the tube to effect erosion. The range of moisture content tested was between 32 and 43 percent for kaolinite, between 29 and 47 percent for grundite, between 33 and 42 percent for kaolinite-sand mix, and between 24 and 36 percent for grundite-sand mixture. The soil indices for kaolinite and grundite used in the tests were given as follows: Liquid limit Plastic limit Plasticity index kaolinite 43 29 14 grundite 51 30 21 clay-sand mixture Data not given. It is clear from the above that the moisture content for these tests was higher than the plastic limit and lower than the liquid limit. (c) Owen (Nov. 1977) concluded that in terms of the time averaged mean shear stress the onset of continuous erosion (i.e. critical shear stress) is almost the same for mud beds of different density, but the rate of erosion is greater for mud beds of lower density. These tests were conducted with natural sediment, viz. Avonmouth mud. Deposited bed was used to study erosion. For zero salinity tests, the low density beds had a density ranging from 187.9 gm/liter to 212 gm/liter whereas the high density beds had a density ranging from 226.0 gm/liter to 261.2 gm/liter. This gives a water content between 79 and 81 percent for the low density bed and between 74 and 77 percent for the high density, both of which are higher than the liquid limit.



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-559-00 -0 run 3 T,=6.0 */ o00 A -Ar run 2 Tw=4.3 / A o500 SERIES I -I S100 --...o -----run I Tw=3.2 dyne/cm? 100 50 100 150 200 Fig. 10 Concentration versus time plots for Series I obtained by Lee (1979) scole A scoa. 90 r un5T 5.9 70 -.-.-.Q.--'.-. ---.n4 TW4.9 150 507 .--6-,, -----e S .-*run 2 Tw= 3.6 100 6 i*-----..__ .___ .S run I Tw 3.0. 2 30 dyne/cm2 scaleA: run 1,2,3,4 50 sale B. run 5 SERIES 17 10 0 100 200 / Imin) Fig. 11 Concentration versus time plot for Series II obtained by Lee (1979)



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-8fabric which would influence development of interparticle force relationships. Property anisotropy refers to strength, compressibility, permeability, conductivity, and other mechanical properties which are not equal in all directions, i.e. the material property demonstrated is a function of the sample tested. The external constraint anisotropy refers particularly to the applied stresses and boundary constraints. While considering properties of fine sediments, the anisotropy needs to be taken into account. 2.2.1 Size, Range, and Definition The maximum size of particles in the clay size grade is defined differently in different disciplines. In geology, the tendency has been to follow the Wentworth Scale to define the clay grade as materials finer than about 4 microns. In soil investigations, the tendency is to use 2 microns as the upper limit of the clay size grade. Although there is no sharp universal boundary between the particle size of clay minerals and non-clay minerals, in argillaceous materials, a large number of analyses have shown that there is a general tendency for the clay minerals to be concentrated in a size less than 2 microns (Grim, 1968). Clays contain varying percentages of clay-grade material and therefore varying relative amounts of non-clay-mineral and clay-mineral constituents. Clays almost always contain some non-clay mineral material coarser than the clay grade, although the amount may be very small. In many materials called clays the clay grade and the claymineral constituents make up considerably less than half the total.



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-51vii) The erosion rates strongly depend on the average shear stress. For both dense beds, the erosion rates increase very rapidly for shear stress greater than 0.478 N/m2 for series I and 1.34 N/m2 for series II. viii) The observed independence of erosion rates from the macroscopic shear strength of the clay and the fact that clay gets eroded at shear stresses which are infinitesimal compared to its strength suggest that the mechanism of failure of clay particles by surface erosion is basically different than the mechanism of failure of clay particles in the interior of the clay mass, when subjected to shear stresses. Krone (1962) conducted studies on San Fransisco Bay mud in order to relate transport and deposition processes to properties of the sediment. Erosion tests were carried out on flow deposited beds. The concentration-time data of erosion tests plotted on log-log coordinates are given in Fig. 7. Concentration as a function of bed shear stress is given in Fig. 8. Results of a 500 hour long erosion test are given in Fig. 9. An arithmetic coordinate plot of these data would produce a curve with a steadily decreasing slope, suggesting an approach to an equilibrium or steady state concentration. However, the continuing straight line plot on log-log coordinates as shown in the figure discourages this suggestion. Krone (1962) found that "the log-log erosion curves and their slopes were difficult to explain." He presented a qualitative hypothesis based on interchange between suspended flocs and the bed and on the dependence of erosion on time and weakly, if at all, on shear (p. 86).



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50-. I 1 I T = 0.9 N/m2 $ Tm = 24 hrs 400.4Td = 0 N/m2 038 z Td = 40 hrs z W 0 N E z Bed Shear Stress 30-in N/m2 0.28 Z U) 0 u LU -L O _z 20-0.2on 0.17 -o Z I a, -C / Values I0cn -W/ n 0 .0 7 *, o 0.015 0 1.0 2.0 3.0 TIME (hrs) Fig. 57 Variation of suspension concentration with bed shear stress as a function of time (Expt. No. 15)



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CHAPTER II FINE SEDIMENTS 2.1 General Description Fine sediments, commonly called clays or muds,are a product of weathering or hydro-thermal action on the rock and other soil on earth's surface. Although the maximum size of particles in the clay grade is defined somewhat differently in different disciplines, the general tendency has been to classify sediments finer than two microns as clays. Mixtures of clays and silt are usually called muds. The classification of fine-grained soil as either a silt or a clay is not merely on the basis of particle size but rather on the plasticity or non-plasticity of the material. Clay soil is plastic over a range of water content; that is, the soil can be remolded or deformed without causing cracking, breaking, or change of volume, and will retain the remolded shape. The clays are frequently cohesive. When dried, a clay soil possesses very high resistance to crushing. A silt soil possesses little or no plasticity and when dried has little or no strength. The basic differences between the elementary particles of noninteracting coarse minerals and the interacting fine minerals such as Kaoline could be briefly described as follows: 1. The non-interacting particles have no electric charge. Hence, they interact only hydrodynamically without any inter-particle attraction. -5-



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i Erosion rate E oc (C2 -) Expt Td Td z -N/m2 hrs 0o 17 0.050 24 -2.0 --x-18 0.015 40 -19 0 135 z // z 0 2 8 Tm = 0.9 N/m z Tm = 24 hrs same for o -each test / z (AT)ex 0.2 w 1.0S/ z K/ L) 0 0 0.2 0.4 BED SHEAR STRESS Te (N/m2) Fig. 87 Variation of erosion rate as a function of bed shear stress for different flow denosited beds



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-10at this level of viewing. The fabric units identified in the microscopic range consist of several particles or groups of particles defined as clusters, also sometimes called flocs. 3. Ultra-microscopic: The fabric units are visually observed in the ultra-microscopic level using electron microscopy (either transmission or scanning electron microscopy). Single or individual clay particles can be distinguished at this level. 2.2.2 Surface Area Colloidal properties such as plasticity and adsorption of molecules arise from the large surface area associated with a small mass. The surface forces are dominant in respect to fine sediments and the influence of gravitational forces is small. The clay minerals are plateshaped or tabular because the layer-lattice structure results in strong bonding along two axes but weak bonding between layers. The clay particle thickness depends upon the magnitude of forces of attraction between the layers. The variation in specific surface area of different minerals is primarily due to different thicknesses of the plate-shaped particles. Variation in the other two dimensions of clay particles is related to the degree of crystallinity of the clay minerals. A well crystallized Kaolinite has large particles. If it is poorly crystalline, the Kaolinite particles may not be larger than those of montmorillonite. The true meaning of the particle size becomes more evident in terms of its specific area. For example, a single sphere 1 cm in diameter has a surface area of 3.14 cm2.The same volume in terms of one micron (1 u diameter spheres has a surface area of 10,000 times greater and for one



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-391964: The present knowledge on the behavior of fine sediments in estuaries was summarized by Partheniades (1964). 1968: Task Committee of ASCE on Erosion of Cohesive Materials prepared a report on literature review (Task Committee, 1968). 1973: State of the art paper on causes and mechanisms of cohesive soil erosion was presented by Paaswell (Paaswell, 1973). 3.2 Review of Literature on Erosion Since the scope of the present study deals with erosion of Kaolinite, a brief account of the work carried out in the past on erosion of cohesive sediments is given here. Effect of various parameters and processes influencing the erosional properties of cohesive sediments has been studied by several research workers. Important findings of these studies have already been given in Chapter II. The earlier experiments conducted to study the erosion of cohesive sediments were oriented to obtain solution to a specific engineering problem such as the model studies performed by the Tennessee Valley Authority for the Fontana Project (1953) and the Fort Patrick Henry Project (1960). Most of the work to understand the basic physical processes related to cohesive sediments has been carried out during the past two decades. Smerdon and Beasley (1959) applied the tractive force theory to the stability of open channels in cohesive soils. Dunn (1959) used a submerged jet to determine the tractive resistance of cohesive soils and correlated it with plasticity index. Masch et al. (1965) conducted studies on remolded cohesive sediments using a rotating cylinder



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CHAPTER VI DISCUSSION AND PROPOSED TEST PROCEDURE 6.1 Bed Density and Other Soil Parameters Correlated to the Shear Strength It has already been mentioned that the shear strength of the bed cannot be estimated correctly by measuring the soil parameters and to obtain the shear strength of the soil by using the available relationships. It is therefore necessary to evolve an experimental procedure for measuring the shear strength of soil at various depth. Justification for not using the already available relationships is given in this section. Attempts made by various investigators to measure the two parameters, viz. the density of the soil and the shear strength of the soil by adopting various methods, have been described in Section 3.5. The conclusions regarding the correlation between the density and the shear strength are contradicting as can be seen from the following: (a) Parthenaides (1962, p. 54) noted that "In spite of the lower overall strength of the bed and its higher water content, the erosion rates of series II were lower than the corresponding rates of series I." However, he has also made the following observation (p. 64): "The most striking results of this study are the independence of the minimum shear stress and erosion rates from bed shear strength and bed density." -132-



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NOMENCLATURE C = Ratio of the consecutive suspension concentrations, e.g. C2 C S2 etc. C' C2 AC = Excess (suspension) concentration, e.g. C2 -C1. C2 -C1 (AC)ex = Normalized excess concentration, e.g. C C1 Co = Suspension concentration at the end of initial mixing. C30 = Suspension concentration at the end of 30 minutes after change of bed shear stress. p = Density of bed. Tm = Bed shear stress for initial mixing. Tm = Duration of initial mixing. Td = Bed shear stress for deposition. Td = Duration of deposition plus consolidation. Te = Bed shear stress for erosion (varied as a function of time). Ts = Time step for Te, i.e. duration over which different magnitudes of Te prevailed. r = Ratio of the consecutive values of bed shear stress, e.g. T T e2 e3 S etc. el 1 e2 xi



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Figure Page 16 Schematic diagrams showing shear strength of cohesive soil related to other parameters 61 17 Relationship between shear strength and bed density observed by Owen (1970) 65 18 Bed density profiles: Owen (1970) 66 19 Definition sketch for notations used to describe experimental conditions 69 20 Size gradation of Kaolinite used for the experiments 72 21 Photograph: The rotating channel facility 75 22 Photograph: Close view of the annular channel and the ring 75 23 Photograph: The motor controllers 76 24 Photograph: The electric motors for the channel and the ring 76 25 Photograph: Millipore filter apparatus assembly 77 26 Photograph: Device for measurement of bed density 77 27 Photograph: Equipment for determining the concentration of sediment suspensions 78 28 Photograph: Sampling bottles 78 29 Operational speeds and controller meter readings for ring and channel at different bed shear stresses 79 30 Correlation between r.p.m. and meter reading for the channel and the ring 80 31 Schematic drawings of apparatus developed for measurement of bed density 84 32 Effect of parameters in Step I on suspension concentration 88 33 Suspension concentration versus time for Expt. 3 89 34 Suspension concentration versus time for Expt. 4 90 35 Suspension concentration versus time for Expt. 5 91 36 Effect of Step II parameters on suspension concentration 92 vii



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I I 1 0.70 / 2 Tm =0.9 N/m 0.6 Tm =24 hrs E -/ Td =0 N/m2 40 -" -Td =40hrs o / I (AT) 1I.0 -g-£__C __A_ M '~-t-



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-136property of the material (type, surface area, etc.) or a function of other factors related to eroding fluid such as pH, temperature, sodium adsorption ratio, etc. A detailed.discussion of the above is given in Chapter II. iii) Pertinent data on soil parameters and type of bed used by different investigators are given below: Type of Soil Parameters Investigator Bed PL LL PI W % % % Partheniades (1962) Deposited Natural Mud 44 99 55 110 Espey (1963) Compacted Taylor Marl 21 47 26 27 Christensen and Das Compacted (1973) Kaolinite 29 43 14 N.R. Compacted Grundite 30 51 21 N.R. Arulanandan (1975) Compacted Yolo Loam ---N.R.--Owen (1977) Deposited Natural Mud ---N.R.--Fukuda (1978) Deposited Natural Mud ---N.R.--61-73 Thorn and Parsons Deposited (1980) Natural Mud ---N.R.--where PL = plastic limit, LL = liquid limit PI = plasticity index W = moisture content N.R. = data not reported by the author Due to the procedure adopted in forming the deposited beds and the compacted beds, the water content in the deposited beds



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-4fine sediment transport, deposition, bed formation and consolidation, and bed resuspension are rather complex, and in the estuarine environment they are inter-linked in a cyclical manner within time-scales imposed typically by the astronomical tides. Investigations of these phenomena under laboratory scales is an important first step towards an elucidation of the mechanics of the transport processes in the prototype, since it is possible to isolate and control the important governing parameters in laboratory tests. In that context, this investigation is concerned with studying the characteristics of resuspension of flocculated cohesive sediment beds. Under an applied bed shear stress, the surficial erodibility of such a bed is contingent upon the structure of the bed, as defined by the inter-particle bond strength of the floc network. Inasmuch as this network is formed under a given set of conditions specified by floc deposition and consolidation of the settling suspension at the bed, the magnitude and the duration of the applied shear stress during bed formation are important governing parameters for the subsequent process of resuspension. Hence the investigation of the erosion of beds formed under a variable shear stress is emphasized in this study. The overall objective of this study was an attempt to establish a laboratory test procedure for specifying the "layer by layer" erodibility of a deposited bed in terms of parameter(s) involving the critical shear stress for the erosion of a particular layer. The observed variation of the erodibility of a given bed with depth has been discussed with reference to the depth-variation of the floc shear strength and the bulk density.



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-19Character Parameter Physical Soil type (clay mineral) Percentage of clay Liquid and plastic limits, and activity Specific gravity Physico-chemical Base exchange capacity Sodium absorption ratio Pore fluid quality Pore fluid environment Mechanical properties Shear strength (surface and body) Cohesion Thixotropy Swelling and shrinkage properties Conditions of environment Weathering (wet-dry) Freezing and thawing Prestress history In addition to the above, Alizadeh (1974) has included the parameters bulk density, water content, effective stress, time (aging), testing methods, and soil preparation methods. Kandiah (1974) mentioned that the erodibility of cohesive soil is controlled by the mineralogical, chemical, and environmental factors. The mineralogical factors include the type and amount of clay mineral present in the soil while the chemical factors include the total salt concentration, sodium absorption ratio, and pH of the pore fluid. The influence of these two factors is mutually independent and they are the "key parameters" of erosion since their effect is far more pronounced than other factors. Attempts have been made to correlate the critical shear stress to various parameters affecting soil erosion. For example, Smerdon and Beasly (1959) presented the following equation between critical shear stress (Tc) and plasticity index (PI): Tc = 0.0017 (PI)0.84(2.3.1)



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-581412-Symbol Shear Stress 0.43 N/m2 10 -x 0.34 N/m2 0 0.24 N/m2 -A 0.15 N/m2 0' z 80 z z 0 4 20 L I I 0 40 80 120 -160 200 TIME (Hours) Fig. 14 Concentration versus time data obtained by Yeh (1979) for erosion of kaolinite



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-11millimicron diameter spheres the surface area is 10 million times as great. The specific surface area of different clay minerals is as follows: Montmorillonite: 800 m2/g Chlorite: 80 m2/g Clay Mica: 80 m2/g Kaolinite: 15 m2/g Kaolinites show the most uniform crystals, often hexagonal plates with a typical diameter of 0.3 to 0.5 p and a thickness of 0.05 to 2 u. Montmorillonite particles are thin plates typically around 30 A thick and 0.1 to 1 p in diameter. Illite particles are plates with a typical thickness of 300 A. Surface area is one of the most important properties of fine sediments. Most of the differences between clay minerals in properties such as water retention, plasticity, or cohesion can be explained by the differences in the surface areas of particles. This explains the high swelling and high liquid limit of montmorillonite. Liquid limit being closely related to the surface area, it is considered as a measure of the nature of the surface as well as the area. 2.2.3 Shape The shape of a particle is also an important factor in determining the specific area. A spherical particle has the least surface area, and a disc or plate shaped particle has the greatest surface area. A sphere of 1 cm3 has an area of 4.836 cm2, while an equi-volume disc one micron thick has 20,000 cm2surface area. Typical plate shaped



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-34reduction in the contribution of structure to apparent viscosity as the shear rate is increased. These indicate pseudo-plastic and plastic behavior, respectively, also known as shear thinning. The intercept T shown in Fig. 2 is the lower yield stress, whereas TB is known as the upper Bingham yield stress which is a measure of the work done in disrupting the floc structure in cohesive suspensions. The content of water in a clay soil determines the consistency in the remolded state. The criteria to determine the various states of consistency are known as Atterberg Limits. At high water content, the soil-water mixture possesses the properties of a liquid; at lesser water content the volume of the mixture is decreased and the material exhibits the properties of a plastic; at still lesser water content, the mixture behaves as a semi-solid and finally as a solid. The water content indicating the division between the liquid and plastic state is called the liquid limit. The division between the plastic and semi-solid state is the plastic limit. The water content at the division between the semi-solid and the solid state is the shrinkage limit. All these three limits are expressed as percentage of water with respect to the weight of solids. Below the shrinkage limit, there is little or no change in volume as water content varies. However, above the shrinkage limit, the change in total volume of soil-water mixture is related to the change in water content. The plasticity index is the numerical difference between the liquid limit and the plastic limit, and indicates the range of water content through which the soil remains plastic. It is necessary to know both the liquid limit and the plasticity index for a proper evaluation of the plasticity properties of soil.



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-62A qualitative comparison of shear strength results for these conditions is shown in Fig. 16(b). Shearing of U-U type is completed relatively quickly because the prevention of volume change results in development of excess pore pressures and consequent reduction of the shear strength. Shearing of C-D type takes place very slowly since both drainage and the volume change are permitted during shearing. The shear strength of clay is essentially composed of two components: i) physical component due to frictional resistance and interlocking between particles, and ii) physico-chemical components due to the inter-particle attractive and repulsive forces. The shear strength of clay soil improves with consolidation, provided that time is available for permitting the necessary pore water drainage to take place. In effect, consolidation results in decreasing the water content of the clay with a subsequent increase in shear strength (Fig. 16(c);McCarthy, 1977, p. 234). 3.5 Shear Strength and Bed Density of Clay Attempts have been made by several research workers to measure the bed density and shear strength of clays and to establish correlations between the two parameters. Ariathurai and Kandiah (1979) have developed an electrical method to measure in situ sediment densities. Dayal et al. (1980) have developed a method for obtaining in situ soil strength by use of low velocity projectile penetration technique. Gularte (1978) measured shear strength with a fall cone device. He also used a modified viscometer for this purpose.



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Abstract of Thesis Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science EFFECT OF BED SHEAR STRESS ON THE EROSIONAL CHARACTERISTICS OF KAOLINITE By T.M. Parchure December 1980 Chairman: Dr. Ashish J. Mehta Major Department: Coastal and Oceanographic Engineering The degree of resistance to the erosion of a cohesive sediment bed under an applied shear stress is controlled by the physico-chemical properties of the sediment and the fluid, as well as by the depthvariation of the bed shear strength characterized by the inter-particle bonds. Previous attempts to correlate the erodibility of cohesive sediment beds with the shear strength determined by such devices as a penetrometer or a viscometer, or with soil indices, have not been successful. The objective of this study was to evolve a test procedure for conducting laboratory experiments to obtain the "layer by layer" shear strength of deposited beds of various density structures. Experiments were conducted using kaolinite with salt water of 35 ppt concentration as the eroding fluid. All the tests were carried out in an annular rotating channel apparatus. New techniques developed for measurement of the density structure of deposited beds have been described. An illustrative example outlining the procedure for the determination of the depth-variation of the bed shear strength is given. Chairman xiii



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LIST OF FIGURES Figure Page 1 Inter-particle forces on clay minerals and the clay micelle 14 2 Rheological models 33 3 Concentration versus time plot obtained in erosion of placed bed: Partheniades (1962), Expt. Series-I 48 4 Concentration versus time plot obtained in erosion of remolded bed: Partheniades (1962), Expt. Series-II 49 5 Concentration versus time plot obtained in erosion of deposited bed: Partheniades (1962), Expt. Series-III 50 6 Relationship between rate of erosion and average bed shear stress: Partheniades (1962), 50 7 Concentration versus time plot: Krone (1962) 52 8 Concentration as a function of bed shear stress: Krone (1962) 52 9 Results of a 500 hour long erosion test: Krone (1962) 53 10 Concentration versus time plots for Series I obtained by Lee (1979) 55 11 Concentration versus time plots for Series II obtained by Lee (1979) 55 12 Lee's (1979) data for Series I re-plotted to indicate variation of suspension concentration as a function of time and bed shear stress 56 13 Lee's (1979) data for Series II re-plotted to indicate variation of suspension concentration as a function of time and bed shear stress 57 14 Concentration versus time data obtained by Yeh (1979) for erosion of Kaolinite 58 15 Concentration as a function of bed shear stress obtained by Yeh 59 vi



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-97T = 30 Min. E 0.6 z n/ LU) w S0.2S2 4 TIME (hrs) T = 60 Min. -n/ E 0.6U) m 0.4U 0.20 m 0 2 4 TIME (hrs) Fig. 38 Representation of a linearly varying bed shear stress by two different discretized time step functions 0: _, S --/ two different discretized time step functions



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-165the soil: Liquid limit Plastic limit Plasticity index Bed density (or moisture content) However these correlations do not give reliable results. iv) The following indices have been used for characterization of the sediment and the fluid: Cation exchange capacity Sodium adsorption ratio Dielectric dispersion constant 7.2 Conclusions of the Present Study The present study on erosion of Kaolinite was conducted in a rotating channel apparatus. Water with a salinity of 35 parts per thousand was used as the eroding fluid. The conclusions of the study are as follows: i) The applied bed shear stress and the duration of time over which it acts during the processes of floc aggregation due to mixing, settling and consolidation were found to have a measurable influence on the bed structure as reflected by the change in bed density and resistance to erosion with depth. ii) The shear strength of the deposited bed as inferred from the erodibility of the bed was found to increase with depth below the interface between the sediment bed and the eroding fluid. The bed shear strength was also found to increase with increasing bed consolidation time. iii) During the erosion tests the bed shear stress was increased by small magnitudes in descretized time steps. Under such an



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-22the ability of clay to store electrical potential energy under the influence of an electric field. The dielectric constant for a soil sample is defined as Scd (2.3.5) VA where c = capacitance d = length of specimen A = cross-sectional area ev = dielectric constant of vacuum = 8.85 x 10-14 farad/cm The dielectric constant of a dry silicate mineral is 4, and of water about 80. Alizadeh (1974) has defined the magnitude of dielectric dispersion (AE') as the total amount of decrease in the measured dielectric constant. The dielectric dispersion depends mainly on the type and amount of clay; the other factors such as pore fluid composition, water content, particle orientation, etc. have a secondary effect. It has been used as a quantitative index for soil characterization. Measurements have shown that for 10 percent Kaolinite and 21.2 percent water content, AE0 has a magnitude 7.5, whereas for 60 percent Kaolinite and 30.4 percent water content, AE' is 18. The shape and size of individual clay particles have an influence on their surface area. Since the surface forces predominate in respect to cohesive sediments, the colloidal properties such as plasticity and adsorption of molecules are governed by the surface area. Natural sediments usually have a wide range of particle sizes. The effect of



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-137is considerably higher than the water content in the compacted bed. This factor influences the rate of erosion of the two types of beds. iv) The amount of moisture content being different than the optimum moisture content corresponding to the maximum density (Proctor density) may have an influence on the erosion rate. With reference to curve shown in Fig. 71.a, the same density of soil can be achieved at two different values of the moisture content. v) The bed density of a deposited bed is a function of the time of consolidation and the elevation below the sediment-water-interface. vi) Various investigators have found that for a given bed, either compacted or deposited, there is a characteristic shear stress above which there is a sudden increase in the erosion rate. vii) In the case of compacted beds, the moisture-density relationship is influenced by the compaction procedure (Fig. 71.b). Attempts to correlate soil parameters other than the bed density with the shear strength of soil have been described in Section 2.3. It may be noted that the Plasticity Index (PI) alone is not adequate to characterize a given soil. It is essential to specify both the Plasticity Index and the Liquid Limit (LL) for a given soil as can be seen from Fig. 71.c. A particular value of PI can have a range of LL between 20 and 100, and depending upon whether the fine sediment is clay or silt, the sediment can have a degree of plasticity ranging from low to high as shown in the figure. In view of the above, the attempt made at U.S. Bureau of Reclamation reported by Espey (1963, p. 3) appears commendable. Various



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Figure Page 37 Effect of Step III parameters on suspension concentration 93 38 Representation of a linearly varying bed shear stress by two different discretized time step functions 97 39 Suspension concentration versus time for Expt. 7 98 40 Suspension concentration versus time for Expt. 8 99 41 Suspension concentration versus time for Expt. 9 100 42 Suspension concentration versus time for Expt. 10 101 43 Suspension concentration versus time for Expt. 11 102 44 Suspension concentration versus time for Expt. 12 103 45 Suspension concentration versus time for Expt. 13 104 46 Suspension concentration versus time for Expt. 14 105 47 Time-step function for bed shear stress 106 48 Suspension concentration versus time for Expt. 15 107 49 Suspension concentration versus time for Expt. 16 108 50 Variation of suspension concentration with bed shear stress as a function of time for Expt. 8 110 51 Variation of suspension concentration with bed shear stress as a function of time for Expt. 9 111 52 Variation of suspension concentration with bed shear stress as a function of time for Expt. 10 112 53 Variation of suspension concentration with bed shear stress as a function of time for Expt. 11 113 54 Variation of suspension concentration with bed shear stress as a function of time for Expt. 12 114 55 Variation of suspension concentration with bed shear stress as a function of time for Expt. 13 115 56 Variation of suspension concentration with bed shear stress as a function of time for Expt. 14 116 57 Variation of suspension concentration with bed shear stress as a function of time for Expt. 15 117 58 Effect of shear stress variation on suspension concentration 118 viii



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-141c) Observations of Yeh (1979) are given in Fig. 14. d) Similar observations made in the present study are given in Fig. 62-64. It is apparent that different notations and definitions need to be used for the three values of bed shear stresses as follows: Tcr = Critical time-mean value of bed shear stress for erosion to begin at the sediment-water interface. This is independent of the density of soil and is related to the floc shear strength of soil. Tch = Characteristic value of the bed shear stress. This changes with the nature of the bed structure. Tch can be as much as an order of magnitude higher than Tcr Tc = Shear stress value obtained by extrapolation of erosion versus shear stress plot. Ts = Shear strength of the bed at certain depth below the top surface of bed (i.e. sediment-fluid interface). After the layers above this depth are eroded, Ts represents the critical shear stress which must be exceeded by the applied shear stress in order to cause erosion of the exposed surface. These notations are shown schematically in Fig. 73. It is apparent that Tcr < T < Tch and rcr < Ts. Since different investigators have used the same term "critical shear stress" to mean different shear stress, it is necessary to be careful in identifying which one out of the above four is being referred to. For instance, Espey (1963, p. 34) is referring to Tch' Parthenaides (1962, p. 64): Tcr' and (p. 66): Tch'



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Figure Page 59 Comparison of suspension concentration obtained under two different discretized time step functions 119 60 Variation of suspension concentration with bed shear stress, all data for Td = 24 hours 120 61 Variation of suspension concentration with bed shear stress, a-i data for Td = 40 hours 121 62 Suspension concentration versus bed shear stress (Expt. 7) 124 63 Variation of suspension concentration as a function of bed shear stress (Expt. 9) 125 64 Variation of suspension concentration as a function of bed shear stress (Expt. 11) 126 65 Variation of suspension concentration with bed shear stress for two different discretized time step functions 127 66 Cr versus Tr at 10 minutes 128 67 Cr versus rr at 20 minutes 129 68 Comparison of Cr versus T for two different bed density structures 69 (AC)ex plotted against the corresponding values of (AT)ex 131 70 Example of erosion test result and critical shear stress for erosion as a function of dry density of mud surface given by Thron and Parsons (1980) 135 71 Parameters influencing bed density and plasticity of soil 138 72 A typical test result reported by Espey (1963) 140 73 Notations for critical shear stress 142 74 Erosion rate versus time for different values of bed shear stress 144 75 Definition sketch for various parameters 145 76 Explanatory sketch for EQ type profile of c-t curve 147 77 Explanatory sketch for ER type profile of c-t curve 148 78 Suspension concentration during deposition under the bed shear stress of 0.05 N/m 153 ix



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-161Expt Td Td -2 N/m2 hrs// ---17 0.050 24 ---x--18 0.015 40 / *-19 0 135 SI I E .I • Tm = 0.9 N/m I o Tm = 24 hrs 6 (T) e 0.2 5z ex -I I n'F-w U 4x o / // 2-f^ o 0o Ii 0 2I 'Oy 0 0.2 0.4 0.6 BED SHEAR STRESS Te (N/m2) Fig. 86 Suspension concentration versus bed shear stress for different flow deposited beds



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-23such heterodisperse sediments is to increase the probability of collision of primary particles. Also, natural sediments typically are not uniform in their shape. Since Brownian motion is rotary in character, the largest dimension is appropriate for the collision diameter, whereas the mean diameter is applicable to the diffusion constant. Consequently, anisodimensional particles have a relatively large collision diameter combined with a relatively large diffusion constant, thus enhancing the collision rate. After flocculation has progressed, however, the flocs tend to be more nearly spherical, and the effect of anisotropy on collision rate is small. Organic matter has an effect on the properties of flocculation of sediment and hence it affects the erosional properties of sediment. Kandiah (1974) found that organic matter strengthens the soil aggregates against slaking. Studies on the erodibility of 30 percent illitic soil showed that the critical shear stress for erosion increased from 1.7 N/m2 to 4.0 N/m2 when the organic content was increased from 0 to about 4 percent. It has been mentioned earlier that the type of clay is one of the factors determining the erosional characteristics. The Cation Exchange Capacity is a property of the soil which can be used to indicate the type while associating it with the corresponding critical shear stress for erosion. Cations from the pore fluid are attracted by the negative charge on clay particles and the anions are repelled in order to maintain electro-neutrality. The number of these exchangeable cations is known as the cation exchange capacity, which is usually expressed as milliequivalent per gram (me/g). The kind and number of exchangeable cations have an important influence on the behavior of soils. For



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-54Lee (1979) conducted studies on resuspension and deposition of Lake Erie sediments, using a circular channel apparatus with an annular ring similar to the apparatus used for the present studies. However, in the case of apparatus used by Lee, the shear stress was produced by rotating the ring alone. The results of concentrationtime curves for series I and II of tests as presented by him are given in Figs. 10 and 11. The same data have been re-plotted in the form of concentration under a time-variant bed shear stress (Figs. 12 and 13). Lee found that the entrainment rate was a strong function of shear stress, water content, and mineralogy. Concentration-time data obtained by Yeh (1979) for different values of bed shear stress are given in Fig. 14, and concentration as a function of bed shear stress given in Fig. 15. 3.4 Shear Strength of Clay The classical Coulomb's equation for shear strength of soils is s = c + p tan ( (3.4.1) where s = shear strength c = cohesion : = angle of internal friction p = pressure normal to the failure plane Since c and are found to depend on the loading rate and drainage condition, the modified Coulomb's equation is given as follows: s = ce + o tan e (3.4.2) where



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o Brisbane mud oi r I -iI-0 + Grangemouth mud I ~ & Belawan mud VNI aIp ,, I z 1 zo I .* I i --.I os -01 -Om I 1 1, -----r ------I -----Ir^ -/01 -. 00 in 0.1 ost.2 Example of erosion test results Reference: Thorn and Parsons (1980) 0-01 10 100 PS: Dry density of mud surface (g/) Critical shear stress for erosion Fig. 70 Example of erosion test results and critical shear stress for erosion as a function of dry density of mud surface, given by Thorn and Parsons.



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-167It was found that for Te < ch the c-t plot gave the EQ type profile provided Te was applied over a sufficiently long duration Ts. For T > ch, the c-t plot always gave the ER type profile. It is postulated that under the ER type profile the erosion of bed would proceed indefinitely because the applied bed shear stress is greater than the shear strength of the bed. viii) The erosion rate is time dependent for the conditions which give EQ type profile of the c-t plot, whereas it is time independent after an initial time dependent phase for the conditions which give ER type profile. ix) In order to determine the shear strength Ts of a deposited bed as a function of the depth below the sediment-eroding fluid interface, the following experimental procedure is recommended: Form a bed in the apparatus using the selected sediment and the fluid and measure the density of the bed as a function of depth. Select a small value of the normalized excess shear stress, (AT)ex, say 0.1, for increasing the bed shear stress in the manner of a discretized time-step. Conduct exploratory tests to determine the Tcr at which surface erosion begins and to determine the minimum duration of the time step Ts which would give the EQ type profile. After reforming the bed under the same conditions, beginning with Tel > T cr increase the bed shear stress for the selected value of (AT)ex and T and erode the bed layer by layer. Calculate the erosion rates from all c-t profiles and determine which profiles satisfy the condition that the erosion rate at the end of the time step be smaller than a pre-selected small percentage of the initial high rate of erosion. Profiles satisfying this condition are the EQ type profiles and hence values of Te used for conducting the experiment



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-170Committee on Tidal Hydraulics, "Soil as a Factor in Shoaling Processes, a Literature Review," Tech. Bulletin No. 4, U. S. Army Corps of Engineers, June 1960. Dayal, U., Allen, J. H., and Reddy, D. V., "Low Velocity Projectile Penetration of Clay," Accepted for publication in the Journal of the Geotechnical Engineering Division, ASCE, 1980. Dunn, I. S., "Tractive Resistance of Cohesive Channels," Journal of the Soil Mechanics and Foundations Division, ASCE, Proc. 85 (SM3) 1959. Einstein, H. A., and Krone, R. B., "Experiments to Determine MOdes of Cohesive Sediment Transport in Salt Water," Journal of the Geophysical Research, Vol. 67, No. 4, April 1962, pp. 1451-1461. Espey, W. H., "A New Test to Measure the Scour of Cohesive Sediment," Tech. Report HYD 01-6301, Hydr. Eng. Lab., Dept. of Civil Engineering, University of Texas, Austin, 1963. Flaxman, E. M., "A Method for Determining the Erosion Potential of Cohesive Soils," Erosion Pub. 59,International Assoc. of Scientific Hydrology, Commission on Land Erosion, October 1962. Fontana Project --Hydraulic Model Study, Monograph 68, Tennessee Valley Authority, 1973, p. 371. Fort Henry Apron Studies, Technical Monograph 87, Tennessee Valley Authority, 1960, pp. 135-141. Fukuda, M. K., "The Entrainment of Cohesive Sediments in Salt Water," Ph.D. Dissertation, Case Western Reserve University, Cleveland, Ohio, 1978. Grim, R. E., Clay Mineralogy, McGraw-Hill Inc., New York, U.S.A., 1968. Grissinger, E. H., "Resistance of Clay Systems to Erosion by Water," Water Resources Research, Vol. 2, No. 1, 1966, pp. 131-138. Grissinger, E. H., "Ephermeral Erosion and the Stability of Cohesive Soils," Special Report 135, Highway Research Board, 1973. Gularte, R. C., Kelley, W. E., and Nacci, V. A., "The Threshold Erosional Velocities and Rates of Erosion for Redeposited Estuarine Dredge Materials," Proceedings of the Second International Symposium on Dredging Technology, Texas A and M University, College Station, Texas, Paper H-3, November 1977. Gularte, R. C., "Erosion of Cohesive Sediment as a Rate Process," Ph.D. Thesis, University of Rhode Island, 1978. Gust, G., "Observations on Turbulent Drag Reduction in a Dilute Suspension of Clay in Sea-water," Journal of Fluid Mechanics, Vol. 75, Part 1, 1976, pp. 29-47.



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-31induced by the weight of the sediment have been dissipated (Committee on Tidal Hydraulics, 1960). The properties of clays in respect to settling and consolidation are very important in connection with the studies for their depositional and erosional properties. 2.5 Clay-Water System Water which can be held by the clay system is grouped into two categories, namely low-temperature water, which can be driven off by heating to about 100C to 150C, and the OH lattice water which is lost at temperatures above about 3000C. The nature of low-temperature water and the factors that control its characteristics are of great importance, since they largely determine the plastic, bonding, compaction, suspension, and other properties of clay materials, which in turn control their behavior under the given flow field. Water, though neutral, has its oxygen and hydrogen atoms spaced in such a manner that the center of gravity of the positive and negative electrical charges do not coincide. The resulting molecule has a positive charge acting at one end and a negative charge acting at the opposite end. Water molecules are thus considered polar molecules. Because of the net positive charge of the cations, they attract negative charges. The negative tips of water molecules are attracted and held to the cation, which in turn is held by the clay particle due to the negative charge on its surface. The resulting effect is that water becomes bonded to the clay. Additional water molecules are also attracted to the clay particle because of a chain-like arrangement of negative ends to positive ends of molecules.



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-86plate 2.5 cm dia. hole was provided in the plate to match with the concentric metal pipe, thus leaving the bottom only for the annular space between the metal tube and the plexiglass canister. After the bed was formed in the rotating channel under the required conditions of Td and Td, a transparent plastic pipe of about 2 cm dia. was placed vertically through the sediment bed over the bottom of rotating channel. The plexiglass canister was then lowered vertically so as to insert the plstic tube through the metal tube of the apparatus. The annular space around the metal tube was filled with commercial grade denatured alcohol and dry ice was added in pieces to the alcohol. In less than about 30 minutes this resulted in freezing of the suspension inside the plastic tube which was then removed and placed horizontally covered with ice cubes in order to keep it frozen. A piston which could be activated by a threaded rod was used to push about 4 mm length of frozen sediment projecting outside the plastic tube. A metal plate held vertically in contact with the projected portion quickly melted the frozen sediment which was collected in a porcelain dish. Next, a 5 mm portion of the sediment was then pushed out and the process was repeated. The density of each 5 mm thick layer could be determined by knowing the volume and the weight. The freezing resulted in swelling of the sample and thus increasing the height of the sediment bed in the tube. The total thickness of frozen sediment was therefore divided into ten equal parts and the density of each layer was measured which was taken to be corresponding to the ten parts of the thickness of the original depth of the bed.



