INVITED PAPER

Fourth IGS-Ferroco Terzaghi Oration: 2014

Soil Clay Mineralogy and Physico-Chemical Mechanisms Governing the Fine-GrainedSoil Behaviour

A. Sridharan

Published online: 30 October 2014

� Indian Geotechnical Society 2014

Abstract Engineering behavior of fine-grained soils

depends upon the clay mineralogical composition of the

soils and the pore medium chemistry as well. A greater part

of the soil–pore liquid interaction is both physical and

physico-chemical in nature. This can be attributed to the

charge deficiency on the surfaces and edges of the clay

platelets and the associated electrical attractive and repul-

sive forces. Clay minerals that are present in the fine-

grained soils can be broadly grouped into kaolinitic and

montmorillonitic types. This paper discusses in detail the

physical and engineering behavior of fine-grained soils as

influenced by the dominant clay minerals composing them

and by the pore medium chemistry. It has been brought that

the liquid limit, sediment volume, undrained shear strength

and compressibility behavior of kaolinitic and montmoril-

lonitic clayey soils are quite opposite to changes in the pore

medium chemistry. The drained strength and secondary

compression coefficient of both kaolinitic and montmoril-

lonitic fine-grained soils are primarily controlled by the

modified effective stress (i.e., net contact stress at the

particle level), which takes into consideration both attrac-

tive and repulsive forces in addition to the conventional

effective stress. The hydraulic conductivity of fine-grained

soils is significantly influenced by the nature of the fluid,

especially so in montmorillonitic soils.

Keywords Clay mineralogy � Fine-grained soils �Physical and engineering properties �Physico-chemical mechanisms � Pore medium chemistry

Introduction

For most of the engineers dealing with physical and engi-

neering behaviour of fine-grained soils, it is necessary to

have a knowledge of the ‘‘how and why’’ of such geo-

technical behaviour. An understanding of why soils behave

as such is important if a proper and correct solution is to be

obtained for any geotechnical problem. Studies on soil

behaviour are concerned with the properties exhibited by

soils and are necessarily interdisciplinary.

Natural fine-grained soils are normally composed of

clay minerals, non-clay minerals and amorphous materials.

The clay minerals have surface charges by virtue of iso-

morphous substitutions that take place during their for-

mation. This charged nature of the clay minerals is

responsible for complex soil behaviour. It also determines

the interactions with the fluid phases. These features alto-

gether determine the plasticity, swelling, compression,

strength and fluid conductivity behaviour of fine-grained

soils. Mineralogy and pore fluid characteristics, therefore,

are fundamental to understand the geotechnical properties,

although mineralogical determination is often not com-

pulsory for many geotechnical investigations. Because of

the clay mineralogical composition of fine-grained soils,

their behaviour under a given pore medium environment is

rather physico-chemical than purely physical. The clay

minerals belong to different groups; but can be broadly

classified into expanding lattice type and non-expanding

lattice type minerals. While the montmorillonites belong to

the former category, the kaolinites come under the latter

group. It is well documented in the geotechnical engi-

neering literature that the response of these two groups of

clay minerals under the prevailing pore medium chemistry

could be quite opposite to each other. This contrasting

response of these mineral groups has an immense influence

A. Sridharan (&)

Indian National Science Academy, New Delhi, India

e-mail: asridhran@yahoo.com

123

Indian Geotech J (October–December 2014) 44(4):371–399

DOI 10.1007/s40098-014-0136-0

on the physico-chemical and engineering properties of the

fine-grained soils they compose. Hence, a knowledge of the

soil clay mineralogy and the mechanisms that could control

the behaviour of clay minerals under a given pore medium

environment is essential before one gets into field problems

related to natural clayey soils.

This paper intends to

• give a brief account of the soil clay mineralogy

• discuss various physical properties of fine-grained soils

(such as Atterberg limits, free swell/sediment volume

behaviour) and various engineering properties (such as

compressibility, drained and undrained shear strengths

and permeability) in the light of dominant clay

minerals’ dependent controlling mechanisms that come

into play under a given pore medium environment.

Soil Clay Mineralogy

In geotechnical engineering, clays are defined as those

soils, which are composed predominantly of clay minerals.

These clay minerals, which are nothing but hydrous alu-

mino silicates, belong to a larger mineral family called

phyllosilicates. There are different classes of clay minerals

such as 2-layered clay minerals (ex: kaolinite mineral),

3-layer clay minerals (ex: montmorillonite mineral),

4-layer minerals (ex: chlorite mineral) and so on. These

clay minerals along with the associated exchangeable cat-

ions play a dominant role in controlling the physico-

chemical and engineering behaviour of fine-grained soils.

Among the various types of clay minerals, kaolinite and

montmorillonite represent the two extreme types and need

major consideration.

Kaolinite

The unit cell of kaolinite clay mineral consists of a silica

sheet and an alumina octahedral sheet. The bonding

between the adjacent unit cells of kaolinite mineral is

through relatively strong hydrogen bond and van der Wa-

als’ forces. Important characteristic features of kaolinite

mineral are listed in Table 1.

Montmorillonite

The unit cell of montmorillonite mineral consists of an

alumina octahedral sheet sandwiched between two silicate

sheets. The bonding between the adjacent unit cells of

montmorillonite mineral is through very weak van der

Waals’ forces. Important characteristic features of mont-

morillonite mineral are listed in Table 1.

Another most commonly occurring clay mineral in fine-

grained soils is illite or hydrous mica. The structure of illite

is similar to that of montmorillonite except that the adja-

cent unit cells of illite are bonded together through non-

exchangeable potassium ion linkage and van der Waals’

forces. Important characteristic features of illite mineral are

listed in Table 1.

The clay mineral platelets carry negative charges on

there surfaces. The nature of the charges on their edges

appear to be dependent on the pH of the prevailing envi-

ronment. Considerable evidences exist to show that the

kaolinite particles carry positive charges on their edges in a

low pH environment and negative charges in a high pH

environment (Fig. 1) [70].

