Title Page
 Table of Contents
 List of Tables
 List of Figures
 Fine sediments
 Previous laboratory studies
 Present investigation
 Test results and analysis
 Discussion and proposed test...
 Summary and conclusions

Group Title: UFL/COEL (University of Florida. Coastal and Oceanographic Engineering Laboratory) ; 80/005
Title: Effect of bed shear stress on the erosional characteristics of kaolinite
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00076161/00001
 Material Information
Title: Effect of bed shear stress on the erosional characteristics of kaolinite
Series Title: UFLCOEL
Physical Description: xiii, 174 leaves : ill. ; 28 cm.
Language: English
Creator: Parchure, T. M ( Trimbak Mukund ), 1943-
University of Florida -- Coastal and Oceanographic Engineering Laboratory
Publication Date: 1980
Subject: Shear strength of soils   ( lcsh )
Kaolinite   ( lcsh )
Erosion   ( lcsh )
Coastal and Oceanographic Engineering thesis M.S
Coastal and Oceanographic Engineering -- Dissertations, Academic -- UF
Genre: bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )
Thesis: Thesis (M.S.)--University of Florida, 1980.
Bibliography: Includes bibliographical references (leaves 169-173).
Statement of Responsibility: by T.M. Parchure.
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.
 Record Information
Bibliographic ID: UF00076161
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: oclc - 07367591

Table of Contents
    Title Page
        Title Page
    Table of Contents
        Table of Contents 1
        Table of Contents 2
    List of Tables
        List of Tables
    List of Figures
        List of Figures 1
        List of Figures 2
        List of Figures 3
        List of Figures 4
        List of Figures 5
        Unnumbered ( 12 )
        Unnumbered ( 13 )
        Page 1
        Page 2
        Page 3
        Page 4
    Fine sediments
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
    Previous laboratory studies
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
    Present investigation
        Page 68
        Page 69
        Page 70
        Page 71
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
    Test results and analysis
        Page 87
        Page 88
        Page 89
        Page 90
        Page 91
        Page 92
        Page 93
        Page 94
        Page 95
        Page 96
        Page 97
        Page 98
        Page 99
        Page 100
        Page 101
        Page 102
        Page 103
        Page 104
        Page 105
        Page 106
        Page 107
        Page 108
        Page 109
        Page 110
        Page 111
        Page 112
        Page 113
        Page 114
        Page 115
        Page 116
        Page 117
        Page 118
        Page 119
        Page 120
        Page 121
        Page 122
        Page 123
        Page 124
        Page 125
        Page 126
        Page 127
        Page 128
        Page 129
        Page 130
        Page 131
    Discussion and proposed test procedure
        Page 132
        Page 133
        Page 134
        Page 135
        Page 136
        Page 137
        Page 138
        Page 139
        Page 140
        Page 141
        Page 142
        Page 143
        Page 144
        Page 145
        Page 146
        Page 147
        Page 148
        Page 149
        Page 150
        Page 151
        Page 152
        Page 153
        Page 154
        Page 155
        Page 156
        Page 157
        Page 158
        Page 159
        Page 160
        Page 161
        Page 162
        Page 163
    Summary and conclusions
        Page 164
        Page 165
        Page 166
        Page 167
        Page 168
        Page 169
        Page 170
        Page 171
        Page 172
        Page 173
Full Text

^ \








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.










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 . . . .


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 . .


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




Apparatus for Measurements of Bed Density .


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 . . . .























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


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

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

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


Specific surface area and liquid limit for typical

Summary of selected studies on cohesive soil erosion

Experimental conditions for tests conducted with a
multiple steps of Te








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


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

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



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
























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



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

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


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

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
Te Te
el e2

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

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



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






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


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


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


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




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.


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.


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,


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


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


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


millimicron diameter spheres the surface area is 10 million times as


The specific surface area of different clay minerals is as


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


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


average electrochemical force exerted on one clay particle is of the

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


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


+ And Indicate Electric Charge

(+ () (W+)

.--- Clay Particle

(A) Cation Bond


---- Water Dipole
E:l Clay F


(B) Water Dipole Linkage

S---o Clay Particle

-- Water Dipole
-- Cation

(C) Dipole Cation Dipole


a @



_-b Diffused
Double Layer

(D) The Clay Micelle

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

I- -


I- -

i_ -- 1-

I -I

G a


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


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


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.


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


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:


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)
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)


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)


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)


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.


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


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)

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


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


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.


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


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


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


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


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


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.


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


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


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.