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-65CONCEN10 0 [ TRATION SALINITY fEXrmg/1 g/1 m 90 ___._/ 16290 32 9 10 06 15 520 17 0 9-36 .O [ 17 775 17 8 6 96 0 a 17280 16-7 4 64 70 6 705 2 7 9 73 0 / x 7 2dd 4 6 9 74 S0 0 392 a8 9-76 6-0 -------0 10272 16 8 1002 a 6 666 33 3 9 72 + 6 B10 0 6 974 5-0 ,.o ,----i-4.0 __ -T 325 -" SHEAR 2-0 N/m2 096 08 --0-0.9 .--Et ----------07 06 0-5 0.4 I 60 90 100 150 200 250 300 350 400 DENSITY g/1 Fig. 17 Relationship between shear strength and bed density observed by Owen (1970)



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-n SUSPENSION CONCENTRATION RATIO Cr (10) --o oo No oW -N n o 0 m 8 o oo 0 00 0 0 0 0 (A 0 (XI



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Shear Stress Te in9 N N/2 0.435 Tm=0.9 N/m 0.281 T= 24 Hrs. Expt.6 d--= 0 N/m2 I20 0071 Td = 40 Hrs. --" 0 I 2 3 0 23 z 16 S 0.379 z 120.281 w 0.169 Expt. 6A ----z -0.071 o_ / 08 z 0 I 2 3 Hours


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-145I -. -f EQ Type Profile of ER Type Profile of c-t Curve I c-t Curve Te = Bed Shear Stress Tr < T2 ch W Te 2 E Tr Tel z Tcr 8 mi E .Ati. 1E -At2 S= Specified Percentage of Em zAz Si ... 7-.......... ........ Fig. 75 efi.........ton sketch for varous parameters....... Fig. 75 Definition sketch for various parameters



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-142


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-150vi) A plot of Ts as a function of depth can be made from the above data. vii) A plot of C(T ) versus e would give the value of the characteristic shear strength of the bed, Tch* Ariathurai (1977, p. 17) has noted the following: "To model the transport process, it is necessary to know the critical shear stress of each stratum of the bed and also the erosion rate if the erosive mechanism is surface erosion. At present laboratory measurements must be made to obtain these parameters." Thorn and Parsons (1980, p. 354) have concluded as follows: "It is possible to treat the data from all the mud tests as a single set and to derive general relationships which might be used for preliminary design calculations of navigation channel improvement and dredged spoil disposal schemes involving cohesive sediments. Nevertheless it is recommended that it is still necessary to determine the specific erosion characteristics of a particular sediment for the purpose of detailed prediction and design calculations. The underlying physical and chemical processes which govern the mud properties are not yet fully understood and the actual behavior of any particular cohesive sediment cannot be absolutely predicted from existing knowledge." The above comments describe very well the up-to-date stage in the understanding of erosional properties of cohesive sediment and justify the need for experimental determination of erosional characteristics. The test procedure recommended in this study would help in meeting the necessary objective.



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-78II Fig. 27 Equipment for determining concentration of sediment suspensions Fig. 28 Sa33-mpli64 mpl65ing bottes 9



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RING CONTROLLER METER READING 0 5 10 15 20 25 30 35 40 45 50 F04 ---. 130 S----,--------(----------1----^ ----I-,---^-------I-__^--.-II.8 4z 8 -.-----.---Water Depth z r 15 cm S-23 cm z o 6 q a .45 : 4) -040--7 15 -i C: I I 1 0~4a ^-Ln0.35 -2 z 0.3 i'u.30 1.:j 0.25 .25 -10 X -0 .2 ----. ----.__ -O^^ .2 0 2 4 6 8 10 12 14 RING SPEED (RPM) Fig. 29 Operational speeds and controller meter readings for ring and channel at different bed shear stresses



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-n c> 10 ) NORMALIZED EXCESS SUSPENSION CONCENTRATION ( AC)ex (D 01 -N 0 SI I I I I I I S. 1 O O x O ~ 0. 0 Oo ar .o -o ( -"'._oo o o I'D 0 0 Srn CD CD o 0 co oo o o X X 0 3 3 3 I> > \ r oo 1\ .-x x N ;13 r u ii ii ii i i -II I I X -* I II



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-734.3 Apparatus for Erosion Tests A system of rotating circular channel and ring was used for conducting experiments. The annular channel was 20 cm wide, 46 cm deep, and had a mean radius of 76 cm. The channel was made of 9.5 mm thick fiberglass. Four windows with a transparent plexiglass were provided on the channel to permit visual observations. The channel was supported on a rigid steel frame. An annular ring made of 6 mm thick plexiglass, having the same mean radius as the channel, was provided within the channel. The width of ring was smaller by 6 mm than the width of channel. The ring could be positioned at any required height within the channel and it could be freely rotated while in contact with the water surface. Taps were provided on the vertical outer wall of the channel for obtaining samples of suspension from the channel. Details of the apparatus assembly are given by Mehta (1973). Accessary equipment consisted of Millipore Filtering Apparatus, an oven, and a Mettler balance having 0.05 mg precision. For determining the concentration of sediment in suspension, the following procedure was adopted: i) Obtain a sample of the eroding fluid having sediment in suspension through the tap provided on the channel and collect it in a sampling bottle. ii) Measure volume of the sample. iii) Filter the sample through pre-weighed Millipore filter paper discs with 0.45 micron pore diameter. iv) Wash the salt using distilled water. v) Dry the filter paper discs in the oven at 600C.



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-1095.4 Correlation Plots of C30 and C60 Values Values of T adopted for various experiments reported under Sections 5.2 and 5.3 were obtained by discretization of bed shear stress varying with time. The data obtained from the tests was also at discrete time intervals. The magnitudes of suspension concentration obtained at the end of each time step wuch as C30, in the case of a 30 minute duration of time step were compared with the corresponding values of bed shear stress. The experimental conditions for various tests have been reported in Table 3 under section 5.3. A comparison of the mean variation of bed shear stress with the variation in magnitudes of C30 or C60 values, both as a function of time is given in Figs. 50 to 57 for experiments 8 to 15 respectively. All the plots indicate a striking similarity in the variation of suspension concentration and the variation of bed shear stress. 5.5 Analysis of Data The basic data available from the various experiments werein the form of concentration of suspended sediment as a function of time for different conditions of bed formation and for different time step functions of Te Superposition of the c-t plots as shown in Figs. 58 and 59 did not appear to give any meaningful results of a general nature. Hence all the c-t data for the beds having Td=24 hours and Td=40 hours was plotted to see if it showed any trend. The plots are shown in Figs. 60 and 61 respectively. Although both the figures show an increase in suspension concentration with increasing bed shear stress, there is considerable scatter in



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(D) Bingham Plastic (C) Plastic ( B) Pseudo-Plastic (A) Newtonian Ty U) I-, SHEARING RATE ( ) Fig. 2 Rheological models



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-12particles may have surface areas as much as ten times as great as the spherical particles of equal volumes. 2.2.4 Electric Charge Substitution of one ion for another in the clay crystal lattice and imperfections at the surface, especially at the edges, lead to negative charges on clay particles. Cations from the pore water are attracted to the particles (and anions repelled) to maintain electroneutrality. These are the exchangeable cations and their number is the cation-exchange capacity (i.e. the amount of negative charge per unit weight or per unit surface area) of the clay. This is usually expressed as milliequivalents per gram (me/g) or per 100 g (me/100 g). The force with which ions are held at the surface varies with the nature of the charge. The amount of charge for different clay minerals is given below: Kaolinite: 5 to 15 me/100 g Clay Mica and Chlorite: 20 to 40 me/100 g Montmorillonite: 80 to 100 me/100 g Vermiculite: 100 to 150 me/100 g The kind and number of exchangeable cations have an important influence on the behavior of soils, e.g. monovalent cations such as sodium increase the activity of the clay, its swelling, etc. 2.2.5 Inter-Particle Forces The behavior of clay particles is controlled more by the surface forces than by the gravity forces. Thus it can be shown that the



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-155100 Location Height Above I of Top Channel Bottom C 40 mm 2N A 125mm Td B 225 mm 60 -, -, ---.--, 40 @ 20\ \ \ 0 2 4 TIME (hrs) Fig. 80 Suspension concentration during deposition under zero bed shear stress



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Table 2. Continued Investigator Mode of Placement of Sample Mode of Measurement of Erodibility Lee (1979) Deposited Measurement of suspension concentration by filtering and weighing Mehta and Partheniades Deposited Measurement of suspension concen(1979) tration by filtering and weighing Yeh (1979) Deposited Measurement of suspension concentration by filtering and weighing Thorn and Parsons (1980) Deposited Measurement of suspension concentration by using photo-absorptiometer



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-110I E .En E (n I I ol 00 CQ tO (:j E o -n 040 IU, d0 c'J w 0 00 N Nr (?/U)6 0OJV-N0O OS~ss o, c'j q ( 2U/ N) SS3dlS ?IVB3HS GJ8 I I I i I I (c>/ui) NOIllJNIN33NO3 NOISN~dSflS



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Tm = 0.9 N/m2 40 Tm 24 hrs 1 Td = 0 N/m2 z o T = 40 hrs U 0 Ho Fz w n z 0 o 0 U z 200 0 o od o o o o S) 0 8 D 8 U) o0o 80 O S0 0 000 0' 9 0 08oO 0 0 0.2 0.4 0.6 0O8 BED SHEAR STRESS T. (N/m2) Fig. 61 Variation of suspension concentration with bed shear stress, all data for Td = 40 hours



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-84T --Top Cylinder 15cm dia. L2 L 22.5 cm Plastic Tubes of various heights, 0.95 cm dia. glued to the bottom plate Bottom T -Cylinder 7.5 cm 15cm dia. Bottom Plate SKETCH OF APPARATUS I ,1--2 cm dia plastic tube jT 15 cm dia. plexiglass cylinder 15cm 2.5 cm dia metal tube -Annular space for mixture of alcohol and dry ice Sediment Metal-! i Plate Filled with ice cbes Porcelein P I iston with Screw Rod Dish SKETCH OF APPARATUS TI Fig. 31 Apparatus developed for measurement of density as a function of depth for deposited beds



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E 12z 0 2-10-5 U 8-4 O z O W o > z 6-3 o S .J -CONCENTRATION S u .-----AVERAGE FLOW VELOCITY ---4 -l L__2 Ext Series I --D n run run run i_ run run run n run run run run -I +2a7' 31 14 -5 1 '6 7 I -T0 I 12 -13 .200 400 600 800 1000 1200 1400 TIME AFTER START OF SERIES -I, hrs Fig. 3 Concentration versus time plot obtained in erosion of placed bed: Partheniades (1962) Expt. Series I



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Ts1 1 I min Bed Shear Stress Te e 3 Ts2 < t2 min T Tcr Te Th -J.. F g ...7. Exp a.r ., "r "pI..-......-..... Fig. 77 Explanatory sketch for ER Type profile of c-t curve



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-29Most soils regain only a fraction of the volume lost during consolidation. This results from the fact that in order to undergo volume change, the soil particles are displaced relative to one another to assume a more closely packed condition, and consequently greater density. In sands these movements are not reversible, and in most clayey soils, they are only slightly reversible. For sand, the volume decrease is proportional to the logarithm of the pressure. For clay, the relation is not linear although at higher pressures, it is approximately so. In a process of continuous sedimentation in water, the soil at any depth is being consolidated under the influence of the weight of the soil above it. Since new material is continually being added, a gradient tending to cause the pore water to flow upward out of the system exists at all points within the stratum. If the material being deposited is sand, consolidation takes place at a rapid rate since the spaces between particles are relatively large and the water can escape readily. Furthermore, the sand particles are essentially inert and are not greatly influenced by adjacent particles, and the initial position of each grain within the mass is relatively stable. Very little volume change can take place in a sand deposit except in certain circumstances as a result of outside influences such as mechanical vibration or shock. Therefore, sands can be considered to be virtually completely consolidated at all times even when the accumulation of sediment is rapid. Consolidation of clayey deposit proceeds at a comparatively much slower rate. The total pore space in a clay mass is large but is composed of a multitude of small channels between the individual particles. The flow of water in the channels is restricted by their small size



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-38Deposited from suspension: Yeh (1979) Compacted: Christensen and Das (1973) d) Characterizing indices: Dielectric dispersion: Alizadeh (1974) Sodium adsorption ratio: Kandiah (1974) Cation exchange capacity: Kandiah (1974) Chemical and electrical parameters: Arulanandan et al. (1973) e) Basic parameters: Bed density and salinity: Owen (1977) Temperature: Gularte (1978) pH: Kandiah (1974) Pore fluid and eroding fluid: Arulanandan et al. (1975) Water quality (pH, conductivity): Migniot (1968) f) Microstructure studies: Kaolin: McConnachie (1974) Marine sediments: Bowels (1969) g) Other studies: Hydrodynamic aspects: Turbulent drag reduction: Gust (1976) Colloidal dispersion: Zeichner and Schowalter (1977) Attempts have been made from time to time in the past to review the information available in respect to cohesive sediments. These are listed below in chronological order: 1960: The Committee on Tidal Hydraulics, U.S. Army Corps of Engineers conducted literature review to study soil as a factor in shoaling processes (Committee on Tidal Hydraulics, 1960).



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-35The liquid limit expresses the overall effect of the inter-particle forces within the clay mass and this soil index varies with clay mineralogy and with the associated cation. The following data given in Table 1 were presented by the Committee on Tidal Hydraulics (1960). Table 1. Specific surface area and liquid limit for typical clays. Primary Specific Clay Associated Surface Liquid Mineral Cation Area Limit Montmorillonite Na 847 710 Montmorillonite H 768 490 Illite (< 2 1) H 79.8 100 Kaolinite H 13.1 53 Kaolinite (< 5 v) H 26.1 110 Many clay soils exhibit the property of rheotropy at water contents above the liquid limit, and also to a lesser degree at water contents in the plastic range. Rheotropy is the change to a more fluid consistency on stirring or disturbance. When the disturbance has ceased, the system reverts to its less fluid or more rigid condition. This is often called thixotropy, although the strict definition of thixotropy is a reversible, isothermal sol-gel transformation. A sol, by definition, has no yield value, while a gel has rigidity. The change in clay-water systems is generally from a system with higher yield value to one with a lower yield value. A sol may be considered as a colloidal dispersion. This restricts sols to liquid-like behavior.



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-rl SUSPENSION CONCENTRATION (gm/e) ro ---o 04 ) v, oa .o 4 roo e o -C I I I I I I I I I -r o 0 0 2 20 0 o .0 02 z 1\ m x N T3 3 (D 8 o U) (D 0( 0



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-Shear Stress Te in N/m2 S220.45 CP 0.20 S0.20 Tm= 0.9 N/m 18SHours Tm =24Hrs. 0 Hours -Td = 0.015 N/m2 S14-Td = 24Hrs. z S10() CL I I I I I I I I z z 6 D I a0 20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 33 Suspension concentration versus time for Experiment 3



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-9In such materials the non-clay is frequently not much coarser than the maximum for the clay grade, and the clay mineral fraction may be particularly potent in causing plasticity. In general, fine grained materials have been called clays so long as they have distinct plasticity and insufficient amounts of coarser material to warrant the appellations "silt" or "sand." If particle size anslyses are made, the term clay would be reserved for a material in which the clay grade dominates. However, names have been and are applied most frequently on the basis of appearance and bulk properties of the material. The expression clay material is used for any fine-grained, earthy, argillaceous, natural material. Clay material includes clays, shales, and argillites. It would also include soils, if such materials were argillaceous and had appreciable contents of clay-size-grade material. Clay particles are usually within a range of diameter smaller than 0.002 mm, but larger than the molecular size (10-6 mm). For the civil engineer, the fine sediments eroded from the earth's crust are of interest and the term clay is primarily a particle size term. For the chemical engineer the interest includes synthetic and other materials in their fine form, rather than the natural clay minerals. There are three levels of first order fabric recognition of clay structure. These are categorized on the basis of the degree of magnification required for a proper observation of the fabric pattern. 1. Macroscopic: The fabric units are distinguishable by the naked eye. They consist of an aggregation of clay particles called peds. 2. Microscopic: The fabric units are visually observed under the light microscope. For clays, single particles are not distinguishable



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I--2I 0.21 t | 2Bed Shear Stress T in N/m2 017 0.175 0.145 5 0.12 750.10 I E 6 --J o I 2 3 4 C-) Ir 5 T= 0.9N/m2 z Tm = 24 hrs co z Td = 0.015 N/m2 0 4 u Td = 40 hrs z / _0.432 o -------(nz 3 w 0.36 J 00. a2 Continued I 0.25 from above I I I I 5 6 7 8 9 TIME (hrs) Fig. 83 Suspension concentration versus time for Experiment 18 \



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-16of the flocs. Brownian motion is the erratic movement of small suspended particles caused by the thermal agitation of the suspending medium, which enables particles to come in contact with each other. The inter-particle forces have been described in the previous paragraph. Whatever the origin of the surface electric charges, any such charged particle in an ion containing water will attract ions of opposite charges, called "counter-ions,' to compensate its own electric charges. At the same time, the ions tend to diffuse away from the surface because of their thermal activity since such a diffusion takes place from a zone of high concentration to a zone of lower concentration in a way analogous to the diffusion of the molecules of the air in the atmosphere. Thus a clay particle idealized by a thin rectangular plate will be surrounded on either side by a diffused layer of counter-ions whose positions will be determined by the balance of the electrostatic attraction and their thermal activity. This layer is known as a "double layer" and it plays a dominant role in the mechanical properties of suspended clays and clay deposits. The system of clay particle and double layer is electrically neutral and is known as "clay micelle." Figurel.D shows a simplified, schematic diagram of a clay micelle. Any form of agitation, including Brownian motion,will eventually cause two particles to approach each other sufficiently close for their double layers to interact. This interaction causes changes in the distribution of the cations in the double layer of both particles. The result will be determined by the potentials of the van der Waals force and the electric forces. Sufficiently far from the particle, the repulsive forces may dominate, whereas closer to the surface the net effect would be attractive which causes flocculation of particles.



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-26Mg ions. Fluids with the same salt concentration can have different SAR. An increase in SAR decreases cohesion, lowers the critical shear stress, and hence increases the erosion rate (Kandiah, 1974). Alizadeh (1974) has presented experimental data to this effect. Kandiah (1974) also studied the effect of pH and concluded that "pH influences the interparticle cohesion which strongly affects aggregation and dispersion of clay soil properties" (p. 118). The effect of water temperature on erosion rate has been studied by Grissinger (1966), Christensen and Das (1973), Randkivi and Hutchison (1974), and Gularte (1978). It is observed that the temperature had a significant effect on the strength of inter-particle cohesion and the rate of erosion increased significantly with increasing temperature. Kandiah (1974) showed from particle-by-particle surface erosion of a remolded illite soil that the critical shear stress for erosion varies as Tc = 1.8 x 10-5 exp[4100/T] (2.3.7) where T is the absolute temperature in the Kelvin scale. The critical shear stress dropped from 3.6 to 0.8 N/m2 over the temperature change from 9.50C to 420C. 2.4 Processes for Deposited Beds The deposited beds are distinguished from the placed or remolded beds because of the fact that they are formed by the process of settling of the sediment in suspension. The settling may take place under quiescent conditions or under a low shear stress which permits



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CHAPTER VII SUMMARY AND CONCLUSIONS 7.1 Summary of Literature Review A review of literature on the parameters influencing the properties of fine sediments and hence their erosion rates indicated the following: i) Each of the following parameters has a measurable effect: Sediment: Composition: Clay minerals Non-clay minerals Percentage of clay in the mixture Organic matter Side gradation Type of bed: Placed/Remolded (Density Compacted structure) Deposited (stress history) Fluid: Composition of pore fluid Composition of eroding fluid pH Temperature Hydrodynamic factors: Bed shear stress Bed-fluid interface properties ii) The results of erosion studies are affected by the following: Type and size of apparatus used Method for measurement of erosion Method of applying shear stress (by a free surface flow of eroding fluid, rotating the apparatus, rotating the ring alone or rotating the ring and the channel both etc. and by the rate of application of bed shear stress). iii) The following soil parameters have been used by the various investigators in order to correlate them with the shear strength of -164-



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-168correspond to Ts at the respective depths of the bed. A plot of the concentration at the end of Ts as a function of Te should give the value of Tch*



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-67iii) Owen (Nov. 1977, p. 11) conducted studies on erosion of Avonmouth mud and concluded the following: "In terms of mean shear stress the onset of continuous erosion is almost simultaneous for mud beds of different density, but the rate of erosion is greater for mud beds of lower density." iv) Thorn and Parsons (July 1977, p. 8) studied properties of Grangemouth mud and made the following observation: "There does not seem to be any strong relationship between bed shear stress and surface density, although further tests would be needed to establish this with confidence. This result is rather surprising as it would seem likely that the thicker or denser the mud the more resistant it should be to erosion. The surface density at equilibrium was divided by mean bed density to give a relative density but this did not give any stronger relationship with bed shear stress. This is an interesting result because an earlier investigation of Avonmouth mud showed that both equilibrium surface density and relative equilibrium surface density were linearly related to bed shear stress." v) Arulanandan et al. (1980) studied the effect of changing the density structure of bed by remolding the soil and found that remolding generally decreased both the critical shear stress and the rate of change of erosion rate. They also found that the salt concentration of eroding fluid influenced the erosion of remolded soil samples. A decrease in salt concentration of eroding fluid decreased the critical shear stress and increased the rate of change of erosion rate. .-E



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-25Owen (1970) reported that the shear strength variation within the bed could be correlated satisfactorily with the variation of density. The shear strength was observed to increase rapidly with density. The method of formation of bed namely, remolded, deposited, or compacted with external force, have an effect on the density structure of the bed and hence on the erosional property of the soil. d) Properties of the pore fluid and the eroding fluid (salt concentration, sodium adsorption ratio, pH, temperature). In the case of a fully saturated soil which has been equilibrated with the eroding fluid over a sufficiently long time, the pore fluid and the eroding fluid are expected to have the same properties. However, in case they have different properties, osmotic pressure gradient is formed which changes the properties of the pore fluid and this in turn may change the erosional properties of the bed. The soil having an aggregated structure resists erosion more effectively than a soil having a dispersed structure. This is because the net force between the particles in an aggregated structure is attractive while in the latter it is repulsive. Since the pore fluid has a substantial influence on the structure, it influences erosion. An increase in the salt concentrations of the pore fluid usually increases resistance to erosion. Sodium Adsorption Ratio (SAR) is defined as Na+ SAR = Na (2.3.6) [(Ca+ + Mg+)]1/ The concentrations of individual ions are in milliequivalents per liter. The SAR is used as an index to characterize the pore fluid and eroding fluid in terms of the relative strength of the Na, Ca, and



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-6The fine sediments are interacting particles which attract or repel each other due to the presence of electric charge. Hence, the noninteracting particles remain separate from each other, whereas the interacting particles can form flocs under suitable environment. 2. If a small sample of moist silt is shaken easily but rapidly, water will appear on the surface but disappear when shaking stops. This phenomenon is referred to as dilatancy. The non-interacting particles show dilatancy with high concentration of sediment, whereas a sample of moist clay when shaken similarly does not show wetting of the surface. 3. The interacting particles exhibit elastic or plastic properties. 4. The non-interacting particles are usually unsaturated, whereas the flocs of interacting particles are saturated with water molecules. 5. With a low concentration of sediment, the suspension of noninteracting sediment is close to a Newtonian fluid, where the deformation is linearly proportional to shear stress. The suspension of interacting particles is non-Newtonian in behavior. 6. The erosional, transport, and depositional characteristics of non-interacting sediments are based mainly on the physical properties such as size, specific gravity, compaction, etc. The properties of the fluid such as salinity, pH, temperature have no substantial effect. On the other hand, the fluid properties have a substantial effect on the formation of flocs and hence on the erosional and depositional properties of interacting particles. 7. Fine sediments have a relatively much higher compressibility than the coarser non-interacting sediments.



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-61-e =True Angle of Internal Friction (a) (Pd =Angle of Drained Shear U, Resistance ST EFFECTIVE NORMAL STRESS CT -.(b) IS, Reference: 5u McCarthy (1977) -," NORMAL STRESS OCD = Consolidated, Drained Soil C-U = Consolidated, Undrained Soil U-U = Unconsolidated, Undrained Soil (c) I-4b Reference: uj z McCarthy (1977) -n -5 % WATER CONTENT Fig. 16 Schematic diagrams showing shear strength of cohesive soil related to other parameters



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-59Symbol Series x -KS o -FS 30* -KD KS= Kaolinite in Salt Water FS = Fernandina Bay Mud in Distilled Water -KD = Kaolinite in Distilled Water z 0 S20cr 10I Z LJ Z 0 / o0 0 0 0O 0.2 0.4 Tb BED SHEAR STRESS (N/m2) Fig. 15 Concentration as a function of bed shear stress obtained by Yeh (1979)



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26 Shear Stress Te in N/m2 Tm = 0.9 N/mr 220.7 T= 24 Hours 0.6 2 S -0 Td =0 N/m S18J--4 Td =40 Hours S180.3 O. 3 z 0.2-0 0.1 140 I 2 3 z -Hours z I 10o i 0 z 6U) 2 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 40 Suspension concentration versus time for Experiment 8



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-60ce = true cohesion, being only a function of the void ratio of the material (Fig. 16(a)) (e = true angle of internal friction, practically independent of the void ratio a = the effective pressure normal to shear plane For normally consolidated clays, the following expression is used: s = pc tan d (3.4.3) where P = consolidation pressure for 100 percent consolidation 6d = angle of drained shear resistance The magnitude of Pd can be much greater than


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$ 0.45 E z 0.20 A 014< O I 2 3 z Bed Shear Stress Te w 10-in N/m2 0 0.37 z Q281 8 -B 0 0169 B /B 0.071 z 6S0 I 2 3 z Hours a. n2c SI I I 3 0 t 0 20 40 60 80 100 120 140 160 TIME (Minutes) Fig. 58 Effect of shear stress variation on suspension concentration



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CHAPTER V TEST RESULTS AND ANALYSIS 5.1 Effects of Parameters in Steps I, II and III It has been mentioned in Section 4.3 (Fig. 19) that the process of bed formation for the present study consisted of Step I: Mixing; plus Step II: Settling and consolidation. The parameters involved were Tm and Tm for Step I, and Td and Td for Step II. It was observed that increase in suspension concentration resulting under a given time-step function of bed shear stress was a function of the shear stress-time history during the stage of bed formation. Change in suspension concentration resulting from a change in magnitudes of parameters under Step I is shown in Fig. 32. The magnitude of shear stress for deposition Td, and the duration of consolidation Td which are the parameters under Step II had a substantial effect on the rate of erosion. Three beds of different density structures were formed for three different combinations of Td and Td. The concentration-time plots obtained for T = 0.2 and 0.45 N/m2 with a Ts = 90 minutes are given in Figs. 33 to 35. The effect of Step II parameters is seen from the superposed results of experiments 3, 4 and 5 given in Fig. 36. For a given bed, a change in sequence of magnitudes of T and T results in different concentration time plots as shown in Fig. 37. -87-



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CHAPTER III PREVIOUS LABORATORY STUDIES 3.1 General Review Over the past two decades, considerable laboratory work has been carried out on cohesive sediments. In order to get an idea about the variety of different ways in which the research work has been carried out, some of the topics under which the literature could be classified are given below along with a typical reference on the same as an illustration. a) Sediment used: Clay mineral alone: Kaolinite: Christensen and Das (1973) Mixture of clay minerals: Yolo Loam: Arulanandan et al. (1975) Mixture of clay and silt: Grundite: Gularte (1978) Natural sediments: Brisbane Mud: Thorn and Parsons (1980) Fernandina Mud: Yeh (1979) San Francisco Bay Mud: Partheniades (1962) b) Fluid used: Salt water: Partheniades (1962) Distilled water: Mehta and Partheniades (1979) Fresh water: Fukuda (1978) c) Type of bed: Remolded: Gularte (1977) -37-



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-27deposition of the suspended material. The settled material may undergo consolidation and form a bed. The aspects related to the settling and consolidation which are the primary processes in formation of deposited beds are briefly described below. 2.4.1 Settling The effect of Brownian motion on a suspension of sediment under apparent quiescent conditions is to induce inter-particle contacts. This may result in bonding of particles through the process of flocculation (Einstein and Krone, 1962). The mechanics and importance of flocculation have already been described. Hence only the parameters which influence settling velocity of the flocs are briefly described here. Owen (1970) conducted detailed study of the settling velocities of an estuary mud and noted the following conclusions: i) Suspended sediment concentration: The median settling velocity increases with concentration up to a value between 4 and 20 grams per liter, depending on the salinity, at which hindered settling begins. For a concentration higher than this, the settling velocity reduces. ii) Salinity: The median settling velocity increases with salinity (except during hindered settling) up to a value between 28 and 43 grams per liter depending on the concentration. For higher values of salinity, the settling velocity decreases. It may however be noted here that certain clays such as Kaolinite flocculate even in distilled water. iii) Depth: The effect of depth of settling is fairly complex, the settling velocity reducing to a minimum at a depth of 1 meter



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26 Shear Stress Te in N/m 2 Tr =0.9 N/m2 0.760 m 2205 T = 24 Hours 0.506 m Td =0 N/m 0.337 d N 18 0225Td = 40Hours 0.15 z 0.1 1 4 ------I-------0 I 2 3 FHours w z 100 z 0 m 6z w a .2Cl) 0 20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 43 Suspension concentration versus time for Experiment 11



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-171Kandiah, A., "Fundamental Aspects of Surface Erosion of Cohesive Soils," Ph.D. Dissertation, University of California, Davis, November 1974. Krone, R. B., "Flume Studies of the Transport of Sediment in Estuarial Shoaling Processes," Final Report, Hydr. Eng. and Sanitary Eng. Res. Lab., University of California, Berkeley, California, June 1962. Krone, R. B., "Aggregation of Suspended Particles," Unpublished Lecture Notes, Dept. of Civil Engineering, University of California, Davis, 1976. Laflen, J. M., and Beasley, R. P., "Effects of Compaction on Critical Tractive Forces in Cohesive Soils," Res. Bull. 749, Agricultural Expt. Station., University of Missouri, September 1960. Lambermont, J., and Lebon, G., "Erosion of Cohesive Soils," Journal of Hydraulic Research, 16, 1978, No. 1. Lee, D. Y., "Resuspension and Deposition of Lake Erie Sediments," M. S. Thesis, Case Western Reserve University, Cleveland, Ohio, August 1979. Liou, Y. D., "Effects of Chemical Additives on Hydraulic Erodibility of Cohesive Soil," M. S. Thesis, Colorado State University, August 1967. Liou, Y. D., "Hydraulic .Erodibility of Two Pure Clay Systems,"Ph.D. Thesis, Colorado State University, 1970. Lutz, J. F., "The Physico-Chemical Properties of Soils Affecting Soil Erosion," Res. Bull. 212, Agricultural Expt. Station, University of Missouri, July 1934. Lyle, W., and Smerdon, E., "Relation of Compaction and Other Soil Properties to the Erosion Resistance of Soils," Trans. American Society of Agri. Engineers, 8, 1965, pp. 419-422. Masch, F. D. Jr., Espey, W. H. Jr., and Moore, W. L., "Measurement of the Shear Resistance of Cohesive Sediments," Publication 970, Agri. Res. Service, 1965. McCarthy, D. F., Essentials of Soil Mechanics and Foundations, Reston Publishing Co. Inc., A Prentice-Hall Company, Reston, Virginia, U.S.A., 1977. McConnachie, I., "Fabric Changes in Consolidated Kaolin," Geotechnique, 24, No. 2, 1974, pp. 207-222. McDowell, D. M., and O'Connor, B. A., Hydraulic Behaviour of Estuaries, John Wiley and Sons Inc., New York, N.Y., 1977, Chapter 4. Mehta, A. J., and Partheniades, E., "Kaolinite Resuspension Properties," Journal of the Hydraulics Division, ASCE, Vol. 105, No. HY4, April 1979, pp. 411-416.



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Shear Stress Te in N/rn2 Tm =0.9 N/m' Tm = 24 Hrs. S0.435 Td =0 N/m2 E 20 0.379 Td =40 Hrs. O 0.281 < 0.015 z a: I 0 1 2 3 Z 12 Hours U Z 8 z 8 o z .4 (I) 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 48 Suspension concentration versus time for Experiment 15



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-18due to the internal shearing of the suspending water. Aggregates formed by fluid shearing are denser and stronger than those formed by the Brownian motion or by differential settling velocities. The aggregates can be dispersed by local high shearing rates and may re-aggregate later. The settling velocities of aggregates and the probability of their sticking to the bed are determined by the size, density, and shear strength. Mineral particles cohering in a cluster with uniform porosity are called primary aggregates or flocs. At lower shearing rates, primary aggregates would collide with each other and bond to form first-order aggregates. At still lower shearing rates, the first order aggregates would collide and bond with each other to form weaker, lessdense second-order aggregates, and so on. Each higher-order aggregate would include water in the new pore volume formed, and because shear stress can be transmitted only through inter-aggregate contacts, the higher order aggregates are weaker. A procedure for determining the order of aggregation has been given by Krone (1976). 2.3 Parameters Influencing the Properties of Cohesive Sediments An attempt wasmade by the Committee on Tidal Hydraulics (1960) to identify the soil parameters which affect the process of shoaling in estuaries. However, the literature review contained in the report is limited. Based on the findings of subsequent research, Paaswell (1973) reviewed the causes and mechanism of cohesive soil erosion and identified the followingparameters used in evaluating the erosion of cohesive beds:



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LIST OF TABLES Table Page 1 Specific surface area and liquid limit for typical clays 35 2 Summary of selected studies on cohesive soil erosion 41 3 Experimental conditions for tests conducted with a multiple steps of e 95 v



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Table 2. Continued Investigator Mode of Placement of Sample Mode of Measurement of Erodibility Partheniades (1965) Remolded natural deposited (salt Measurement of suspended sediment water) in duct concentrating with time Grissinger (1966) Remolded in channel Rate of erosion by weighing Masch, Espey, and Moore Unspecified but trimmed as hollow Weight loss versus rotating shear; (1965) cylinder visual correlated with shear Mirtskhulava (1966) Remolded in flume Weight of floc loss Liou (1967) Remolded in flume Point-gauge measurement of erosion depth Liou (1970) Remolded in flume Arulanandan et al. (1973) Molded in ring Weight comparison Christensen and Das Remolded in tube Weight comparison (1973) Grissinger (1973) Natural samples remolded in channel Rate of erosion by weighing Sargunam et al. (1973) Remolded (compacted) in rotating Weight loss of sample cylinder test apparatus Alizadeh (1974) Remolded (compacted) in rotating Weight loss of sample cylinder test apparatus



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-49E -10 z o uw r88-40z ou 0 < 6 60: w 4 -2 S2rn run run -CONCENTRATION w S.--VELOCITY (1 0r--___,_____________________________,____, 0 200 400 600 800 TIME AFTER STARTOF SERIES -I,hrs E ) 4 0 z w run 22 tn **^ 23 0 20 40 60 80 100 TIME AFTER START OF RUN, hrs Fig. 4 Concentration versus time plot obtained in erosion of remolded bed: Partheniades (1962), Expt. Series II



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-146Thus EQ profile would be identified by a definition such as say for S= 5 percent of m By increasing the magnitude of re at every Ts, the bed would be eroded layer by layer. In order to obtain useful information, it is necessary to increase re in small increments. Since it is observed from the experiments described under Chapter V that the suspension concentration C2 is a function of not only Te2 but also that of C1 and Te, it is proposed to use the normalized excess shear stress ee as the incremental factor. Thus (AT)ex = 0.2 would mean that the next value of Te is greater by 20 percent the previous value of re. When the c-t graph is in the form of an EQ profile, it indicates that the erosion rate at the end of Ts is a very small percentage of the maximum erosion rate and hence it is apparent that under these conditions the bed shear stress Te must be only very slightly greater than the shear strength of the bed. It may thus be assumed that Te = s (Fig. 76). When the c-t graph is in the form of an ER profile, it indicates that either Ts was too small or Te > Tch. In the first case, the ER profile would take the form of an EQ profile if sufficiently long duration of Ts is provided. In the second case, no matter what the duration of Ts is, the bed will continue to erode indefinitely until the entire depth of the bed is eroded. It is apparent that this condition results under re > Tch which is the maximum shear strength of the bed (Fig. 77).



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(A) AFTER 250 MINUTES (B) AFTER 500 MINUTES 10 -1 i09 .-----------0 ------07 0 7 _.. ..... 06 ---0 --04 -----------------04 ---------S0L -------" 4--01 1-70N Ig/l 05 00 o 0 02 04 06 08 1.0 I I 14 16 8210 22 2. 26 2.8 300 02 04 0-6 08 10 1.2 I1 1.6 10 20 22 24 26 28 30 0O (C) AFTER 1000 MINUTES (0) AFTER 2000 MINUTES I.0 .SUSPENSWOV Li 09 ---09 ---Oes 08 ----------------os -----7 4 0 0 -------0 O I Wp" OENSITY/MEAN DENSITY CALCULATED DENSITY PROFILES FOR VARIOUS BED THICKNESSES Fig. 18 Bed density profiles: Owen (1970) 04 (---I-1--II--l----I------I 04 .0 -----------(D -MI Sa 01 I----I---W \-t--~----1--I 01 --0 0 0V4 ENSITY/HEAN D0NSITY



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TABLE OF CONTENTS Page ACKNOWLEDGEMENTS ................... .... ..ii LIST OF TABLES ................... .......v LIST OF FIGURES ................... ......vi NOMENCLATURE ................... ..... ...xi ABSTRACT ................... .......... .xiii CHAPTER I INTRODUCTION ....................1 II FINE SEDIMENTS ................... ... 5 2.1 General Description ...............5 2.2 Properties of Fine Sediments ..........7 2.3 Parameters Influencing the Properties of Cohesive Sediments ....................18 2.4 Processes for Deposited Beds .......... 26 2.5 Clay-Water System ................ .31 III PREVIOUS LABORATORY STUDIES .............37 3.1 General Review .................37 3.2 Review of Literature on Erosion .........39 3.3 Review of Literature Pertinent to the Present Study .....................45 3.4 Shear Strength of Clay .............54 3.5 Shear Strength and Bed Density of Clay ..... 62 IV PRESENT INVESTIGATION ................ .68 4.1 Objective ................... ...68 4.2 Material ....................71 4.3 Apparatus for Erosion Tests ........... 73 4.4 Experimental Procedure ............. 74 4.5 Apparatus for Measurements of Bed Density .... 83 V TEST RESULTS AND ANALYSIS .............. .87 5.1 Effects of Parameters in Steps I, II and III .87 5.2 Multiple Steps of T ..94 5.3 Discretized Sinusoidal Velocity Variation 94 5.4 Correlation Plots of C30 and C60 Values ..... 109 5.5 Analysis of Data ................109 iii



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Tm= 0.9 N/m2 Expt. 14 E 40Tm = 24hrs --Expt. 13 / r Td = 0 N/m2 Td = 24hrs a / / z / z .. W 70 0 20/ Vaues60) Values 0" ,n P/ 0 0.2 04 0.6 0.8 BED SHEAR STRESS Te (N/m2) Fig. 65 Variation of suspension concentration with bed shear stress for two different discretized time step functions II



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-17It has long been observed that gentle stirring promotes flocculation. This is due to the velocity gradients which are induced in the liquid causing relative motion of the particles. Such velocity gradient-controlled flocculation is called orthokinetic flocculation. A simple theory of flocculation kinetics can be derived for a uniform liquid shear field, giving a constant velocity gradient. Such constant velocity gradients are difficult to achieve in practice; the closest experimental form has been in the annular gap between coaxial rotating cylinders, also known as Couette apparatus. Consequently, the theory has to be extended to velocity gradients created in turbulent flow conditions. The orthokinetic rate of flocculation has a high dependence on initial particle size, is linearly dependent on velocity gradient, and it is independent of temperature. While considering Brownian motion, it is useful to consider one particle (the collector) as stationary, and to calculate the diffusion rate caused by Brownian movement of other particles to this collector. Because particles become attached to the collector, and are therefore removed from the suspension, a concentration gradient is formed radially outwards from the collector. This diffusion-controlled flocculation is called perikinetic flocculation. Temperature and viscosity effects are significant under perikinetic flocculation, and the rate of flocculation is independent of particle size. In the benthic boundary layer, orthokinetic (i.e. velocity gradient-controlled) flocculation has a much greater influence on the frequency of collision between particles than perikinetic (i.e. diffusion-controlled) flocculation (Williams, 1980). The effect of flocculation is formation of aggregates. Repeated inter-particle collisions in a turbulent flow field are predominantly



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-Step I Step U -Step III ---Mixing Deposition and Consolidation Erosion Tm Cn Te3 w T in Td F Tm ---'Td ---Ts --Tsetc. -Step I and Step IH = Pre-erosion stress history STm= Bed Shear Stress for Initial Mixing Tm = Duration of Initial Mixing Td = Bed Shear Stress for Deposition and Td = Duration of Deposition and Consolidation of Sediment Consolidation Tel Te2, = Bed Shear Stress for Erosion Ts = Duration of Time Step Fig. 19 Definition sketch for notations used to describe experimental conditions '*1 I,, ^ ^ ^ ^ ^ „ ......