Large variations in specific surface and surface charge

characteristics of different clay minerals result in a variety

of complex arrangements of clay particles, termed as clay/

soil fabric. The clay fabric together with the inter particle

forces, termed as clay/soil structure, governs the fine-

grained soil behaviour such as consistency limits, volume

change, permeability, swelling and shear strength. The clay

minerals that are present in natural soils can influence their

behaviour to an extent much greater than their proportion

in the soil mineralogical composition. Thus, the soil clay

mineralogy can be considered fundamental to the under-

standing of geotechnical properties of fine-grained soils. In

this paper, the role of clay minerals together with pore

medium chemistry in governing the physico-chemical and

engineering properties of fine-grained soils has been dis-

cussed at length.

Electrical Forces of Attraction and Repulsion

It is well established that both electrical attractive and

repulsive forces exist between clay particles. Many com-

plex factors are responsible for the net attractive and

repulsive forces between these particles. Number of

investigators have attempted to better the understanding of

the nature of these forces. Lambe [12–14], Rosenqvist [36],

Bolt [2], Mitchell [23], Quirk [34], Seed et al. [40],

Sridharan [41] are among the many who have contributed

to the understanding of the nature of electrical forces from

the view point of the fine-grained soil behaviour.

Attractive Forces

A number of phenomena are responsible for the existence

of electrical attractive forces between clay particles. They

can be primarily grouped as those relating to primary

valence bonds and secondary valence forces. The Cou-

lombic attraction, hydrogen bonds and other possible

attractions such as the ion–dipole linkage or induced dipole

372 Indian Geotech J (October–December 2014) 44(4):371–399

123

interaction or dipole–dipole interaction are inversely pro-

portional to the dielectric constant of the pore medium and

the distance between the clay platelets [13, 36]. The

secondary valence forces are of more concern to the geo-

technical engineer, since they are greatly influenced by the

applied stresses and by the changes in the nature of the

clay-water system and also due to the fact that they can act

over relatively large distances. It is likely that the principal

contribution to the secondary valence forces is from the

mutual influence of the electronic motion between two

atoms (London forces). According to London’s theory,

these forces are universal forces which act between all

pairs of atoms or molecules, varying inversely as the sev-

enth power of the distance between them. Hamaker [8]

derived an equation for the attractive force between two

plates from London’s [19] theory as

F ¼ A

6pd3ð1Þ

where F is the force in dyne per cm2, A is the Hamaker

constant and d is the distance between the plates in cm.

There has been much discussion concerning a suitable

value for the constant A, and values ranging from

5 9 10-14 to 10-12 ergs have been estimated by various

Table 1 Characteristic features of clay minerals

Distinctive features Kaolinite clay mineral Illite clay mineral Montmorillonite clay mineral

Unit cell Silicon tetrahedral sheet connected

with aluminum octahedral sheet

Aluminum octahedral sheet

sandwiched between two silicon

tetrahedral sheet

Aluminum octahedral sheet

sandwiched between two silicon

tetrahedral sheet

Bonding between unit cells Relatively strong hydrogen bond Non-exchangeable potassium ion

linkage

Very weak van der Waals’ force of

attraction.

Isomorphous substitution In silica sheet

Very less

In silica sheet

moderate

In Gibbsite sheet

extensive

Cation exchange capacity 3–15 meq./100 g 10–40 meq./100 g 80–150 meq./100 g

Specific surface 5–20 m2/g 20–80 m2/g 400–800 m2/g

Liquid limit (%) 30–50 40–80 300–800

Plastic limit (%) 20–30 15–30 40–60

Shrinkage limit (%) 20–30 15–20 6–14

Activity 0.5–1 1–2 6–12

Fig. 1 The development of charges on the edges of kaolin clay

particles, together with the resulting distribution of charges on the

particles (Source: [70])

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123

researchers [34]. The value of A can be calculated by

summing up the pair potentials between volume elements

having polarisability (a1), ionisation potential (I1) and the

number of inter acting elements per unit volume (N1). The

summation by Hamaker [8] given for the plates in vac-

uum is as per Eq. (2a).

A ¼ A1 ¼3

4p2N2

1a21I2

1 ð2aÞ

If the medium is other than vacuum, then

A ¼ A12 ¼ffiffiffiffiffi

A1

p

�ffiffiffiffiffi

A2

p

h i2

ð2bÞ

where the subscripts 1 and 2 refer to the soil substance and

the fluid medium respectively [4]. Using the approach of

Fowkes [4] and assuming that the number of inter acting

volume elements is directly proportional to the degree of

saturation (i.e. between the two plates, there is a uniform

and homogeneous distribution of water molecules, which is

directly proportional to the degree of saturation), Sridharan

[41] computed the values of A with degree of saturation as

shown in Fig. 2a. Sridharan and Rao [64] calculated the

values of A for different soil–liquid systems for which N, aand I values readily available as shown in Fig. 2b. It is

clear that A parameter is inversely proportional to the

dielectric constant of the medium.

Thus, if the soil system consists of parallel plates, the

contribution of the dispersion force to the London -van der

Waals’ force, which directly varies with Hamaker’s A, is

inversely proportional to the dielectric constant, when the

change in the dielectric constant is brought about by the use

of different pore fluids.

The clay water system as used in soil engineering, is not an

ideal system. The nature of the inter particle contacts are not

well understood. It cannot be authoritatively stated as to

whether any specific attractive force predominantly con-

tributes to the net attractive force or not. The system is so

complex that individual effects cannot be readily separated.

However, it can be stated that the attractive forces vary

inversely with the dielectric constant of the pore medium and

with the distance between particles. It increases with the

concentration of the cation and its valence. As the hydrated

size of the cation decreases, the attractive force increases. In

view of the negative charges present on the surface of the

clay platelets, cations accommodate themselves in the

vicinity of the clay platelets and hence, their influence is

predominant. Information on the effect of anion type and its

concentration on the attractive forces are scanty and is a

potential area for further work.