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


Fig. 2 Rheological models



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.


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.


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


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




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


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)



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


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).


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


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-


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

Soil loss and runoff tables

Correlation of erodibility with
shear measurements

Jet to produce erosion; visual

Visual observation of bed movement

Visual correlation or erosion with
calculated inactive stress

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

Measurement of scour depth and
weight loss

Visual; measurement of erosion

Table 2. Continued

Investigator Mode of Placement of Sample Mode of Measurement of Erodibility

Partheniades (1965)

Grissinger (1966)

Masch, Espey, and Moore

Mirtskhulava (1966)

Liou (1967)

Liou (1970)

Arulanandan et al. (1973)

Christensen and Das

Grissinger (1973)

Sargunam et al. (1973)

Alizadeh (1974)

Remolded natural deposited (salt
water) in duct

Remolded in channel

Unspecified but trimmed as hollow

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

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

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-

recirculating refri-

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

Measurement of suspension con-
centration by using laser-photocell

Fukuda (1978)

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

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-


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.


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


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


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

pendent of the macroscopic shear strength of the bed.

E 12-

0 -
I- 10-5 -

u 8-4 O
o w
o >

z 6-3 o
o w .

o w

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

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


E -
o uw

0 < -
r- 4
8 6- 0

S2- rn run run CONCENTRATION
w _

0 200 400 600 800


8 -

( ) 4

C )

run 22

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


Concentration versus
Partheniades (1962),.

200 400 600 800
time plot obtained in erosion of deposited bed:
Expt. Series III

O 002 0.04 006
Fig. 6 Relationship between rate of erosion and average bed shear stress:
Partheniades (1962)

Fig. 5


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).


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






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)


S, 20 0
-. 0


0 0 v 0.34


' ' '

" '"


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

t '




St VELOCITY, 1.14 ft/sec
j I



0.01 0.1 1.0 10 100 1000

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

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


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)


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)





Fig. 10 Concentration
Lee (1979)




6 owl

run 3 Tw=6.0

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


- --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

Bed Shear Stress in N/m2 0.43

S0.5- T AT (AT)ex
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


- A LV.J I 11/ItI

z 8




0I- IL
0 40 80 120 160 200

TIME (Hours)

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


KD = Kaolinite in Distilled Water




OO 0.2 0.4


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


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


s = Pc tan d (3.4.3)


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.


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



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




RPfiro nr'. :

Partheniades (1962)



McCarthy (1977)


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


McCarthy (1977)


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




m m I

1- 01



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.


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


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."


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
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

4.0 _

3.0 x -

2 -5

09 -


0-5 -

0. I 4
60 90 100 150 200 250 300 350 400

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

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


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

Fig. 18 Bed density profiles: Owen (1970)


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.



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):



H--Step I

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 ----

+Ts _+TS H_


Tm = Duration of Initial Mixing
Td = Duration of Deposition and
Ts = Duration of Time Step

Fig. 19 Definition sketch for notations used to describe experimental conditions


Te I
Td I


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


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


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.


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


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


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.



<0 50



100 50 10 5 I 0.4
Fiq. 20 Si7p qradation of Kaolinite tisd for thp experiments
Fiq,. 2(? Si7p qrarlation of Kaolinite nlspd for the experiments


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.


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


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


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


Fig. 23 The motor controllers
Fig. 23 The motor controllers


*-- .i- ,a $ '
..o'-. o?

Fig. 24 The electric

motors for the channel and the ring


Fig. 25 Millipore filter apparatus assembly

Fig. 26 Device for measurement of bed density


c ~-I


Fig. 27 Equipment for determining concentration

of sediment suspensions

Fig. 28 Sampling bottles


15 20





Fig. 29 Operational speeds and controller meter readings for
bed shear stresses

ring and channel at different

30 2






5 0


30- Ring

- 20 -


5 10 15

Fiq. 30 Correlation hbtween r.p.m. and mnter reading for the channel and the ring


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,


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
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


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


I--Top Cylinder 15cm dia.2 2 I

Plastic Tubes of various heights,
0.95 cm dia. glued to the
bottom plate
In fl II 1

Bottom Plate


-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

Dish -

Piston with Screw Rod


Fig. 31 Apparatus developed for measurement of density as a function
of depth for deposited beds

5 cm
225 cm





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


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


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.

University of Florida Home Page
© 2004 - 2010 University of Florida George A. Smathers Libraries.
All rights reserved.

Acceptable Use, Copyright, and Disclaimer Statement
Last updated October 10, 2010 - - mvs