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-134(d) After studying the properties of Grangemouth mud, Thorn and Parsons (1977) concluded that "There does not seem to be any strong relationship between bed shear stress and surface density" (p. 8). However, later on (1980) based on the studies of the properties of three different cohesive muds using the same experimental and analytical technique, the same authors suggested the following relationship (p. 352): -6 2.28 T = 5.42 x 106 228 (6.1.1) c s where Tc = critical shear stress for erosion at the exposed surface Ps = surface density The correlation observed by Thorn and Parsons is given in Fig. 70. The contradicting or rather confusing conclusions drawn from the correlation of bed density and bed shear strength of fine sediments are due to the following reasons: i) The shear strength depends upon the condition of soil, viz. U-U (unconsolidated-undrained), C-U (consolidated-undarined), or C-D (consolidated-drained). This has been discussed under Section 3.4. ii) The shear strength of cohesive soil is dependent upon two factors: physical component due to frictional resistance and interlocking between particles, and the physico-chemical component due to interparticle attractive and repulsive forces. The contribution of each component cannot be measured separately. The inter-particle forces include electrostatic forces, inter-molecular forces, cation bonds, water dipole linkaage, chemical cementation, hydrogen bond, and van der Waals Forces. These forces are either a



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CHAPTER I INTRODUCTION Study of the properties of transportation and deposition of sediments has engaged the attention of several research workers. The motive behind these studies has primarily been to assist in the design of dams and irrigation canals, water treatment and sewage disposal works, navigation channels, docks and harbors, etc. Special phenomena such as mud-banks along the shorelines, density flows at docks, flocculation and deposition of sediments due to mixing of salt water and fresh water in estuaries are associated with the fine sediments. Among various ways of classifying the sediments based on their geological origin, chemical properties, physical properties, etc., one of the classifications has been to treat the fine sediments of various compositions as a class in itself due to their special properties which differ considerably from the other sediments, and hence they need to be studied separately. The aspects related to fine sediments in some of the engineering projects are illustrated by case studies. In the Mersey Estuary, England, because of the abundance of fine cohesive sediment, attempts to increase the depths in one channel by removing 2.3 million m3 of material each year failed utterly. The channel in fact became shallower after five years of intensive dredging than it was before. Another example is Savannah harbor in the U.S.A., where in spite of continuous -1-



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-81followed and the notation used. Initial mixing was carried out at a shear stress Tm over a mixing duration of Tm. The bed shear stress was then reduced to Td to permit settlement of the suspended sediment and form a bed. The time for settling plus consolidation is indicated by Td in the sketch. All the tests reported in this study were conducted with a total depth of 30.5 cm in the channel which was kept constant. The quantity of individual sample withdrawn from the rotating channel was of the order 20 c.c. out of a total volume of 300 liters in the channel. With a suspension concentration of say 2 percent, the amount of sediment withdrawn at each sample was about 0.4 grams out of a total of 12 kg of sediment in the channel. The quantity of water and sediment withdrawn from the channel were small enough not to have any measurable effect on the fundamental processes taking place during the experiment. A large quantity of the eroding fluid having properties identical to that in the channel was kept in stock. This was used to replenish the eroding fluid by adding small quantities from time to time over the duration of experiment in order to keep a constant depth. Sediment was replenished at the mixing stage of the next experiment. The sediment and the eroding fluid was the same throughout the study. All the tests were conducted on the deposited bed. No remolding or compaction with external force was carried out. ii) Erosion of bed: The bed shear stress (re) varying in its duration and magnitude was applied by rotating the ring and the channel in accordance with the calibration curves. The time-step function of bed shear stress is shown schematically in Fig. 19. For any one experiment, the duration of time step (T ) was kept constant (such as 30 min,



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-83time graph for each experiment. Further analysis of these data was used to study the variation in suspension concentration as a function of bed shear stress and for computation of erosion rates. Although the room housing the rotating channel assembly was airconditioned, facilities to maintain a constant temperature of water in the channel were not available. Hence, typically a change in the water temperature of the order of 2 to 30C took place over the duration of the experiment. 4.5 Apparatus for Measurements of Bed Density During the course of the present study, two different apparatus were developed for the following measurements: (a) Measurement of bed density for sediment deposited under quiescent conditions (Td = 0). The apparatus developed for this purpose consisted of a 30 cm high, 15 cm dia. polyvinyl chloride cylinder provided with a bottom plate. Ten plastic tubes of various heights ranging from 0.95 to 6.35 cm, all having a 0.95 cm inner diameter, were glued to the bottom plate (Fig. 31). The cylinder was made in two pieces, the bottom cylinder being 7.5 cm high (photograph in Fig. 26). After placing the 22.5 cm tall piece of cylinder on the bottom cylinder, the circumferencial joint was sealed with a tape to make it water-tight. The cylinder was then filled with a sediment suspension of known concentration. The sediment was allowed to deposit under quiescent conditions (rd = 0) for the required consolidation time (Td = 24 hrs, 40 hrs, etc.). The supernatant water was siphoned out, and the top cylinder was removed after peeling the tape off. All the



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o At Elevation A(125 mm Above 30 -Channel Bottom) SA x At Elevation B (225 mm Above Channel Bottom) E Td =0.015 N/m2 Z < 20z \ o \\ 0 \ \ Or) S10 Q\ 9 0 2 4 6 TIME (hrs Fig. 79 Suspension concentration during dpeosition under the bed shear stress of 0.015 N/m2



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-1561.00D Td =0.05 N/m2 Td = 24 hrs : h 0.5 -: :. h 0 4 8 12 P/ Po Po = Initial Concentration of Suspension 10 = Density of Deposited Bed at Elevation h above h/hBottom 2h ho= Thickness of the Deposited 0.5Td= 0.015 N/m Bed Td = 40 hrs -p=41.3 g/0 --' I I I I I I I I I 0 4 8 12 P/Po 1.09 ) Td = 0 N/m2 Td = 135 hrs ho Po =41.1 g/. 0.50 II I I I I l I l 1 0 4 8 12 P/Po Fig. 81 Variation of bed density with depth for three different conditions of flow deposited beds



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Shear Stress Te in N/m Tm =0.9 N/m2 50 T =24 Hrs. Td =0 N/m2 Td =24 Hrs. 0.7 0.6 o30H 0.2 SO.I w -Hours z z 0 m 10z 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 46 Suspension concentration versus time for Experiment 14



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Table 2. Summary of selected studies on cohesive soil erosion. Investigator Mode of Placement of Sample Mode of Measurement of Erodibility Lutz (1934) Comparison of physical tests with Use of qualitative physicochemical erosive properties of natural soils analyses Peele (1937) In-place topsoils Soil loss and runoff tables Anderson (1951) In-place topsoils Correlation of erodibility with shear measurements Dunn (1959) Remolded, subjected to jet Jet to produce erosion; visual measures Smerdon and Beasley (1959) Slightly recompacted natural soil, Visual observation of bed movement top leveled Laflen and Beasley (1960) Remolded at unspecified percentage Visual correlation or erosion with of water, then saturated calculated inactive stress Flaxman (1962) Natural soils Correlation of permeability and unconfined compressive strength with natural erosion (channel measures) Moore and Masch (1962) Remolded and natural (trimmed) jet Measurement of scour depth and weight loss Abdel-Rahman (1964) Remolded in duct Visual; measurement of erosion depth '_ __ .. I''



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-0.4 40E / -0.3 / '30z/ \\ \ Z rU) C / \ \-CD -0.2 ( / o-o Sinusoidal velocity variation \ 200 F-o J -xCorresponding shear stress 0/ ---Discretised 30 min. time step \ 0. variation of shear stress S/ adopted for experiments. I I I I I I I 0 40 80 120 160 200 240 280 320 360 TI ME (Minutes) Fig. 47 Time step function for bed shear stress



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9 0.21..----2Bed Shear Stress 0.175 J T in N/m2 0. 145 I ~ -00.120 I. to --_r---_/ ,cp 70 2 z Tm 2 3h 5 S5 --Td 0.05 N/m o 5U Td = 24 hrs z 0.432 0r----n 4 z 0.36 .I / rI S.0.305 Continued from above 5 6 7 8 9 TIME (hrs) Fig. 82 Suspension concentration versus time for Experiment 17



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-24instance, monovalent cations such as sodium increase the activity of clay and its swelling. The predominant exchangeable ions in a soil are usually Na+, Ca+, and Mg .The rate of cation exchange reaction varies with the clay mineral, the concentration of the cations, and with the nature and concentration of the anions. In general, the reaction for Kaolinite is most rapid, slower for Illite, and still slower for Montmorillonite. The CEC is independent of the ionic strength of the solution with which the clay is.in contact or the physical structure of clay. However, it is a function of pH of the fluid in contact. Kandiah (1974) has shown that the critical shear stress of cohesive soil increased from 1.3 N/m2 to about 2.7 N/m2 with a change in CEC from 2 me/100 g to 34 me/100 g. These results were with an average sodium absorption ratio of 2.5. A decrease in critical shear stress was observed for the same range of CEC values when the average sodium absorption ratio was 48.0 (Fig. 4). c) Properties of the bed (moisture content, density). Attempts have been made to study the effect of moisture content in the bed which can be compared with standard soil indices such as plastic limit and liquid limit. Lutz (1934) noted that soils with high plasticity and low plasticity had different resistance to erosion. The low plasticity soil was more erosive than the high plasticity soil. Fukuda (1978) has shown that an increase in suspension concentration can be expected as the water content of the sediment increases, with the stress being held constant. A very small change in the water content of the sediment may give a large increase in the concentration of suspension. For instance, for a 12 percent increase in water content in his tests, the concentration of suspended sediment increased from 50 to 3000 mg/l.



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-2dredging over thirty years, the siltation rate has almost doubled and displaced the major deposition zone 20 km upstream along the river to an inconvenient location. Construction of a dam across a river transporting fine sediments in suspension results in the deposition of sediments immediately upstream of the structure and may, as in the case of the Aswan Dam, Egypt, deprive farmers downstream of their annual replenishment of fertile sediment during the flood season. Density currents created by the presence of fine sediments in sea water cause excessive siltation of navigation basins as experienced at the Tilbury tidal basin on the Thames River, England. Cohesive sediments also form an important consideration in the nuclear power generation planning and disposal of radioactive waste. Research has shown that the transuranic elements such as plutonium and americium are reconcentrated strongly by marine sediments. Their presence in sediment has focused attention on routes by which contaminated sediment might present a route of exposure to man especially in the longer term in the form of airborne dust in a respirable form or uptake through crops grown in soils reclaimed from areas of contaminated sediments. In the more recent years the study of fine sediments has assumed an important place in the context of pollution control. The transport and ultimate fate of contaminants is a complex process involving physical, chemical, and biological aspects, all of which play an important role. Sediments in a way are pollutants themselves since they increase turbidity. Their deposition may pose serious engineering and environmental problems. However, the more important aspect is their property to adsorb other pollutants very effectively and transport them



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-173Sargunam, A., Riley, P., Arulanandan, K., and Krone, R. B., "Effect of Physico-Chemical Factors on the Erosion of Cohesive Soils," Journal of the Hydraulics Division, Proc. ASCE, Tech. Paper, 1973. Smerdon, E. T., and Beasley, R. P., "The Tractive Force Theory APplied to Stability of Open Channels in Cohesive Soils," Res. Bull. 715, Agri. Expt. Station, Missouri University, October 1959. Task Committee on Erosion of Cohesive Materials, Committee on Sedimentation, "Erosion of Cohesive Sediments," Journal of the Hydraulics Division, ASCE, Vol. 94, No. HY4, July 1968, pp. 1017-1049. Thorn, M. F. C., and Parsons, J. G., "Properties of Grangemouth Mud," Report No. EX 781, Hydraulics Research Station, Wallingford, England, July 1977. Thorn, M. F. C., and Parsons, J. G., "Erosion of Cohesive Sediments in Estuaries: An Engineering Guide," Proc. of Third International Symposium on Dredging Technology, Paper Fl, March 1980. Williams, D. J. A., "Physical Properties of Cohesive Suspensions and Liquid Muds," Lecture Notes, Short Course on Engineering and Environmental Applications of Cohesive Sediment Studies, University of Florida, Gainesville, March 31-April 1, 1980. Yeh, H. Y., "Resuspension Properties of Flow Deposited Cohesive Sediment Beds," M.S. Thesis, University of Florida, Gainesville, Florida, 1979. Zeichner, G. R., and Schowalter, W. R., "Use of Trjectory Analysis to Study Stability of Colloidal Dispersions in Flow Fields," Am. Inst. Chem. Eng., Vol. 23, No. 3, 1977.



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-21a) Hydrodynamic factors (bed shear stress). These are principally embodied in the instantaneous bed shear stress and its frequency distribution, as specified by the flow characteristics, including the surface roughness of the bed-fluid interface. The studies reported later in this thesis have indicated that the concentration of suspension resulting from the erosion of bed is not only a function of the applied shear stress but also the shear stress previously acting on the bed. b) Properties of sediment (composition, shape, size gradation, organic matter, cation exchange capacity, moisture content). Fine sediments include interacting particles such as clays, as well as noninteracting fine particles such as silt. The differences in the physical properties of these have already been described. The clays may be composed of different clay minerals each of which has its own physical properties such as shape, size, surface area, liquid limit, etc. Since these properties in turn influence the erosional properties, the basic composition of fine sediments in terms of individual clay minerals or their mixtures, and the clays alone or mixture of clay and silt has a considerable influence on the erosional properties. Sediment composition is specified by the clay mineral, its weight fraction, and the amount and type of organic matter. Recent attempts at the University of California, Davis, have been directed toward characterizing the composition of clays through measurements of the dielectric constant at selected frequencies. A "dielectric dispersion parameter" is derived from these measurements. Each clay tested seemed to have a characteristic value of this parameter (Alizadeh, 1974; Arulanandan et al., 1973). The dielectric constant is a measure of



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-152The density structure of the three beds was determined by using the apparatus described under section 4.5. The results of measurement are given in Fig. 81. In order to erode the bed layer by layer, the bed shear stress starting from 0.1 N/m2 was increased each time by 20 percent of the previous shear stress, i.e. (AT)ex = 0.2 was used. The time step Ts had 60 minute duration. The concentration-time plots for the three erosion tests are given in Fig. 82 to 84. Concentration of suspension at the end of each Ts, i.e. C(60) values, are plotted in Fig. 85 as a function of time along with the discretized bed shear variation. Suspension concentration as a function of the bed shear stress is plotted in Fig. 86 which gives the magnitude of ch for the three different beds as follows: bed 1, 0.21 N/m2; bed 2, 0.29 N/m2 bed 3, 0.34 N/m2.Since Ts = 60 min. throughout the test, the erosion rate is proportional to the difference between the consecutive values of the concentration. These are plotted as a function of bed shear stress in Fig. 87. The shear strength of bed as a function of depth for the three beds is shown in Fig. 88.



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50(Y)=3.75 (X)-1.25 7 404 7(Y)= 3.33(X) -Channel 7 z7 S3Ring 30I r-20 105 10 15 REVOLUTIONS PER MINUTE (rpm) Fiq. 30 Cnrrelation hbtwoen r.p.m. and mnter reading for the channel and the ring



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( Sa-



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Table 3 (continued) Expt. Figure T T T Td TS e AT (AT) Comment No m2 m 2 e 2r ex N/m hours N/m hours min. N/m 11 43 0.9 24 0 40 30 0.100 0 1.5 .5 0.150 0.075 1.5 0.5 ex 0.225 =cons0.225 0.112 1.5 0.5 =constant 0.337 0.169 1.5 0.5 = 0.5 0.506 0.254 1.5 0.5 0.760 12 44 0.9 24 0 40 30 0.100 0.150 0.05 1.50 0.50 0.150 0 0 0.05 1.33 0.33 =cons0.200 t 0.250 0.05 1.25 0.25 0.250 0.300 0.05 1.20 0.20 13 45 0.9 24 0 24 60 0.11 0.19 2.72 1.72 A 0.30 0.20 1.67 0.67 =cons0.50 0.20 1.40 0.40 tant 0.70 0.2 14 46 0.9 24 0 24 30 0.10 0.10 2.00 1.00 (AT) 0.20 ex 0.20 0.10 1.50 0.50 dee decrea0.30 0.10 1.33 0.33 sing 0.40 0.10 1.25 0.25 0.50 50 0.10 1.20 0.20 0.60 60 0.10 1.17 0.17 0.70



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-172Migniot, P. C., "Etude des Proprietes Physiques de Differents Sediments Tres Fins et de Leur Comportment Sous de Actions Hydrodynamiques," La Houille Blanche, 7, 591, 1968. Mirtskhulava, Ts. E., "Erosional Stability of Cohesive Soils," Journal of Hydraulic Research, Vol. 4, No. 1, 1966. Moore, W. M., and Masch, F. D. Jr., "Experiments on the Scour Resistance of Cohesive Materials," Journal of Geophysical Research, Vol. 67, No. 4, n, April 1962, pp. 1437-1499. Owen, M. W., "Properties of Consolidating Mud," Report NO. INT 83, Hydraulics Research Station, Wallingford, England, December 1970. Owen, M. W., "Erosion of Avonmouth Mud," Report NO. INT 150, Hydraulics Research Station, Wallingford, England, September 1975, Second Impression, November 1977. Partheniades, E., "A Study of Erosion and Deposition of Cohesive Soils in Salt Water," Ph.D. Thesis, University of California, Berkeley, 1962. Partheniades, E., "Erosion and Deposition of Cohesive Soils," Journal of the Hydraulics Division, Proc. ASCE, Vol. 91, No. HY 1, Paper 4204, January 1965. Partheniades, E., "A Summary of the Present Knowledge on the Behavior of Fine Sediments in Estuaries," Tech. Note No. 8, Hydrodynamics Lab., M.I.T., Cambridge, Mass., 1964. Paaswell, R. E., "Causes and Mechanisms of Cohesive Soil Erosion, The State of the Art," in "Soil Erosion: Causes and Mechanisms; Prevention and Control," Special Report No. 135, Highway Research Board, Washington D.C., 1973, pp. 52-74. Partheniades, E., "Cohesive Sediment Transport Mechanics and Estuarial Sedimentation," Unpublished Lecture Notes, University of Florida, Gainesville, Florida, 1979. Peck, R. B., Foundation Engineering, John Wiley and Sons, Second Edition, 1953. Peele, T. C., "The Relation of Certain Physical Characteristics to the Erodibility of Soils," Proc. Soil Sciences Soc. of America, Vol. 2, 1937. Randkivi, A. J., and Hutchison, D. L., "Erosion of Kaolinite by Flowing Water," Proc. Royal Society, London, A337, 1974, pp. 537-554. Sargunam, A., "Influence of Mineralogy, Pore Fluid Composition and Structure of the Erosion of Cohesive Soils," Ph.D. Dissertation, University of California, Davis, 1973.



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-47strength of the placed bed. The entire experimental work consisting of 32 runs was divided into three series. Series I: Tests on placed bed by changing the flow'velocity by small positive and negative increment. Results are given in Fig. 3 (runs 1 to 13 only). Series II: The same bed was used as for series I except that it was remolded after the upper surfacing was removed. Results are given in Fig. 4. Series III: Tests on flow deposited bed. Results of these tests are given in Fig. 5. Comparison of the test results of series I and series III are given in Fig. 6 in terms of rate of erosion. Important conclusions drawn by Partheniades (1962) from his erosion tests were as follows: i) The rates of erosion were independent of concentration. ii) The erosion rates for the flocculated bed changed abruptly several times. This change was proven to be caused by changes of the bed properties. iii) The eroded surface did not cause any measurable increase of the frictional resistance of the bed. iv) The minimum shear stress to start erosion was about 0.05 N/m2 for both the placed and the flocculated bed, although they had different densities. v) Erosion rates for both the beds were of the same order of magnitude. vi) The overall resistance to erosion of a cohesive bed is independent of the macroscopic shear strength of the bed.



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-28and then increasing with depth to reach its terminal value at about 2 meters. For a fixed depth of settling, there is an absolute maximum value of settling velocity, which is attained at a fixed salinity and concentration. iv) Temperature: The effect of temperature is not very clear. It is largely limited to the effect temperature changes have on the viscosity of suspension. There appears to be a slight tendency to increased flocculation at higher temperatures, generally accompanied by slightly greater median settling velocities. However, at temperatures above 150C, the settling velocities of flocs formed in low salinity suspensions decreases. Effect of other parameters is as follows: v) pH: High pH contributes to dispersion, whereas low pH enhances flocculation. vi) Organic matter: Usually flocculation is promoted by the organic matter. vii) Dissolved chemicals: Only those chemicals which enter in some way into the physico-chemical reaction with soil can probably have an effect on flocculation and settling. 2.4.2 Consolidation Consolidation is the term used to refer to that portion of the compressibility of a soil that is essentially inelastic, i.e. its volume changes under load. Since the pore water and the soil grains in a saturated system are relatively incompressible, the volume change observed under load is the result of the expulsion of water from the interstices between soil grains.



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Shear Stress Te in N/m2 Tm = 0.9 N/m2 24 05Tm = 24 Hours 045 m 2 Td =0 N/m 20 0.20 Td.=40 Hours E 0 1.5 3 16 Hours -* z o0 < 12125 mm obove S2channel bottom J --225 mm above z channel bottom o 8 z 0 -4 20 40 60 80 100 120 140 160 18 TIME (Minutes) Fig. 34 Suspension concentration versus time for Experiment 4



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-46c) Measurement of erosion: By photo-electric cell, filtration, and weighing weight loss of sample before and after test. Although the importance of several parameters in influencing the erosion of cohesive sediments has now been established, data in respect to these are not available for each study. The variation in respect to fluid used, sediment used, and type of bed have already been mentioned under paragraph 3.1. Also there exists considerable variation in duration of test from 1 minute (Espey, 1963) to 500 hours (Krone, 1962). Sampling time for measurement of concentration is substantially different from one test to another. Hence, some of the observations made at small intervals of concentration-time history are not available in respect to studies where observation of concentration was made only at long time intervals. Visual observations of erosion permitted by the transparency of apparatus help in a more realistic interpretation of data than in the case of an opaque apparatus where erosional characteristics are indirectly inferred from the observations available. It is therefore necessary to exercise caution while comparing the results of various studies. In view of the implications mentioned above, results of only those studies where the size of the apparatus was comparable in order of magnitude, the bed was of a deposited type, and the emphasis was on the bed shear stress and the bed density are primarily considered here. Partheniades (1962) conducted erosion tests on San Fransisco Bay mud using a straight flume. He tested two types of beds, viz. i) placed bed at natural density and water content and ii) flow deposited bed. The shear strength of the flow deposited bed was 1/136 to 1/14th of the



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-30and also by the affinity of the particle surfaces for water, which in effect reduces even further the dimensions of the available flow channels. Also, unlike sand grains, clay particles, due to their shape and the interactions of their force systems, do not tend to fall into stable positions. As a result, the upper portion of a clay deposit is very porous and contains a large percentage of water. The actual porosity at the clay surface can vary considerably, depending upon the amount and type of clay mineral present, and on the factors that affect the interparticle forces, but it is always large in comparison with that of sand. Soil concentrations of the order of 10 to 20 percent by weight may be expected in newly formed clayey deposits. It is likely that at about this concentration, a continuous, interdependent network of soil particles is formed. This condition has been referred to as the "hindered settling." It might as well serve as a useful dividing line between the processes of sedimentation and consolidation. At lesser concentrations, settling occurs as the individual particles of flocs independently move downward through water. At higher concentrations, where the units can no longer move independently, the downward movement is accomplished by water moving up through the soil voids. In continually accumulating deposits, hydraulic gradients indicative of incomplete consolidation are present throughout the clay layer. If the accumulation is relatively rapid, the degree of consolidation at any depth is slight and it will be found that the density of the clay deposit will be virtually independent of the depth. When sedimentation ceases, the thickness of the layer will continue to decrease for some time probably for many years, until the water pressures



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-70a) The process of bed formation in the case of deposited bed starts with an initial concentration of suspension, C .Under the laboratory conditions, the sediment and the eroding fluid are mixed under a comparatively high shear stress in order to obtain a suspension with C as its uniform concentration throughout the depth of fluid. b) The shear stress Tm under which the initial mixing of the sediment and the eroding fluid are carried out. c) The duration of mixing, T .With a sufficiently long duration of Tm, the maximum size of the flocs in suspension is controlled by the balance between the local shear stress and the floc shear strength. d) The bed shear stress Td which is sufficiently small in its magnitude so as to permit deposition of most of the material in suspension. e) Duration of the total time for deposition plus bed consolidation, Td, which influences the density of the bed. If C0 is kept constant, the erosion of bed will depend upon the following two important processes. Formation of bed: influenced by Tm, Tm, Td, and Td' Erosion of bed: influenced by Te which may vary in its magnitude and duration. Attempts made by previous research workers to directly measure the shear strength of the bed or to correlate it to the bed density have not been satisfactory. The overall objective of the present study was to develop a laboratory test procedure which would enable the determination of the variation of the shear strength of a deposited bed over the depth. This was accomplished by increasing the applied bed shear stress in small increments of selected short time periods.



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Table 3: Experimental Conditions for Tests Conducted with a Multiple Steps of T Expt. Figure T T T Td T T AT T (AT)e Comment No. m 2 m d2 d s e2 r ex N/m hours N/m hours min. N/m 7 39 0.9 6 0 40 30 0.1 0.1 2.00 1.00 0.2 0.1 1.50 0.50 ( ex 0.3 3 0.1 1.33 0.33 decrea0.4 0.1 1.25 0.25 sing 0.5 0.1 1.20 0.20 0.6 0.1 1.17 0.17 0.7 8 40 0.9 24 0 40 30 --same as for Expt. 7 9 41 0.9 24 0 40 30 0.100 0.017 0.17 0.17 0.117 (ATex 0 7 0.022 1.20 0.20 ex 0.139 increa139 0.033 1.24 0.24 in 0.172 sing 0.055 1.32 0.32 0.227 0.123 1.54 0.54 0.350 350 0.350 2.00 1.00 0.700 10 42 0.9 24 0 40 30 0.09 0. .. 0.10 2.11 1.11 0.19 0.19 2.00 1.00 ex 0.38 0.32 1.84 0.84 1.0 0.70



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-149The following test procedure is recommended for conducting the experiments: i) Form a bed in the apparatus by using the sediment and the fluid having the required properties and measure the density of bed as a function of depth. The density structure of the deposited bed can be changed for each experiment for selecting the proper values of Tm, Tm' Td' and Td' ii) Select a small value of (AT) ex say 0.1 or 0.2. Conduct exploratory tests to determine the minimum duration of Ts which would give the EQ type profiles for the concentration versus time plots on a linear scale. During the same tests determine the value of Tcr at which the surface erosion begins. iii) Starting with T el TCr increase the bed shear stress in the form of time step function of duration Ts, and successive magnitudes determined by (AT)ex, so as to erode the bed layer by layer. iv) Calculate erosion rates for all the c-t profiles and determine which profiles satisfy the condition E < 5. The profiles satisfying this condition are the EQ type profiles and hence the various values of re used for conducting the experiments give the corresponding values of Ts at the respective depths of the bed. v) The depth of erosion for each re can be calculated from the measured bed density p, as a function of depth; the suspension concentration at the end of Ts, C(Ts ; the total surface area of the bed; and the total volume of the eroding fluid.



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-71Different types of bed structures were formed by using different combinations of Tm, Tm, Td, and Td. Concentration of suspended sediment resulting from the different values of Te was measured as a function of time. The term "resuspension" is usually used in the case of erosion of a flow deposited bed. Although all the experiments reported under the present study were for the deposited beds, the terms erosion and resuspension are considered to be synonymous. 4.2 Material Commercially available Kaolinite was used as sediment in the studies. Size gradation curve for the material is given in Fig. 20. The median diameter was 1.4 microns. Ninety-five percent of the material was within the size range of 1 to 7 microns. Seventy-four percent of the material was finer than 2 microns. The maximum size was 15 microns. This size distribution was obtained by using "Sedigraph" Particle Size Analyzer. Before using Kaolinite for conducting tests, it was kept submerged under the eroding fluid for a period of three months for the purpose of equilibration. The pore fluid and the eroding fluid was identical in these studies. The fluid was prepared by dissolving commercial salt in tap water and was adjusted to have a concentration of 35 parts per 1000 by weight. The pH of the eroding fluid was 7.6.



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-8260 min, 90 min) and only the magnitude of shear stress was varied. The values of Te were obtained by discretization procedure of a linearly increasing shear stress or the one equivalent to a sihusoidal velocity variation, etc. If T and T are the two consecutive magnitudes of bed shear stress, then excess shear stress is given by AT = T -(4.4.1) e2 e The normalized excess shear stress was defined as T -T (AT) 2 1 (4.4.2) ex T e For example, (ATr)e = 0.2 represents magnitude of r which is 20 percent ex e2 greater than the magnitude of T, and so forth. Different values of e1 AT as well as (AT)ex were selected for variation of bed shear stress. iii) Data analysis: Data collection consisted of obtaining samples of suspension at pre-determined time intervals after every change of the bed shear stress. The sampling time used was 1, 2, 3, 5, 10, 15, 20, and 30 minutes in the case of Ts = 30 min. For Ts of longer duration such as 60 and 90 minutes, additional samples were taken at every 10 minutes after the first 8 samples were collected in 30 minutes. In order to study the variation of suspension concentration over the water depth, samples were taken from two locations, viz. Tap A located 125 mm above the channel bottom and Tap B located 225 mm above the channel bottom. The concentration of sediment in suspension for each sample was determined by following the procedure described earlier under section 4.3. The basic data consisted of plotting of a concentration versus



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-85sediment outside of the tubes was removed. Entire sediment from each tube was taken out in porcelain dishes by using a hypodermic syringe with repeated washing by small quantities of distilled water. The weight of the sediment was determined after evaporating water in an oven at 500C temperature. From the height and diameter, the volume of sediment in each tube was calculated and by knowing the weight, the density of sediment in each tube was calculated. Further calculations were made as follows: Let L1 and L2 be the heights of two adjacent tubes 1 and 2 with a small change in heights (of the order of 0.3 cm). Let p, and P2 be the densities of sediment in each tube, calculated as above. Let V1 and V2 be the volumes of sediment in each tube. Let L2 < L1 and hence V2 < V1. It was assumed that the bottom sediment of height L2 in tube L1 had the same density as that of tube 1, viz. pI. The reason p2 is not equal to p1 is the fact that the sediment contained in the upper portion of tube L1, viz. in the incremental height (L1 -L2), has a different density (Ap)1-2 which was calculated as follows: A) pV1 -P2V 2 (4.5.1) plV1 -P2V2 (Ap)1-2 V -V (4.5.1) (b) Measurement of bed density for sediment deposited in the 2 rotating channel under a low bed shear stress (Td = 0.015 N/m2 0.05 N/m2, etc.). The apparatus consisted of a 2.5 cm dia. metal tube 15 cm high placed concentric in a 15 cm dia., 15 cm high circular plexiglass cylinder having a sealed bottom. At the center of bottom



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-64established (Fig. 17). The tests were carried out in a perspex settling column 10 meters high and 99 mm internal diameter. The variation of the density at various depths for different consolidation times is given in Fig. 18. He concluded that the shear strength variation within the bed could be correlated satisfactorily with the variation of density and that the shear strength increased rapidly with density. It has been mentioned earlier that consolidation results in decreasing the water content of the clay with a subsequent increase in shear strength (McCarthy, 1977, p. 234). However, the following interesting observations have been made regarding the erosion rate as a function of moisture content: i) Partheniades (1962, p. 54) noted that "in spite of the lower overall strength of the bed and its higher water content, the erosion rates of series II were lower than the corresponding rates of series I." ii) Christensen and Das (1973, p. 13) noted the following: "It is generally assumed that under similar conditions, the rate of erosion will decrease with increasing density. However the evidence in previous studies has not been conclusive. For this phase of the laboratory investigation, saturated soil samples were parepared at varying densities and moisture content and subjected to a constant hydraulic tractive stress. Because the soil samples were saturated, the density decreases with increasing moisture content. The duration of the test and the temperature of the water were kept constant for each type of soil. The laboratory test results exhibit a sharp decrease in erosion with increasing moisture content."