Repulsive Forces

The primary force responsible for the repulsion between

two clay platelets is the interaction between the diffuse

double layers. Extensive investigations have been carried

Fig. 2 a Variation of Hamaker’s ‘A’ coefficient with degree of saturation (Data Source: [41]). b Variation of Hamaker’s ‘A’ coefficient with

dielectric constant (Data Source: [64])

374 Indian Geotech J (October–December 2014) 44(4):371–399

123

out by many investigators on the application of Gouy–

Chapman diffuse double layer theory to understand the

nature of water next to the clay platelets and the repulsive

forces operating between the parallel clay platelets ([2, 24,

38, 74, 75] to name a few). Sridharan and Jayadeva [50]

studied in detail the Gouy–Chapman diffuse double layer

repulsion and compressibility of clays. They presented the

solution to the governing differential equation of diffuse

double layer repulsive forces in a simple form. The

important equations that occur in the calculation of repul-

sive pressure are:

p ¼ 2nkTðcosh u� 1Þ ð3Þ

e ¼ GcwSd ð4ÞZ u

z

2 cosh y� 2 cosh uð Þ�12dy ¼ �

Z a

0

dn ¼ �Kd ð5Þ

dy

dn

� �

x¼0

¼ B

S

ffiffiffiffiffiffiffiffiffiffi

2penkT

r

ð6Þ

where p is repulsive pressure, G is specific gravity of soil,

n is concentration of cation in the bulk fluid, cw is unit

weight of water, k is Boltzmann constant, S is specific

surface of the soil, T is absolute temperature, d is half

space distance between the parallel platelets, u is non

dimensional mid plane potential =ve0/m

kT; z is non dimen-

sional surface potential =ve0/0

kT; v is valance of the cation,

/m is mid plane potential, e0 is unit electrostatic charge,

e is void ratio, /0 is surface potential, n = Kx, B is base

exchange capacity, e is dielectric constant of the pore

medium, x is the distance from the clay platelets (x = d at

mid plane distance between the platelets) and

K ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

8p e0ð Þ2v2n

ekT

q

:

Equation 3 is valid for parallel plates [2]. In a very

simple form, solution to Eq. 3 by numerical integration

was given by Sridharan and Jayadeva [50] and is presented

in Fig. 3. Knowing dydn

� �

x¼0from Eq. 6 (i.e. soil and pore

fluid characteristics), d from Eq. 4 and K from the given

fluid properties, u can be obtained from Fig. 3. The

repulsive pressure (p) is calculated from Eq. 3 knowing u.

For the ranges of base exchange capacities (3–100 meq/100 g),

specific surface (10–800 m2/g), concentration of ions

(10-1 to 10-5 M) and pressure range (0.1–10 kg/cm2) and

for water as pore medium, Sridharan and Jayadeva [50]

showed that u–Kd relationship could be treated as unique

as in Eq. 7, without loss of accuracy:

u ¼ 2:35� 4:375 log10Kd ð7Þ

Considering the various limitations involved in the

calculation of the repulsive pressure, Eq. 7 can be used for

clay–water system satisfactorily.

Thus, the repulsive pressure (p) is related to soil related

quantities such as void ratio, specific surface & specific

gravity and to the fluid properties such as concentration of

ions (n), valance (v), dielectric constant (n) & temperature

(T). As per the Gouy–Chapman theory, specific surface and

void ratio of the soil play a dominant role than the other

soil properties.

Figure 4 relating dffiffiffi

np

and log (p/n) shows the influence

of electrolyte concentration from which it can be seen that

the theoretical curve is essentially independent of clay type

and cation concentration for the range of values involved.

Although all the experimental results do not coincide with

the theoretical line, it is seen that there is an almost parallel

shift. Considering the variations in the concentration (100

times) and clay type (illite and montmorillonite), the results

obtained by Bolt [2] and Mesri and Olsen [21] can be

considered to agree satisfactorily with the prediction from

the diffuse double layer theory.

Low [20] reported the consolidation test results on 35

sodium saturated montmorillonitic clays with surface areas

varying from 288 to 800 m2/g. Figure 5a shows six typical

void ratio–pressure curves obtained from the data of Low

[20]. On the log d–log p plot, Low’s results lie in a narrow

band (Fig. 5b).

Figure 5b also shows the theoretical line and the line

representing the average of the experimental points. Thus,

the results of Low [20] support the predictions from the

diffuse double layer theory. Sridharan and Jayadeva [50]

presented the experimental results from Bolt [2] as in

Fig. 6 in the form of vd–log p relationship for sodium and

calcium montmorillonites and for sodium illite. As per the

double layer theory, this is supposed to be a linear rela-

tionship. Considering the various assumptions made in the

development of diffuse double layer theory, the results

included in Fig. 6 can be considered to agree reasonably

well with the theory.

Fig. 3 Relationship between u and Kd for various values of

(dy/dn)x=0 (Data Source: [50])

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Sridharan and Choudhury [46] analysed the results

obtained by Bolt [2], Mesri and Olsen [21] and Low [20]

and compared the same with the theoretically obtained

relationship between U and Kd (i.e., Eqs. 8a, 8b) (Fig. 7).

It can be seen from this figure that the comparison between

the theory and experimental results is quite satisfactory.

When the value of the valency of exchangeable cation

varies in the clay, Tripathy et al. [72] adopted weighted

average valency in the predictions. Using this approach,

they compared their experimental results with the theo-

retical values for MX 80 bentonite (Fig. 8). They also

obtained similar equations for MX 80 compacted bentonite.

It can be seen that the comparison between the theory and

the experimental results is satisfactory.

It can also be seen that the Eqs. 8a, 8b, 8c, and 8d are similar

to Eq. 7, but are specific to certain cases indicated. It may be

seen in these equations that the intercept does not vary much;

but the slope varies to some extent for the case of compacted

bentonites (Eq. 8d), where the slope is vastly different.

Equation 8a is the theoretical equation for Na-mont-

morillonite [46]

u ¼ 2:10 � 4:583 log10Kd ð8aÞ

Equation 8b is the experimental equation for Na-

montmorillonite [46]

u ¼ 2:81� 3:375 log10Kd ð8bÞ

Equation 8c is the theoretical equation for MX 80

Bentonite [72]

u ¼ 2:77� 3:804 log10Kd ð8cÞ

Equation 8d is the experimental equation for compacted

MX 80 Bentonite [72]

u ¼ 2:91� 7:277 log10Kd ð8dÞ

Soil Structure

The term soil structure refers to inter particle force oper-

ative in a clay water system as well as the geometric

arrangement of clay particles (i.e. soil fabric). A knowledge

of the soil structure is essential in soil engineering as the

inter particle forces arising from physico-chemical mech-

anisms have been observed to have a profound influence on

a wide range of soil engineering properties, which include

consistency, consolidation, permeability and shear strength

([13, 18, 26, 34, 81] to name a few).

Fig. 4 Comparison of experimental and theoretical relationship

between log dffiffiffi

np

and log p/n (Data Source: [50])

Fig. 5 a Void ratio plotted against log p using Low’s [20] data (Data Source: [50]). b Comparison of experimental results with theory using

Low’s [20] data (Data Source: [50])

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123

Pore Spaces and Fabric

Two kinds of pores or pore spaces can readily be identified

in clayey soils. The pore spaces between fabric units are

larger than the pore spaces between particles constituting

the fabric units. The pores between the fabric units are

termed as macro pores, and the pore spaces between par-

ticles within the fabric unit as micro pores [82]. In iden-

tifying and characterising the clay fabrics and defining the

solid network for fabric units, it is necessary to take into

account the spacing between the particles and between the

fabric units. A knowledge of distribution of pore spaces

provides an appreciation of packing of fabric units and

their gradation [43]. Soil behaviour concerning to the water

flow, pore water exclusion, soil deformation and

consolidation requires a knowledge of characteristics of

water movement in the macro and micro pores and also of

rearrangement of fabric units. For example, Sridharan [41]

and Sridharan et al. [45, 69] showed for partly saturated

soils that the finer pores had significant bearing on the soil

shear strength characteristics. The narrow pores mobilise

significant matric suction leading to larger shear strength

values. They also showed that the soil compacted dry of

optimum had wider pores, which could have a significant

bearing on the permeability of compacted clays.

Effective Stress Concept

It has been now widely accepted that Terzaghi’s concept of

effective stress provides a satisfactory basis for under-

standing the strength and deformation characteristics of

saturated soils, which can be stated as

r0 ¼ r� u ð9Þ

where r0 is effective stress, r is applied external stress or

the total stress and u is pore water pressure. It may be

noted that r0 is the contact stress at mineral to mineral

contact zone, which is also called inter granular stress.

While one can discuss at greater length the nature of this

contact, for purpose of brevity, it can be said that the role

of contact is to transfer the stress. It has been brought out

earlier that both electrical attractive and repulsive forces

exist between clay particles. Since the fine grained soils are

normally composed of clays, the existence of attractive and

repulsive forces in the soil–water system is inevitable. In

this context, the conventional effective stress concept needs

a critical examination. The studies of Sridharan [41] and

Fig. 7 Relaionship of u and log10 Kd for sodium montmorillonite

clay–water electrolyte systems using experimental results (Data

Source: [46])

Fig. 8 Theoretical and experimental u–Kd relationship for MX80

bentonite (weighted average valency, v = 1.14) (Data Source: [72])

Fig. 6 md plotted against log p using Bolt’s [2] data

(1 kg/cm2 = 98.1 kN/m2) (Data Source: [50])

Indian Geotech J (October–December 2014) 44(4):371–399 377

123

Sridharan and Rao [62, 64] resulted in the proposition of

modified effective stress concept as given in Eq. 10.

�c ¼ ram ¼ r� u� Rþ A ð10Þ

For a saturated system, where �c is the average contact

stress, r is actual contact stress at mineral-to-mineral level,

am is area fraction over which r acts or percentage area

through which r acts (non dimensional), r is the external

applied stress, u is pore water pressure, R is average

repulsive pressure acting throughout the area,

A is average attractive pressure acting throughout the area.

Equation (10) can be written as

�c ¼ ram ¼ r0 þ r00 ð11Þ

where r00 is the intrinsic effective stress and r0 is the

conventional effective stress.

It may further be stated that the effect of r and A is to bring

the particles closer to each other. The effect of positive pore

water pressure and R is to keep the particles away from each

other. If the pore water pressure is negative (i.e., capillary

pressure operative in partly saturated soils), its role is to bring

the particles closer to each other. The average contact stress

or the inter granular stress (�c) between particles, is defined as

the modified effective stress and it is hypothesised that �c is

the stress controlling the shearing resistance and volume

changes that take place in soil–water system. In fine-grained

soils/clayey soils, the attractive and repulsive forces cannot

be neglected, especially when the water content and the soil

plasticity are high. Since the clay-water system is complex,

quantitative determination of R and A becomes difficult for

real systems. However, qualitative evaluation could be done.

The validity of Eqs. (10) and (11) have been qualitatively

studied extensively considering the volume change behav-

iour [62, 65], the strength behaviour [1, 45, 54, 60, 64, 69],

the shrinkage phenomena [61], the secondary compression

behaviour [59] and the sediment formation [53].

Atterberg Limits

Liquid Limit

Although the Atterberg limits (perhaps the oldest and most

commonly used tests in the field of soil engineering) were

devised originally for the purpose of soil classification,

various attempts have been made to correlate them with

various soil properties like surface area, cation exchange

capacity, swelling and compressibility characteristics in the

recent past [42, 43). Hence, understanding the mecha-

nism(s) controlling the liquid limit behaviour assumes

importance. It is widely accepted that the liquid limit test is

essentially a limiting water content separating the viscous

liquid state and plastic state of soil consistency. The soils at

their liquid limit possess definite but small shear strength,

which is considered to be nearly same for all soil types.

The results obtained by the use of direct shear or vane shear

tests indicate that the strength at liquid limit is of the range

0.5–5.6 kPa ([51, 63, 78, 80]; to name a few).

Sridharan and Rao [63] discussed the possible mecha-

nisms governing the liquid limit of kaolinite and mont-

morillonite type of clays. Kaolinite and montmorillonite,

the two extreme types of clay minerals, behave quite dif-

ferently from each other under any given set of physico-

chemical environment. Hence, the mechanisms governing

the liquid limit of kaolinitic and montmorillonitic soils

cannot be the same.

Extensive studies conducted at the Indian Institute of

Science, Bangalore, revealed the existence of two different

mechanisms governing the liquid limit of soils, taking into

account the clay mineralogy and the pore medium chem-

istry [63, 66, 68]. These mechanisms are:

• the thickness of diffuse double layer controlling the

liquid limit

• mode of particle arrangement as determined by the

inter-particle forces (i.e. fabric) controlling the liquid

limit.

According to the first mechanism, the liquid limit of

soils is mainly due to the diffuse double layer held water. A

detailed study of the double layer theory has shown that the

thickness of the double layer is a function of dielectric

constant of the pore fluid, electrolyte concentration and the

cation valency. The diffuse double layer thickness gets

suppressed when:

• the dielectric constant of the pore fluid decreases.

• the electrolyte concentration of the pore medium

increases

• the cation valency increases.