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30 10 E L 20 o u u") r'.1 2 3 4 5 6 Bed Shear Stress Fig. 72 A typical test result reported by Espey (1963)



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-13810 E Proctor Density E (a) Reference: McCarthy (1977) p. 315 z o, % WATER CONTENT (a) Schematic MoistureDensity Curve obtained from Laboratory Compaction Test Trials Heavy Zero Air Voids Curve n Compaction _Zr E (b) Reference: McCarthy S(1977) p. 317 z S % WATER CONTENT (b) Schematic Moisture -Density Curve showing the Effect of Compaction Procedure Plasticity (Clays) Compressibility (Silts) x 60 low rred high 0 6 -40 (c) Reference Peck(1953) FS 0 20 40 60 80 100 LIQUID LIMIT (c) The Plasticity Chart Fig. 71 Parameters influencing bed density and plasticity of soil



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1.0 I I I Reference Lee (1979) Series I 0.60 E Z 0 iBed Shear Stress in N/m2 0.43 z z S0.5 T AT ( AT)ex 0.32 z 0.11 0.34 0 0.43 (n 0.17 0.39 z 0.60 U)J S00.32 0 I I I i I I -I I 0 2 4 6 8 TIME (hrs) Fig. 12 Lee's (1979) data re-plotted to indicate variation of suspension concentration as a function of time and bed shear stress



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1 I I Tm = 0.9 N/m AT : Increasing Tm = 24 hrs (AT)ex Constant SII ex S20Td 0 N/m2 =0.5 STd = 40 hrs -d z 0 z w (n (-z o z C(0 Values 0I I i I I I 0 0.2 0.4 0.6 0.8 BED SHEAR STRESS Tc (N/m2) Fig. 64 Variation of suspension concentration as a function of bed shear stress (Expt.ll )



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Shear Stress Tein N/m2 /2 Tm= 0.9 N/m Tm=24 Hrs. Td= N/mr Td =40 Hrs. E 0.25 0.20|_ 4 -IO 0.15 I4S 0 I 2 3 r7 3Hours z 0 O 0 20 (n Oz I I I I I I I I I I 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 44 Suspension concentration versus time for Experiment 12



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50Tm = 0.9 N/m2 Tm = 24 hrs 2 -Td = 0 N/m2 o .40Td = 24 hrs Z o o o F30w 30 u o z O o z 0 o 200 0 IC0 0 0 0o 0,00 00 I I I 0 0.2 0.4 0.6 0.8 BED SHEAR STRESS Te (N/m2) C3f£D vIaria+inn nF cijnpnrinn rnnrntr~atinn with had chpar c+r-cc all data for T. =2 4 hnurc



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-32The term diffused double layer has already been described. With the water and clay molecules in contact with each other, it is believed that immediately surrounding the clay particle, there'is a thin, very tightly held layer of water, perhaps 1 x 10-6 mm (10 A) thick, and a second, more mobile, diffused layer extends beyond the first layer to the limit of attraction. The molecular movement occurs continually in both the layers. The water which is held in the diffused double layer is frequently termed adsorbed water or oriented water to differentiate it from normal pore water which is not oriented. The plasticity possessed by clay soils is attributed to the water which is attracted and held by the clay particles. Experiments performed with clay using non-polar liquid in place of water do not indicate plasticity and the particles act similar to those of a coarse grained sandy soil (McCarthy, 1977). A dispersion of particles subject only to hydrodynamic interaction will exhibit Newtonian flow characteristics, i.e. the shear stress and shear strain have a linear relationship no matter how small the magnitude. A clay-water suspension of high concentration on the other hand shows properties of a non-Newtonian fluid as shown in Fig. 2 which is a plot of equilibrium shear stress (T) versus shear rate (v). Curve A represents Newtonian behavior. Very dilute cohesive suspensions may exhibit this flow behavior, particularly if they are weakly flocculated. Curve D describes Bingham plastic behavior. A Bingham fluid will not flow at all until the yield stress is exceeded. This implies that the soil structure fails at some critical stress Ty and for stresses in excess of this the dispersion flows in a quasi-Newtonian manner. Curves B and C show more realistic rheograms where there is a gradual



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Shear Stress Te in N/m2 24 Trn =0.9 N/mZ Tm =4 Hrs. Expt. 2 0.435 T =0.9 N/m2 T = 24Hrs. Expt. I 20 28 1 d =0 N/m2 Td =40Hrs. Expt. I S ..and 2 E 0.071 -16z 0 a: 12 S2_----Expt. I z w --Expt. 2 S8z 4LUx ---------------------q20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 32 Effect of parameters in Step I on suspension concentration



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I.-OL ~ .. .. .." .I..'. ...| --' -, ''. .I ...-'-, -." ,. t -' Z W o C 0.316 MARE ISLAND STRAIT SEDIMENT 0 SUSPENDED SOLIDS 2 0. 0 .0 -J L10 IL tVELOCITY, 1.14 ft/sec S -T Ao OPTICAL DENSITY 0 M SUSPENDED SOLIDS w a. 0.1 0.01 0.1 1.0 10 100 1000 / TIME AFTER VELOCITY CHANGE, hr Fig. 9 Results of 1 500 hour long erosion test: Krone (1962) ... ........ .. ... ..... ..'. ........ tI



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22 Shear Stress Tein N/m9 e 0.45 T =0.9 N/m2 Ep Tm = 24 Hours 18 0.20 Td = 0 N/m2 Z -Td = 115 Hours 0 I 140 1.5 3.0 .." 0 Hours z w z o 100 1 z 0 ( 6U) U)2S20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 35 Suspension concentration versus time for Experiment 5



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100i-" z w w < 50I-2 w 3 -D 0 100 50 10 5 I 0.4 EQUIVALENT SPHERICAL DIAMETER, L.m Fiq. 2( Si7p qrarlation of Kaolinite tlsd for the experiments


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-144T c 0.6-15 0.43 0.4-100.38 _j 0.28 ] r -0.25-0.17 F 01.0 2.0 Z o 0.5z D 0.4 Sa O. o 0.2 I 0.10LI 0a 0 1.0 2.0 TIME (hrs) Te =Bed Shear Stress (N/m2) C = Suspension Concentration (gm/.) E = Erosion Rate (gm/cm2/min) c= Rate of Change of Concentration (gm/-i/min) Experimental Conditions : Tm = 0.9 N/m T AT (AT)ex Tm = 24 hrs 0.17 0.11 0.65 Td = N/m2 0.28 0.10 0.36 0.38 : 0.05 0. I 3 Td = 40 hrs 0.43 05 3 Fig. 74 Erosion rate versus time for Expt. for different values of bed shear stress



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262 Shear Stress Te in N/m2 0.7 m0.9 N/m2 22 3 Tm= 24 Hours STd = 0 N/m2 0.172 0.139 Td =40 Hours 0.117 E 18 0.10 S0 I 2 3 z Hours 0 14z 6 5 10C) z 0I I I I I I I I 0 20 40 60 80 100 120 140 160 180 200 z 60 2 -0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 41 Suspension concentration versus time for Experiment 9



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[------------I-----I----( ^ J----2Bed Shear Stress T in N/m2 L75 I 0.145 -0.12 70 .IO z 6oO I I I I I SI 2 3 4 5 5 2 S -m Tm = 0.9 N/m (oz T m = 2 4 h rs S4Td = ON/m z Td = 135 hrs 0 0.432 S0.36 SI) 0.30 _1 2r----Continued 2 from above 5 6 7 8 9 TIME (hrs) Fig. 84 Suspension concentration versus time for Experiment 19 I I



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-50E Z "" o 10-5 Io z 8 -4j 6 -3 u -CONCENTRATION I -----VELOCITY 4 4 -2 r S-ru~n^Z -run27428-run 2 39+run30 run3lrun (n .... ...-' y ^ ---i*'-'* ----rL 0 200 400 600 800 TIME AFTER START OF SERIES-m, hrs Fig. 5 Concentration versus time plot obtained in erosion of deposited bed: Partheniades (1962),. Expt...Series III .01 (-6 E o 0 --< Series I a: Series Ir 2 Computed on the Basis of Equation 1-33 Port -VTM C 0 002 0.04 006 AVERAGE BOTTOM SHEAR STRESS (lbs/ft2) Fig. 6 Relationship between rate of erosion and average bed shear stress: Partheniades (1962)



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-0.7 -0I I 0.7 I I 0.7-/ / 400.6t Bed Shear Stress N/m2 / E / N0.5 z E 0.5z // S30 z z -/Tm = 0.9 N/m z cn Tm = 24 hrs S0.3 / o 20 cr 0.3 -N/m2 W Td 0 u o / Tm = 24 hrs S200303 / z w 0.2a a2 / .C(60)Values // -/ / 0/ O -0 I I I I I I I 0 1.0 2.0 3.0 4.0 TIME (hrs) Fig. 55 Variation of suspension concentration with bed shear stress as a function of time (Expt.13 )



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30I I 0.76 o/ / 06/ I 0.6 / / SBed Shear Stress / I co1 in NAn2 0.50/ Tm = 0.9 N/m2 20-E -i z / / Tm = 24 hrs o -/ o2 S/ / Td = 0 N/m S ,,0.4/ FW z / / Td 40 hrs U 0.34 / z / (AT)e 0.5 z / ex o /zO U) /.23./ (n z0-" C (Values z 00.2 C a G. 50 a. m 0..15/ 0O0. O -I I I I-0 1.0 2.0 3.0 4.0 TIME (hrs) Fig. 53 Variation of suspension concentration with bed shear stress as a function of time (Expt. 11)



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Tm = 0.9 N/m2 AT : Increasing 20S Tm = 24 hrs (AT)ex: Increasing Td = 0 N/m2 E Td = 40 hrs / 0.100 / W : 0.017 0.17 0.117 z : 0.022 0.20 w / S 0.139 0.033 0.24 o 10 0.172 o : 0.055 0.32 0.227 z -0.123 0.54 / 0 0.350 0.350 1.00 / z 0.700 / LU / 0) C (30 Vlues 0 0.2 0.4 0.6 0.8 BED SHEAR STRESS Te (N/m2) Fig. 63 Variation of suspension concentration as a function of bed shear stress (Expt. 9 ) >



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S>I Bed Shear Stress Te Ts >I min T (cr /2 min (Tcr


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020 I I Reference Lee (1979) Series TI e x "E T AT (AT)ex z 0.150.30 o 0.06 0.20 0.36 0 < -0.07 0.19 0.56 X 0.43 0.06 0. 14 z 0.49 L 0.07 0.14 o 0.100.56 O 0 z Bed Shear Stress in N/m2 0.49 S0.43 M 0.36 y-w 0.05c. .0.30 / I I I I 0 2 4 6 8 10 12 14 16 TIME (hrs) Fig. 13 Lee's data re-plotted to indicate variation of suspension concentration as a function of time and bed shear stress



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( Sa-

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EFFECT OF BED SHEAR STRESS ON THE EROSIONAL CHARACTERISTICS OF KAOLINITE By T.M. PARCHURE A THESIS PRESENTED TO THE GRADUATE COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 1980

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ACKNOWLEDGEMENTS My deepest gratitude goes to Dr. Ashish J. Mehta, Associate Professor, Department of Coastal and Oceanographic Engineering, who has been my advisor and chairman of the committee, for his valuable guidance, encouragement, and support during the course of studies. I thank Dr. J. L. Eades, Professor, Department of Geology, and Dr. B. A. Benedict, Professor, Department of Civil Engineering, for serving on my supervisory committee. I have to thank Dr. E. Partheniades, Professor, Department of Engineering Sciences, for his advice regarding measurement of bed density. I am grateful to the personnel at the Coastal Engineering Laboratory, Mr. George Jones in particular, for their excellent cooperation and for extending every possible help for a successful completion of the experimental work. I am very thankful to Adele Koehler for careful typing of the manuscript and to Lillean Pieter for the excellent drafting work. I would like to express gratitude to my loving wife Aparna and my mother Indira for their continued encouragement in my endeavor and for accepting to bear the hardships caused during my long stay away from them. The present study was conducted as a part of the research project entitled "Deposition of Fine Sediments in Turbulent Flows" supported by the National Science Foundation under Grant Number GK-31259. Support was also received partially from the Environmental Protection Agency under Grant Number R806684010. This support from both the agencies is sincerely acknowledged. ii

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TABLE OF CONTENTS Page ACKNOWLEDGEMENTS ................... .... ..ii LIST OF TABLES ................... .......v LIST OF FIGURES ................... ......vi NOMENCLATURE ................... ..... ...xi ABSTRACT ................... .......... .xiii CHAPTER I INTRODUCTION ....................1 II FINE SEDIMENTS ................... ... 5 2.1 General Description ...............5 2.2 Properties of Fine Sediments ..........7 2.3 Parameters Influencing the Properties of Cohesive Sediments ....................18 2.4 Processes for Deposited Beds .......... 26 2.5 Clay-Water System ................ .31 III PREVIOUS LABORATORY STUDIES .............37 3.1 General Review .................37 3.2 Review of Literature on Erosion .........39 3.3 Review of Literature Pertinent to the Present Study .....................45 3.4 Shear Strength of Clay .............54 3.5 Shear Strength and Bed Density of Clay ..... 62 IV PRESENT INVESTIGATION ................ .68 4.1 Objective ................... ...68 4.2 Material ....................71 4.3 Apparatus for Erosion Tests ........... 73 4.4 Experimental Procedure ............. 74 4.5 Apparatus for Measurements of Bed Density .... 83 V TEST RESULTS AND ANALYSIS .............. .87 5.1 Effects of Parameters in Steps I, II and III .87 5.2 Multiple Steps of T ..94 5.3 Discretized Sinusoidal Velocity Variation 94 5.4 Correlation Plots of C30 and C60 Values ..... 109 5.5 Analysis of Data ................109 iii

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CHAPTER Page VI DISCUSSION AND PROPOSED TEST PROCEDURE ......... .132 6.1 Bed Density and Other Soil Parameters Correlated to the Shear Strength ............... .132 6.2 Critical Shear Stress ............... .139 6.3 Proposed Test Procedure .............. .143 6.4 Illustrative Example ................ 151 VII SUMMARY AND CONCLUSIONS ................. .164 7.1 Summary of Literature Review ............ .164 7.2 Conclusions of the Present Study .......... 165 REFERENCES ................... .......... .169 BIOGRAPHICAL SKETCH ................... ..... .174 iv

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LIST OF TABLES Table Page 1 Specific surface area and liquid limit for typical clays 35 2 Summary of selected studies on cohesive soil erosion 41 3 Experimental conditions for tests conducted with a multiple steps of e 95 v

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LIST OF FIGURES Figure Page 1 Inter-particle forces on clay minerals and the clay micelle 14 2 Rheological models 33 3 Concentration versus time plot obtained in erosion of placed bed: Partheniades (1962), Expt. Series-I 48 4 Concentration versus time plot obtained in erosion of remolded bed: Partheniades (1962), Expt. Series-II 49 5 Concentration versus time plot obtained in erosion of deposited bed: Partheniades (1962), Expt. Series-III 50 6 Relationship between rate of erosion and average bed shear stress: Partheniades (1962), 50 7 Concentration versus time plot: Krone (1962) 52 8 Concentration as a function of bed shear stress: Krone (1962) 52 9 Results of a 500 hour long erosion test: Krone (1962) 53 10 Concentration versus time plots for Series I obtained by Lee (1979) 55 11 Concentration versus time plots for Series II obtained by Lee (1979) 55 12 Lee's (1979) data for Series I re-plotted to indicate variation of suspension concentration as a function of time and bed shear stress 56 13 Lee's (1979) data for Series II re-plotted to indicate variation of suspension concentration as a function of time and bed shear stress 57 14 Concentration versus time data obtained by Yeh (1979) for erosion of Kaolinite 58 15 Concentration as a function of bed shear stress obtained by Yeh 59 vi

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Figure Page 16 Schematic diagrams showing shear strength of cohesive soil related to other parameters 61 17 Relationship between shear strength and bed density observed by Owen (1970) 65 18 Bed density profiles: Owen (1970) 66 19 Definition sketch for notations used to describe experimental conditions 69 20 Size gradation of Kaolinite used for the experiments 72 21 Photograph: The rotating channel facility 75 22 Photograph: Close view of the annular channel and the ring 75 23 Photograph: The motor controllers 76 24 Photograph: The electric motors for the channel and the ring 76 25 Photograph: Millipore filter apparatus assembly 77 26 Photograph: Device for measurement of bed density 77 27 Photograph: Equipment for determining the concentration of sediment suspensions 78 28 Photograph: Sampling bottles 78 29 Operational speeds and controller meter readings for ring and channel at different bed shear stresses 79 30 Correlation between r.p.m. and meter reading for the channel and the ring 80 31 Schematic drawings of apparatus developed for measurement of bed density 84 32 Effect of parameters in Step I on suspension concentration 88 33 Suspension concentration versus time for Expt. 3 89 34 Suspension concentration versus time for Expt. 4 90 35 Suspension concentration versus time for Expt. 5 91 36 Effect of Step II parameters on suspension concentration 92 vii

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Figure Page 37 Effect of Step III parameters on suspension concentration 93 38 Representation of a linearly varying bed shear stress by two different discretized time step functions 97 39 Suspension concentration versus time for Expt. 7 98 40 Suspension concentration versus time for Expt. 8 99 41 Suspension concentration versus time for Expt. 9 100 42 Suspension concentration versus time for Expt. 10 101 43 Suspension concentration versus time for Expt. 11 102 44 Suspension concentration versus time for Expt. 12 103 45 Suspension concentration versus time for Expt. 13 104 46 Suspension concentration versus time for Expt. 14 105 47 Time-step function for bed shear stress 106 48 Suspension concentration versus time for Expt. 15 107 49 Suspension concentration versus time for Expt. 16 108 50 Variation of suspension concentration with bed shear stress as a function of time for Expt. 8 110 51 Variation of suspension concentration with bed shear stress as a function of time for Expt. 9 111 52 Variation of suspension concentration with bed shear stress as a function of time for Expt. 10 112 53 Variation of suspension concentration with bed shear stress as a function of time for Expt. 11 113 54 Variation of suspension concentration with bed shear stress as a function of time for Expt. 12 114 55 Variation of suspension concentration with bed shear stress as a function of time for Expt. 13 115 56 Variation of suspension concentration with bed shear stress as a function of time for Expt. 14 116 57 Variation of suspension concentration with bed shear stress as a function of time for Expt. 15 117 58 Effect of shear stress variation on suspension concentration 118 viii

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Figure Page 59 Comparison of suspension concentration obtained under two different discretized time step functions 119 60 Variation of suspension concentration with bed shear stress, all data for Td = 24 hours 120 61 Variation of suspension concentration with bed shear stress, a-i data for Td = 40 hours 121 62 Suspension concentration versus bed shear stress (Expt. 7) 124 63 Variation of suspension concentration as a function of bed shear stress (Expt. 9) 125 64 Variation of suspension concentration as a function of bed shear stress (Expt. 11) 126 65 Variation of suspension concentration with bed shear stress for two different discretized time step functions 127 66 Cr versus Tr at 10 minutes 128 67 Cr versus rr at 20 minutes 129 68 Comparison of Cr versus T for two different bed density structures 69 (AC)ex plotted against the corresponding values of (AT)ex 131 70 Example of erosion test result and critical shear stress for erosion as a function of dry density of mud surface given by Thron and Parsons (1980) 135 71 Parameters influencing bed density and plasticity of soil 138 72 A typical test result reported by Espey (1963) 140 73 Notations for critical shear stress 142 74 Erosion rate versus time for different values of bed shear stress 144 75 Definition sketch for various parameters 145 76 Explanatory sketch for EQ type profile of c-t curve 147 77 Explanatory sketch for ER type profile of c-t curve 148 78 Suspension concentration during deposition under the bed shear stress of 0.05 N/m 153 ix

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Figure Paoe 79 Suspension concentration during deposition under the bed shear stress of 0.015 N/m2 154 80 Suspension concentration during deposition under zero bed shear stress 155 81 Variation of bed density with depth for three different conditions of flow deposited beds 156 82 Suspension concentration versus time for Expt. 17 157 83 Suspension concentration versus time for Expt. 18 158 84 Suspension concentration versus time for Expt. 19 159 85 Variation of suspension concentration with bed shear for different flow deposited beds 160 86 Suspension concentration versus bed shear stress for different flow deposited beds 161 87 Variation of erosion rate as a function of bed shear stress for different flow deposited beds 162 88 Shear strength of bed as a function of depth 163 x

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NOMENCLATURE C = Ratio of the consecutive suspension concentrations, e.g. C2 C S2 etc. C' C2 AC = Excess (suspension) concentration, e.g. C2 -C1. C2 -C1 (AC)ex = Normalized excess concentration, e.g. C C1 Co = Suspension concentration at the end of initial mixing. C30 = Suspension concentration at the end of 30 minutes after change of bed shear stress. p = Density of bed. Tm = Bed shear stress for initial mixing. Tm = Duration of initial mixing. Td = Bed shear stress for deposition. Td = Duration of deposition plus consolidation. Te = Bed shear stress for erosion (varied as a function of time). Ts = Time step for Te, i.e. duration over which different magnitudes of Te prevailed. r = Ratio of the consecutive values of bed shear stress, e.g. T T e2 e3 S etc. el 1 e2 xi

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AT = Excess shear stress, e.g. T -Te 22 e re2 -T (AT)ex = Normalized excess shear stress, e.g. e xii

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Abstract of Thesis Presented to the Graduate Council of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science EFFECT OF BED SHEAR STRESS ON THE EROSIONAL CHARACTERISTICS OF KAOLINITE By T.M. Parchure December 1980 Chairman: Dr. Ashish J. Mehta Major Department: Coastal and Oceanographic Engineering The degree of resistance to the erosion of a cohesive sediment bed under an applied shear stress is controlled by the physico-chemical properties of the sediment and the fluid, as well as by the depthvariation of the bed shear strength characterized by the inter-particle bonds. Previous attempts to correlate the erodibility of cohesive sediment beds with the shear strength determined by such devices as a penetrometer or a viscometer, or with soil indices, have not been successful. The objective of this study was to evolve a test procedure for conducting laboratory experiments to obtain the "layer by layer" shear strength of deposited beds of various density structures. Experiments were conducted using kaolinite with salt water of 35 ppt concentration as the eroding fluid. All the tests were carried out in an annular rotating channel apparatus. New techniques developed for measurement of the density structure of deposited beds have been described. An illustrative example outlining the procedure for the determination of the depth-variation of the bed shear strength is given. Chairman xiii

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CHAPTER I INTRODUCTION Study of the properties of transportation and deposition of sediments has engaged the attention of several research workers. The motive behind these studies has primarily been to assist in the design of dams and irrigation canals, water treatment and sewage disposal works, navigation channels, docks and harbors, etc. Special phenomena such as mud-banks along the shorelines, density flows at docks, flocculation and deposition of sediments due to mixing of salt water and fresh water in estuaries are associated with the fine sediments. Among various ways of classifying the sediments based on their geological origin, chemical properties, physical properties, etc., one of the classifications has been to treat the fine sediments of various compositions as a class in itself due to their special properties which differ considerably from the other sediments, and hence they need to be studied separately. The aspects related to fine sediments in some of the engineering projects are illustrated by case studies. In the Mersey Estuary, England, because of the abundance of fine cohesive sediment, attempts to increase the depths in one channel by removing 2.3 million m3 of material each year failed utterly. The channel in fact became shallower after five years of intensive dredging than it was before. Another example is Savannah harbor in the U.S.A., where in spite of continuous -1-

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-2dredging over thirty years, the siltation rate has almost doubled and displaced the major deposition zone 20 km upstream along the river to an inconvenient location. Construction of a dam across a river transporting fine sediments in suspension results in the deposition of sediments immediately upstream of the structure and may, as in the case of the Aswan Dam, Egypt, deprive farmers downstream of their annual replenishment of fertile sediment during the flood season. Density currents created by the presence of fine sediments in sea water cause excessive siltation of navigation basins as experienced at the Tilbury tidal basin on the Thames River, England. Cohesive sediments also form an important consideration in the nuclear power generation planning and disposal of radioactive waste. Research has shown that the transuranic elements such as plutonium and americium are reconcentrated strongly by marine sediments. Their presence in sediment has focused attention on routes by which contaminated sediment might present a route of exposure to man especially in the longer term in the form of airborne dust in a respirable form or uptake through crops grown in soils reclaimed from areas of contaminated sediments. In the more recent years the study of fine sediments has assumed an important place in the context of pollution control. The transport and ultimate fate of contaminants is a complex process involving physical, chemical, and biological aspects, all of which play an important role. Sediments in a way are pollutants themselves since they increase turbidity. Their deposition may pose serious engineering and environmental problems. However, the more important aspect is their property to adsorb other pollutants very effectively and transport them

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-3along. The content of heavy metals adsorbed to the sediments is found to depend on the grain size of the sediment. The smaller the grain size, the higher is the contamination with heavy metals. The sediments less than 16 microns in size are likely to have very high contaminants. The bulk of the pollutants may be carried on the sediments rather than the water. Hence, the dispersal of pollutants cannot be dissociated from the dispersal of sediments. Study of dispersal of sediments alone, however, cannot be expected to provide the required information related to pollutants since the chemical, biological, and physical aspects involving oxidation, reduction, exchange of elements, decay, etc. considerably change the properties of pollutants. An understanding of the depositional and resuspension characteristics of the fine sediments would therefore be beneficial in obtaining better solutions to engineering problems and in exercising a more effective pollution control. Over the last about twenty years, studies have been carried out to investigate the erosional and depositional properties of fine sediments. The influence of different parameters connected with the sediment and the eroding fluid on the characteristics of erosion and deposition has also been studied. These have been described in Chapter III. The experimental work reported in this thesis was carried out to study the resuspension of flow deposited bed of Kaolinite under varying shear stress and to study the effect of bed structure on the process of resuspension. Saline water with 35 parts per thousand concentration was used as the eroding fluid. An understanding of the physical processes associated with the movement of fine, cohesive sediments is clearly essential for obtaining improved engineering solutions to estuarine problems. The phenomena of

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-4fine sediment transport, deposition, bed formation and consolidation, and bed resuspension are rather complex, and in the estuarine environment they are inter-linked in a cyclical manner within time-scales imposed typically by the astronomical tides. Investigations of these phenomena under laboratory scales is an important first step towards an elucidation of the mechanics of the transport processes in the prototype, since it is possible to isolate and control the important governing parameters in laboratory tests. In that context, this investigation is concerned with studying the characteristics of resuspension of flocculated cohesive sediment beds. Under an applied bed shear stress, the surficial erodibility of such a bed is contingent upon the structure of the bed, as defined by the inter-particle bond strength of the floc network. Inasmuch as this network is formed under a given set of conditions specified by floc deposition and consolidation of the settling suspension at the bed, the magnitude and the duration of the applied shear stress during bed formation are important governing parameters for the subsequent process of resuspension. Hence the investigation of the erosion of beds formed under a variable shear stress is emphasized in this study. The overall objective of this study was an attempt to establish a laboratory test procedure for specifying the "layer by layer" erodibility of a deposited bed in terms of parameter(s) involving the critical shear stress for the erosion of a particular layer. The observed variation of the erodibility of a given bed with depth has been discussed with reference to the depth-variation of the floc shear strength and the bulk density.

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CHAPTER II FINE SEDIMENTS 2.1 General Description Fine sediments, commonly called clays or muds,are a product of weathering or hydro-thermal action on the rock and other soil on earth's surface. Although the maximum size of particles in the clay grade is defined somewhat differently in different disciplines, the general tendency has been to classify sediments finer than two microns as clays. Mixtures of clays and silt are usually called muds. The classification of fine-grained soil as either a silt or a clay is not merely on the basis of particle size but rather on the plasticity or non-plasticity of the material. Clay soil is plastic over a range of water content; that is, the soil can be remolded or deformed without causing cracking, breaking, or change of volume, and will retain the remolded shape. The clays are frequently cohesive. When dried, a clay soil possesses very high resistance to crushing. A silt soil possesses little or no plasticity and when dried has little or no strength. The basic differences between the elementary particles of noninteracting coarse minerals and the interacting fine minerals such as Kaoline could be briefly described as follows: 1. The non-interacting particles have no electric charge. Hence, they interact only hydrodynamically without any inter-particle attraction. -5-

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-6The fine sediments are interacting particles which attract or repel each other due to the presence of electric charge. Hence, the noninteracting particles remain separate from each other, whereas the interacting particles can form flocs under suitable environment. 2. If a small sample of moist silt is shaken easily but rapidly, water will appear on the surface but disappear when shaking stops. This phenomenon is referred to as dilatancy. The non-interacting particles show dilatancy with high concentration of sediment, whereas a sample of moist clay when shaken similarly does not show wetting of the surface. 3. The interacting particles exhibit elastic or plastic properties. 4. The non-interacting particles are usually unsaturated, whereas the flocs of interacting particles are saturated with water molecules. 5. With a low concentration of sediment, the suspension of noninteracting sediment is close to a Newtonian fluid, where the deformation is linearly proportional to shear stress. The suspension of interacting particles is non-Newtonian in behavior. 6. The erosional, transport, and depositional characteristics of non-interacting sediments are based mainly on the physical properties such as size, specific gravity, compaction, etc. The properties of the fluid such as salinity, pH, temperature have no substantial effect. On the other hand, the fluid properties have a substantial effect on the formation of flocs and hence on the erosional and depositional properties of interacting particles. 7. Fine sediments have a relatively much higher compressibility than the coarser non-interacting sediments.

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-78. Surface forces are dominant in respect to fine sediments, whereas gravitational forces predominate in the case of non-interacting particles. Due to the tendency of fine sediments to attach to each other due to surface forces, fine sediments are also sometimes referred to as cohesive sediments and the other sediments as noncohesive sediments. 9. Fine sediments are transported in the form of the suspended load or wash load, whereas the coarser sediments are predominantly transplanted as bed load. 2.2 Properties of Fine Sediments Several parameters affect the properties of clay materials, particularly the following: 1. Clay mineral composition. 2. Non-clay mineral composition. 3. Organic matter. 4. Exchangeable ions and soluble salts. 5. Texture, i.e. the particle size distribution of the constituent particles, the shape of the particles, their orientation in space relative to each other, and the forces tending to bind the particles together. In the context of clays, it is necessary to distinguish between material structure and property anisotropy. In general, "anisotropy" refers to the material structure and/or properties which do not exhibit the same characteristics and/or properties in every direction. The material structure anisotropy relates primarily to the anisotropy of

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-8fabric which would influence development of interparticle force relationships. Property anisotropy refers to strength, compressibility, permeability, conductivity, and other mechanical properties which are not equal in all directions, i.e. the material property demonstrated is a function of the sample tested. The external constraint anisotropy refers particularly to the applied stresses and boundary constraints. While considering properties of fine sediments, the anisotropy needs to be taken into account. 2.2.1 Size, Range, and Definition The maximum size of particles in the clay size grade is defined differently in different disciplines. In geology, the tendency has been to follow the Wentworth Scale to define the clay grade as materials finer than about 4 microns. In soil investigations, the tendency is to use 2 microns as the upper limit of the clay size grade. Although there is no sharp universal boundary between the particle size of clay minerals and non-clay minerals, in argillaceous materials, a large number of analyses have shown that there is a general tendency for the clay minerals to be concentrated in a size less than 2 microns (Grim, 1968). Clays contain varying percentages of clay-grade material and therefore varying relative amounts of non-clay-mineral and clay-mineral constituents. Clays almost always contain some non-clay mineral material coarser than the clay grade, although the amount may be very small. In many materials called clays the clay grade and the claymineral constituents make up considerably less than half the total.

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-9In such materials the non-clay is frequently not much coarser than the maximum for the clay grade, and the clay mineral fraction may be particularly potent in causing plasticity. In general, fine grained materials have been called clays so long as they have distinct plasticity and insufficient amounts of coarser material to warrant the appellations "silt" or "sand." If particle size anslyses are made, the term clay would be reserved for a material in which the clay grade dominates. However, names have been and are applied most frequently on the basis of appearance and bulk properties of the material. The expression clay material is used for any fine-grained, earthy, argillaceous, natural material. Clay material includes clays, shales, and argillites. It would also include soils, if such materials were argillaceous and had appreciable contents of clay-size-grade material. Clay particles are usually within a range of diameter smaller than 0.002 mm, but larger than the molecular size (10-6 mm). For the civil engineer, the fine sediments eroded from the earth's crust are of interest and the term clay is primarily a particle size term. For the chemical engineer the interest includes synthetic and other materials in their fine form, rather than the natural clay minerals. There are three levels of first order fabric recognition of clay structure. These are categorized on the basis of the degree of magnification required for a proper observation of the fabric pattern. 1. Macroscopic: The fabric units are distinguishable by the naked eye. They consist of an aggregation of clay particles called peds. 2. Microscopic: The fabric units are visually observed under the light microscope. For clays, single particles are not distinguishable

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-10at this level of viewing. The fabric units identified in the microscopic range consist of several particles or groups of particles defined as clusters, also sometimes called flocs. 3. Ultra-microscopic: The fabric units are visually observed in the ultra-microscopic level using electron microscopy (either transmission or scanning electron microscopy). Single or individual clay particles can be distinguished at this level. 2.2.2 Surface Area Colloidal properties such as plasticity and adsorption of molecules arise from the large surface area associated with a small mass. The surface forces are dominant in respect to fine sediments and the influence of gravitational forces is small. The clay minerals are plateshaped or tabular because the layer-lattice structure results in strong bonding along two axes but weak bonding between layers. The clay particle thickness depends upon the magnitude of forces of attraction between the layers. The variation in specific surface area of different minerals is primarily due to different thicknesses of the plate-shaped particles. Variation in the other two dimensions of clay particles is related to the degree of crystallinity of the clay minerals. A well crystallized Kaolinite has large particles. If it is poorly crystalline, the Kaolinite particles may not be larger than those of montmorillonite. The true meaning of the particle size becomes more evident in terms of its specific area. For example, a single sphere 1 cm in diameter has a surface area of 3.14 cm2.The same volume in terms of one micron (1 u diameter spheres has a surface area of 10,000 times greater and for one

PAGE 25

-11millimicron diameter spheres the surface area is 10 million times as great. The specific surface area of different clay minerals is as follows: Montmorillonite: 800 m2/g Chlorite: 80 m2/g Clay Mica: 80 m2/g Kaolinite: 15 m2/g Kaolinites show the most uniform crystals, often hexagonal plates with a typical diameter of 0.3 to 0.5 p and a thickness of 0.05 to 2 u. Montmorillonite particles are thin plates typically around 30 A thick and 0.1 to 1 p in diameter. Illite particles are plates with a typical thickness of 300 A. Surface area is one of the most important properties of fine sediments. Most of the differences between clay minerals in properties such as water retention, plasticity, or cohesion can be explained by the differences in the surface areas of particles. This explains the high swelling and high liquid limit of montmorillonite. Liquid limit being closely related to the surface area, it is considered as a measure of the nature of the surface as well as the area. 2.2.3 Shape The shape of a particle is also an important factor in determining the specific area. A spherical particle has the least surface area, and a disc or plate shaped particle has the greatest surface area. A sphere of 1 cm3 has an area of 4.836 cm2, while an equi-volume disc one micron thick has 20,000 cm2surface area. Typical plate shaped

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-12particles may have surface areas as much as ten times as great as the spherical particles of equal volumes. 2.2.4 Electric Charge Substitution of one ion for another in the clay crystal lattice and imperfections at the surface, especially at the edges, lead to negative charges on clay particles. Cations from the pore water are attracted to the particles (and anions repelled) to maintain electroneutrality. These are the exchangeable cations and their number is the cation-exchange capacity (i.e. the amount of negative charge per unit weight or per unit surface area) of the clay. This is usually expressed as milliequivalents per gram (me/g) or per 100 g (me/100 g). The force with which ions are held at the surface varies with the nature of the charge. The amount of charge for different clay minerals is given below: Kaolinite: 5 to 15 me/100 g Clay Mica and Chlorite: 20 to 40 me/100 g Montmorillonite: 80 to 100 me/100 g Vermiculite: 100 to 150 me/100 g The kind and number of exchangeable cations have an important influence on the behavior of soils, e.g. monovalent cations such as sodium increase the activity of the clay, its swelling, etc. 2.2.5 Inter-Particle Forces The behavior of clay particles is controlled more by the surface forces than by the gravity forces. Thus it can be shown that the

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-13average electrochemical force exerted on one clay particle is of the order of one million times greater than the average weight of the particle. The inter-particle forces are both repulsive and attractive in nature. The most important repulsive forces generated by the electrical charges are i) Repulsion caused by the negatively charged particle faces. ii) Repulsion of adsorbed positively charged cations. iii) Osmotic pressure resulting from the high concentration of cations near the surface of the particles in the pore water. The attractive forces binding the particles together in clay minerals are the following: i) Forces due to the attraction of the mass of one clay mineral particle and the mass of another. ii) Inter-molecular forces resulting from the nearness of one particle to another with the overlap of fields of force of molecules in the surface layers of adjacent particles. iii) Electrostatic forces due to changes in the lattice resulting from unbalanced substitution within the lattice, broken bonds on the edges of the lattice, and the attractive force of cation ions adsorbed on the clay-mineral surfaces. iv) Chemical cementation between particles by various compounds. v) Cation bonds: Forces exerted by cations attracting and trying to neutralize negatively charged particles (Fig. 1.A). vi) Water dipole linkage is the bonding action of adsorbed polar molecules (Fig. 1.B). Oriented water molecules between two claymineral surfaces may form a bridge of considerable strength if

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-14+ And -Indicate Electric Charge ~~C--Clay Particle (+) (+) () () (+ (A) Cation Bond S-Water Dipole EL E ] -Clay Particle (B) Water Dipole Linkage |4 --1Clay Particle F 1 Ii Water Dipole s c--Cation (C) Dipole -Cation -Dipole @11 0 O 0 SE G I1 e Diffused Double Layer (D) The Clay Micelle Fig. 1 Inter-particle forces on clay minerals and clay micelle

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-15only a few molecules thick, and of practically no strength if more than a few molecules thick. Similarly, adsorbed polar organic molecules could serve as a bond between clay-mineral particles. vii) Dipole-cation-dipole linkage (Fig. 1.C). viii) Hydrogen bond occurs when an atom of hydrogen is strongly attracted by two other atoms. ix) van der Waals forces are secondary valance forces of an electrochemical nature. They are generated by the mutual influence of the motion of electrons of the atoms and they are always attractive. These forces acting between all units are property of the matter and are independent of the chemical characteristics of water. Although other attractive forces of electrical nature may exist, the van der Waals forces are the main cause for bonding together of clay particles. An important characteristic of these forces is that they decay very rapidly with distance and hence the particles must come very close to each other so that the forces can be effective. 2.2.6 Flocculation The bonding of clay particles when they are brought together is known as flocculation. Both collision and cohesion are essential to flocculation. Cohesion is believed to result from the predominance of attractive forces on the surface of clay particles. Collision of particles may be caused by the Brownian motion of the suspended particles, by internal shear of water, and by the differential settling velocities

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-16of the flocs. Brownian motion is the erratic movement of small suspended particles caused by the thermal agitation of the suspending medium, which enables particles to come in contact with each other. The inter-particle forces have been described in the previous paragraph. Whatever the origin of the surface electric charges, any such charged particle in an ion containing water will attract ions of opposite charges, called "counter-ions,' to compensate its own electric charges. At the same time, the ions tend to diffuse away from the surface because of their thermal activity since such a diffusion takes place from a zone of high concentration to a zone of lower concentration in a way analogous to the diffusion of the molecules of the air in the atmosphere. Thus a clay particle idealized by a thin rectangular plate will be surrounded on either side by a diffused layer of counter-ions whose positions will be determined by the balance of the electrostatic attraction and their thermal activity. This layer is known as a "double layer" and it plays a dominant role in the mechanical properties of suspended clays and clay deposits. The system of clay particle and double layer is electrically neutral and is known as "clay micelle." Figurel.D shows a simplified, schematic diagram of a clay micelle. Any form of agitation, including Brownian motion,will eventually cause two particles to approach each other sufficiently close for their double layers to interact. This interaction causes changes in the distribution of the cations in the double layer of both particles. The result will be determined by the potentials of the van der Waals force and the electric forces. Sufficiently far from the particle, the repulsive forces may dominate, whereas closer to the surface the net effect would be attractive which causes flocculation of particles.

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-17It has long been observed that gentle stirring promotes flocculation. This is due to the velocity gradients which are induced in the liquid causing relative motion of the particles. Such velocity gradient-controlled flocculation is called orthokinetic flocculation. A simple theory of flocculation kinetics can be derived for a uniform liquid shear field, giving a constant velocity gradient. Such constant velocity gradients are difficult to achieve in practice; the closest experimental form has been in the annular gap between coaxial rotating cylinders, also known as Couette apparatus. Consequently, the theory has to be extended to velocity gradients created in turbulent flow conditions. The orthokinetic rate of flocculation has a high dependence on initial particle size, is linearly dependent on velocity gradient, and it is independent of temperature. While considering Brownian motion, it is useful to consider one particle (the collector) as stationary, and to calculate the diffusion rate caused by Brownian movement of other particles to this collector. Because particles become attached to the collector, and are therefore removed from the suspension, a concentration gradient is formed radially outwards from the collector. This diffusion-controlled flocculation is called perikinetic flocculation. Temperature and viscosity effects are significant under perikinetic flocculation, and the rate of flocculation is independent of particle size. In the benthic boundary layer, orthokinetic (i.e. velocity gradient-controlled) flocculation has a much greater influence on the frequency of collision between particles than perikinetic (i.e. diffusion-controlled) flocculation (Williams, 1980). The effect of flocculation is formation of aggregates. Repeated inter-particle collisions in a turbulent flow field are predominantly

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-18due to the internal shearing of the suspending water. Aggregates formed by fluid shearing are denser and stronger than those formed by the Brownian motion or by differential settling velocities. The aggregates can be dispersed by local high shearing rates and may re-aggregate later. The settling velocities of aggregates and the probability of their sticking to the bed are determined by the size, density, and shear strength. Mineral particles cohering in a cluster with uniform porosity are called primary aggregates or flocs. At lower shearing rates, primary aggregates would collide with each other and bond to form first-order aggregates. At still lower shearing rates, the first order aggregates would collide and bond with each other to form weaker, lessdense second-order aggregates, and so on. Each higher-order aggregate would include water in the new pore volume formed, and because shear stress can be transmitted only through inter-aggregate contacts, the higher order aggregates are weaker. A procedure for determining the order of aggregation has been given by Krone (1976). 2.3 Parameters Influencing the Properties of Cohesive Sediments An attempt wasmade by the Committee on Tidal Hydraulics (1960) to identify the soil parameters which affect the process of shoaling in estuaries. However, the literature review contained in the report is limited. Based on the findings of subsequent research, Paaswell (1973) reviewed the causes and mechanism of cohesive soil erosion and identified the followingparameters used in evaluating the erosion of cohesive beds:

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-19Character Parameter Physical Soil type (clay mineral) Percentage of clay Liquid and plastic limits, and activity Specific gravity Physico-chemical Base exchange capacity Sodium absorption ratio Pore fluid quality Pore fluid environment Mechanical properties Shear strength (surface and body) Cohesion Thixotropy Swelling and shrinkage properties Conditions of environment Weathering (wet-dry) Freezing and thawing Prestress history In addition to the above, Alizadeh (1974) has included the parameters bulk density, water content, effective stress, time (aging), testing methods, and soil preparation methods. Kandiah (1974) mentioned that the erodibility of cohesive soil is controlled by the mineralogical, chemical, and environmental factors. The mineralogical factors include the type and amount of clay mineral present in the soil while the chemical factors include the total salt concentration, sodium absorption ratio, and pH of the pore fluid. The influence of these two factors is mutually independent and they are the "key parameters" of erosion since their effect is far more pronounced than other factors. Attempts have been made to correlate the critical shear stress to various parameters affecting soil erosion. For example, Smerdon and Beasly (1959) presented the following equation between critical shear stress (Tc) and plasticity index (PI): Tc = 0.0017 (PI)0.84(2.3.1)

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-20Dunn (1959) found that not only the plasticity index but vane shear strength (S ) needs to be included in the expression as follows: Tc = 0.0098 + 0.00049 (Su + 180) tan (30 + 1.73 PI) (2.3.2) Carlson and Enger (1963) suggested the following relationship: Tc = -0.017 + 0.000181(PI) + 0.000186(v) + 0.00268(K) + 0.000465(LL) (2.3.3) where v = sample density K = phi-skewness of the grain size distribution LL = liquid limit Sargunam (1973) presented the following expression related to the composition of pore fluid C, and sodium adsorption ratio (SAR) Tc = C1 + (C2 -n log SAR) log C (2.3.4) where C = pore fluid concentration C1, C2, and n = constants which vary with the type and amount of clay minerals For describing the influence of various parameters the following classification appears more appropriate: a) Hydrodynamic factors. b) Properties of sediment. c) Properties of bed. d) Properties of pore fluid and eroding fluid.

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-21a) Hydrodynamic factors (bed shear stress). These are principally embodied in the instantaneous bed shear stress and its frequency distribution, as specified by the flow characteristics, including the surface roughness of the bed-fluid interface. The studies reported later in this thesis have indicated that the concentration of suspension resulting from the erosion of bed is not only a function of the applied shear stress but also the shear stress previously acting on the bed. b) Properties of sediment (composition, shape, size gradation, organic matter, cation exchange capacity, moisture content). Fine sediments include interacting particles such as clays, as well as noninteracting fine particles such as silt. The differences in the physical properties of these have already been described. The clays may be composed of different clay minerals each of which has its own physical properties such as shape, size, surface area, liquid limit, etc. Since these properties in turn influence the erosional properties, the basic composition of fine sediments in terms of individual clay minerals or their mixtures, and the clays alone or mixture of clay and silt has a considerable influence on the erosional properties. Sediment composition is specified by the clay mineral, its weight fraction, and the amount and type of organic matter. Recent attempts at the University of California, Davis, have been directed toward characterizing the composition of clays through measurements of the dielectric constant at selected frequencies. A "dielectric dispersion parameter" is derived from these measurements. Each clay tested seemed to have a characteristic value of this parameter (Alizadeh, 1974; Arulanandan et al., 1973). The dielectric constant is a measure of

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-22the ability of clay to store electrical potential energy under the influence of an electric field. The dielectric constant for a soil sample is defined as Scd (2.3.5) VA where c = capacitance d = length of specimen A = cross-sectional area ev = dielectric constant of vacuum = 8.85 x 10-14 farad/cm The dielectric constant of a dry silicate mineral is 4, and of water about 80. Alizadeh (1974) has defined the magnitude of dielectric dispersion (AE') as the total amount of decrease in the measured dielectric constant. The dielectric dispersion depends mainly on the type and amount of clay; the other factors such as pore fluid composition, water content, particle orientation, etc. have a secondary effect. It has been used as a quantitative index for soil characterization. Measurements have shown that for 10 percent Kaolinite and 21.2 percent water content, AE0 has a magnitude 7.5, whereas for 60 percent Kaolinite and 30.4 percent water content, AE' is 18. The shape and size of individual clay particles have an influence on their surface area. Since the surface forces predominate in respect to cohesive sediments, the colloidal properties such as plasticity and adsorption of molecules are governed by the surface area. Natural sediments usually have a wide range of particle sizes. The effect of

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-23such heterodisperse sediments is to increase the probability of collision of primary particles. Also, natural sediments typically are not uniform in their shape. Since Brownian motion is rotary in character, the largest dimension is appropriate for the collision diameter, whereas the mean diameter is applicable to the diffusion constant. Consequently, anisodimensional particles have a relatively large collision diameter combined with a relatively large diffusion constant, thus enhancing the collision rate. After flocculation has progressed, however, the flocs tend to be more nearly spherical, and the effect of anisotropy on collision rate is small. Organic matter has an effect on the properties of flocculation of sediment and hence it affects the erosional properties of sediment. Kandiah (1974) found that organic matter strengthens the soil aggregates against slaking. Studies on the erodibility of 30 percent illitic soil showed that the critical shear stress for erosion increased from 1.7 N/m2 to 4.0 N/m2 when the organic content was increased from 0 to about 4 percent. It has been mentioned earlier that the type of clay is one of the factors determining the erosional characteristics. The Cation Exchange Capacity is a property of the soil which can be used to indicate the type while associating it with the corresponding critical shear stress for erosion. Cations from the pore fluid are attracted by the negative charge on clay particles and the anions are repelled in order to maintain electro-neutrality. The number of these exchangeable cations is known as the cation exchange capacity, which is usually expressed as milliequivalent per gram (me/g). The kind and number of exchangeable cations have an important influence on the behavior of soils. For

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-24instance, monovalent cations such as sodium increase the activity of clay and its swelling. The predominant exchangeable ions in a soil are usually Na+, Ca+, and Mg .The rate of cation exchange reaction varies with the clay mineral, the concentration of the cations, and with the nature and concentration of the anions. In general, the reaction for Kaolinite is most rapid, slower for Illite, and still slower for Montmorillonite. The CEC is independent of the ionic strength of the solution with which the clay is.in contact or the physical structure of clay. However, it is a function of pH of the fluid in contact. Kandiah (1974) has shown that the critical shear stress of cohesive soil increased from 1.3 N/m2 to about 2.7 N/m2 with a change in CEC from 2 me/100 g to 34 me/100 g. These results were with an average sodium absorption ratio of 2.5. A decrease in critical shear stress was observed for the same range of CEC values when the average sodium absorption ratio was 48.0 (Fig. 4). c) Properties of the bed (moisture content, density). Attempts have been made to study the effect of moisture content in the bed which can be compared with standard soil indices such as plastic limit and liquid limit. Lutz (1934) noted that soils with high plasticity and low plasticity had different resistance to erosion. The low plasticity soil was more erosive than the high plasticity soil. Fukuda (1978) has shown that an increase in suspension concentration can be expected as the water content of the sediment increases, with the stress being held constant. A very small change in the water content of the sediment may give a large increase in the concentration of suspension. For instance, for a 12 percent increase in water content in his tests, the concentration of suspended sediment increased from 50 to 3000 mg/l.

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-25Owen (1970) reported that the shear strength variation within the bed could be correlated satisfactorily with the variation of density. The shear strength was observed to increase rapidly with density. The method of formation of bed namely, remolded, deposited, or compacted with external force, have an effect on the density structure of the bed and hence on the erosional property of the soil. d) Properties of the pore fluid and the eroding fluid (salt concentration, sodium adsorption ratio, pH, temperature). In the case of a fully saturated soil which has been equilibrated with the eroding fluid over a sufficiently long time, the pore fluid and the eroding fluid are expected to have the same properties. However, in case they have different properties, osmotic pressure gradient is formed which changes the properties of the pore fluid and this in turn may change the erosional properties of the bed. The soil having an aggregated structure resists erosion more effectively than a soil having a dispersed structure. This is because the net force between the particles in an aggregated structure is attractive while in the latter it is repulsive. Since the pore fluid has a substantial influence on the structure, it influences erosion. An increase in the salt concentrations of the pore fluid usually increases resistance to erosion. Sodium Adsorption Ratio (SAR) is defined as Na+ SAR = Na (2.3.6) [(Ca+ + Mg+)]1/ The concentrations of individual ions are in milliequivalents per liter. The SAR is used as an index to characterize the pore fluid and eroding fluid in terms of the relative strength of the Na, Ca, and

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-26Mg ions. Fluids with the same salt concentration can have different SAR. An increase in SAR decreases cohesion, lowers the critical shear stress, and hence increases the erosion rate (Kandiah, 1974). Alizadeh (1974) has presented experimental data to this effect. Kandiah (1974) also studied the effect of pH and concluded that "pH influences the interparticle cohesion which strongly affects aggregation and dispersion of clay soil properties" (p. 118). The effect of water temperature on erosion rate has been studied by Grissinger (1966), Christensen and Das (1973), Randkivi and Hutchison (1974), and Gularte (1978). It is observed that the temperature had a significant effect on the strength of inter-particle cohesion and the rate of erosion increased significantly with increasing temperature. Kandiah (1974) showed from particle-by-particle surface erosion of a remolded illite soil that the critical shear stress for erosion varies as Tc = 1.8 x 10-5 exp[4100/T] (2.3.7) where T is the absolute temperature in the Kelvin scale. The critical shear stress dropped from 3.6 to 0.8 N/m2 over the temperature change from 9.50C to 420C. 2.4 Processes for Deposited Beds The deposited beds are distinguished from the placed or remolded beds because of the fact that they are formed by the process of settling of the sediment in suspension. The settling may take place under quiescent conditions or under a low shear stress which permits

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-27deposition of the suspended material. The settled material may undergo consolidation and form a bed. The aspects related to the settling and consolidation which are the primary processes in formation of deposited beds are briefly described below. 2.4.1 Settling The effect of Brownian motion on a suspension of sediment under apparent quiescent conditions is to induce inter-particle contacts. This may result in bonding of particles through the process of flocculation (Einstein and Krone, 1962). The mechanics and importance of flocculation have already been described. Hence only the parameters which influence settling velocity of the flocs are briefly described here. Owen (1970) conducted detailed study of the settling velocities of an estuary mud and noted the following conclusions: i) Suspended sediment concentration: The median settling velocity increases with concentration up to a value between 4 and 20 grams per liter, depending on the salinity, at which hindered settling begins. For a concentration higher than this, the settling velocity reduces. ii) Salinity: The median settling velocity increases with salinity (except during hindered settling) up to a value between 28 and 43 grams per liter depending on the concentration. For higher values of salinity, the settling velocity decreases. It may however be noted here that certain clays such as Kaolinite flocculate even in distilled water. iii) Depth: The effect of depth of settling is fairly complex, the settling velocity reducing to a minimum at a depth of 1 meter

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-28and then increasing with depth to reach its terminal value at about 2 meters. For a fixed depth of settling, there is an absolute maximum value of settling velocity, which is attained at a fixed salinity and concentration. iv) Temperature: The effect of temperature is not very clear. It is largely limited to the effect temperature changes have on the viscosity of suspension. There appears to be a slight tendency to increased flocculation at higher temperatures, generally accompanied by slightly greater median settling velocities. However, at temperatures above 150C, the settling velocities of flocs formed in low salinity suspensions decreases. Effect of other parameters is as follows: v) pH: High pH contributes to dispersion, whereas low pH enhances flocculation. vi) Organic matter: Usually flocculation is promoted by the organic matter. vii) Dissolved chemicals: Only those chemicals which enter in some way into the physico-chemical reaction with soil can probably have an effect on flocculation and settling. 2.4.2 Consolidation Consolidation is the term used to refer to that portion of the compressibility of a soil that is essentially inelastic, i.e. its volume changes under load. Since the pore water and the soil grains in a saturated system are relatively incompressible, the volume change observed under load is the result of the expulsion of water from the interstices between soil grains.

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-29Most soils regain only a fraction of the volume lost during consolidation. This results from the fact that in order to undergo volume change, the soil particles are displaced relative to one another to assume a more closely packed condition, and consequently greater density. In sands these movements are not reversible, and in most clayey soils, they are only slightly reversible. For sand, the volume decrease is proportional to the logarithm of the pressure. For clay, the relation is not linear although at higher pressures, it is approximately so. In a process of continuous sedimentation in water, the soil at any depth is being consolidated under the influence of the weight of the soil above it. Since new material is continually being added, a gradient tending to cause the pore water to flow upward out of the system exists at all points within the stratum. If the material being deposited is sand, consolidation takes place at a rapid rate since the spaces between particles are relatively large and the water can escape readily. Furthermore, the sand particles are essentially inert and are not greatly influenced by adjacent particles, and the initial position of each grain within the mass is relatively stable. Very little volume change can take place in a sand deposit except in certain circumstances as a result of outside influences such as mechanical vibration or shock. Therefore, sands can be considered to be virtually completely consolidated at all times even when the accumulation of sediment is rapid. Consolidation of clayey deposit proceeds at a comparatively much slower rate. The total pore space in a clay mass is large but is composed of a multitude of small channels between the individual particles. The flow of water in the channels is restricted by their small size

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-30and also by the affinity of the particle surfaces for water, which in effect reduces even further the dimensions of the available flow channels. Also, unlike sand grains, clay particles, due to their shape and the interactions of their force systems, do not tend to fall into stable positions. As a result, the upper portion of a clay deposit is very porous and contains a large percentage of water. The actual porosity at the clay surface can vary considerably, depending upon the amount and type of clay mineral present, and on the factors that affect the interparticle forces, but it is always large in comparison with that of sand. Soil concentrations of the order of 10 to 20 percent by weight may be expected in newly formed clayey deposits. It is likely that at about this concentration, a continuous, interdependent network of soil particles is formed. This condition has been referred to as the "hindered settling." It might as well serve as a useful dividing line between the processes of sedimentation and consolidation. At lesser concentrations, settling occurs as the individual particles of flocs independently move downward through water. At higher concentrations, where the units can no longer move independently, the downward movement is accomplished by water moving up through the soil voids. In continually accumulating deposits, hydraulic gradients indicative of incomplete consolidation are present throughout the clay layer. If the accumulation is relatively rapid, the degree of consolidation at any depth is slight and it will be found that the density of the clay deposit will be virtually independent of the depth. When sedimentation ceases, the thickness of the layer will continue to decrease for some time probably for many years, until the water pressures

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-31induced by the weight of the sediment have been dissipated (Committee on Tidal Hydraulics, 1960). The properties of clays in respect to settling and consolidation are very important in connection with the studies for their depositional and erosional properties. 2.5 Clay-Water System Water which can be held by the clay system is grouped into two categories, namely low-temperature water, which can be driven off by heating to about 100C to 150C, and the OH lattice water which is lost at temperatures above about 3000C. The nature of low-temperature water and the factors that control its characteristics are of great importance, since they largely determine the plastic, bonding, compaction, suspension, and other properties of clay materials, which in turn control their behavior under the given flow field. Water, though neutral, has its oxygen and hydrogen atoms spaced in such a manner that the center of gravity of the positive and negative electrical charges do not coincide. The resulting molecule has a positive charge acting at one end and a negative charge acting at the opposite end. Water molecules are thus considered polar molecules. Because of the net positive charge of the cations, they attract negative charges. The negative tips of water molecules are attracted and held to the cation, which in turn is held by the clay particle due to the negative charge on its surface. The resulting effect is that water becomes bonded to the clay. Additional water molecules are also attracted to the clay particle because of a chain-like arrangement of negative ends to positive ends of molecules.

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-32The term diffused double layer has already been described. With the water and clay molecules in contact with each other, it is believed that immediately surrounding the clay particle, there'is a thin, very tightly held layer of water, perhaps 1 x 10-6 mm (10 A) thick, and a second, more mobile, diffused layer extends beyond the first layer to the limit of attraction. The molecular movement occurs continually in both the layers. The water which is held in the diffused double layer is frequently termed adsorbed water or oriented water to differentiate it from normal pore water which is not oriented. The plasticity possessed by clay soils is attributed to the water which is attracted and held by the clay particles. Experiments performed with clay using non-polar liquid in place of water do not indicate plasticity and the particles act similar to those of a coarse grained sandy soil (McCarthy, 1977). A dispersion of particles subject only to hydrodynamic interaction will exhibit Newtonian flow characteristics, i.e. the shear stress and shear strain have a linear relationship no matter how small the magnitude. A clay-water suspension of high concentration on the other hand shows properties of a non-Newtonian fluid as shown in Fig. 2 which is a plot of equilibrium shear stress (T) versus shear rate (v). Curve A represents Newtonian behavior. Very dilute cohesive suspensions may exhibit this flow behavior, particularly if they are weakly flocculated. Curve D describes Bingham plastic behavior. A Bingham fluid will not flow at all until the yield stress is exceeded. This implies that the soil structure fails at some critical stress Ty and for stresses in excess of this the dispersion flows in a quasi-Newtonian manner. Curves B and C show more realistic rheograms where there is a gradual

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(D) Bingham Plastic (C) Plastic ( B) Pseudo-Plastic (A) Newtonian Ty U) I-, SHEARING RATE ( ) Fig. 2 Rheological models

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-34reduction in the contribution of structure to apparent viscosity as the shear rate is increased. These indicate pseudo-plastic and plastic behavior, respectively, also known as shear thinning. The intercept T shown in Fig. 2 is the lower yield stress, whereas TB is known as the upper Bingham yield stress which is a measure of the work done in disrupting the floc structure in cohesive suspensions. The content of water in a clay soil determines the consistency in the remolded state. The criteria to determine the various states of consistency are known as Atterberg Limits. At high water content, the soil-water mixture possesses the properties of a liquid; at lesser water content the volume of the mixture is decreased and the material exhibits the properties of a plastic; at still lesser water content, the mixture behaves as a semi-solid and finally as a solid. The water content indicating the division between the liquid and plastic state is called the liquid limit. The division between the plastic and semi-solid state is the plastic limit. The water content at the division between the semi-solid and the solid state is the shrinkage limit. All these three limits are expressed as percentage of water with respect to the weight of solids. Below the shrinkage limit, there is little or no change in volume as water content varies. However, above the shrinkage limit, the change in total volume of soil-water mixture is related to the change in water content. The plasticity index is the numerical difference between the liquid limit and the plastic limit, and indicates the range of water content through which the soil remains plastic. It is necessary to know both the liquid limit and the plasticity index for a proper evaluation of the plasticity properties of soil.

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-35The liquid limit expresses the overall effect of the inter-particle forces within the clay mass and this soil index varies with clay mineralogy and with the associated cation. The following data given in Table 1 were presented by the Committee on Tidal Hydraulics (1960). Table 1. Specific surface area and liquid limit for typical clays. Primary Specific Clay Associated Surface Liquid Mineral Cation Area Limit Montmorillonite Na 847 710 Montmorillonite H 768 490 Illite (< 2 1) H 79.8 100 Kaolinite H 13.1 53 Kaolinite (< 5 v) H 26.1 110 Many clay soils exhibit the property of rheotropy at water contents above the liquid limit, and also to a lesser degree at water contents in the plastic range. Rheotropy is the change to a more fluid consistency on stirring or disturbance. When the disturbance has ceased, the system reverts to its less fluid or more rigid condition. This is often called thixotropy, although the strict definition of thixotropy is a reversible, isothermal sol-gel transformation. A sol, by definition, has no yield value, while a gel has rigidity. The change in clay-water systems is generally from a system with higher yield value to one with a lower yield value. A sol may be considered as a colloidal dispersion. This restricts sols to liquid-like behavior.

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-36When hardening of the sol occurs, a gel is formed. This requires a change of state from a semi-liquid substance (sol) to a semi-solid (gel). Rheotropy of clay soils can be measured by a vane shear or at higher water contents by means of a viscometer. The property of thixotropy (or rest-hardening) has been explained either by changes in particle rearrangement and inter-particle forces, or by changes in adsorbed water. On stirring, the particles and fabric units are rearranged and the bonds between particles and units are broken. Also, the structure of the adsorbed water is broken up and the clay mass will be more susceptible to deformation under selfweight. After deformation, the clay fabric will seek a status of minimum energy with maximum attraction between particles and fabric units. The adsorbed water also regains its quasi-crystalline form to give the system sufficient rigidity to have a yield value. There are several factors which contribute to the regaining of part or all of the strength. These are original structure, activity of the clay minerals, and the degree of disturbance. The activity is a characteristic parameter of the electrochemical action of the colloids and is defined as the ratio of plasticity index and clay fraction less than two microns.

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CHAPTER III PREVIOUS LABORATORY STUDIES 3.1 General Review Over the past two decades, considerable laboratory work has been carried out on cohesive sediments. In order to get an idea about the variety of different ways in which the research work has been carried out, some of the topics under which the literature could be classified are given below along with a typical reference on the same as an illustration. a) Sediment used: Clay mineral alone: Kaolinite: Christensen and Das (1973) Mixture of clay minerals: Yolo Loam: Arulanandan et al. (1975) Mixture of clay and silt: Grundite: Gularte (1978) Natural sediments: Brisbane Mud: Thorn and Parsons (1980) Fernandina Mud: Yeh (1979) San Francisco Bay Mud: Partheniades (1962) b) Fluid used: Salt water: Partheniades (1962) Distilled water: Mehta and Partheniades (1979) Fresh water: Fukuda (1978) c) Type of bed: Remolded: Gularte (1977) -37-

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-38Deposited from suspension: Yeh (1979) Compacted: Christensen and Das (1973) d) Characterizing indices: Dielectric dispersion: Alizadeh (1974) Sodium adsorption ratio: Kandiah (1974) Cation exchange capacity: Kandiah (1974) Chemical and electrical parameters: Arulanandan et al. (1973) e) Basic parameters: Bed density and salinity: Owen (1977) Temperature: Gularte (1978) pH: Kandiah (1974) Pore fluid and eroding fluid: Arulanandan et al. (1975) Water quality (pH, conductivity): Migniot (1968) f) Microstructure studies: Kaolin: McConnachie (1974) Marine sediments: Bowels (1969) g) Other studies: Hydrodynamic aspects: Turbulent drag reduction: Gust (1976) Colloidal dispersion: Zeichner and Schowalter (1977) Attempts have been made from time to time in the past to review the information available in respect to cohesive sediments. These are listed below in chronological order: 1960: The Committee on Tidal Hydraulics, U.S. Army Corps of Engineers conducted literature review to study soil as a factor in shoaling processes (Committee on Tidal Hydraulics, 1960).

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-391964: The present knowledge on the behavior of fine sediments in estuaries was summarized by Partheniades (1964). 1968: Task Committee of ASCE on Erosion of Cohesive Materials prepared a report on literature review (Task Committee, 1968). 1973: State of the art paper on causes and mechanisms of cohesive soil erosion was presented by Paaswell (Paaswell, 1973). 3.2 Review of Literature on Erosion Since the scope of the present study deals with erosion of Kaolinite, a brief account of the work carried out in the past on erosion of cohesive sediments is given here. Effect of various parameters and processes influencing the erosional properties of cohesive sediments has been studied by several research workers. Important findings of these studies have already been given in Chapter II. The earlier experiments conducted to study the erosion of cohesive sediments were oriented to obtain solution to a specific engineering problem such as the model studies performed by the Tennessee Valley Authority for the Fontana Project (1953) and the Fort Patrick Henry Project (1960). Most of the work to understand the basic physical processes related to cohesive sediments has been carried out during the past two decades. Smerdon and Beasley (1959) applied the tractive force theory to the stability of open channels in cohesive soils. Dunn (1959) used a submerged jet to determine the tractive resistance of cohesive soils and correlated it with plasticity index. Masch et al. (1965) conducted studies on remolded cohesive sediments using a rotating cylinder

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-40apparatus and found that the critical shear stress was related to water content and vane shear strength. Flume study of natural soils conducted by Lyle and Smerdon (1965) showed that the critical tractive force correlated to void ratio, cation exchange capacity, and plasticity index. Grissinger (1966) found that the erosion rate of soil decreases with increasing clay content and decreasing void ratio and temperature. Findings of various research workers related to determining the effect of various parameters and characterization indices on erosion have already been described in Chapter II. Paaswell (1973) summarized selected studies on cohesive soil erosion. The same are reproduced from his paper along with addition of subsequent investigations. It may be noted that the mode of formation of bed in the laboratory equipment is an important factor. This can be done in the following 3 different ways. a) Placed or remolded or uniform bed: Formed by mixing the sediment thoroughly with required water content and placed evenly in the apparatus so as to have uniform density without any external compaction. b) Flow deposited or flocculated or stratified bed: Formed by allowing a sediment suspension with high concentration under a low flow velocity which would permit most of the material to slowly deposit on the bed. When the flow velocity is zero, the term deposited bed is used. Such a bed is flocculated and has density stratification over depth. c) Compacted bed: Sediment with low moisture content is compacted with external pressure.

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Table 2. Summary of selected studies on cohesive soil erosion. Investigator Mode of Placement of Sample Mode of Measurement of Erodibility Lutz (1934) Comparison of physical tests with Use of qualitative physicochemical erosive properties of natural soils analyses Peele (1937) In-place topsoils Soil loss and runoff tables Anderson (1951) In-place topsoils Correlation of erodibility with shear measurements Dunn (1959) Remolded, subjected to jet Jet to produce erosion; visual measures Smerdon and Beasley (1959) Slightly recompacted natural soil, Visual observation of bed movement top leveled Laflen and Beasley (1960) Remolded at unspecified percentage Visual correlation or erosion with of water, then saturated calculated inactive stress Flaxman (1962) Natural soils Correlation of permeability and unconfined compressive strength with natural erosion (channel measures) Moore and Masch (1962) Remolded and natural (trimmed) jet Measurement of scour depth and weight loss Abdel-Rahman (1964) Remolded in duct Visual; measurement of erosion depth '_ __ .. I''

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Table 2. Continued Investigator Mode of Placement of Sample Mode of Measurement of Erodibility Partheniades (1965) Remolded natural deposited (salt Measurement of suspended sediment water) in duct concentrating with time Grissinger (1966) Remolded in channel Rate of erosion by weighing Masch, Espey, and Moore Unspecified but trimmed as hollow Weight loss versus rotating shear; (1965) cylinder visual correlated with shear Mirtskhulava (1966) Remolded in flume Weight of floc loss Liou (1967) Remolded in flume Point-gauge measurement of erosion depth Liou (1970) Remolded in flume Arulanandan et al. (1973) Molded in ring Weight comparison Christensen and Das Remolded in tube Weight comparison (1973) Grissinger (1973) Natural samples remolded in channel Rate of erosion by weighing Sargunam et al. (1973) Remolded (compacted) in rotating Weight loss of sample cylinder test apparatus Alizadeh (1974) Remolded (compacted) in rotating Weight loss of sample cylinder test apparatus

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Table 2. Continued Investigator Mode of Placement of Sample Mode of Measurement of Erodibility Kandiah (1974) Remolded (compacted) in a rotating Weight loss of sample cylinder test apparatus Raudkivi and Hutchison Remolded in a recirculating refriWeight loss of sample before and (1974) gerated water tunnel after test Thorn and Parsons (July 1977) Deposited in a flume Withdrawal of samples, filtering and weighing or use of photoabsorptiometer to determine suspension concentration Owen (Nov. 1977) Deposited in a flume Withdrawal of samples, filtering and weighing or use of photoabsorptiometer to determine suspension concentration Gularte et al. (1977) Remolded in a recirculating refriMeasurement of suspension congerated water tunnel centration by using laser-photocell system Gularte (1978) Remolded in a recirculating refriMeasurement of suspension congerated water tunnel centration by using laser-photocell system Fukuda (1978) Deposited Measurement of suspension concentration by using laser-photocell system ___ 'aJ

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Table 2. Continued Investigator Mode of Placement of Sample Mode of Measurement of Erodibility Lee (1979) Deposited Measurement of suspension concentration by filtering and weighing Mehta and Partheniades Deposited Measurement of suspension concen(1979) tration by filtering and weighing Yeh (1979) Deposited Measurement of suspension concentration by filtering and weighing Thorn and Parsons (1980) Deposited Measurement of suspension concentration by using photo-absorptiometer

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-45Types (a) and (b) mentioned above are usually used in flumes, whereas type (c) is adopted in rotating cylinder type apparatus. 3.3 Review of Literature Pertinent to the Present Study It may be noticed from 3.2 above that the literature even on the erosion aspects of fine sediments is quite extensive. Comparative study of the results obtained by various research workers may pose some problem due to the experimental technique involved in these studies. Details of some of these are given below. a) Type and size of apparatus used: Christensen and Das (1973): Rotating circular cylinder: 2.5 cm dia, 10 cm long Gularte (1978): Water tunnel: 5.5 m long, 2.0 m wide, 1.5 m high Yeh (1979): Circular flume: 20 cm wide, 1.5 m centerline dia Krone (1962): Straight steel flume: 0.9 m wide, 30.5 m long Partheniades (1962): Straight steel flume: 0.3 m wide, 0.45 m deep, 18 m long Owen (1977): Straight flume: 0.3 m wide, 0.2 m deep, 17.6 m long Thorn and Parsons (1980): Straight flume: 0.3 m wide, 0.2 m deep, 17.6 m long b) Method of reproducing shear stress: Rotating the apparatus in the case of circular cylinders. Flow of fluid in the case of straight flumes and water tunnels. Rotation of ring alone or both ring and channel in the case of circular flumes.

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-46c) Measurement of erosion: By photo-electric cell, filtration, and weighing weight loss of sample before and after test. Although the importance of several parameters in influencing the erosion of cohesive sediments has now been established, data in respect to these are not available for each study. The variation in respect to fluid used, sediment used, and type of bed have already been mentioned under paragraph 3.1. Also there exists considerable variation in duration of test from 1 minute (Espey, 1963) to 500 hours (Krone, 1962). Sampling time for measurement of concentration is substantially different from one test to another. Hence, some of the observations made at small intervals of concentration-time history are not available in respect to studies where observation of concentration was made only at long time intervals. Visual observations of erosion permitted by the transparency of apparatus help in a more realistic interpretation of data than in the case of an opaque apparatus where erosional characteristics are indirectly inferred from the observations available. It is therefore necessary to exercise caution while comparing the results of various studies. In view of the implications mentioned above, results of only those studies where the size of the apparatus was comparable in order of magnitude, the bed was of a deposited type, and the emphasis was on the bed shear stress and the bed density are primarily considered here. Partheniades (1962) conducted erosion tests on San Fransisco Bay mud using a straight flume. He tested two types of beds, viz. i) placed bed at natural density and water content and ii) flow deposited bed. The shear strength of the flow deposited bed was 1/136 to 1/14th of the

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-47strength of the placed bed. The entire experimental work consisting of 32 runs was divided into three series. Series I: Tests on placed bed by changing the flow'velocity by small positive and negative increment. Results are given in Fig. 3 (runs 1 to 13 only). Series II: The same bed was used as for series I except that it was remolded after the upper surfacing was removed. Results are given in Fig. 4. Series III: Tests on flow deposited bed. Results of these tests are given in Fig. 5. Comparison of the test results of series I and series III are given in Fig. 6 in terms of rate of erosion. Important conclusions drawn by Partheniades (1962) from his erosion tests were as follows: i) The rates of erosion were independent of concentration. ii) The erosion rates for the flocculated bed changed abruptly several times. This change was proven to be caused by changes of the bed properties. iii) The eroded surface did not cause any measurable increase of the frictional resistance of the bed. iv) The minimum shear stress to start erosion was about 0.05 N/m2 for both the placed and the flocculated bed, although they had different densities. v) Erosion rates for both the beds were of the same order of magnitude. vi) The overall resistance to erosion of a cohesive bed is independent of the macroscopic shear strength of the bed.

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E 12z 0 2-10-5 U 8-4 O z O W o > z 6-3 o S .J -CONCENTRATION S u .-----AVERAGE FLOW VELOCITY ---4 -l L__2 Ext Series I --D n run run run i_ run run run n run run run run -I +2a7' 31 14 -5 1 '6 7 I -T0 I 12 -13 .200 400 600 800 1000 1200 1400 TIME AFTER START OF SERIES -I, hrs Fig. 3 Concentration versus time plot obtained in erosion of placed bed: Partheniades (1962) Expt. Series I

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-49E -10 z o uw r88-40z ou 0 < 6 60: w 4 -2 S2rn run run -CONCENTRATION w S.--VELOCITY (1 0r--___,_____________________________,____, 0 200 400 600 800 TIME AFTER STARTOF SERIES -I,hrs E ) 4 0 z w run 22 tn **^ 23 0 20 40 60 80 100 TIME AFTER START OF RUN, hrs Fig. 4 Concentration versus time plot obtained in erosion of remolded bed: Partheniades (1962), Expt. Series II

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-50E Z "" o 10-5 Io z 8 -4j 6 -3 u -CONCENTRATION I -----VELOCITY 4 4 -2 r S-ru~n^Z -run27428-run 2 39+run30 run3lrun (n .... ...-' y ^ ---i*'-'* ----rL 0 200 400 600 800 TIME AFTER START OF SERIES-m, hrs Fig. 5 Concentration versus time plot obtained in erosion of deposited bed: Partheniades (1962),. Expt...Series III .01 (-6 E o 0 --< Series I a: Series Ir 2 Computed on the Basis of Equation 1-33 Port -VTM C 0 002 0.04 006 AVERAGE BOTTOM SHEAR STRESS (lbs/ft2) Fig. 6 Relationship between rate of erosion and average bed shear stress: Partheniades (1962)

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-51vii) The erosion rates strongly depend on the average shear stress. For both dense beds, the erosion rates increase very rapidly for shear stress greater than 0.478 N/m2 for series I and 1.34 N/m2 for series II. viii) The observed independence of erosion rates from the macroscopic shear strength of the clay and the fact that clay gets eroded at shear stresses which are infinitesimal compared to its strength suggest that the mechanism of failure of clay particles by surface erosion is basically different than the mechanism of failure of clay particles in the interior of the clay mass, when subjected to shear stresses. Krone (1962) conducted studies on San Fransisco Bay mud in order to relate transport and deposition processes to properties of the sediment. Erosion tests were carried out on flow deposited beds. The concentration-time data of erosion tests plotted on log-log coordinates are given in Fig. 7. Concentration as a function of bed shear stress is given in Fig. 8. Results of a 500 hour long erosion test are given in Fig. 9. An arithmetic coordinate plot of these data would produce a curve with a steadily decreasing slope, suggesting an approach to an equilibrium or steady state concentration. However, the continuing straight line plot on log-log coordinates as shown in the figure discourages this suggestion. Krone (1962) found that "the log-log erosion curves and their slopes were difficult to explain." He presented a qualitative hypothesis based on interchange between suspended flocs and the bed and on the dependence of erosion on time and weakly, if at all, on shear (p. 86).

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-52I-I BED NO. 7 w .I S 0.1 0 0 z 00 v 0 0.34 /O 0.01 .* I* t r .1 I I I* I I r r r .0 10 100 1000 10000o TIME AFTER VELOCITY INCREASE, rin Fig. 7 Concentration versus time plot: Krone (1962) 14 SCURVE A 2 -I U .. o 44 .-b se s s o I / ". \.^ i2 2 0 I 0 | .6 ? a 9 10 I SHEAR ON SEDIMENT BED Ir,), dynes/sq cm Fig. 8 Concentration as a function of bed shear stress: Krone (1962)

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I.-OL ~ .. .. .." .I..'. ...| --' -, ''. .I ...-'-, -." ,. t -' Z W o C 0.316 MARE ISLAND STRAIT SEDIMENT 0 SUSPENDED SOLIDS 2 0. 0 .0 -J L10 IL tVELOCITY, 1.14 ft/sec S -T Ao OPTICAL DENSITY 0 M SUSPENDED SOLIDS w a. 0.1 0.01 0.1 1.0 10 100 1000 / TIME AFTER VELOCITY CHANGE, hr Fig. 9 Results of 1 500 hour long erosion test: Krone (1962) ... ........ .. ... ..... ..'. ........ tI

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-54Lee (1979) conducted studies on resuspension and deposition of Lake Erie sediments, using a circular channel apparatus with an annular ring similar to the apparatus used for the present studies. However, in the case of apparatus used by Lee, the shear stress was produced by rotating the ring alone. The results of concentrationtime curves for series I and II of tests as presented by him are given in Figs. 10 and 11. The same data have been re-plotted in the form of concentration under a time-variant bed shear stress (Figs. 12 and 13). Lee found that the entrainment rate was a strong function of shear stress, water content, and mineralogy. Concentration-time data obtained by Yeh (1979) for different values of bed shear stress are given in Fig. 14, and concentration as a function of bed shear stress given in Fig. 15. 3.4 Shear Strength of Clay The classical Coulomb's equation for shear strength of soils is s = c + p tan ( (3.4.1) where s = shear strength c = cohesion : = angle of internal friction p = pressure normal to the failure plane Since c and are found to depend on the loading rate and drainage condition, the modified Coulomb's equation is given as follows: s = ce + o tan e (3.4.2) where

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-559-00 -0 run 3 T,=6.0 */ o00 A -Ar run 2 Tw=4.3 / A o500 SERIES I -I S100 --...o -----run I Tw=3.2 dyne/cm? 100 50 100 150 200 Fig. 10 Concentration versus time plots for Series I obtained by Lee (1979) scole A scoa. 90 r un5T 5.9 70 -.-.-.Q.--'.-. ---.n4 TW4.9 150 507 .--6-,, -----e S .-*run 2 Tw= 3.6 100 6 i*-----..__ .___ .S run I Tw 3.0. 2 30 dyne/cm2 scaleA: run 1,2,3,4 50 sale B. run 5 SERIES 17 10 0 100 200 / Imin) Fig. 11 Concentration versus time plot for Series II obtained by Lee (1979)

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1.0 I I I Reference Lee (1979) Series I 0.60 E Z 0 iBed Shear Stress in N/m2 0.43 z z S0.5 T AT ( AT)ex 0.32 z 0.11 0.34 0 0.43 (n 0.17 0.39 z 0.60 U)J S00.32 0 I I I i I I -I I 0 2 4 6 8 TIME (hrs) Fig. 12 Lee's (1979) data re-plotted to indicate variation of suspension concentration as a function of time and bed shear stress

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020 I I Reference Lee (1979) Series TI e x "E T AT (AT)ex z 0.150.30 o 0.06 0.20 0.36 0 < -0.07 0.19 0.56 X 0.43 0.06 0. 14 z 0.49 L 0.07 0.14 o 0.100.56 O 0 z Bed Shear Stress in N/m2 0.49 S0.43 M 0.36 y-w 0.05c. .0.30 / I I I I 0 2 4 6 8 10 12 14 16 TIME (hrs) Fig. 13 Lee's data re-plotted to indicate variation of suspension concentration as a function of time and bed shear stress

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-581412-Symbol Shear Stress 0.43 N/m2 10 -x 0.34 N/m2 0 0.24 N/m2 -A 0.15 N/m2 0' z 80 z z 0 4 20 L I I 0 40 80 120 -160 200 TIME (Hours) Fig. 14 Concentration versus time data obtained by Yeh (1979) for erosion of kaolinite

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-59Symbol Series x -KS o -FS 30* -KD KS= Kaolinite in Salt Water FS = Fernandina Bay Mud in Distilled Water -KD = Kaolinite in Distilled Water z 0 S20cr 10I Z LJ Z 0 / o0 0 0 0O 0.2 0.4 Tb BED SHEAR STRESS (N/m2) Fig. 15 Concentration as a function of bed shear stress obtained by Yeh (1979)

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-60ce = true cohesion, being only a function of the void ratio of the material (Fig. 16(a)) (e = true angle of internal friction, practically independent of the void ratio a = the effective pressure normal to shear plane For normally consolidated clays, the following expression is used: s = pc tan d (3.4.3) where P = consolidation pressure for 100 percent consolidation 6d = angle of drained shear resistance The magnitude of Pd can be much greater than
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-61-e =True Angle of Internal Friction (a) (Pd =Angle of Drained Shear U, Resistance ST EFFECTIVE NORMAL STRESS CT -.(b) IS, Reference: 5u McCarthy (1977) -," NORMAL STRESS OCD = Consolidated, Drained Soil C-U = Consolidated, Undrained Soil U-U = Unconsolidated, Undrained Soil (c) I-4b Reference: uj z McCarthy (1977) -n -5 % WATER CONTENT Fig. 16 Schematic diagrams showing shear strength of cohesive soil related to other parameters

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-62A qualitative comparison of shear strength results for these conditions is shown in Fig. 16(b). Shearing of U-U type is completed relatively quickly because the prevention of volume change results in development of excess pore pressures and consequent reduction of the shear strength. Shearing of C-D type takes place very slowly since both drainage and the volume change are permitted during shearing. The shear strength of clay is essentially composed of two components: i) physical component due to frictional resistance and interlocking between particles, and ii) physico-chemical components due to the inter-particle attractive and repulsive forces. The shear strength of clay soil improves with consolidation, provided that time is available for permitting the necessary pore water drainage to take place. In effect, consolidation results in decreasing the water content of the clay with a subsequent increase in shear strength (Fig. 16(c);McCarthy, 1977, p. 234). 3.5 Shear Strength and Bed Density of Clay Attempts have been made by several research workers to measure the bed density and shear strength of clays and to establish correlations between the two parameters. Ariathurai and Kandiah (1979) have developed an electrical method to measure in situ sediment densities. Dayal et al. (1980) have developed a method for obtaining in situ soil strength by use of low velocity projectile penetration technique. Gularte (1978) measured shear strength with a fall cone device. He also used a modified viscometer for this purpose.