Correspondingly, there should be a decrease in the

liquid limit of a soil in all these cases. This has been proved

true for montmorillonitic soils [66] At the same time, these

changes in the pore medium chemistry has a dominant

effect on the inter particle attractive and repulsive forces.

In all the cases mentioned above, the repulsive forces

reduce and the magnitude of the attractive forces increase,

resulting in an increase in the shearing resistance at the

particle level [36, 41, 61, 62]. These conditions favour

flocculation, resulting in an increase in the liquid limit

values in kaolinitic soils (mechanism 2) [68]. With both the

mechanisms being operative in clays, the liquid limit of the

soil depends upon the predominant mechanism of the two

that would be decided by the dominant clay mineral type

present in the natural soils.

The literature has documented detailed discussions and/

or data on the effect of dielectric constant, electrolyte

378 Indian Geotech J (October–December 2014) 44(4):371–399

123

concentration, cation valency and hydrated size of the

cation on the liquid limit of kaolinitic and montmorillonitic

soils [9, 27, 28, 42, 43, 67, 76, 79]. It has indicated that any

decrease in the dielectric constant or increase in the elec-

trolyte concentration or increase in the cation valency,

results in an increase in the liquid limit of kaolinite soils

and a decrease in the liquid limit of montmorillonitic soils.

Figure 9 presents the liquid limits (determined by cone

method) of kaolinite and montmorillonite determined with

eight organic pore fluids, hexane, heptane, carbontetra

chloride, benzin, ethyl acetate, acetone, ethanol, methanol

and water, mainly with a view to study the variation in the

force field governing the particulate system. The liquid

limit value have been presented on volume basis (the ratio

of the volume of fluid to the volume of solids expressed as

percentage) since the unit weight of the fluids used differs

from one another. From Fig. 9, it is distinctly seen that the

two clays viz: kaolinite and montmorillonite behave in a

strikingly opposite manner when there is an identical

change in the pore medium. Interestingly, the liquid limit

values of kaolinite with hexane, carbontetra chloride and

ethyl acetate are more than the corresponding values of

montmorillonite. At the outset, these observations may

look paradoxical, but the following study into the mecha-

nisms controlling the liquid limit behaviour would show

that these results are in order.

Sridharan and Rao [63] demonstrated that for both

kaolinite and montmorillonite, the shear strength decreases

rapidly as the dielectric constant increases. Thus, for kao-

linite, the net effect of increase in dielectric constant is to

decrease the attractive force and hence, shear strength

decreases resulting in the reduction of liquid limit values.

For montmorillonite, the increase in diffuse double layer

thickness over shadows the effect of the decrease in the

shear strength as the dielectric constant increases, resulting

in an increase in the liquid limit. In addition to its influence

and the shearing resistance at the inter particle level, a

decrease in dielectric constant also promotes the extent of

particle flocculation increasing the water holding capacities

of kaolinitic clays. While both mechanisms operate

simultaneously, the strength and fabric effect dominate the

kaolinite behaviour whereas the thickness of diffuse double

layer dominates the montmorillonite behaviour. Compari-

son of liquid limits of kaolinite and montmorillonite clays

at very low dielectric constant (for example hexane and

carbon tetra chloride) brings out further evidence to sup-

port the proposed mechanisms. Because of the low

dielectric constant, these fluids develop very thin or prac-

tically no double layer at all on the clay particles. Hence,

the liquid limit values should primarily be governed by the

inter particle shearing resistance. Because of its relatively

higher shearing resistance and enhanced particle floccula-

tion, kaolinite has a higher liquid limit than montmoril-

lonite for these fluids.

Figure 10 provides is complementary to the data pre-

sented in Fig. 9. It presents the comparison of liquid limits

of kaolinitic and montmorillonitic soils obtained with dis-

tilled water and carbon tetra chloride as the test liquids.

From this figure, it can be seen that the liquid limits

obtained with carbon tetra chloride are more than in water

for kaolinitic soils, whereas for montmorillonitic soils, they

are much more in water than in carbontetra chloride. This

behaviour can also be made use of in distinguishing the

montmorillonitic soils from kaolinitic soils [33].

Figure 11 shows that, for natural montmorillonitic soils,

there is a good correlation between liquid limit and

exchangeable sodium. The plausible reasons for the

dependence of the double layer thickness and in

Fig. 10 Comparison of liquid limits in water and CCl4 (Data Source:

[44])Fig. 9 Effect of dielectric constant on liquid limit (Data Source: [63])

Indian Geotech J (October–December 2014) 44(4):371–399 379

123

consequence, of the liquid limit, on the exchangeable cat-

ion type may be explained as follows. The exchangeable

cations usually present in the natural soils are calcium,

magnesium, sodium and potassium. The divalent calcium

and magnesium ions, by virtue of their higher valency, are

strongly adsorbed by the clay surface [2, 43] and do not

undergo appreciable dissociation in the presence of water

to contribute significantly to the number of ions in the

double layer. It is also known that contribution of the

divalent ions to the double layer thickness is much less. In

the case of potassium ions, their size (unhydrated diameter,

0.266 nm) is such that they fit partly into the hexagonal

holes in the surface configuration of the silicate layers. As

they are close to the seat of negative charges, they are held

tightly by electro static bonding. The resultant high

adsorption affinity and also the minimal concentration of

potassium ions prevent them from contributing signifi-

cantly to the thickness of diffuse double layer. Unlike

potassium ions, sodium ions cannot be fixed, partly because

of their smaller size (unhydrated diameter, 0. 19 nm) and

partly because of their greater hydration energy, which

prevents their close approach to the surface [67]. As a

result, the sodium ions are weakly held by the surface and

readily dissociate to contribute significantly to the thick-

ness of diffuse double layer.

Figure 12 shows the variation of liquid limits of kao-

linitic soils with the exchangeable sodium content. Wide

scatter between the exchangeable sodium content and the

liquid limit suggests that the diffuse double layer does not

contribute to the liquid limit of kaolinitic soils. In view of

these observations, it is likely that geometric arrangement

of clay particles (clay fabric) regulates the liquid limit of

kaolinitic soils. Soils with a relatively greater degree of

particle flocculation will enclose larger void spaces for

water entrapment and exhibit higher liquid limit values,

while soils with lesser degree of particle flocculation and

with smaller void spaces will possess lower liquid limit

values.