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-63Krone (1962) measured shear strength of the bed by using a screen penetrometer for consolidation time of 8, 24, 46, 72, 97, 120, 146, 168, 240, and 312 hours, and also measured densities. He'concluded that the ultimate density appears to be independent of total depth, i.e. consolidation occurs independently of the weight of material above a consolidating layer. Partheniades (1962) used a conventional vane shear test apparatus as well as a simple penetration test device for measurement of the shear strength of soil in connection with the erosion tests conducted on a flocculated bed and arrived at the following important conclusion (p. 108): "The observed independence of erosion rates from the macroscopic shear strength of the clay and the fact that clay gets eroded at shear stresses which are infinitesimal compared to its strength suggests that the mechanism of failure of clay particles by surface erosion is basically different than the mechanism of failure of clay particles in the interior of the clay mass, when subjected to shear stresses." He also observed that the erosion resistance of the flocculated bed seemed to increase with depth, and attributed this to the heterogeneity of the bed which was deposited from a suspension of high initial concentration containing a wide range of particles from clay size to fine sand. Owen (1970) measured the shear strength of the surface layers of the bed by using a Brookfield viscometer. After measuring the shear strength of the top layer, that portion of the bed was allowed to spill slowly. The shear strength of the next layer thus exposed was again determined by using the viscometer. Samples of the bed were taken simultaneously and a correlation of shear strength with bed density was

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-64established (Fig. 17). The tests were carried out in a perspex settling column 10 meters high and 99 mm internal diameter. The variation of the density at various depths for different consolidation times is given in Fig. 18. He concluded that the shear strength variation within the bed could be correlated satisfactorily with the variation of density and that the shear strength increased rapidly with density. It has been mentioned earlier that consolidation results in decreasing the water content of the clay with a subsequent increase in shear strength (McCarthy, 1977, p. 234). However, the following interesting observations have been made regarding the erosion rate as a function of moisture content: i) Partheniades (1962, p. 54) noted that "in spite of the lower overall strength of the bed and its higher water content, the erosion rates of series II were lower than the corresponding rates of series I." ii) Christensen and Das (1973, p. 13) noted the following: "It is generally assumed that under similar conditions, the rate of erosion will decrease with increasing density. However the evidence in previous studies has not been conclusive. For this phase of the laboratory investigation, saturated soil samples were parepared at varying densities and moisture content and subjected to a constant hydraulic tractive stress. Because the soil samples were saturated, the density decreases with increasing moisture content. The duration of the test and the temperature of the water were kept constant for each type of soil. The laboratory test results exhibit a sharp decrease in erosion with increasing moisture content."

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-65CONCEN10 0 [ TRATION SALINITY fEXrmg/1 g/1 m 90 ___._/ 16290 32 9 10 06 15 520 17 0 9-36 .O [ 17 775 17 8 6 96 0 a 17280 16-7 4 64 70 6 705 2 7 9 73 0 / x 7 2dd 4 6 9 74 S0 0 392 a8 9-76 6-0 -------0 10272 16 8 1002 a 6 666 33 3 9 72 + 6 B10 0 6 974 5-0 ,.o ,----i-4.0 __ -T 325 -" SHEAR 2-0 N/m2 096 08 --0-0.9 .--Et ----------07 06 0-5 0.4 I 60 90 100 150 200 250 300 350 400 DENSITY g/1 Fig. 17 Relationship between shear strength and bed density observed by Owen (1970)

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(A) AFTER 250 MINUTES (B) AFTER 500 MINUTES 10 -1 i09 .-----------0 ------07 0 7 _.. ..... 06 ---0 --04 -----------------04 ---------S0L -------" 4--01 1-70N Ig/l 05 00 o 0 02 04 06 08 1.0 I I 14 16 8210 22 2. 26 2.8 300 02 04 0-6 08 10 1.2 I1 1.6 10 20 22 24 26 28 30 0O (C) AFTER 1000 MINUTES (0) AFTER 2000 MINUTES I.0 .SUSPENSWOV Li 09 ---09 ---Oes 08 ----------------os -----7 4 0 0 -------0 O I Wp" OENSITY/MEAN DENSITY CALCULATED DENSITY PROFILES FOR VARIOUS BED THICKNESSES Fig. 18 Bed density profiles: Owen (1970) 04 (---I-1--II--l----I------I 04 .0 -----------(D -MI Sa 01 I----I---W \-t--~----1--I 01 --0 0 0V4 ENSITY/HEAN D0NSITY

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-67iii) Owen (Nov. 1977, p. 11) conducted studies on erosion of Avonmouth mud and concluded the following: "In terms of mean shear stress the onset of continuous erosion is almost simultaneous for mud beds of different density, but the rate of erosion is greater for mud beds of lower density." iv) Thorn and Parsons (July 1977, p. 8) studied properties of Grangemouth mud and made the following observation: "There does not seem to be any strong relationship between bed shear stress and surface density, although further tests would be needed to establish this with confidence. This result is rather surprising as it would seem likely that the thicker or denser the mud the more resistant it should be to erosion. The surface density at equilibrium was divided by mean bed density to give a relative density but this did not give any stronger relationship with bed shear stress. This is an interesting result because an earlier investigation of Avonmouth mud showed that both equilibrium surface density and relative equilibrium surface density were linearly related to bed shear stress." v) Arulanandan et al. (1980) studied the effect of changing the density structure of bed by remolding the soil and found that remolding generally decreased both the critical shear stress and the rate of change of erosion rate. They also found that the salt concentration of eroding fluid influenced the erosion of remolded soil samples. A decrease in salt concentration of eroding fluid decreased the critical shear stress and increased the rate of change of erosion rate. .-E

PAGE 82

CHAPTER IV PRESENT INVESTIGATION 4.1 Objective The parameters and processes influencing the behavior of fine sediments in contact with water have been described in Chapter II. Also, the results of important investigations carried out to study the erosional properties of fine sediments have been presented in Chapter III. It is clear from the presented information that a range of physical and chemical parameters are necessary for characterizing the properties of the sediment bed as well as the properties of the eroding and the pore fluids. When a given sediment is equilibrated with the eroding fluid over a sufficiently long time, the pore fluid and the eroding fluid have the same properties. The erosion process is then predominantly governed by the following parameters: i) The structure of the bed in terms of its floc shear strength variation over the depth, which is a function of the type of bed, viz. placed, deposited, or compacted. ii) The bed shear stress Te which causes erosion when it has a magnitude greater than the critical shear stress for erosion. The moisture content is an important parameter in the case of the placed bed and the compacted bed. In the case of the deposited bed, which is of interest in this study, the process of bed formation is important, involving the following parameters (Fig. 19): -68-

PAGE 83

-Step I Step U -Step III ---Mixing Deposition and Consolidation Erosion Tm Cn Te3 w T in Td F Tm ---'Td ---Ts --Tsetc. -Step I and Step IH = Pre-erosion stress history STm= Bed Shear Stress for Initial Mixing Tm = Duration of Initial Mixing Td = Bed Shear Stress for Deposition and Td = Duration of Deposition and Consolidation of Sediment Consolidation Tel Te2, = Bed Shear Stress for Erosion Ts = Duration of Time Step Fig. 19 Definition sketch for notations used to describe experimental conditions '*1 I,, ^ ^ ^ ^ ^ „ ......

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-70a) The process of bed formation in the case of deposited bed starts with an initial concentration of suspension, C .Under the laboratory conditions, the sediment and the eroding fluid are mixed under a comparatively high shear stress in order to obtain a suspension with C as its uniform concentration throughout the depth of fluid. b) The shear stress Tm under which the initial mixing of the sediment and the eroding fluid are carried out. c) The duration of mixing, T .With a sufficiently long duration of Tm, the maximum size of the flocs in suspension is controlled by the balance between the local shear stress and the floc shear strength. d) The bed shear stress Td which is sufficiently small in its magnitude so as to permit deposition of most of the material in suspension. e) Duration of the total time for deposition plus bed consolidation, Td, which influences the density of the bed. If C0 is kept constant, the erosion of bed will depend upon the following two important processes. Formation of bed: influenced by Tm, Tm, Td, and Td' Erosion of bed: influenced by Te which may vary in its magnitude and duration. Attempts made by previous research workers to directly measure the shear strength of the bed or to correlate it to the bed density have not been satisfactory. The overall objective of the present study was to develop a laboratory test procedure which would enable the determination of the variation of the shear strength of a deposited bed over the depth. This was accomplished by increasing the applied bed shear stress in small increments of selected short time periods.

PAGE 85

-71Different types of bed structures were formed by using different combinations of Tm, Tm, Td, and Td. Concentration of suspended sediment resulting from the different values of Te was measured as a function of time. The term "resuspension" is usually used in the case of erosion of a flow deposited bed. Although all the experiments reported under the present study were for the deposited beds, the terms erosion and resuspension are considered to be synonymous. 4.2 Material Commercially available Kaolinite was used as sediment in the studies. Size gradation curve for the material is given in Fig. 20. The median diameter was 1.4 microns. Ninety-five percent of the material was within the size range of 1 to 7 microns. Seventy-four percent of the material was finer than 2 microns. The maximum size was 15 microns. This size distribution was obtained by using "Sedigraph" Particle Size Analyzer. Before using Kaolinite for conducting tests, it was kept submerged under the eroding fluid for a period of three months for the purpose of equilibration. The pore fluid and the eroding fluid was identical in these studies. The fluid was prepared by dissolving commercial salt in tap water and was adjusted to have a concentration of 35 parts per 1000 by weight. The pH of the eroding fluid was 7.6.

PAGE 86

100i-" z w w < 50I-2 w 3 -D 0 100 50 10 5 I 0.4 EQUIVALENT SPHERICAL DIAMETER, L.m Fiq. 2( Si7p qrarlation of Kaolinite tlsd for the experiments

PAGE 87

-734.3 Apparatus for Erosion Tests A system of rotating circular channel and ring was used for conducting experiments. The annular channel was 20 cm wide, 46 cm deep, and had a mean radius of 76 cm. The channel was made of 9.5 mm thick fiberglass. Four windows with a transparent plexiglass were provided on the channel to permit visual observations. The channel was supported on a rigid steel frame. An annular ring made of 6 mm thick plexiglass, having the same mean radius as the channel, was provided within the channel. The width of ring was smaller by 6 mm than the width of channel. The ring could be positioned at any required height within the channel and it could be freely rotated while in contact with the water surface. Taps were provided on the vertical outer wall of the channel for obtaining samples of suspension from the channel. Details of the apparatus assembly are given by Mehta (1973). Accessary equipment consisted of Millipore Filtering Apparatus, an oven, and a Mettler balance having 0.05 mg precision. For determining the concentration of sediment in suspension, the following procedure was adopted: i) Obtain a sample of the eroding fluid having sediment in suspension through the tap provided on the channel and collect it in a sampling bottle. ii) Measure volume of the sample. iii) Filter the sample through pre-weighed Millipore filter paper discs with 0.45 micron pore diameter. iv) Wash the salt using distilled water. v) Dry the filter paper discs in the oven at 600C.

PAGE 88

-74vi) Weigh the filter papers containing sediment. vii) Obtain concentration by calculating the weight of sediment from the difference in the weight of the filter paper with and without the sediment, and dividing it by the volume of the sample. A mercury thermometer was used to measure water temperature. Photographs of the apparatus and accessory equipment are given in Figs. 21 to 28. The rotating channel facility was previously calibrated for measurement of bed shear stress, details of which are given by Mehta (1973). The required bed shear stress could be attained by adjusting the speeds of rotation of the ring and the channel. Calibration curves used for this purpose are given in Figs. 29 and 30. The fing and the channel are rotated in directions opposite to each other in order to minimize the effects of the radial secondary currents (Mehta, 1973). 4.4 Experimental Procedure The experimental procedure consisted of the following three parts: i) Formation of bed in the rotating channel: Kaolinite equilibrated with the eroding fluid for a period of 3 months was put in the channel. The quantities of Kaolinite and the eroding fluid (which was saline water with 35 parts per 1000 concentration) were adjusted in order to have a sediment suspension of the order of 40 parts per 1000 concentration by weight when fully mixed (C ). The bed was formed in the channel by initial mixing and allowing the sediment to deposit on the channel bottom. Figure 19 schematically indicates the procedure

PAGE 89

-75Fig. 21 The rotating channel facility Faa Fig. 22 Close view of the annular channel and the ring

PAGE 90

-76L i .. CI Fig. 23 The motor controllers F i g .. ......... F.c Fig. 24 The electric motors for the channel and the ring

PAGE 91

-77Fig. 25 Millipore filter apparatus assembly Fig. 26 Device for measurement of bed density Fig. 26 Device for measurement of bed density

PAGE 92

-78II Fig. 27 Equipment for determining concentration of sediment suspensions Fig. 28 Sa33-mpli64 mpl65ing bottes 9

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RING CONTROLLER METER READING 0 5 10 15 20 25 30 35 40 45 50 F04 ---. 130 S----,--------(----------1----^ ----I-,---^-------I-__^--.-II.8 4z 8 -.-----.---Water Depth z r 15 cm S-23 cm z o 6 q a .45 : 4) -040--7 15 -i C: I I 1 0~4a ^-Ln0.35 -2 z 0.3 i'u.30 1.:j 0.25 .25 -10 X -0 .2 ----. ----.__ -O^^ .2 0 2 4 6 8 10 12 14 RING SPEED (RPM) Fig. 29 Operational speeds and controller meter readings for ring and channel at different bed shear stresses

PAGE 94

50(Y)=3.75 (X)-1.25 7 404 7(Y)= 3.33(X) -Channel 7 z7 S3Ring 30I r-20 105 10 15 REVOLUTIONS PER MINUTE (rpm) Fiq. 30 Cnrrelation hbtwoen r.p.m. and mnter reading for the channel and the ring

PAGE 95

-81followed and the notation used. Initial mixing was carried out at a shear stress Tm over a mixing duration of Tm. The bed shear stress was then reduced to Td to permit settlement of the suspended sediment and form a bed. The time for settling plus consolidation is indicated by Td in the sketch. All the tests reported in this study were conducted with a total depth of 30.5 cm in the channel which was kept constant. The quantity of individual sample withdrawn from the rotating channel was of the order 20 c.c. out of a total volume of 300 liters in the channel. With a suspension concentration of say 2 percent, the amount of sediment withdrawn at each sample was about 0.4 grams out of a total of 12 kg of sediment in the channel. The quantity of water and sediment withdrawn from the channel were small enough not to have any measurable effect on the fundamental processes taking place during the experiment. A large quantity of the eroding fluid having properties identical to that in the channel was kept in stock. This was used to replenish the eroding fluid by adding small quantities from time to time over the duration of experiment in order to keep a constant depth. Sediment was replenished at the mixing stage of the next experiment. The sediment and the eroding fluid was the same throughout the study. All the tests were conducted on the deposited bed. No remolding or compaction with external force was carried out. ii) Erosion of bed: The bed shear stress (re) varying in its duration and magnitude was applied by rotating the ring and the channel in accordance with the calibration curves. The time-step function of bed shear stress is shown schematically in Fig. 19. For any one experiment, the duration of time step (T ) was kept constant (such as 30 min,

PAGE 96

-8260 min, 90 min) and only the magnitude of shear stress was varied. The values of Te were obtained by discretization procedure of a linearly increasing shear stress or the one equivalent to a sihusoidal velocity variation, etc. If T and T are the two consecutive magnitudes of bed shear stress, then excess shear stress is given by AT = T -(4.4.1) e2 e The normalized excess shear stress was defined as T -T (AT) 2 1 (4.4.2) ex T e For example, (ATr)e = 0.2 represents magnitude of r which is 20 percent ex e2 greater than the magnitude of T, and so forth. Different values of e1 AT as well as (AT)ex were selected for variation of bed shear stress. iii) Data analysis: Data collection consisted of obtaining samples of suspension at pre-determined time intervals after every change of the bed shear stress. The sampling time used was 1, 2, 3, 5, 10, 15, 20, and 30 minutes in the case of Ts = 30 min. For Ts of longer duration such as 60 and 90 minutes, additional samples were taken at every 10 minutes after the first 8 samples were collected in 30 minutes. In order to study the variation of suspension concentration over the water depth, samples were taken from two locations, viz. Tap A located 125 mm above the channel bottom and Tap B located 225 mm above the channel bottom. The concentration of sediment in suspension for each sample was determined by following the procedure described earlier under section 4.3. The basic data consisted of plotting of a concentration versus

PAGE 97

-83time graph for each experiment. Further analysis of these data was used to study the variation in suspension concentration as a function of bed shear stress and for computation of erosion rates. Although the room housing the rotating channel assembly was airconditioned, facilities to maintain a constant temperature of water in the channel were not available. Hence, typically a change in the water temperature of the order of 2 to 30C took place over the duration of the experiment. 4.5 Apparatus for Measurements of Bed Density During the course of the present study, two different apparatus were developed for the following measurements: (a) Measurement of bed density for sediment deposited under quiescent conditions (Td = 0). The apparatus developed for this purpose consisted of a 30 cm high, 15 cm dia. polyvinyl chloride cylinder provided with a bottom plate. Ten plastic tubes of various heights ranging from 0.95 to 6.35 cm, all having a 0.95 cm inner diameter, were glued to the bottom plate (Fig. 31). The cylinder was made in two pieces, the bottom cylinder being 7.5 cm high (photograph in Fig. 26). After placing the 22.5 cm tall piece of cylinder on the bottom cylinder, the circumferencial joint was sealed with a tape to make it water-tight. The cylinder was then filled with a sediment suspension of known concentration. The sediment was allowed to deposit under quiescent conditions (rd = 0) for the required consolidation time (Td = 24 hrs, 40 hrs, etc.). The supernatant water was siphoned out, and the top cylinder was removed after peeling the tape off. All the

PAGE 98

-84T --Top Cylinder 15cm dia. L2 L 22.5 cm Plastic Tubes of various heights, 0.95 cm dia. glued to the bottom plate Bottom T -Cylinder 7.5 cm 15cm dia. Bottom Plate SKETCH OF APPARATUS I ,1--2 cm dia plastic tube jT 15 cm dia. plexiglass cylinder 15cm 2.5 cm dia metal tube -Annular space for mixture of alcohol and dry ice Sediment Metal-! i Plate Filled with ice cbes Porcelein P I iston with Screw Rod Dish SKETCH OF APPARATUS TI Fig. 31 Apparatus developed for measurement of density as a function of depth for deposited beds

PAGE 99

-85sediment outside of the tubes was removed. Entire sediment from each tube was taken out in porcelain dishes by using a hypodermic syringe with repeated washing by small quantities of distilled water. The weight of the sediment was determined after evaporating water in an oven at 500C temperature. From the height and diameter, the volume of sediment in each tube was calculated and by knowing the weight, the density of sediment in each tube was calculated. Further calculations were made as follows: Let L1 and L2 be the heights of two adjacent tubes 1 and 2 with a small change in heights (of the order of 0.3 cm). Let p, and P2 be the densities of sediment in each tube, calculated as above. Let V1 and V2 be the volumes of sediment in each tube. Let L2 < L1 and hence V2 < V1. It was assumed that the bottom sediment of height L2 in tube L1 had the same density as that of tube 1, viz. pI. The reason p2 is not equal to p1 is the fact that the sediment contained in the upper portion of tube L1, viz. in the incremental height (L1 -L2), has a different density (Ap)1-2 which was calculated as follows: A) pV1 -P2V 2 (4.5.1) plV1 -P2V2 (Ap)1-2 V -V (4.5.1) (b) Measurement of bed density for sediment deposited in the 2 rotating channel under a low bed shear stress (Td = 0.015 N/m2 0.05 N/m2, etc.). The apparatus consisted of a 2.5 cm dia. metal tube 15 cm high placed concentric in a 15 cm dia., 15 cm high circular plexiglass cylinder having a sealed bottom. At the center of bottom

PAGE 100

-86plate 2.5 cm dia. hole was provided in the plate to match with the concentric metal pipe, thus leaving the bottom only for the annular space between the metal tube and the plexiglass canister. After the bed was formed in the rotating channel under the required conditions of Td and Td, a transparent plastic pipe of about 2 cm dia. was placed vertically through the sediment bed over the bottom of rotating channel. The plexiglass canister was then lowered vertically so as to insert the plstic tube through the metal tube of the apparatus. The annular space around the metal tube was filled with commercial grade denatured alcohol and dry ice was added in pieces to the alcohol. In less than about 30 minutes this resulted in freezing of the suspension inside the plastic tube which was then removed and placed horizontally covered with ice cubes in order to keep it frozen. A piston which could be activated by a threaded rod was used to push about 4 mm length of frozen sediment projecting outside the plastic tube. A metal plate held vertically in contact with the projected portion quickly melted the frozen sediment which was collected in a porcelain dish. Next, a 5 mm portion of the sediment was then pushed out and the process was repeated. The density of each 5 mm thick layer could be determined by knowing the volume and the weight. The freezing resulted in swelling of the sample and thus increasing the height of the sediment bed in the tube. The total thickness of frozen sediment was therefore divided into ten equal parts and the density of each layer was measured which was taken to be corresponding to the ten parts of the thickness of the original depth of the bed.

PAGE 101

CHAPTER V TEST RESULTS AND ANALYSIS 5.1 Effects of Parameters in Steps I, II and III It has been mentioned in Section 4.3 (Fig. 19) that the process of bed formation for the present study consisted of Step I: Mixing; plus Step II: Settling and consolidation. The parameters involved were Tm and Tm for Step I, and Td and Td for Step II. It was observed that increase in suspension concentration resulting under a given time-step function of bed shear stress was a function of the shear stress-time history during the stage of bed formation. Change in suspension concentration resulting from a change in magnitudes of parameters under Step I is shown in Fig. 32. The magnitude of shear stress for deposition Td, and the duration of consolidation Td which are the parameters under Step II had a substantial effect on the rate of erosion. Three beds of different density structures were formed for three different combinations of Td and Td. The concentration-time plots obtained for T = 0.2 and 0.45 N/m2 with a Ts = 90 minutes are given in Figs. 33 to 35. The effect of Step II parameters is seen from the superposed results of experiments 3, 4 and 5 given in Fig. 36. For a given bed, a change in sequence of magnitudes of T and T results in different concentration time plots as shown in Fig. 37. -87-

PAGE 102

Shear Stress Te in N/m2 24 Trn =0.9 N/mZ Tm =4 Hrs. Expt. 2 0.435 T =0.9 N/m2 T = 24Hrs. Expt. I 20 28 1 d =0 N/m2 Td =40Hrs. Expt. I S ..and 2 E 0.071 -16z 0 a: 12 S2_----Expt. I z w --Expt. 2 S8z 4LUx ---------------------q20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 32 Effect of parameters in Step I on suspension concentration

PAGE 103

-Shear Stress Te in N/m2 S220.45 CP 0.20 S0.20 Tm= 0.9 N/m 18SHours Tm =24Hrs. 0 Hours -Td = 0.015 N/m2 S14-Td = 24Hrs. z S10() CL I I I I I I I I z z 6 D I a0 20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 33 Suspension concentration versus time for Experiment 3

PAGE 104

Shear Stress Te in N/m2 Tm = 0.9 N/m2 24 05Tm = 24 Hours 045 m 2 Td =0 N/m 20 0.20 Td.=40 Hours E 0 1.5 3 16 Hours -* z o0 < 12125 mm obove S2channel bottom J --225 mm above z channel bottom o 8 z 0 -4 20 40 60 80 100 120 140 160 18 TIME (Minutes) Fig. 34 Suspension concentration versus time for Experiment 4

PAGE 105

22 Shear Stress Tein N/m9 e 0.45 T =0.9 N/m2 Ep Tm = 24 Hours 18 0.20 Td = 0 N/m2 Z -Td = 115 Hours 0 I 140 1.5 3.0 .." 0 Hours z w z o 100 1 z 0 ( 6U) U)2S20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 35 Suspension concentration versus time for Experiment 5

PAGE 106

28 Shear Stress Te in N/m2 E EXPT. 3 0.4 5 240 d = 0.015 N/m2 0.2O S -/ T = 24 Hours E20 o> 0 1.5 3.0 Hours z EXPT. 4 0 2 16 Tm =0.9 N/m < -n Tm = 24 Hours T =0 N/m z Td =40 Hours o 12z 0 O 0 8O EXPT. 5 _L 2 U Td =O N/ m U) 4r Td = 115 Hours O f-I I I I i I I I I 0 20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 36 Effect of Step II parameters on suspension concentration

PAGE 107

Shear Stress Te in9 N N/2 0.435 Tm=0.9 N/m 0.281 T= 24 Hrs. Expt.6 d--= 0 N/m2 I20 0071 Td = 40 Hrs. --" 0 I 2 3 0 23 z 16 S 0.379 z 120.281 w 0.169 Expt. 6A ----z -0.071 o_ / 08 z 0 I 2 3 Hours
PAGE 108

-945.2 Multiple Steps of T e In order to study the effect of varying bed shear stress on the erosion of bed of a given density sturcture, experiments were conducted with various combinations of parameters related to the formation of bed (viz.T m, Tm, Td and Td) and the parameters related to the erosion of bed (viz. Te and T ). The selection of Te values was based on the following: i) AT, the excess shear stress ii) Tr, the ratio of the consecutive values of shear stress iii) (AT)ex, the normalized excess shear stress. The duration of time-step Ts was either 30 or 60 minutes. The procedure for discretization of a varying shear stress is shown in Fig. 38. The results of experiments consisting of a multiple steps of Te are given in Figs. 39 to 46. The experimental conditions for these tests are given in Table 3. 5.3 Discretized Sinusoidal Velocity Variation In a typical estuarine environment, the variation of tidal stream velocity is close to sinusoidal as shown in Fig. 47. The bed shear stress is proportional to the square of the velocity. Delecting a maximum value of 0.45 N/m2 for the bed shear stress, its variation as a function of time corresponding to the sinuisoidal velocity variation was computed and this variation was discretized with time steps of 30 minute duration as shown in Fig. 47. Results of experiment conducted with these discretized values of Te are given in Fig. 48, and those obtained with a higher starting value of are given in Fig. 49.

PAGE 109

Table 3: Experimental Conditions for Tests Conducted with a Multiple Steps of T Expt. Figure T T T Td T T AT T (AT)e Comment No. m 2 m d2 d s e2 r ex N/m hours N/m hours min. N/m 7 39 0.9 6 0 40 30 0.1 0.1 2.00 1.00 0.2 0.1 1.50 0.50 ( ex 0.3 3 0.1 1.33 0.33 decrea0.4 0.1 1.25 0.25 sing 0.5 0.1 1.20 0.20 0.6 0.1 1.17 0.17 0.7 8 40 0.9 24 0 40 30 --same as for Expt. 7 9 41 0.9 24 0 40 30 0.100 0.017 0.17 0.17 0.117 (ATex 0 7 0.022 1.20 0.20 ex 0.139 increa139 0.033 1.24 0.24 in 0.172 sing 0.055 1.32 0.32 0.227 0.123 1.54 0.54 0.350 350 0.350 2.00 1.00 0.700 10 42 0.9 24 0 40 30 0.09 0. .. 0.10 2.11 1.11 0.19 0.19 2.00 1.00 ex 0.38 0.32 1.84 0.84 1.0 0.70

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Table 3 (continued) Expt. Figure T T T Td TS e AT (AT) Comment No m2 m 2 e 2r ex N/m hours N/m hours min. N/m 11 43 0.9 24 0 40 30 0.100 0 1.5 .5 0.150 0.075 1.5 0.5 ex 0.225 =cons0.225 0.112 1.5 0.5 =constant 0.337 0.169 1.5 0.5 = 0.5 0.506 0.254 1.5 0.5 0.760 12 44 0.9 24 0 40 30 0.100 0.150 0.05 1.50 0.50 0.150 0 0 0.05 1.33 0.33 =cons0.200 t 0.250 0.05 1.25 0.25 0.250 0.300 0.05 1.20 0.20 13 45 0.9 24 0 24 60 0.11 0.19 2.72 1.72 A 0.30 0.20 1.67 0.67 =cons0.50 0.20 1.40 0.40 tant 0.70 0.2 14 46 0.9 24 0 24 30 0.10 0.10 2.00 1.00 (AT) 0.20 ex 0.20 0.10 1.50 0.50 dee decrea0.30 0.10 1.33 0.33 sing 0.40 0.10 1.25 0.25 0.50 50 0.10 1.20 0.20 0.60 60 0.10 1.17 0.17 0.70

PAGE 111

-97T = 30 Min. E 0.6 z n/ LU) w S0.2S2 4 TIME (hrs) T = 60 Min. -n/ E 0.6U) m 0.4U 0.20 m 0 2 4 TIME (hrs) Fig. 38 Representation of a linearly varying bed shear stress by two different discretized time step functions 0: _, S --/ two different discretized time step functions

PAGE 112

Shear Stress Tein N/m2 Tm =0.9 N/m2 24 Tm = 6 Hrs. S 7Td = 0 N/m2 0.420 03Td = 40 Hrs. 0.2 S0.1 160 I 2 3 z Hours F, 5 12z O U z 0 U 8 z 0 z 4CL (I) 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fi(. 31 Simn, nsn n nncntrtn ntvprStIS timno for Fyxnorimnnt 7

PAGE 113

26 Shear Stress Te in N/m2 Tm = 0.9 N/mr 220.7 T= 24 Hours 0.6 2 S -0 Td =0 N/m S18J--4 Td =40 Hours S180.3 O. 3 z 0.2-0 0.1 140 I 2 3 z -Hours z I 10o i 0 z 6U) 2 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 40 Suspension concentration versus time for Experiment 8

PAGE 114

262 Shear Stress Te in N/m2 0.7 m0.9 N/m2 22 3 Tm= 24 Hours STd = 0 N/m2 0.172 0.139 Td =40 Hours 0.117 E 18 0.10 S0 I 2 3 z Hours 0 14z 6 5 10C) z 0I I I I I I I I 0 20 40 60 80 100 120 140 160 180 200 z 60 2 -0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 41 Suspension concentration versus time for Experiment 9

PAGE 115

-rl SUSPENSION CONCENTRATION (gm/e) ro ---o 04 ) v, oa .o 4 roo e o -C I I I I I I I I I -r o 0 0 2 20 0 o .0 02 z 1\ m x N T3 3 (D 8 o U) (D 0( 0

PAGE 116

26 Shear Stress Te in N/m 2 Tr =0.9 N/m2 0.760 m 2205 T = 24 Hours 0.506 m Td =0 N/m 0.337 d N 18 0225Td = 40Hours 0.15 z 0.1 1 4 ------I-------0 I 2 3 FHours w z 100 z 0 m 6z w a .2Cl) 0 20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 43 Suspension concentration versus time for Experiment 11

PAGE 117

Shear Stress Tein N/m2 /2 Tm= 0.9 N/m Tm=24 Hrs. Td= N/mr Td =40 Hrs. E 0.25 0.20|_ 4 -IO 0.15 I4S 0 I 2 3 r7 3Hours z 0 O 0 20 (n Oz I I I I I I I I I I 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 44 Suspension concentration versus time for Experiment 12

PAGE 118

Shear Stress Tein N/m2 -T =O0. 9 N/m2 Q) J 40T = 24Hrs. 1 40 0.70 2 Td =0 N/m E S0.50 Td =24 Hrs. S0.1 I -SI 2 3 z 20Hours O i z CL (C) S0 I I I I I I I I I w a. V) 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 45 Suspension concentration versus time for Experiment 13

PAGE 119

Shear Stress Te in N/m Tm =0.9 N/m2 50 T =24 Hrs. Td =0 N/m2 Td =24 Hrs. 0.7 0.6 o30H 0.2 SO.I w -Hours z z 0 m 10z 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 46 Suspension concentration versus time for Experiment 14

PAGE 120

-0.4 40E / -0.3 / '30z/ \\ \ Z rU) C / \ \-CD -0.2 ( / o-o Sinusoidal velocity variation \ 200 F-o J -xCorresponding shear stress 0/ ---Discretised 30 min. time step \ 0. variation of shear stress S/ adopted for experiments. I I I I I I I 0 40 80 120 160 200 240 280 320 360 TI ME (Minutes) Fig. 47 Time step function for bed shear stress

PAGE 121

Shear Stress Te in N/rn2 Tm =0.9 N/m' Tm = 24 Hrs. S0.435 Td =0 N/m2 E 20 0.379 Td =40 Hrs. O 0.281 < 0.015 z a: I 0 1 2 3 Z 12 Hours U Z 8 z 8 o z .4 (I) 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 48 Suspension concentration versus time for Experiment 15

PAGE 122

Shear Stress Te in N/m2 Tm=0.9 N/mZ2 0435 Tm 24 Hrs. -200.379 Td =0 N/rn ST0281 = 40 Hrs. 160.16 z 0 I 2 S Hours O -12 z 0 o 8z 0 w z 4.) 0 20 40 60 80 100 120 140 TIME (Minutes) Fig. 49 Suspension concentration versus time for Experiment 16

PAGE 123

-1095.4 Correlation Plots of C30 and C60 Values Values of T adopted for various experiments reported under Sections 5.2 and 5.3 were obtained by discretization of bed shear stress varying with time. The data obtained from the tests was also at discrete time intervals. The magnitudes of suspension concentration obtained at the end of each time step wuch as C30, in the case of a 30 minute duration of time step were compared with the corresponding values of bed shear stress. The experimental conditions for various tests have been reported in Table 3 under section 5.3. A comparison of the mean variation of bed shear stress with the variation in magnitudes of C30 or C60 values, both as a function of time is given in Figs. 50 to 57 for experiments 8 to 15 respectively. All the plots indicate a striking similarity in the variation of suspension concentration and the variation of bed shear stress. 5.5 Analysis of Data The basic data available from the various experiments werein the form of concentration of suspended sediment as a function of time for different conditions of bed formation and for different time step functions of Te Superposition of the c-t plots as shown in Figs. 58 and 59 did not appear to give any meaningful results of a general nature. Hence all the c-t data for the beds having Td=24 hours and Td=40 hours was plotted to see if it showed any trend. The plots are shown in Figs. 60 and 61 respectively. Although both the figures show an increase in suspension concentration with increasing bed shear stress, there is considerable scatter in

PAGE 124

-110I E .En E (n I I ol 00 CQ tO (:j E o -n 040 IU, d0 c'J w 0 00 N Nr (?/U)6 0OJV-N0O OS~ss o, c'j q ( 2U/ N) SS3dlS ?IVB3HS GJ8 I I I i I I (c>/ui) NOIllJNIN33NO3 NOISN~dSflS

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I I I I '10.70 -Tm = 0.9 N/m2 25 25 5 0.6T = 24 hrs Td =0N/m2 STd = 40 hrs z F-~ S/ w 030.4 Bed Shear Stress zo15 -in N/m2 0.35/ o H 0 / n 0 -.I z 0.23 0 SI I0.14 5 0.0 10 -,//C(3o)Values oL -----I___o.-----o---'------10 1.0 2.0 3.0 4.0 TIME (hrs) Fig. 51 Variation of suspension concentration with bed shear stress as a function of time (Expt.9 )

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I I 1 0.70 / 2 Tm =0.9 N/m 0.6 Tm =24 hrs E -/ Td =0 N/m2 40 -" -Td =40hrs o / I (AT) 1I.0 -g-£__C __A_ M '~-t-

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30I I 0.76 o/ / 06/ I 0.6 / / SBed Shear Stress / I co1 in NAn2 0.50/ Tm = 0.9 N/m2 20-E -i z / / Tm = 24 hrs o -/ o2 S/ / Td = 0 N/m S ,,0.4/ FW z / / Td 40 hrs U 0.34 / z / (AT)e 0.5 z / ex o /zO U) /.23./ (n z0-" C (Values z 00.2 C a G. 50 a. m 0..15/ 0O0. O -I I I I-0 1.0 2.0 3.0 4.0 TIME (hrs) Fig. 53 Variation of suspension concentration with bed shear stress as a function of time (Expt. 11)

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I I I I I Tr 0.9 N/m2 Bed Shear Stress min N/m2 0.3Tm =24hrs 0 N2 E Td 0 N/m Ed % 4 -Td 40 hrs 0.25 0 -" z 2 Im 20 z 0 0 .15 ) al z / / rO I-'--0 .0 2.0 3.0 TIME (hrs) Ss s a a f o e L. 045 a. n" -n O 00 1.0 2.0 3.0 TIME (hrs) Fig. 54 Variation nf quspension concentration with bed shear stress as a function of time (Expt. 12)

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-0.7 -0I I 0.7 I I 0.7-/ / 400.6t Bed Shear Stress N/m2 / E / N0.5 z E 0.5z // S30 z z -/Tm = 0.9 N/m z cn Tm = 24 hrs S0.3 / o 20 cr 0.3 -N/m2 W Td 0 u o / Tm = 24 hrs S200303 / z w 0.2a a2 / .C(60)Values // -/ / 0/ O -0 I I I I I I I 0 1.0 2.0 3.0 4.0 TIME (hrs) Fig. 55 Variation of suspension concentration with bed shear stress as a function of time (Expt.13 )

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T= 0.9 N/m 0.70 Tm =0.9 N/m2 Bed Shear Stress STm = 24 hrs in N/m2 0.60,/ 0.6 E Td = 0 N/m / Td = 24 hrs 0.50/ z 40 -E o/ rz 0.40 / S0.4'0 W / z w / o a 0.30 / o __ 0 0.2 yi w Fn m 0I. 2.0 3.0 4.0 TIME (hrs) TIME (hrs)

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50-. I 1 I T = 0.9 N/m2 $ Tm = 24 hrs 400.4Td = 0 N/m2 038 z Td = 40 hrs z W 0 N E z Bed Shear Stress 30-in N/m2 0.28 Z U) 0 u LU -L O _z 20-0.2on 0.17 -o Z I a, -C / Values I0cn -W/ n 0 .0 7 *, o 0.015 0 1.0 2.0 3.0 TIME (hrs) Fig. 57 Variation of suspension concentration with bed shear stress as a function of time (Expt. No. 15)

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$ 0.45 E z 0.20 A 014< O I 2 3 z Bed Shear Stress Te w 10-in N/m2 0 0.37 z Q281 8 -B 0 0169 B /B 0.071 z 6S0 I 2 3 z Hours a. n2c SI I I 3 0 t 0 20 40 60 80 100 120 140 160 TIME (Minutes) Fig. 58 Effect of shear stress variation on suspension concentration

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Shear Stress T in N/m2 0,7 Tm =0.9 N/m 05 Tm = 24 Hrs 50 Expt. 13 Td = 0 N/m? 0.3 0 60 min.time step Td =24 Hrs 0.11 S40 E 0 I 2 3 o 0.61 / -0.5 <30 0.4 Expt.14 ---.-31 30min. time step 0 02 z 0.1 / o n III--------------I--> S20 0 I 2 3 z Hours 0 LIi o10 010 --I i i I iIi 20 40 60 80 100 120 140 160 180 200 TIl kI h I A-:.... ^ Fig. 59 Comparison of suspension concentration obtained under two different discretized time step functions i I

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50Tm = 0.9 N/m2 Tm = 24 hrs 2 -Td = 0 N/m2 o .40Td = 24 hrs Z o o o F30w 30 u o z O o z 0 o 200 0 IC0 0 0 0o 0,00 00 I I I 0 0.2 0.4 0.6 0.8 BED SHEAR STRESS Te (N/m2) C3f£D vIaria+inn nF cijnpnrinn rnnrntr~atinn with had chpar c+r-cc all data for T. =2 4 hnurc

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Tm = 0.9 N/m2 40 Tm 24 hrs 1 Td = 0 N/m2 z o T = 40 hrs U 0 Ho Fz w n z 0 o 0 U z 200 0 o od o o o o S) 0 8 D 8 U) o0o 80 O S0 0 000 0' 9 0 08oO 0 0 0.2 0.4 0.6 0O8 BED SHEAR STRESS T. (N/m2) Fig. 61 Variation of suspension concentration with bed shear stress, all data for Td = 40 hours

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-122data points and a relationship of a general nature was not seen. However, for individual experiments, plots of C30 versus Te did show a definite trend. Results of experiments 7, 9, and 11 are given in Figs. 62, 63 and 64 where the magnitude of normalized bed shear stress (AT)ex was decreasing, increasing and constant respectively. Results of two experiments having the same bed formation but which were eroded with two different values of Ts viz. 30 minutes and 60 minutes are shown in Fig. 65. All the data for a given bed formation such as for Td=40 hours seemed to indicate variation of a general nature when the ratio of the magnitudes of suspension concentration of two consecutive time steps were plotted against the corresponding ratios of the bed shear stress. For example let the consecutive values of Te be Tel and Te2 and let C10(2) and C(10)2 be the magnitudes of suspension concentration 10 minutes after applying the Tel and Te2 respectively. The the parameters were taken as C10(2)/C(10)1 plotted against Te2/Tel. This data is plotted in Fig. 66. Similar data was plotted corresponding to 20 minutes and 30 minutes after each change of Te and has been shown in Figs. 67 and 68 respectively. All such data available for the bed formation with Td=24 hours is plotted for C(30) values in Fig. 68 in order to compare it with results for Td=40 hours. In order to eliminate the variations due to the initial values of Te and C, a non-dimensional plot of all the values for Td=24 hours as well as Td=40 hours was made with the normalized excess concentration (Ac)ex plotted as a function of normalized excess bed shear stress (AT)ex, where

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-123(C2-C1) (Te2-el) (Ac) = 2-C) and (AT) e= eTe (C)ex C1 and ()ex : el 1 el as an example for the consecutive values of Tel and te2 with the corresponding values of C1 and C2.Fig. 69 shows a plotting of this data. For all the test results reported under Chapter V, the data was collected at 1, 2, 3, 5, 10, 15, 20 and 30 minutes after each new value of T in the case of Ts=30 minutes. Additional data points in continuation of these were obtained at 40, 50, 60 and 90 minutes in the case of Ts= greater than 30 minutes. These individual data points are not shown on the plots of experimental results. Instead, the lines joining the observed data points are shown on all the figures giving experimental data of the present study.

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Tm = 0.9 N/m2 AT= Constant = 0.1 N/m2 Tm = 24 hrs (A T)ex :Decreasing / 4 Td = 0 N/m E Td = 24 hrs / o 20/ z Je (A T)ex / -0.I / <0 1.00 £0.2 z 0.50 L 0.3 o 0.33 z 0.4 / o 0.25 / U 0.5 z 10 -0 0.20 o 0.6 / S07 70.17 7z 0.7 o0/ S C(3o)Values /O 0 0.2 0.4 0.6 0.8 BED SHEAR STRESS Te (N/m2) Fig. 62 Suspension concentration versus bed shear stress (Expt.14 )

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Tm = 0.9 N/m2 AT : Increasing 20S Tm = 24 hrs (AT)ex: Increasing Td = 0 N/m2 E Td = 40 hrs / 0.100 / W : 0.017 0.17 0.117 z : 0.022 0.20 w / S 0.139 0.033 0.24 o 10 0.172 o : 0.055 0.32 0.227 z -0.123 0.54 / 0 0.350 0.350 1.00 / z 0.700 / LU / 0) C (30 Vlues 0 0.2 0.4 0.6 0.8 BED SHEAR STRESS Te (N/m2) Fig. 63 Variation of suspension concentration as a function of bed shear stress (Expt. 9 ) >

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1 I I Tm = 0.9 N/m AT : Increasing Tm = 24 hrs (AT)ex Constant SII ex S20Td 0 N/m2 =0.5 STd = 40 hrs -d z 0 z w (n (-z o z C(0 Values 0I I i I I I 0 0.2 0.4 0.6 0.8 BED SHEAR STRESS Tc (N/m2) Fig. 64 Variation of suspension concentration as a function of bed shear stress (Expt.ll )

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Tm= 0.9 N/m2 Expt. 14 E 40Tm = 24hrs --Expt. 13 / r Td = 0 N/m2 Td = 24hrs a / / z / z .. W 70 0 20/ Vaues60) Values 0" ,n P/ 0 0.2 04 0.6 0.8 BED SHEAR STRESS Te (N/m2) Fig. 65 Variation of suspension concentration with bed shear stress for two different discretized time step functions II

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-n SUSPENSION CONCENTRATION RATIO Cr (10) --o oo No oW -N n o 0 m 8 o oo 0 00 0 0 0 0 (A 0 (XI

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-n "11 SUSPENSION CONCENTRATION RATIO Cr (20) _0 0 0 00 o O O OO O 0 0 0 0 0 OJ

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-Cy u0 0 -SUSPENSION CONCENTRATION RATIO Cr (30) I -01 N CoJ 0 x 00 00 -I II I o \0 %I0 < .-"O D 0 0 0 S00 .0 0 r> o) 'm o --O S 0 O cr o r+ CD +In (-S CD Il

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-n c> 10 ) NORMALIZED EXCESS SUSPENSION CONCENTRATION ( AC)ex (D 01 -N 0 SI I I I I I I S. 1 O O x O ~ 0. 0 Oo ar .o -o ( -"'._oo o o I'D 0 0 Srn CD CD o 0 co oo o o X X 0 3 3 3 I> > \ r oo 1\ .-x x N ;13 r u ii ii ii i i -II I I X -* I II

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CHAPTER VI DISCUSSION AND PROPOSED TEST PROCEDURE 6.1 Bed Density and Other Soil Parameters Correlated to the Shear Strength It has already been mentioned that the shear strength of the bed cannot be estimated correctly by measuring the soil parameters and to obtain the shear strength of the soil by using the available relationships. It is therefore necessary to evolve an experimental procedure for measuring the shear strength of soil at various depth. Justification for not using the already available relationships is given in this section. Attempts made by various investigators to measure the two parameters, viz. the density of the soil and the shear strength of the soil by adopting various methods, have been described in Section 3.5. The conclusions regarding the correlation between the density and the shear strength are contradicting as can be seen from the following: (a) Parthenaides (1962, p. 54) noted that "In spite of the lower overall strength of the bed and its higher water content, the erosion rates of series II were lower than the corresponding rates of series I." However, he has also made the following observation (p. 64): "The most striking results of this study are the independence of the minimum shear stress and erosion rates from bed shear strength and bed density." -132-

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-133(b) A significant decrease in the erosion rate was noticed with increasing moisture content (Christensen and Das, 1973). These tests were performed on soils containing kaolinite and grundite as basic clay minerals. The studies were conducted by providing a lining of compacted clay inside a small brass tube (25 mm diameter x 100 mm length). A steady flow of water was maintained through the tube to effect erosion. The range of moisture content tested was between 32 and 43 percent for kaolinite, between 29 and 47 percent for grundite, between 33 and 42 percent for kaolinite-sand mix, and between 24 and 36 percent for grundite-sand mixture. The soil indices for kaolinite and grundite used in the tests were given as follows: Liquid limit Plastic limit Plasticity index kaolinite 43 29 14 grundite 51 30 21 clay-sand mixture Data not given. It is clear from the above that the moisture content for these tests was higher than the plastic limit and lower than the liquid limit. (c) Owen (Nov. 1977) concluded that in terms of the time averaged mean shear stress the onset of continuous erosion (i.e. critical shear stress) is almost the same for mud beds of different density, but the rate of erosion is greater for mud beds of lower density. These tests were conducted with natural sediment, viz. Avonmouth mud. Deposited bed was used to study erosion. For zero salinity tests, the low density beds had a density ranging from 187.9 gm/liter to 212 gm/liter whereas the high density beds had a density ranging from 226.0 gm/liter to 261.2 gm/liter. This gives a water content between 79 and 81 percent for the low density bed and between 74 and 77 percent for the high density, both of which are higher than the liquid limit.

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-134(d) After studying the properties of Grangemouth mud, Thorn and Parsons (1977) concluded that "There does not seem to be any strong relationship between bed shear stress and surface density" (p. 8). However, later on (1980) based on the studies of the properties of three different cohesive muds using the same experimental and analytical technique, the same authors suggested the following relationship (p. 352): -6 2.28 T = 5.42 x 106 228 (6.1.1) c s where Tc = critical shear stress for erosion at the exposed surface Ps = surface density The correlation observed by Thorn and Parsons is given in Fig. 70. The contradicting or rather confusing conclusions drawn from the correlation of bed density and bed shear strength of fine sediments are due to the following reasons: i) The shear strength depends upon the condition of soil, viz. U-U (unconsolidated-undrained), C-U (consolidated-undarined), or C-D (consolidated-drained). This has been discussed under Section 3.4. ii) The shear strength of cohesive soil is dependent upon two factors: physical component due to frictional resistance and interlocking between particles, and the physico-chemical component due to interparticle attractive and repulsive forces. The contribution of each component cannot be measured separately. The inter-particle forces include electrostatic forces, inter-molecular forces, cation bonds, water dipole linkaage, chemical cementation, hydrogen bond, and van der Waals Forces. These forces are either a

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o Brisbane mud oi r I -iI-0 + Grangemouth mud I ~ & Belawan mud VNI aIp ,, I z 1 zo I .* I i --.I os -01 -Om I 1 1, -----r ------I -----Ir^ -/01 -. 00 in 0.1 ost.2 Example of erosion test results Reference: Thorn and Parsons (1980) 0-01 10 100 PS: Dry density of mud surface (g/) Critical shear stress for erosion Fig. 70 Example of erosion test results and critical shear stress for erosion as a function of dry density of mud surface, given by Thorn and Parsons.

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-136property of the material (type, surface area, etc.) or a function of other factors related to eroding fluid such as pH, temperature, sodium adsorption ratio, etc. A detailed.discussion of the above is given in Chapter II. iii) Pertinent data on soil parameters and type of bed used by different investigators are given below: Type of Soil Parameters Investigator Bed PL LL PI W % % % Partheniades (1962) Deposited Natural Mud 44 99 55 110 Espey (1963) Compacted Taylor Marl 21 47 26 27 Christensen and Das Compacted (1973) Kaolinite 29 43 14 N.R. Compacted Grundite 30 51 21 N.R. Arulanandan (1975) Compacted Yolo Loam ---N.R.--Owen (1977) Deposited Natural Mud ---N.R.--Fukuda (1978) Deposited Natural Mud ---N.R.--61-73 Thorn and Parsons Deposited (1980) Natural Mud ---N.R.--where PL = plastic limit, LL = liquid limit PI = plasticity index W = moisture content N.R. = data not reported by the author Due to the procedure adopted in forming the deposited beds and the compacted beds, the water content in the deposited beds

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-137is considerably higher than the water content in the compacted bed. This factor influences the rate of erosion of the two types of beds. iv) The amount of moisture content being different than the optimum moisture content corresponding to the maximum density (Proctor density) may have an influence on the erosion rate. With reference to curve shown in Fig. 71.a, the same density of soil can be achieved at two different values of the moisture content. v) The bed density of a deposited bed is a function of the time of consolidation and the elevation below the sediment-water-interface. vi) Various investigators have found that for a given bed, either compacted or deposited, there is a characteristic shear stress above which there is a sudden increase in the erosion rate. vii) In the case of compacted beds, the moisture-density relationship is influenced by the compaction procedure (Fig. 71.b). Attempts to correlate soil parameters other than the bed density with the shear strength of soil have been described in Section 2.3. It may be noted that the Plasticity Index (PI) alone is not adequate to characterize a given soil. It is essential to specify both the Plasticity Index and the Liquid Limit (LL) for a given soil as can be seen from Fig. 71.c. A particular value of PI can have a range of LL between 20 and 100, and depending upon whether the fine sediment is clay or silt, the sediment can have a degree of plasticity ranging from low to high as shown in the figure. In view of the above, the attempt made at U.S. Bureau of Reclamation reported by Espey (1963, p. 3) appears commendable. Various

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-13810 E Proctor Density E (a) Reference: McCarthy (1977) p. 315 z o, % WATER CONTENT (a) Schematic MoistureDensity Curve obtained from Laboratory Compaction Test Trials Heavy Zero Air Voids Curve n Compaction _Zr E (b) Reference: McCarthy S(1977) p. 317 z S % WATER CONTENT (b) Schematic Moisture -Density Curve showing the Effect of Compaction Procedure Plasticity (Clays) Compressibility (Silts) x 60 low rred high 0 6 -40 (c) Reference Peck(1953) FS 0 20 40 60 80 100 LIQUID LIMIT (c) The Plasticity Chart Fig. 71 Parameters influencing bed density and plasticity of soil

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-139properties of soil such as plasticity index, liquid limit, soil density, mechanical analysis, shrinkage limit, vane shear strength, and percent of maximum Procor density were measured. For correlating the critical bed shear stress with the soil properties, a multiple linear correlation method programmed for the digital computer was used. The results of these studies indicated that the best correlation was obtained when the plasticity, gradation, and density were included as independent variables. 6.2 Critical Shear Stress While correlating the suspension concentration or the erosion rate with the bed shear stress, it appears the term critical shear stress has been used either for the minimum bed stress to initiate erosion or for that value of the bed shear stress beyond which there is a sudden increase in erosion rates as would be seen from the following: a) Parthenaides (1962, p. 66): "After a certain critical value of the shear stress had been reached, the erosion rates increase much more rapidly with increasing shear stress. This critical value was found to be 0.010 Ibs/ft2 (0.47 N/m2) for series I and 0.028 Ibs/ft2 (1.33 N/m2) for series II." These are shown in Fig. 6 b) Espey (1963): A typical test result is given in Fig. 72. Espey reports the following (p. 34): "As the critical shear stress is approached, the rate of scour increases. At some critical point, a large amount of material is suddenly ripped loose from the sample, resulting in a high rate of scour for that particular shear stress."

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30 10 E L 20 o u u") r'.1 2 3 4 5 6 Bed Shear Stress Fig. 72 A typical test result reported by Espey (1963)

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-141c) Observations of Yeh (1979) are given in Fig. 14. d) Similar observations made in the present study are given in Fig. 62-64. It is apparent that different notations and definitions need to be used for the three values of bed shear stresses as follows: Tcr = Critical time-mean value of bed shear stress for erosion to begin at the sediment-water interface. This is independent of the density of soil and is related to the floc shear strength of soil. Tch = Characteristic value of the bed shear stress. This changes with the nature of the bed structure. Tch can be as much as an order of magnitude higher than Tcr Tc = Shear stress value obtained by extrapolation of erosion versus shear stress plot. Ts = Shear strength of the bed at certain depth below the top surface of bed (i.e. sediment-fluid interface). After the layers above this depth are eroded, Ts represents the critical shear stress which must be exceeded by the applied shear stress in order to cause erosion of the exposed surface. These notations are shown schematically in Fig. 73. It is apparent that Tcr < T < Tch and rcr < Ts. Since different investigators have used the same term "critical shear stress" to mean different shear stress, it is necessary to be careful in identifying which one out of the above four is being referred to. For instance, Espey (1963, p. 34) is referring to Tch' Parthenaides (1962, p. 64): Tcr' and (p. 66): Tch'

PAGE 156

-142
PAGE 157

-1436.3 Proposed Test Procedure In order to obtain the shear strength as a function of depth below the sediment-water interface of a deposited bed, it is necessary to erode the bed layer by layer in an experimental apparatus and obtain concentration-time plots. Figure 74 shows results of a typical experiment indicating the bed shear stress Te as a function of time, and the corresponding suspension concentration. The erosion rates, e, plotted from these data are also given as a function of time. The experimental conditions in terms of rm, Tm, Td, Td, 'e' AT, and (AT)ex have been defined on the figure. It is seen that every time the bed shear stress is increased at Ts = 30 minutes, the erosion rate is high in the beginning and decreases rapidly with time. At the end of T E is some fraction of the maximum erosion rate. It is important to note the two different types of the concentration-time (c-t) curve plotted with linear scales on both axes. In one type the concentration "appears to be" tending towards an equilibrium concentration whereas in the other type the concentration appears to be increasing at the end of Ts. The first type may be denoted as EQ type profile of the c-t curve whereas the second type may be denoted as ER profile (Fig. 75). The characteristics related to the two types of profiles are as follows: EQ type profile: i) Tcr < Te < Tch' ii) c < ER type profile: i) Tcr < Te > Tch, ii) E > 6 where 6 is a specified small percentage of maximum erosion rate Em.

PAGE 158

-144T c 0.6-15 0.43 0.4-100.38 _j 0.28 ] r -0.25-0.17 F 01.0 2.0 Z o 0.5z D 0.4 Sa O. o 0.2 I 0.10LI 0a 0 1.0 2.0 TIME (hrs) Te =Bed Shear Stress (N/m2) C = Suspension Concentration (gm/.) E = Erosion Rate (gm/cm2/min) c= Rate of Change of Concentration (gm/-i/min) Experimental Conditions : Tm = 0.9 N/m T AT (AT)ex Tm = 24 hrs 0.17 0.11 0.65 Td = N/m2 0.28 0.10 0.36 0.38 : 0.05 0. I 3 Td = 40 hrs 0.43 05 3 Fig. 74 Erosion rate versus time for Expt. for different values of bed shear stress

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-145I -. -f EQ Type Profile of ER Type Profile of c-t Curve I c-t Curve Te = Bed Shear Stress Tr < T2 ch W Te 2 E Tr Tel z Tcr 8 mi E .Ati. 1E -At2 S= Specified Percentage of Em zAz Si ... 7-.......... ........ Fig. 75 efi.........ton sketch for varous parameters....... Fig. 75 Definition sketch for various parameters

PAGE 160

-146Thus EQ profile would be identified by a definition such as say for S= 5 percent of m By increasing the magnitude of re at every Ts, the bed would be eroded layer by layer. In order to obtain useful information, it is necessary to increase re in small increments. Since it is observed from the experiments described under Chapter V that the suspension concentration C2 is a function of not only Te2 but also that of C1 and Te, it is proposed to use the normalized excess shear stress ee as the incremental factor. Thus (AT)ex = 0.2 would mean that the next value of Te is greater by 20 percent the previous value of re. When the c-t graph is in the form of an EQ profile, it indicates that the erosion rate at the end of Ts is a very small percentage of the maximum erosion rate and hence it is apparent that under these conditions the bed shear stress Te must be only very slightly greater than the shear strength of the bed. It may thus be assumed that Te = s (Fig. 76). When the c-t graph is in the form of an ER profile, it indicates that either Ts was too small or Te > Tch. In the first case, the ER profile would take the form of an EQ profile if sufficiently long duration of Ts is provided. In the second case, no matter what the duration of Ts is, the bed will continue to erode indefinitely until the entire depth of the bed is eroded. It is apparent that this condition results under re > Tch which is the maximum shear strength of the bed (Fig. 77).

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S>I Bed Shear Stress Te Ts >I min T (cr /2 min (Tcr
PAGE 162

Ts1 1 I min Bed Shear Stress Te e 3 Ts2 < t2 min T Tcr Te Th -J.. F g ...7. Exp a.r ., "r "pI..-......-..... Fig. 77 Explanatory sketch for ER Type profile of c-t curve

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-149The following test procedure is recommended for conducting the experiments: i) Form a bed in the apparatus by using the sediment and the fluid having the required properties and measure the density of bed as a function of depth. The density structure of the deposited bed can be changed for each experiment for selecting the proper values of Tm, Tm' Td' and Td' ii) Select a small value of (AT) ex say 0.1 or 0.2. Conduct exploratory tests to determine the minimum duration of Ts which would give the EQ type profiles for the concentration versus time plots on a linear scale. During the same tests determine the value of Tcr at which the surface erosion begins. iii) Starting with T el TCr increase the bed shear stress in the form of time step function of duration Ts, and successive magnitudes determined by (AT)ex, so as to erode the bed layer by layer. iv) Calculate erosion rates for all the c-t profiles and determine which profiles satisfy the condition E < 5. The profiles satisfying this condition are the EQ type profiles and hence the various values of re used for conducting the experiments give the corresponding values of Ts at the respective depths of the bed. v) The depth of erosion for each re can be calculated from the measured bed density p, as a function of depth; the suspension concentration at the end of Ts, C(Ts ; the total surface area of the bed; and the total volume of the eroding fluid.

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-150vi) A plot of Ts as a function of depth can be made from the above data. vii) A plot of C(T ) versus e would give the value of the characteristic shear strength of the bed, Tch* Ariathurai (1977, p. 17) has noted the following: "To model the transport process, it is necessary to know the critical shear stress of each stratum of the bed and also the erosion rate if the erosive mechanism is surface erosion. At present laboratory measurements must be made to obtain these parameters." Thorn and Parsons (1980, p. 354) have concluded as follows: "It is possible to treat the data from all the mud tests as a single set and to derive general relationships which might be used for preliminary design calculations of navigation channel improvement and dredged spoil disposal schemes involving cohesive sediments. Nevertheless it is recommended that it is still necessary to determine the specific erosion characteristics of a particular sediment for the purpose of detailed prediction and design calculations. The underlying physical and chemical processes which govern the mud properties are not yet fully understood and the actual behavior of any particular cohesive sediment cannot be absolutely predicted from existing knowledge." The above comments describe very well the up-to-date stage in the understanding of erosional properties of cohesive sediment and justify the need for experimental determination of erosional characteristics. The test procedure recommended in this study would help in meeting the necessary objective.

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-1516.4 Illustrative Example In order to illustrate the recommended procedure, an example of tests conducted to study the erosion of kaolinite for three different density structures has been given in this section. The three different density structures of the bed were obtained with the following magnitudes of parameters influencing the formation of bed: Bed Expt. T T Td Td No. m m d d No. No. (N/m2) (Hrs) (N/m2) (Hrs) 1 19 0.9 24 0.05 24 2 20 0.9 24 0.015 40 3 21 0.9 24 0 135 While the sediment in suspension was depositing on the bottom of rotating channel, samples were collected at 125 mm and 225 mm above the bottom of the channel. It may be seen from Fig. 78 that with a shear stress of 0.05 N/m2, the change in concentration at two elevations was small, indicating a near-complete vertical mixing of the sediment. Under shear stress of 0.015 N/m2, a concentration gradient was formed in the channel as shown in Fig. 79 and the settling was almost complete within 5 hours. The change in concentration at three different elevations when the material was depositing under quiescent conditions is given in Fig. 80. Samples were collected at elevations A, B, and C shown in the figure. A distinct sediment-water interface was quickly formed and it settled down as a function of time. The interface passed vertically downward of elevation B, the uppermost of the three elevations, within 34 hours. It passed elevation A within 7 hours. The sediment settling from the upper layers was continuously increasing the concentration at elevation C, the bottom-most level of measurement.

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-152The density structure of the three beds was determined by using the apparatus described under section 4.5. The results of measurement are given in Fig. 81. In order to erode the bed layer by layer, the bed shear stress starting from 0.1 N/m2 was increased each time by 20 percent of the previous shear stress, i.e. (AT)ex = 0.2 was used. The time step Ts had 60 minute duration. The concentration-time plots for the three erosion tests are given in Fig. 82 to 84. Concentration of suspension at the end of each Ts, i.e. C(60) values, are plotted in Fig. 85 as a function of time along with the discretized bed shear variation. Suspension concentration as a function of the bed shear stress is plotted in Fig. 86 which gives the magnitude of ch for the three different beds as follows: bed 1, 0.21 N/m2; bed 2, 0.29 N/m2 bed 3, 0.34 N/m2.Since Ts = 60 min. throughout the test, the erosion rate is proportional to the difference between the consecutive values of the concentration. These are plotted as a function of bed shear stress in Fig. 87. The shear strength of bed as a function of depth for the three beds is shown in Fig. 88.

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SAt Elevation A (125 mm Above Channel Bottom) 0 x At Elevation B (225 mm Above Channel Bottom) S Td =0.05 N/m2 E z S20z0 Ui O O LC) o EO Z (f 108 01 0 4 8 12 16 20 24 TIME (hrs) Fig. 78 Suspension concentration during deposition under the bed shear stress of 0.05 N/m2 II r

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o At Elevation A(125 mm Above 30 -Channel Bottom) SA x At Elevation B (225 mm Above Channel Bottom) E Td =0.015 N/m2 Z < 20z \ o \\ 0 \ \ Or) S10 Q\ 9 0 2 4 6 TIME (hrs Fig. 79 Suspension concentration during dpeosition under the bed shear stress of 0.015 N/m2

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-155100 Location Height Above I of Top Channel Bottom C 40 mm 2N A 125mm Td B 225 mm 60 -, -, ---.--, 40 @ 20\ \ \ 0 2 4 TIME (hrs) Fig. 80 Suspension concentration during deposition under zero bed shear stress

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-1561.00D Td =0.05 N/m2 Td = 24 hrs : h 0.5 -: :. h 0 4 8 12 P/ Po Po = Initial Concentration of Suspension 10 = Density of Deposited Bed at Elevation h above h/hBottom 2h ho= Thickness of the Deposited 0.5Td= 0.015 N/m Bed Td = 40 hrs -p=41.3 g/0 --' I I I I I I I I I 0 4 8 12 P/Po 1.09 ) Td = 0 N/m2 Td = 135 hrs ho Po =41.1 g/. 0.50 II I I I I l I l 1 0 4 8 12 P/Po Fig. 81 Variation of bed density with depth for three different conditions of flow deposited beds

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9 0.21..----2Bed Shear Stress 0.175 J T in N/m2 0. 145 I ~ -00.120 I. to --_r---_/ ,cp 70 2 z Tm 2 3h 5 S5 --Td 0.05 N/m o 5U Td = 24 hrs z 0.432 0r----n 4 z 0.36 .I / rI S.0.305 Continued from above 5 6 7 8 9 TIME (hrs) Fig. 82 Suspension concentration versus time for Experiment 17

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I--2I 0.21 t | 2Bed Shear Stress T in N/m2 017 0.175 0.145 5 0.12 750.10 I E 6 --J o I 2 3 4 C-) Ir 5 T= 0.9N/m2 z Tm = 24 hrs co z Td = 0.015 N/m2 0 4 u Td = 40 hrs z / _0.432 o -------(nz 3 w 0.36 J 00. a2 Continued I 0.25 from above I I I I 5 6 7 8 9 TIME (hrs) Fig. 83 Suspension concentration versus time for Experiment 18 \

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[------------I-----I----( ^ J----2Bed Shear Stress T in N/m2 L75 I 0.145 -0.12 70 .IO z 6oO I I I I I SI 2 3 4 5 5 2 S -m Tm = 0.9 N/m (oz T m = 2 4 h rs S4Td = ON/m z Td = 135 hrs 0 0.432 S0.36 SI) 0.30 _1 2r----Continued 2 from above 5 6 7 8 9 TIME (hrs) Fig. 84 Suspension concentration versus time for Experiment 19 I I

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10 -Expt. Tm Tm Td Td N/m2 hrs N/m2 hrs --17 o 0.9 24 0.050 24 0.432 18 x 0.9 24 0.015 40 84 19 a 0.9 24 O 135 Bed Shear Stress o in N/m2 0.38 z --ex S-t (arT, / / z 0.2 5ex 2 o a 0.21 I I z 4 2z 00 0.175 -..-6 *) _o o -V-/ 0 2 4 6 8 TIME (hrs) Fig. 85 Variation of suspension concentration with bed shear for different flow deposited beds

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-161Expt Td Td -2 N/m2 hrs// ---17 0.050 24 ---x--18 0.015 40 / *-19 0 135 SI I E .I • Tm = 0.9 N/m I o Tm = 24 hrs 6 (T) e 0.2 5z ex -I I n'F-w U 4x o / // 2-f^ o 0o Ii 0 2I 'Oy 0 0.2 0.4 0.6 BED SHEAR STRESS Te (N/m2) Fig. 86 Suspension concentration versus bed shear stress for different flow deposited beds

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i Erosion rate E oc (C2 -) Expt Td Td z -N/m2 hrs 0o 17 0.050 24 -2.0 --x-18 0.015 40 -19 0 135 z // z 0 2 8 Tm = 0.9 N/m z Tm = 24 hrs same for o -each test / z (AT)ex 0.2 w 1.0S/ z K/ L) 0 0 0.2 0.4 BED SHEAR STRESS Te (N/m2) Fig. 87 Variation of erosion rate as a function of bed shear stress for different flow denosited beds

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-1630.1 0.2 0.3 Expt. 17 o I= Td =0.05 N/m2 Td = 24 hrs E E 20Tch=O0.21 N/m w L 40 W z 0.1 0.2 0.3 Expt. 18 w 2 Td =0.015 N/mr S -Td =40hrs Tch =0.28 N/m z 20 w U) 40 o 0.1 0.2 0.3 Expt. 19 0 -0 Td 0 N/m2 STd = 135 hrs 020Th=0.34N/m w 0 4 -2 SBED SHEAR STRENGTH (N/m ) Fig. 88 Bed shear strength observed as a function of depth

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CHAPTER VII SUMMARY AND CONCLUSIONS 7.1 Summary of Literature Review A review of literature on the parameters influencing the properties of fine sediments and hence their erosion rates indicated the following: i) Each of the following parameters has a measurable effect: Sediment: Composition: Clay minerals Non-clay minerals Percentage of clay in the mixture Organic matter Side gradation Type of bed: Placed/Remolded (Density Compacted structure) Deposited (stress history) Fluid: Composition of pore fluid Composition of eroding fluid pH Temperature Hydrodynamic factors: Bed shear stress Bed-fluid interface properties ii) The results of erosion studies are affected by the following: Type and size of apparatus used Method for measurement of erosion Method of applying shear stress (by a free surface flow of eroding fluid, rotating the apparatus, rotating the ring alone or rotating the ring and the channel both etc. and by the rate of application of bed shear stress). iii) The following soil parameters have been used by the various investigators in order to correlate them with the shear strength of -164-

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-165the soil: Liquid limit Plastic limit Plasticity index Bed density (or moisture content) However these correlations do not give reliable results. iv) The following indices have been used for characterization of the sediment and the fluid: Cation exchange capacity Sodium adsorption ratio Dielectric dispersion constant 7.2 Conclusions of the Present Study The present study on erosion of Kaolinite was conducted in a rotating channel apparatus. Water with a salinity of 35 parts per thousand was used as the eroding fluid. The conclusions of the study are as follows: i) The applied bed shear stress and the duration of time over which it acts during the processes of floc aggregation due to mixing, settling and consolidation were found to have a measurable influence on the bed structure as reflected by the change in bed density and resistance to erosion with depth. ii) The shear strength of the deposited bed as inferred from the erodibility of the bed was found to increase with depth below the interface between the sediment bed and the eroding fluid. The bed shear strength was also found to increase with increasing bed consolidation time. iii) During the erosion tests the bed shear stress was increased by small magnitudes in descretized time steps. Under such an

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-166application of a time-step function. iv) For a given bed structure, the rate of erosion was found to depend upon the time step function of the bed shear stress, whereas for a given time-step function the erosion rate was found to vary with the structure of the bed. The rate of erosion resulting from the application of two consecutive bed shear stresses Tel and Te2 acting over corresponding durations Tsl and Ts2, was found to be different from the rate of erosion resulting under a bed shear stress (Tel + Te2) acting over a duration of (Ts1 + Ts2). Which of these two applications would give a greater erosion, depends upon the relative magnitudes of Tel' Te2' Tsl and Ts2. v) The rate of increase of concentration was found to be comparatively high during the first approximately five minutes every time after the application of a given bed shear stress. vi) Sediment concentration gave a better correlation with shear stress than the correlation of erosion rate with bed shear stress. The concentration versus bed shear stress plot indicated the existence of a characteristic bed shear stress Tch. For magnitudes of Te smaller than Tch the rate of erosion was considerably lower than for the values of Te higher than Tch. This is also revealed in the erosion studies carried out by several previous investigators. vii) The concentration C, versus time t relationship plotted on a linear scale indicated two types of profiles. In one type the rate of erosion continuously decreased with time whereas in the other the rate of erosion became practically constant after an initial time-variant erosion. These two types are denoted as EQ and ER profiles respectively.

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-167It was found that for Te < ch the c-t plot gave the EQ type profile provided Te was applied over a sufficiently long duration Ts. For T > ch, the c-t plot always gave the ER type profile. It is postulated that under the ER type profile the erosion of bed would proceed indefinitely because the applied bed shear stress is greater than the shear strength of the bed. viii) The erosion rate is time dependent for the conditions which give EQ type profile of the c-t plot, whereas it is time independent after an initial time dependent phase for the conditions which give ER type profile. ix) In order to determine the shear strength Ts of a deposited bed as a function of the depth below the sediment-eroding fluid interface, the following experimental procedure is recommended: Form a bed in the apparatus using the selected sediment and the fluid and measure the density of the bed as a function of depth. Select a small value of the normalized excess shear stress, (AT)ex, say 0.1, for increasing the bed shear stress in the manner of a discretized time-step. Conduct exploratory tests to determine the Tcr at which surface erosion begins and to determine the minimum duration of the time step Ts which would give the EQ type profile. After reforming the bed under the same conditions, beginning with Tel > T cr increase the bed shear stress for the selected value of (AT)ex and T and erode the bed layer by layer. Calculate the erosion rates from all c-t profiles and determine which profiles satisfy the condition that the erosion rate at the end of the time step be smaller than a pre-selected small percentage of the initial high rate of erosion. Profiles satisfying this condition are the EQ type profiles and hence values of Te used for conducting the experiment

PAGE 182

-168correspond to Ts at the respective depths of the bed. A plot of the concentration at the end of Ts as a function of Te should give the value of Tch*

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REFERENCES Abdel-Rahman, N. N., "The Effect of Flowing Water on Cohesive Beds," Lab. for Hydr. Res. and Soil Mech., Thesis, 1962, Swiss Federal Institute of Technology, Zurich. Alizadeh, A., "Amount and Type of Clay and Pore Fluid Influences on the Critical Shear Stress and Swelling of Cohesive Soils," Ph.D. Dissertation, 1974, University of California, Davis. Anderson, H. W., "Physical Charcteristics of Soils Related to Erosion," Journal of Soils and Water Conservation, July 1951. Ariathurai, R., MacArthur, R. C., and Krone, R. B., "Mathematical Model of Estuarial Sediment Transport," Technical Report D-77-12, U. S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi, October 1977. Ariathurai, R., and Kandiah, A., "An Electrical Method to Measure In-situ Sediment Densities," Final Report NEAR TR 172, Nielsen Engineering and Research Inc., California, 1979. Arulanandan, K., Sargunam, A., Loganathan, P., and Krone, R. B., "Application of Chemical and Electrical Parameters to Prediction of Erodibility," Special Report 135, Highway Research Board, Washington D.C., 1973, pp. 42-51. Arulanandan, K., Loganathan, P., and Krone, R. B., "Pore and Eroding Fluid Influences on Surface Erosion of Soil," Journal of Geotechnical Engineering Division, ASCE, Vol. 101, No. GT 1, January 1975, pp. 51-66. Arulanandan, K., Gillogley, E., and Tully, R., "Development of a Quantitative Method of Predict Critical Shear Stress and Rate of Erosion of Natural Undisturbed Cohesive Soils," Tech. Report GL-80-5, U. S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi, July 1980. Bowels, F. A., "Microstructure of Unconsolidated and Consolidated Marine Sediments," Journal of the Sedimentary Petrology, Vol. 39, No. 4, December 1969, pp. 1546-1551. Carlson, E. J., and Enger, P. F., "Tractive Force Studies of Cohesive Soils for Design of Earth Canals," Report No. HYD-504, Hydraulic Branch, U. S. Dept. of Interior, Bureau of Reclamation, Denver, Colorado, 1962. Christensen, R. W., and Das, B. M., "Hydraulic Erosion of Remolded Cohesive Soils," Special Report 135, Highway Research Board, Washington D.C., 1973, pp. 8-19. -169-

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-170Committee on Tidal Hydraulics, "Soil as a Factor in Shoaling Processes, a Literature Review," Tech. Bulletin No. 4, U. S. Army Corps of Engineers, June 1960. Dayal, U., Allen, J. H., and Reddy, D. V., "Low Velocity Projectile Penetration of Clay," Accepted for publication in the Journal of the Geotechnical Engineering Division, ASCE, 1980. Dunn, I. S., "Tractive Resistance of Cohesive Channels," Journal of the Soil Mechanics and Foundations Division, ASCE, Proc. 85 (SM3) 1959. Einstein, H. A., and Krone, R. B., "Experiments to Determine MOdes of Cohesive Sediment Transport in Salt Water," Journal of the Geophysical Research, Vol. 67, No. 4, April 1962, pp. 1451-1461. Espey, W. H., "A New Test to Measure the Scour of Cohesive Sediment," Tech. Report HYD 01-6301, Hydr. Eng. Lab., Dept. of Civil Engineering, University of Texas, Austin, 1963. Flaxman, E. M., "A Method for Determining the Erosion Potential of Cohesive Soils," Erosion Pub. 59,International Assoc. of Scientific Hydrology, Commission on Land Erosion, October 1962. Fontana Project --Hydraulic Model Study, Monograph 68, Tennessee Valley Authority, 1973, p. 371. Fort Henry Apron Studies, Technical Monograph 87, Tennessee Valley Authority, 1960, pp. 135-141. Fukuda, M. K., "The Entrainment of Cohesive Sediments in Salt Water," Ph.D. Dissertation, Case Western Reserve University, Cleveland, Ohio, 1978. Grim, R. E., Clay Mineralogy, McGraw-Hill Inc., New York, U.S.A., 1968. Grissinger, E. H., "Resistance of Clay Systems to Erosion by Water," Water Resources Research, Vol. 2, No. 1, 1966, pp. 131-138. Grissinger, E. H., "Ephermeral Erosion and the Stability of Cohesive Soils," Special Report 135, Highway Research Board, 1973. Gularte, R. C., Kelley, W. E., and Nacci, V. A., "The Threshold Erosional Velocities and Rates of Erosion for Redeposited Estuarine Dredge Materials," Proceedings of the Second International Symposium on Dredging Technology, Texas A and M University, College Station, Texas, Paper H-3, November 1977. Gularte, R. C., "Erosion of Cohesive Sediment as a Rate Process," Ph.D. Thesis, University of Rhode Island, 1978. Gust, G., "Observations on Turbulent Drag Reduction in a Dilute Suspension of Clay in Sea-water," Journal of Fluid Mechanics, Vol. 75, Part 1, 1976, pp. 29-47.