Direct measurements of particle flocculation within a

clay sample are difficult to make. Lambe [13] observed that

the amount of shrinkage upon drying could be used as a

measure of average particle orientation and that any soil

with a parallel arrangement of particles should undergo

more volume reduction upon drying than the same soil with

its particles in a random/flocculent array. It has been shown

that more the parallel particles are, greater is the shrinkage

of the soil upon drying. It was, therefore, thought that a

kaolinitic soil with greater degree of particle flocculation

and higher liquid limit should undergo lesser shrinkage

than a soil with a lesser extent of particle flocculation and

lower liquid limit.

Figure 13 shows the effect of sodium ion concentration

on the liquid limit of Isahaya marine clay, which is a ka-

olinitic clay. It is seen that the liquid limit increases almost

linearly with the sodium ion concentration, the increase

being almost to an extent of 100 % [48].

Figure 14 shows that the liquid limits of Kawazoe and

Kubato marine clays, which are montmorillonitic soils,

decrease with an increase in the salt concentration.

Tables 2 and 3 present the effect of salt concentration on

the liquid limit of montmorillonitic and kaolinitic soils

Fig. 11 Effect of exchangeable sodium ions on the liquid limit of

montmorillonitic soils (Data Source: [66])

Fig. 12 Effect of exchangeable sodium ions on the liquid limit of

kaolinitic soils (Data Source: [43])

380 Indian Geotech J (October–December 2014) 44(4):371–399

123

respectively. From this data, it is evident that the effect of

cation concentration in the pore fluid is to increase the

liquid limit of kaolinitic soils and to decrease the liquid

limit of montmorillonitic soils.

Even though the Gouy–Chapman theory can explain

qualitatively the variation in the liquid limit of montmo-

rillonitic soils fairly satisfactorily with regard to the vari-

ations in the dielectric constant and electrolyte

concentration, it is inadequate to explain the effect of

cationic valency completely. The deviations can be attrib-

uted to the idealisation that the cations are point charges.

However, the hydrated cationic radius appreciably affects

the liquid limit of montmorillonitic soils, valency being the

same. This effect is more pronounced with monovalent

cations than the cations of higher valency (Table 4)

(Fig. 15) [57]. In general, for a given valency the liquid

limit of montmorillonitic clays increases with an increase

in the hydrated radius of the adsorbed cations.

Plastic Limit

Very less work has been reported in the literature on the

effect of soil clay mineralogy on plastic limits of soils.

Table 5 presents some data showing the variation of plastic

limit of soils with the cation concentration in the pore fluid

for kaolinitic and montmorillonitic soils. The observed

behaviour is very much identical to that of variation of

liquid limit of soils with the cation concentration.

Shrinkage Limit

Shrinkage limit of natural fine-grained soils has been

observed to be affected by many factors, out of which the

effect of relative grain size distribution appears to be more

dominant [52, 55]. Soils with well graded particle distri-

bution would exhibit lesser shrinkage limits, and soils with

poor gradation of particles would have higher shrinkage

limits. However, for pure kaolinite mineral, the shrinkage

limit has been observed to be much higher than that of pure

montmorillonite mineral. This can be explained on two

counts.

• Kaolinite mineral is known for its flocculent structure.

With the result, its shrinkage limit is obviously higher

as a result of higher strength at the particle level.

• Kaolinite particles are normally of uniform, silt sized.

Hence, their shrinkage limits are expected to be more.

• Kaolinitic and montmorillonitic clays and also the soils

with mixed clay mineralogy (CH and CI clays) show

higher shrinkage limits on the addition of lime as a

consequence of relative change of fabric towards

flocculation.

Effect of dielectric constant of the pore fluid from very

low of 1.89 (Hexane) to as high as 80.4 (water) has shown

a decrease in shrinkage void ratio from 1.7 to 0.75 for

kaolinite [61].

Sediment Volume

The soil particles settle under gravity either as discreet

particles or as flocs. As more and more soil solids settle, the

underlying soil layers get compressed due to self weight.

The sediment thus formed is very soft in nature with very

high water content. The nature of the sediment so formed is

a function of depositional environment, which can be

understood by the study of various forces that exist in the

settling system and the changes to which they are sub-

jected. Three main forces that exist in the fine-grained soil

Fig. 13 Effect of sodium ion concentration on liquid limit for

Isahaya (Kaolinitic) clay (Data Source: [48])

Fig. 14 Effect of salt concentration on liquid limit of montmorillon-

itic marine clays (Data Source: Author’s file)

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123

water system are: (i) the forces due to self weight, (ii)

electrical forces of attraction (i.e. distance forces), (iii)

electrical forces of repulsion (i.e. distance forces). Even

though, the latter two forces are negligibly small compared

to the contact forces in coarse grained soils, their influence

is all the more important in a system with very high water

content wherein the effect of mechanical forces like inter

particle friction is appreciably less.

Table 2 Effect of salt concentration on liquid limit of montmorillonitic soils

Sl. no. Description of soil Salt concentration (N) Liquid limit (%) Reference

1. Na-montmorillonite 00 950 Yong and Warkentin [82]

0.01 N NaCl 870

1.00 N NaCl 350

2. Ca-montmorillonite 00 360

1.00 N CaCl2 310

3. Bentonite 00 332 Sridharan and Prakash [54]

0.5 N NaCl 94

4. Black cotton soil 00 92

0.5 N NaCl 85

5. W-179-3: Na-soil 0.01 N 90 Sridharan et al. [49]

1.00 N 74

6. W-179-3: Ca-soil 0.01 N 78

1.00 N 73

Table 3 Effect of salt concentration on liquid limit of kaolinitic soils

Sl. no. Description of soil Salt concentration (N) Liquid limit (%) Reference

1. Na-kaolinite 0.01 N NaCl 34 Yong and Warkentin [82]

1.00 N NaCl 40

2. Kundara clay 00 38 Sridharan and Prakash [54]

0.50 N NaCl 55

3. E-17: Na-soil 0.01 N 89 Sridharan et al. [49]

1.00 N 147

4. E-17: Ca-soil 0.01 N 135

1.00 N 140

Table 4 Influence of valency and size of the adsorbed cations on the liquid limit of bentonite [67]

Bentonite type Specific gravity Liquid limit (%) Plastic limit (%) Hydrated ionic radius (A)a