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-171Kandiah, A., "Fundamental Aspects of Surface Erosion of Cohesive Soils," Ph.D. Dissertation, University of California, Davis, November 1974. Krone, R. B., "Flume Studies of the Transport of Sediment in Estuarial Shoaling Processes," Final Report, Hydr. Eng. and Sanitary Eng. Res. Lab., University of California, Berkeley, California, June 1962. Krone, R. B., "Aggregation of Suspended Particles," Unpublished Lecture Notes, Dept. of Civil Engineering, University of California, Davis, 1976. Laflen, J. M., and Beasley, R. P., "Effects of Compaction on Critical Tractive Forces in Cohesive Soils," Res. Bull. 749, Agricultural Expt. Station., University of Missouri, September 1960. Lambermont, J., and Lebon, G., "Erosion of Cohesive Soils," Journal of Hydraulic Research, 16, 1978, No. 1. Lee, D. Y., "Resuspension and Deposition of Lake Erie Sediments," M. S. Thesis, Case Western Reserve University, Cleveland, Ohio, August 1979. Liou, Y. D., "Effects of Chemical Additives on Hydraulic Erodibility of Cohesive Soil," M. S. Thesis, Colorado State University, August 1967. Liou, Y. D., "Hydraulic .Erodibility of Two Pure Clay Systems,"Ph.D. Thesis, Colorado State University, 1970. Lutz, J. F., "The Physico-Chemical Properties of Soils Affecting Soil Erosion," Res. Bull. 212, Agricultural Expt. Station, University of Missouri, July 1934. Lyle, W., and Smerdon, E., "Relation of Compaction and Other Soil Properties to the Erosion Resistance of Soils," Trans. American Society of Agri. Engineers, 8, 1965, pp. 419-422. Masch, F. D. Jr., Espey, W. H. Jr., and Moore, W. L., "Measurement of the Shear Resistance of Cohesive Sediments," Publication 970, Agri. Res. Service, 1965. McCarthy, D. F., Essentials of Soil Mechanics and Foundations, Reston Publishing Co. Inc., A Prentice-Hall Company, Reston, Virginia, U.S.A., 1977. McConnachie, I., "Fabric Changes in Consolidated Kaolin," Geotechnique, 24, No. 2, 1974, pp. 207-222. McDowell, D. M., and O'Connor, B. A., Hydraulic Behaviour of Estuaries, John Wiley and Sons Inc., New York, N.Y., 1977, Chapter 4. Mehta, A. J., and Partheniades, E., "Kaolinite Resuspension Properties," Journal of the Hydraulics Division, ASCE, Vol. 105, No. HY4, April 1979, pp. 411-416.

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-172Migniot, P. C., "Etude des Proprietes Physiques de Differents Sediments Tres Fins et de Leur Comportment Sous de Actions Hydrodynamiques," La Houille Blanche, 7, 591, 1968. Mirtskhulava, Ts. E., "Erosional Stability of Cohesive Soils," Journal of Hydraulic Research, Vol. 4, No. 1, 1966. Moore, W. M., and Masch, F. D. Jr., "Experiments on the Scour Resistance of Cohesive Materials," Journal of Geophysical Research, Vol. 67, No. 4, n, April 1962, pp. 1437-1499. Owen, M. W., "Properties of Consolidating Mud," Report NO. INT 83, Hydraulics Research Station, Wallingford, England, December 1970. Owen, M. W., "Erosion of Avonmouth Mud," Report NO. INT 150, Hydraulics Research Station, Wallingford, England, September 1975, Second Impression, November 1977. Partheniades, E., "A Study of Erosion and Deposition of Cohesive Soils in Salt Water," Ph.D. Thesis, University of California, Berkeley, 1962. Partheniades, E., "Erosion and Deposition of Cohesive Soils," Journal of the Hydraulics Division, Proc. ASCE, Vol. 91, No. HY 1, Paper 4204, January 1965. Partheniades, E., "A Summary of the Present Knowledge on the Behavior of Fine Sediments in Estuaries," Tech. Note No. 8, Hydrodynamics Lab., M.I.T., Cambridge, Mass., 1964. Paaswell, R. E., "Causes and Mechanisms of Cohesive Soil Erosion, The State of the Art," in "Soil Erosion: Causes and Mechanisms; Prevention and Control," Special Report No. 135, Highway Research Board, Washington D.C., 1973, pp. 52-74. Partheniades, E., "Cohesive Sediment Transport Mechanics and Estuarial Sedimentation," Unpublished Lecture Notes, University of Florida, Gainesville, Florida, 1979. Peck, R. B., Foundation Engineering, John Wiley and Sons, Second Edition, 1953. Peele, T. C., "The Relation of Certain Physical Characteristics to the Erodibility of Soils," Proc. Soil Sciences Soc. of America, Vol. 2, 1937. Randkivi, A. J., and Hutchison, D. L., "Erosion of Kaolinite by Flowing Water," Proc. Royal Society, London, A337, 1974, pp. 537-554. Sargunam, A., "Influence of Mineralogy, Pore Fluid Composition and Structure of the Erosion of Cohesive Soils," Ph.D. Dissertation, University of California, Davis, 1973.

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-173Sargunam, A., Riley, P., Arulanandan, K., and Krone, R. B., "Effect of Physico-Chemical Factors on the Erosion of Cohesive Soils," Journal of the Hydraulics Division, Proc. ASCE, Tech. Paper, 1973. Smerdon, E. T., and Beasley, R. P., "The Tractive Force Theory APplied to Stability of Open Channels in Cohesive Soils," Res. Bull. 715, Agri. Expt. Station, Missouri University, October 1959. Task Committee on Erosion of Cohesive Materials, Committee on Sedimentation, "Erosion of Cohesive Sediments," Journal of the Hydraulics Division, ASCE, Vol. 94, No. HY4, July 1968, pp. 1017-1049. Thorn, M. F. C., and Parsons, J. G., "Properties of Grangemouth Mud," Report No. EX 781, Hydraulics Research Station, Wallingford, England, July 1977. Thorn, M. F. C., and Parsons, J. G., "Erosion of Cohesive Sediments in Estuaries: An Engineering Guide," Proc. of Third International Symposium on Dredging Technology, Paper Fl, March 1980. Williams, D. J. A., "Physical Properties of Cohesive Suspensions and Liquid Muds," Lecture Notes, Short Course on Engineering and Environmental Applications of Cohesive Sediment Studies, University of Florida, Gainesville, March 31-April 1, 1980. Yeh, H. Y., "Resuspension Properties of Flow Deposited Cohesive Sediment Beds," M.S. Thesis, University of Florida, Gainesville, Florida, 1979. Zeichner, G. R., and Schowalter, W. R., "Use of Trjectory Analysis to Study Stability of Colloidal Dispersions in Flow Fields," Am. Inst. Chem. Eng., Vol. 23, No. 3, 1977.



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-Cy u0 0 -SUSPENSION CONCENTRATION RATIO Cr (30) I -01 N CoJ 0 x 00 00 -I II I o \0 %I0 < .-"O D 0 0 0 S00 .0 0 r> o) 'm o --O S 0 O cr o r+ CD +In (-S CD Il



PAGE 1

-15only a few molecules thick, and of practically no strength if more than a few molecules thick. Similarly, adsorbed polar organic molecules could serve as a bond between clay-mineral particles. vii) Dipole-cation-dipole linkage (Fig. 1.C). viii) Hydrogen bond occurs when an atom of hydrogen is strongly attracted by two other atoms. ix) van der Waals forces are secondary valance forces of an electrochemical nature. They are generated by the mutual influence of the motion of electrons of the atoms and they are always attractive. These forces acting between all units are property of the matter and are independent of the chemical characteristics of water. Although other attractive forces of electrical nature may exist, the van der Waals forces are the main cause for bonding together of clay particles. An important characteristic of these forces is that they decay very rapidly with distance and hence the particles must come very close to each other so that the forces can be effective. 2.2.6 Flocculation The bonding of clay particles when they are brought together is known as flocculation. Both collision and cohesion are essential to flocculation. Cohesion is believed to result from the predominance of attractive forces on the surface of clay particles. Collision of particles may be caused by the Brownian motion of the suspended particles, by internal shear of water, and by the differential settling velocities



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Figure Paoe 79 Suspension concentration during deposition under the bed shear stress of 0.015 N/m2 154 80 Suspension concentration during deposition under zero bed shear stress 155 81 Variation of bed density with depth for three different conditions of flow deposited beds 156 82 Suspension concentration versus time for Expt. 17 157 83 Suspension concentration versus time for Expt. 18 158 84 Suspension concentration versus time for Expt. 19 159 85 Variation of suspension concentration with bed shear for different flow deposited beds 160 86 Suspension concentration versus bed shear stress for different flow deposited beds 161 87 Variation of erosion rate as a function of bed shear stress for different flow deposited beds 162 88 Shear strength of bed as a function of depth 163 x



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-75Fig. 21 The rotating channel facility Faa Fig. 22 Close view of the annular channel and the ring



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-3along. The content of heavy metals adsorbed to the sediments is found to depend on the grain size of the sediment. The smaller the grain size, the higher is the contamination with heavy metals. The sediments less than 16 microns in size are likely to have very high contaminants. The bulk of the pollutants may be carried on the sediments rather than the water. Hence, the dispersal of pollutants cannot be dissociated from the dispersal of sediments. Study of dispersal of sediments alone, however, cannot be expected to provide the required information related to pollutants since the chemical, biological, and physical aspects involving oxidation, reduction, exchange of elements, decay, etc. considerably change the properties of pollutants. An understanding of the depositional and resuspension characteristics of the fine sediments would therefore be beneficial in obtaining better solutions to engineering problems and in exercising a more effective pollution control. Over the last about twenty years, studies have been carried out to investigate the erosional and depositional properties of fine sediments. The influence of different parameters connected with the sediment and the eroding fluid on the characteristics of erosion and deposition has also been studied. These have been described in Chapter III. The experimental work reported in this thesis was carried out to study the resuspension of flow deposited bed of Kaolinite under varying shear stress and to study the effect of bed structure on the process of resuspension. Saline water with 35 parts per thousand concentration was used as the eroding fluid. An understanding of the physical processes associated with the movement of fine, cohesive sediments is clearly essential for obtaining improved engineering solutions to estuarine problems. The phenomena of



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-76L i .. CI Fig. 23 The motor controllers F i g .. ......... F.c Fig. 24 The electric motors for the channel and the ring



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-52I-I BED NO. 7 w .I S 0.1 0 0 z 00 v 0 0.34 /O 0.01 .* I* t r .1 I I I* I I r r r .0 10 100 1000 10000o TIME AFTER VELOCITY INCREASE, rin Fig. 7 Concentration versus time plot: Krone (1962) 14 SCURVE A 2 -I U .. o 44 .-b se s s o I / ". \.^ i2 2 0 I 0 | .6 ? a 9 10 I SHEAR ON SEDIMENT BED Ir,), dynes/sq cm Fig. 8 Concentration as a function of bed shear stress: Krone (1962)



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Shear Stress T in N/m2 0,7 Tm =0.9 N/m 05 Tm = 24 Hrs 50 Expt. 13 Td = 0 N/m? 0.3 0 60 min.time step Td =24 Hrs 0.11 S40 E 0 I 2 3 o 0.61 / -0.5 <30 0.4 Expt.14 ---.-31 30min. time step 0 02 z 0.1 / o n III--------------I--> S20 0 I 2 3 z Hours 0 LIi o10 010 --I i i I iIi 20 40 60 80 100 120 140 160 180 200 TIl kI h I A-:.... ^ Fig. 59 Comparison of suspension concentration obtained under two different discretized time step functions i I



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T= 0.9 N/m 0.70 Tm =0.9 N/m2 Bed Shear Stress STm = 24 hrs in N/m2 0.60,/ 0.6 E Td = 0 N/m / Td = 24 hrs 0.50/ z 40 -E o/ rz 0.40 / S0.4'0 W / z w / o a 0.30 / o __ 0 0.2 yi w Fn m 0I. 2.0 3.0 4.0 TIME (hrs) TIME (hrs)



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-63Krone (1962) measured shear strength of the bed by using a screen penetrometer for consolidation time of 8, 24, 46, 72, 97, 120, 146, 168, 240, and 312 hours, and also measured densities. He'concluded that the ultimate density appears to be independent of total depth, i.e. consolidation occurs independently of the weight of material above a consolidating layer. Partheniades (1962) used a conventional vane shear test apparatus as well as a simple penetration test device for measurement of the shear strength of soil in connection with the erosion tests conducted on a flocculated bed and arrived at the following important conclusion (p. 108): "The observed independence of erosion rates from the macroscopic shear strength of the clay and the fact that clay gets eroded at shear stresses which are infinitesimal compared to its strength suggests that the mechanism of failure of clay particles by surface erosion is basically different than the mechanism of failure of clay particles in the interior of the clay mass, when subjected to shear stresses." He also observed that the erosion resistance of the flocculated bed seemed to increase with depth, and attributed this to the heterogeneity of the bed which was deposited from a suspension of high initial concentration containing a wide range of particles from clay size to fine sand. Owen (1970) measured the shear strength of the surface layers of the bed by using a Brookfield viscometer. After measuring the shear strength of the top layer, that portion of the bed was allowed to spill slowly. The shear strength of the next layer thus exposed was again determined by using the viscometer. Samples of the bed were taken simultaneously and a correlation of shear strength with bed density was



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-166application of a time-step function. iv) For a given bed structure, the rate of erosion was found to depend upon the time step function of the bed shear stress, whereas for a given time-step function the erosion rate was found to vary with the structure of the bed. The rate of erosion resulting from the application of two consecutive bed shear stresses Tel and Te2 acting over corresponding durations Tsl and Ts2, was found to be different from the rate of erosion resulting under a bed shear stress (Tel + Te2) acting over a duration of (Ts1 + Ts2). Which of these two applications would give a greater erosion, depends upon the relative magnitudes of Tel' Te2' Tsl and Ts2. v) The rate of increase of concentration was found to be comparatively high during the first approximately five minutes every time after the application of a given bed shear stress. vi) Sediment concentration gave a better correlation with shear stress than the correlation of erosion rate with bed shear stress. The concentration versus bed shear stress plot indicated the existence of a characteristic bed shear stress Tch. For magnitudes of Te smaller than Tch the rate of erosion was considerably lower than for the values of Te higher than Tch. This is also revealed in the erosion studies carried out by several previous investigators. vii) The concentration C, versus time t relationship plotted on a linear scale indicated two types of profiles. In one type the rate of erosion continuously decreased with time whereas in the other the rate of erosion became practically constant after an initial time-variant erosion. These two types are denoted as EQ and ER profiles respectively.



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-14+ And -Indicate Electric Charge ~~C--Clay Particle (+) (+) () () (+ (A) Cation Bond S-Water Dipole EL E ] -Clay Particle (B) Water Dipole Linkage |4 --1Clay Particle F 1 Ii Water Dipole s c--Cation (C) Dipole -Cation -Dipole @11 0 O 0 SE G I1 e Diffused Double Layer (D) The Clay Micelle Fig. 1 Inter-particle forces on clay minerals and clay micelle



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Shear Stress Tein N/m2 -T =O0. 9 N/m2 Q) J 40T = 24Hrs. 1 40 0.70 2 Td =0 N/m E S0.50 Td =24 Hrs. S0.1 I -SI 2 3 z 20Hours O i z CL (C) S0 I I I I I I I I I w a. V) 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fig. 45 Suspension concentration versus time for Experiment 13


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-1436.3 Proposed Test Procedure In order to obtain the shear strength as a function of depth below the sediment-water interface of a deposited bed, it is necessary to erode the bed layer by layer in an experimental apparatus and obtain concentration-time plots. Figure 74 shows results of a typical experiment indicating the bed shear stress Te as a function of time, and the corresponding suspension concentration. The erosion rates, e, plotted from these data are also given as a function of time. The experimental conditions in terms of rm, Tm, Td, Td, 'e' AT, and (AT)ex have been defined on the figure. It is seen that every time the bed shear stress is increased at Ts = 30 minutes, the erosion rate is high in the beginning and decreases rapidly with time. At the end of T E is some fraction of the maximum erosion rate. It is important to note the two different types of the concentration-time (c-t) curve plotted with linear scales on both axes. In one type the concentration "appears to be" tending towards an equilibrium concentration whereas in the other type the concentration appears to be increasing at the end of Ts. The first type may be denoted as EQ type profile of the c-t curve whereas the second type may be denoted as ER profile (Fig. 75). The characteristics related to the two types of profiles are as follows: EQ type profile: i) Tcr < Te < Tch' ii) c < ER type profile: i) Tcr < Te > Tch, ii) E > 6 where 6 is a specified small percentage of maximum erosion rate Em.



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CHAPTER IV PRESENT INVESTIGATION 4.1 Objective The parameters and processes influencing the behavior of fine sediments in contact with water have been described in Chapter II. Also, the results of important investigations carried out to study the erosional properties of fine sediments have been presented in Chapter III. It is clear from the presented information that a range of physical and chemical parameters are necessary for characterizing the properties of the sediment bed as well as the properties of the eroding and the pore fluids. When a given sediment is equilibrated with the eroding fluid over a sufficiently long time, the pore fluid and the eroding fluid have the same properties. The erosion process is then predominantly governed by the following parameters: i) The structure of the bed in terms of its floc shear strength variation over the depth, which is a function of the type of bed, viz. placed, deposited, or compacted. ii) The bed shear stress Te which causes erosion when it has a magnitude greater than the critical shear stress for erosion. The moisture content is an important parameter in the case of the placed bed and the compacted bed. In the case of the deposited bed, which is of interest in this study, the process of bed formation is important, involving the following parameters (Fig. 19): -68-



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-945.2 Multiple Steps of T e In order to study the effect of varying bed shear stress on the erosion of bed of a given density sturcture, experiments were conducted with various combinations of parameters related to the formation of bed (viz.T m, Tm, Td and Td) and the parameters related to the erosion of bed (viz. Te and T ). The selection of Te values was based on the following: i) AT, the excess shear stress ii) Tr, the ratio of the consecutive values of shear stress iii) (AT)ex, the normalized excess shear stress. The duration of time-step Ts was either 30 or 60 minutes. The procedure for discretization of a varying shear stress is shown in Fig. 38. The results of experiments consisting of a multiple steps of Te are given in Figs. 39 to 46. The experimental conditions for these tests are given in Table 3. 5.3 Discretized Sinusoidal Velocity Variation In a typical estuarine environment, the variation of tidal stream velocity is close to sinusoidal as shown in Fig. 47. The bed shear stress is proportional to the square of the velocity. Delecting a maximum value of 0.45 N/m2 for the bed shear stress, its variation as a function of time corresponding to the sinuisoidal velocity variation was computed and this variation was discretized with time steps of 30 minute duration as shown in Fig. 47. Results of experiment conducted with these discretized values of Te are given in Fig. 48, and those obtained with a higher starting value of are given in Fig. 49.



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-1630.1 0.2 0.3 Expt. 17 o I= Td =0.05 N/m2 Td = 24 hrs E E 20Tch=O0.21 N/m w L 40 W z 0.1 0.2 0.3 Expt. 18 w 2 Td =0.015 N/mr S -Td =40hrs Tch =0.28 N/m z 20 w U) 40 o 0.1 0.2 0.3 Expt. 19 0 -0 Td 0 N/m2 STd = 135 hrs 020Th=0.34N/m w 0 4 -2 SBED SHEAR STRENGTH (N/m ) Fig. 88 Bed shear strength observed as a function of depth



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-122data points and a relationship of a general nature was not seen. However, for individual experiments, plots of C30 versus Te did show a definite trend. Results of experiments 7, 9, and 11 are given in Figs. 62, 63 and 64 where the magnitude of normalized bed shear stress (AT)ex was decreasing, increasing and constant respectively. Results of two experiments having the same bed formation but which were eroded with two different values of Ts viz. 30 minutes and 60 minutes are shown in Fig. 65. All the data for a given bed formation such as for Td=40 hours seemed to indicate variation of a general nature when the ratio of the magnitudes of suspension concentration of two consecutive time steps were plotted against the corresponding ratios of the bed shear stress. For example let the consecutive values of Te be Tel and Te2 and let C10(2) and C(10)2 be the magnitudes of suspension concentration 10 minutes after applying the Tel and Te2 respectively. The the parameters were taken as C10(2)/C(10)1 plotted against Te2/Tel. This data is plotted in Fig. 66. Similar data was plotted corresponding to 20 minutes and 30 minutes after each change of Te and has been shown in Figs. 67 and 68 respectively. All such data available for the bed formation with Td=24 hours is plotted for C(30) values in Fig. 68 in order to compare it with results for Td=40 hours. In order to eliminate the variations due to the initial values of Te and C, a non-dimensional plot of all the values for Td=24 hours as well as Td=40 hours was made with the normalized excess concentration (Ac)ex plotted as a function of normalized excess bed shear stress (AT)ex, where



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28 Shear Stress Te in N/m2 E EXPT. 3 0.4 5 240 d = 0.015 N/m2 0.2O S -/ T = 24 Hours E20 o> 0 1.5 3.0 Hours z EXPT. 4 0 2 16 Tm =0.9 N/m < -n Tm = 24 Hours T =0 N/m z Td =40 Hours o 12z 0 O 0 8O EXPT. 5 _L 2 U Td =O N/ m U) 4r Td = 115 Hours O f-I I I I i I I I I 0 20 40 60 80 100 120 140 160 180 TIME (Minutes) Fig. 36 Effect of Step II parameters on suspension concentration



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-1516.4 Illustrative Example In order to illustrate the recommended procedure, an example of tests conducted to study the erosion of kaolinite for three different density structures has been given in this section. The three different density structures of the bed were obtained with the following magnitudes of parameters influencing the formation of bed: Bed Expt. T T Td Td No. m m d d No. No. (N/m2) (Hrs) (N/m2) (Hrs) 1 19 0.9 24 0.05 24 2 20 0.9 24 0.015 40 3 21 0.9 24 0 135 While the sediment in suspension was depositing on the bottom of rotating channel, samples were collected at 125 mm and 225 mm above the bottom of the channel. It may be seen from Fig. 78 that with a shear stress of 0.05 N/m2, the change in concentration at two elevations was small, indicating a near-complete vertical mixing of the sediment. Under shear stress of 0.015 N/m2, a concentration gradient was formed in the channel as shown in Fig. 79 and the settling was almost complete within 5 hours. The change in concentration at three different elevations when the material was depositing under quiescent conditions is given in Fig. 80. Samples were collected at elevations A, B, and C shown in the figure. A distinct sediment-water interface was quickly formed and it settled down as a function of time. The interface passed vertically downward of elevation B, the uppermost of the three elevations, within 34 hours. It passed elevation A within 7 hours. The sediment settling from the upper layers was continuously increasing the concentration at elevation C, the bottom-most level of measurement.



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-123(C2-C1) (Te2-el) (Ac) = 2-C) and (AT) e= eTe (C)ex C1 and ()ex : el 1 el as an example for the consecutive values of Tel and te2 with the corresponding values of C1 and C2.Fig. 69 shows a plotting of this data. For all the test results reported under Chapter V, the data was collected at 1, 2, 3, 5, 10, 15, 20 and 30 minutes after each new value of T in the case of Ts=30 minutes. Additional data points in continuation of these were obtained at 40, 50, 60 and 90 minutes in the case of Ts= greater than 30 minutes. These individual data points are not shown on the plots of experimental results. Instead, the lines joining the observed data points are shown on all the figures giving experimental data of the present study.



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I I I I I Tr 0.9 N/m2 Bed Shear Stress min N/m2 0.3Tm =24hrs 0 N2 E Td 0 N/m Ed % 4 -Td 40 hrs 0.25 0 -" z 2 Im 20 z 0 0 .15 ) al z / / rO I-'--0 .0 2.0 3.0 TIME (hrs) Ss s a a f o e L. 045 a. n" -n O 00 1.0 2.0 3.0 TIME (hrs) Fig. 54 Variation nf quspension concentration with bed shear stress as a function of time (Expt. 12)



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CHAPTER Page VI DISCUSSION AND PROPOSED TEST PROCEDURE ......... .132 6.1 Bed Density and Other Soil Parameters Correlated to the Shear Strength ............... .132 6.2 Critical Shear Stress ............... .139 6.3 Proposed Test Procedure .............. .143 6.4 Illustrative Example ................ 151 VII SUMMARY AND CONCLUSIONS ................. .164 7.1 Summary of Literature Review ............ .164 7.2 Conclusions of the Present Study .......... 165 REFERENCES ................... .......... .169 BIOGRAPHICAL SKETCH ................... ..... .174 iv



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-77Fig. 25 Millipore filter apparatus assembly Fig. 26 Device for measurement of bed density Fig. 26 Device for measurement of bed density



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AT = Excess shear stress, e.g. T -Te 22 e re2 -T (AT)ex = Normalized excess shear stress, e.g. e xii



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-45Types (a) and (b) mentioned above are usually used in flumes, whereas type (c) is adopted in rotating cylinder type apparatus. 3.3 Review of Literature Pertinent to the Present Study It may be noticed from 3.2 above that the literature even on the erosion aspects of fine sediments is quite extensive. Comparative study of the results obtained by various research workers may pose some problem due to the experimental technique involved in these studies. Details of some of these are given below. a) Type and size of apparatus used: Christensen and Das (1973): Rotating circular cylinder: 2.5 cm dia, 10 cm long Gularte (1978): Water tunnel: 5.5 m long, 2.0 m wide, 1.5 m high Yeh (1979): Circular flume: 20 cm wide, 1.5 m centerline dia Krone (1962): Straight steel flume: 0.9 m wide, 30.5 m long Partheniades (1962): Straight steel flume: 0.3 m wide, 0.45 m deep, 18 m long Owen (1977): Straight flume: 0.3 m wide, 0.2 m deep, 17.6 m long Thorn and Parsons (1980): Straight flume: 0.3 m wide, 0.2 m deep, 17.6 m long b) Method of reproducing shear stress: Rotating the apparatus in the case of circular cylinders. Flow of fluid in the case of straight flumes and water tunnels. Rotation of ring alone or both ring and channel in the case of circular flumes.



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SAt Elevation A (125 mm Above Channel Bottom) 0 x At Elevation B (225 mm Above Channel Bottom) S Td =0.05 N/m2 E z S20z0 Ui O O LC) o EO Z (f 108 01 0 4 8 12 16 20 24 TIME (hrs) Fig. 78 Suspension concentration during deposition under the bed shear stress of 0.05 N/m2 II r



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-36When hardening of the sol occurs, a gel is formed. This requires a change of state from a semi-liquid substance (sol) to a semi-solid (gel). Rheotropy of clay soils can be measured by a vane shear or at higher water contents by means of a viscometer. The property of thixotropy (or rest-hardening) has been explained either by changes in particle rearrangement and inter-particle forces, or by changes in adsorbed water. On stirring, the particles and fabric units are rearranged and the bonds between particles and units are broken. Also, the structure of the adsorbed water is broken up and the clay mass will be more susceptible to deformation under selfweight. After deformation, the clay fabric will seek a status of minimum energy with maximum attraction between particles and fabric units. The adsorbed water also regains its quasi-crystalline form to give the system sufficient rigidity to have a yield value. There are several factors which contribute to the regaining of part or all of the strength. These are original structure, activity of the clay minerals, and the degree of disturbance. The activity is a characteristic parameter of the electrochemical action of the colloids and is defined as the ratio of plasticity index and clay fraction less than two microns.



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ACKNOWLEDGEMENTS My deepest gratitude goes to Dr. Ashish J. Mehta, Associate Professor, Department of Coastal and Oceanographic Engineering, who has been my advisor and chairman of the committee, for his valuable guidance, encouragement, and support during the course of studies. I thank Dr. J. L. Eades, Professor, Department of Geology, and Dr. B. A. Benedict, Professor, Department of Civil Engineering, for serving on my supervisory committee. I have to thank Dr. E. Partheniades, Professor, Department of Engineering Sciences, for his advice regarding measurement of bed density. I am grateful to the personnel at the Coastal Engineering Laboratory, Mr. George Jones in particular, for their excellent cooperation and for extending every possible help for a successful completion of the experimental work. I am very thankful to Adele Koehler for careful typing of the manuscript and to Lillean Pieter for the excellent drafting work. I would like to express gratitude to my loving wife Aparna and my mother Indira for their continued encouragement in my endeavor and for accepting to bear the hardships caused during my long stay away from them. The present study was conducted as a part of the research project entitled "Deposition of Fine Sediments in Turbulent Flows" supported by the National Science Foundation under Grant Number GK-31259. Support was also received partially from the Environmental Protection Agency under Grant Number R806684010. This support from both the agencies is sincerely acknowledged. ii



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-n "11 SUSPENSION CONCENTRATION RATIO Cr (20) _0 0 0 00 o O O OO O 0 0 0 0 0 OJ



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Shear Stress Tein N/m2 Tm =0.9 N/m2 24 Tm = 6 Hrs. S 7Td = 0 N/m2 0.420 03Td = 40 Hrs. 0.2 S0.1 160 I 2 3 z Hours F, 5 12z O U z 0 U 8 z 0 z 4CL (I) 0 20 40 60 80 100 120 140 160 180 200 TIME (Minutes) Fi(. 31 Simn, nsn n nncntrtn ntvprStIS timno for Fyxnorimnnt 7



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Tm = 0.9 N/m2 AT= Constant = 0.1 N/m2 Tm = 24 hrs (A T)ex :Decreasing / 4 Td = 0 N/m E Td = 24 hrs / o 20/ z Je (A T)ex / -0.I / <0 1.00 £0.2 z 0.50 L 0.3 o 0.33 z 0.4 / o 0.25 / U 0.5 z 10 -0 0.20 o 0.6 / S07 70.17 7z 0.7 o0/ S C(3o)Values /O 0 0.2 0.4 0.6 0.8 BED SHEAR STRESS Te (N/m2) Fig. 62 Suspension concentration versus bed shear stress (Expt.14 )



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-13average electrochemical force exerted on one clay particle is of the order of one million times greater than the average weight of the particle. The inter-particle forces are both repulsive and attractive in nature. The most important repulsive forces generated by the electrical charges are i) Repulsion caused by the negatively charged particle faces. ii) Repulsion of adsorbed positively charged cations. iii) Osmotic pressure resulting from the high concentration of cations near the surface of the particles in the pore water. The attractive forces binding the particles together in clay minerals are the following: i) Forces due to the attraction of the mass of one clay mineral particle and the mass of another. ii) Inter-molecular forces resulting from the nearness of one particle to another with the overlap of fields of force of molecules in the surface layers of adjacent particles. iii) Electrostatic forces due to changes in the lattice resulting from unbalanced substitution within the lattice, broken bonds on the edges of the lattice, and the attractive force of cation ions adsorbed on the clay-mineral surfaces. iv) Chemical cementation between particles by various compounds. v) Cation bonds: Forces exerted by cations attracting and trying to neutralize negatively charged particles (Fig. 1.A). vi) Water dipole linkage is the bonding action of adsorbed polar molecules (Fig. 1.B). Oriented water molecules between two claymineral surfaces may form a bridge of considerable strength if



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REFERENCES Abdel-Rahman, N. N., "The Effect of Flowing Water on Cohesive Beds," Lab. for Hydr. Res. and Soil Mech., Thesis, 1962, Swiss Federal Institute of Technology, Zurich. Alizadeh, A., "Amount and Type of Clay and Pore Fluid Influences on the Critical Shear Stress and Swelling of Cohesive Soils," Ph.D. Dissertation, 1974, University of California, Davis. Anderson, H. W., "Physical Charcteristics of Soils Related to Erosion," Journal of Soils and Water Conservation, July 1951. Ariathurai, R., MacArthur, R. C., and Krone, R. B., "Mathematical Model of Estuarial Sediment Transport," Technical Report D-77-12, U. S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi, October 1977. Ariathurai, R., and Kandiah, A., "An Electrical Method to Measure In-situ Sediment Densities," Final Report NEAR TR 172, Nielsen Engineering and Research Inc., California, 1979. Arulanandan, K., Sargunam, A., Loganathan, P., and Krone, R. B., "Application of Chemical and Electrical Parameters to Prediction of Erodibility," Special Report 135, Highway Research Board, Washington D.C., 1973, pp. 42-51. Arulanandan, K., Loganathan, P., and Krone, R. B., "Pore and Eroding Fluid Influences on Surface Erosion of Soil," Journal of Geotechnical Engineering Division, ASCE, Vol. 101, No. GT 1, January 1975, pp. 51-66. Arulanandan, K., Gillogley, E., and Tully, R., "Development of a Quantitative Method of Predict Critical Shear Stress and Rate of Erosion of Natural Undisturbed Cohesive Soils," Tech. Report GL-80-5, U. S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi, July 1980. Bowels, F. A., "Microstructure of Unconsolidated and Consolidated Marine Sediments," Journal of the Sedimentary Petrology, Vol. 39, No. 4, December 1969, pp. 1546-1551. Carlson, E. J., and Enger, P. F., "Tractive Force Studies of Cohesive Soils for Design of Earth Canals," Report No. HYD-504, Hydraulic Branch, U. S. Dept. of Interior, Bureau of Reclamation, Denver, Colorado, 1962. Christensen, R. W., and Das, B. M., "Hydraulic Erosion of Remolded Cohesive Soils," Special Report 135, Highway Research Board, Washington D.C., 1973, pp. 8-19. -169-



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-20Dunn (1959) found that not only the plasticity index but vane shear strength (S ) needs to be included in the expression as follows: Tc = 0.0098 + 0.00049 (Su + 180) tan (30 + 1.73 PI) (2.3.2) Carlson and Enger (1963) suggested the following relationship: Tc = -0.017 + 0.000181(PI) + 0.000186(v) + 0.00268(K) + 0.000465(LL) (2.3.3) where v = sample density K = phi-skewness of the grain size distribution LL = liquid limit Sargunam (1973) presented the following expression related to the composition of pore fluid C, and sodium adsorption ratio (SAR) Tc = C1 + (C2 -n log SAR) log C (2.3.4) where C = pore fluid concentration C1, C2, and n = constants which vary with the type and amount of clay minerals For describing the influence of various parameters the following classification appears more appropriate: a) Hydrodynamic factors. b) Properties of sediment. c) Properties of bed. d) Properties of pore fluid and eroding fluid.



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-40apparatus and found that the critical shear stress was related to water content and vane shear strength. Flume study of natural soils conducted by Lyle and Smerdon (1965) showed that the critical tractive force correlated to void ratio, cation exchange capacity, and plasticity index. Grissinger (1966) found that the erosion rate of soil decreases with increasing clay content and decreasing void ratio and temperature. Findings of various research workers related to determining the effect of various parameters and characterization indices on erosion have already been described in Chapter II. Paaswell (1973) summarized selected studies on cohesive soil erosion. The same are reproduced from his paper along with addition of subsequent investigations. It may be noted that the mode of formation of bed in the laboratory equipment is an important factor. This can be done in the following 3 different ways. a) Placed or remolded or uniform bed: Formed by mixing the sediment thoroughly with required water content and placed evenly in the apparatus so as to have uniform density without any external compaction. b) Flow deposited or flocculated or stratified bed: Formed by allowing a sediment suspension with high concentration under a low flow velocity which would permit most of the material to slowly deposit on the bed. When the flow velocity is zero, the term deposited bed is used. Such a bed is flocculated and has density stratification over depth. c) Compacted bed: Sediment with low moisture content is compacted with external pressure.



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Shear Stress Te in N/m2 Tm=0.9 N/mZ2 0435 Tm 24 Hrs. -200.379 Td =0 N/rn ST0281 = 40 Hrs. 160.16 z 0 I 2 S Hours O -12 z 0 o 8z 0 w z 4.) 0 20 40 60 80 100 120 140 TIME (Minutes) Fig. 49 Suspension concentration versus time for Experiment 16



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Table 2. Continued Investigator Mode of Placement of Sample Mode of Measurement of Erodibility Kandiah (1974) Remolded (compacted) in a rotating Weight loss of sample cylinder test apparatus Raudkivi and Hutchison Remolded in a recirculating refriWeight loss of sample before and (1974) gerated water tunnel after test Thorn and Parsons (July 1977) Deposited in a flume Withdrawal of samples, filtering and weighing or use of photoabsorptiometer to determine suspension concentration Owen (Nov. 1977) Deposited in a flume Withdrawal of samples, filtering and weighing or use of photoabsorptiometer to determine suspension concentration Gularte et al. (1977) Remolded in a recirculating refriMeasurement of suspension congerated water tunnel centration by using laser-photocell system Gularte (1978) Remolded in a recirculating refriMeasurement of suspension congerated water tunnel centration by using laser-photocell system Fukuda (1978) Deposited Measurement of suspension concentration by using laser-photocell system ___ 'aJ



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-74vi) Weigh the filter papers containing sediment. vii) Obtain concentration by calculating the weight of sediment from the difference in the weight of the filter paper with and without the sediment, and dividing it by the volume of the sample. A mercury thermometer was used to measure water temperature. Photographs of the apparatus and accessory equipment are given in Figs. 21 to 28. The rotating channel facility was previously calibrated for measurement of bed shear stress, details of which are given by Mehta (1973). The required bed shear stress could be attained by adjusting the speeds of rotation of the ring and the channel. Calibration curves used for this purpose are given in Figs. 29 and 30. The fing and the channel are rotated in directions opposite to each other in order to minimize the effects of the radial secondary currents (Mehta, 1973). 4.4 Experimental Procedure The experimental procedure consisted of the following three parts: i) Formation of bed in the rotating channel: Kaolinite equilibrated with the eroding fluid for a period of 3 months was put in the channel. The quantities of Kaolinite and the eroding fluid (which was saline water with 35 parts per 1000 concentration) were adjusted in order to have a sediment suspension of the order of 40 parts per 1000 concentration by weight when fully mixed (C ). The bed was formed in the channel by initial mixing and allowing the sediment to deposit on the channel bottom. Figure 19 schematically indicates the procedure



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-139properties of soil such as plasticity index, liquid limit, soil density, mechanical analysis, shrinkage limit, vane shear strength, and percent of maximum Procor density were measured. For correlating the critical bed shear stress with the soil properties, a multiple linear correlation method programmed for the digital computer was used. The results of these studies indicated that the best correlation was obtained when the plasticity, gradation, and density were included as independent variables. 6.2 Critical Shear Stress While correlating the suspension concentration or the erosion rate with the bed shear stress, it appears the term critical shear stress has been used either for the minimum bed stress to initiate erosion or for that value of the bed shear stress beyond which there is a sudden increase in erosion rates as would be seen from the following: a) Parthenaides (1962, p. 66): "After a certain critical value of the shear stress had been reached, the erosion rates increase much more rapidly with increasing shear stress. This critical value was found to be 0.010 Ibs/ft2 (0.47 N/m2) for series I and 0.028 Ibs/ft2 (1.33 N/m2) for series II." These are shown in Fig. 6 b) Espey (1963): A typical test result is given in Fig. 72. Espey reports the following (p. 34): "As the critical shear stress is approached, the rate of scour increases. At some critical point, a large amount of material is suddenly ripped loose from the sample, resulting in a high rate of scour for that particular shear stress."



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10 -Expt. Tm Tm Td Td N/m2 hrs N/m2 hrs --17 o 0.9 24 0.050 24 0.432 18 x 0.9 24 0.015 40 84 19 a 0.9 24 O 135 Bed Shear Stress o in N/m2 0.38 z --ex S-t (arT, / / z 0.2 5ex 2 o a 0.21 I I z 4 2z 00 0.175 -..-6 *) _o o -V-/ 0 2 4 6 8 TIME (hrs) Fig. 85 Variation of suspension concentration with bed shear for different flow deposited beds