Lithium 2.61 675 49.1 7.30–10.30

Sodium 2.81 495 49.2 5.60–7.90

Ammonium 2.59 223 55.8 5.37

Potassium 2.72 233 57.8 3.80–5.32

Magnesium 2.65 129 49.9 10.80

Calcium 2.65 125 40.6 9,60

Barium 2.73 108 45.8 8.80

Aluminumb 2.43 108 60.5 –

Ironb 2.70 120 63.5 –

a Mitchell [25]b Owing to the hydrolysis of the Al3? and Fe3? ions in the presence of water, the hydrated radii of these ions cannot be evaluated

382 Indian Geotech J (October–December 2014) 44(4):371–399

123

The inter particle attractive and repulsive forces being

the predominant forces in a settling clay- electrolyte sys-

tem, any changes in them are likely to control the process

of formation of the sediments, their nature and the resulting

equilibrium sediment volume. In addition, soil clay min-

eralogy plays an important roll on the equilibrium sediment

volume of fine-grained soils under different physico-

chemical environments [15, 53].

Similar to liquid limit behaviour, the sediment volume

behaviour of kaolinitic and montmorillonitic soils are quite

contradictory to each other (Table 6).

Figure 16 presents the plasticity chart showing the

positions occupied by a number of natural Kaolinitic and

montmorillonitic soils, from which it can be seen that

irrespective of the soil clay mineralogy, they fall along the

Casagrande A-line. Figure 17 shows the effect of dielectric

constant of the pore medium on the equilibrium sediment

volume of the soils represented in Fig. 16. It may be seen

that the equilibrium sediment volume behaviour of kao-

linitic and montmorillonitic soils are distinctly different.

Figure 18a, b provide complementary data to support this

behaviour. The sediment volume in water is dominated by

diffuse double layer water, whereas the sediment volume in

kerosine is dominated by attractive forces leading to floc-

culent clay fabric. With the result, the montmorillonitic

soils exhibit increasing equilibrium sediment volumes with

an increase in the dielectric constant of the pore liquid as

their behaviour is controlled by diffuse double layer

thickness than in kerosene or carbon tetra chloride, whereas

the kaolinitic soils show increasing equilibrium sediment

volumes with a decrease in the dielectric constant of the

pore liquid as a result of increasing flocculation.

Figure 19 shows the effect of cation concentration on

the equilibrium sediment volume behaviour of kaolinitic

and montmorillonitic soils. It can be seen that the equi-

librium sediment volume of the montmorillonitic soils

decreases with an increase in the electrolyte concentration

whereas it increases for kaolinitic soils.

Table 7 presents the effect of valency of cations and

their hydrated radius. The observations made from this

table with respect to montmorillonitic soils are given

below.

(i) Valency being the same, equilibrium sediment volume

increases with an increase in the hydrated radius of the

exchangeable cation, the size effect being more

pronounced for the monovalent cations than for

divalent cations. This observation depicts one of the

limitations of the Gouy–Chapman diffuse double layer

theory, which does not consider the effect of hydrated

cationic radius, as it assumes the cations to be point

charges.

(ii) As the valency and hydrated ionic radius cannot be

treated as independent parameters, the effect of

valency has to be studied at the same level of

hydrated ionic radius. In this context, equilibrium

sediment volume of the montmorillonitic soils with

monovalent lithium as the exchangeable cation is

higher compared with those of divalent cations, the

hydrated radii of lithium, barium, calcium and

magnesium being more or less of the same order.

Fig. 15 Effect of hydrated radius of absorbed cations on the liquid

limit of montmorillonitic soils (Data Source: [57])

Table 5 Influence of cation concentration on the plastic limit of soils

[53]

Soil Water 0.5 N NaCl

Montmorillonitic soils:

Bentonite 50 48

Black cotton soil 52 43

Kaolinitic soils:

Kundara clay 34 39

Table 6 Effect of pore medium chemistry on the sediment volume

and liquid limit of fine-grained soils

Increase In Sediment volume/liquid

limit

Kaolinitic

soils

Montmorillonitic

soil

Dielectric constant Decrease Increase

Concentration of ions in pore medium Increase Decrease

Valency of cation Increase Decrease

Hydrated size of ions in pore medium Decrease Increase

Indian Geotech J (October–December 2014) 44(4):371–399 383

123

These observations indicate that the influence of

hydrated ionic radius is significant and in some cases can

override the influence of valency. Any generalisation of the

equilibrium sediment volume behaviour based only on

cationic valency can be misleading or is not tenable.

The observations made from Table 7 with respect to

kaolinitic soil (i.e., Kundara clay) are indicated below.

Any increase in the cationic valency will increase the

inter-particle attractive forces, which favour higher floc-

culation. At the same time, an increase in the hydrated

cationic radius reduces the inter particle attraction, which

leads to lower level of flocculation. Hence, the variation in

the equilibrium sediment volume of a kaolinitic soil

depends upon which of the two factors dominate.

(i) when the cationic valency is one, Kundara clay with

pottasium and ammonium as the exchangeable cations

exhibited appreciably higher equilibrium sediment

volumes than those obtained with sodium and lithium.

This can be attributed to the effect of hydrated

cationic radius.

(ii) With the divalent cations, the effect of hydrated

radius appears to be negligible. Like montmorillonitic

soils, the size effect is more pronounced with

monovalent cations than with divalent cations.

(iii) The equilibrium sediment volumes of Kundara clay

with lithium as the exchangeable cation is relatively

less than those with barium, calcium and magnesium

as the cations. (Note that the comparison is made at

about the same level of hydrated cationic radius).

This shows that the equilibrium sediment volume of

kaolinitic soils increases with the valency, at the

same level of hydrated cationic radius.

It may be mentioned here that the process of settling of

soil particles is physico-chemical in nature as the soils are

composed of chemically active clay minerals. The extent to

which the clay mineralogy affects the settling process

depends upon the initial water content of soil–water sus-

pension [58]. For montmorillonitic soils, the limiting water

content at which the nature of the settling changes from

discreet free type to a flocculated free type increases with

an increase in the soil plasticity, the process being con-

trolled by the diffuse double layer repulsion. In case of

kaolinitic soils, the limiting water content at which the

nature of settling changes from discreet free type to floc-

culated free type decreases with the soil plasticity due to

the effect attractive forces and soil fabric [58].

Since the same diffuse double layer related mechanism

controls both sediment volume and the liquid limit behav-

iour of montmorillonitic soils and the same interparticle

attractive force and fabric related mechanism governs both

sediment volume and liquid limit behaviour of kaolinitic

soils, one can expect a good correlation between sediment

volume and liquid limit [32]. Figure 20 shows such a cor-

relation based on the results for natural soils obtained from

different parts of India with very wide variations in clay

mineralogy. Based on this good correlation obtained, a

simple method has been proposed by Prakash and Sridharan

[32] for obtaining the liquid limit of fine-grained soils. The

method essentially consists in determining the equilibrium

sediment volume of 100 ml of soil–water suspension con-

taining 10 g of dry soil passing 425 lm after an equilibra-

tion period of 24 h. The equilibrium sediment volume so

obtained has correlated very well with the liquid limit

determined either by the Casagrande percussion method or

fall cone method (Eqs. 12 and 13).

Fig. 16 Position of kaolinitic and montmorillonitic soils on the

plasticity chart (Data Source: Sridharan et al. [44])

Fig. 17 Effect of dielectric constant of the pore medium on the

equilibrium sediment volume of the soils for the data in Fig. 16 (Data

Source: Sridharan et al. [44])

384 Indian Geotech J (October–December 2014) 44(4):371–399

123

wL Casagrandeð Þ% ¼ 37:17 Sv ð12ÞwL coneð Þ% ¼ 32:74 Sv ð13Þ

where Sv is the sediment volume in cm3/g.

It has already been illustrated that the sediment volume

behaviour of montmorillonitic and kaolinitic soils in dis-

tilled water and in kerosene or carbon tetra chloride are of

quite opposite nature. This can be effectively used for

classifying the degree of expansivity of natural fine-grained

soils and also to obtain the clay mineralogical composition

of such soils. Sridharan and Prakash [56] have defined a

term Free Swell Ratio (FSR) as the ratio of equilibrium

sediment volume of 10 g of oven dry soil passing 425 lm

sieve in distilled water (Vd) to that in carbon tetra chloride

(Vk) after an equilibration period of 24 h.

FSR ¼ Vd= Vk ð14Þ

Making use of the values of FSR so obtained, the degree of

expansivity of fine-grained soils can be obtained by the

guidelines proposed by Sridharan and Prakash [56] after

validating the same with the oedometer swell test results

(Table 8). The works of Prakash and Sridharan [33] have

shown that the values of FSR can also be used to predict the

soil clay mineralogical composition (Fig. 21) (Table 8).

Extensive works done at Indian Institute of Science,

Bengaluru have indicated that the clay mineralogical

composition predicted by the FSR method has one-to-one

match with the actual clay mineralogical composition of the

soils obtained by X-ray diffraction analysis.

Volume Change Behaviour

Terzaghi as referred by Taylor [71] indicated that high

compressibility was due to the presence of ‘scale shaped’

particles, adsorbed water being the reason for low perme-

ability and secondary compression. Leonards and Alts-

chaeffl [18] proposed that the sliding between the particles

Fig. 18 a Variation of

equilibrium sediment volume of

montmorillonitic soils with

dielectric constant (Data

Source: [53]). b Variation of of

equilibrium sediment volume of

kaolinitic soils with dielectric

constant (Data Source: [53])

Table 7 Effect of valency and hydrated radius of cation on equilibrium sediment volume of the soils [53]

Salt solution Valency of cation Hydrated cationic radius (A)a Equilibrium sediment volume (cm3/g)

Bentonite Black cotton soil Kundara clay

Potassium chloride 1 3.80–5.32 3.0 2.2 4.7

Ammonium chloride 1 5.37 3.5 2.3 5.8

Sodium chloride 1 5.60–7.90 3.8 2.8 2.9

Lithium chloride 1 7.30–1,030 7.4 3.3 2.5

Barium chloride 2 8.8 3.3 2.7 3.6

Calcium chloride 2 9.6 3.4 2.7 3.8

Magnesium chloride 2 10.8 3.4 2.9 3.7

Iron oxideb 3 – 2.8 2.6 3.6

a After Mitchell [25]b Due to hydrolysis of Fe3? ions in the presence of water, the hydrated radius of these ions could not be evaluated

Indian Geotech J (October–December 2014) 44(4):371–399 385

123

resulted in volume changes. Kenny et al. [11] proposed that

shearing resistance at contact points controlled the defor-

mation. Gouy–Chapman theory was examined critically for

the prediction of volume change behaviour. Bolt [2]

explained the compressibility of pure clays by considering

long range repulsive forces between the particles. Due to

diffuse type of ion distribution around a clay particle in a

clay–electrolytic system, the system could be regarded as an

osmometer, and the compressibility would essentially be a

function of the double layer repulsive force, which was pri-

marily dependent on the type of clay and the electrolyte

content of the system. Bolt [2] noticed that the compression

curves as observed and as calculated from the theoretical

considerations from diffuse double layer theory of Gouy–

Chapman indicated that, in the case of pure clays, the com-

pressibility could be accounted for quantitatively by con-

sidering the long range forces only. The results of Warkentin

et al. [77] showed a good agreement between theoretical and

experimental values of inter particle spacing and pressure for

montmorillonite in 10-4 NaCl solution. Less good agree-

ment was obtained by Mitchell [24] for tests on unfraction-

ated montmorillonite and for montmorillonite–silt mixtures

in 10-1 and 10-3 N NaCl solutions.

Olsen and Mesri [29] proposed that physico-chemical

mechanisms control the volume change behaviour in

montmorillonitic clays. Sridharan and Rao [62] brought out

in detail two basic mechanisms controlling the volume

change behaviour of clays.

Mechanism 1: Volume change of a clay is primarily

controlled by the shear resistance at the near contact points

and the volume changes occur by the shear displacements

or by the sliding between the particles or by both. Equi-

librium takes place when the shear stress is equal to the

shear strength, which is controlled by the modified effec-

tive stress concept (Eq. 10).

Mechanism 2: Volume change is primarily governed by

the long range electrical repulsive forces, which are

essentially double layer repulsive forces. Equilibrium takes

place when the sum of the self weight and the attractive

forces is equal to the repulsive forces.

It has been brought out through conventional consoli-

dation experiments with various organic fluids that mech-

anism 1 primarily controls the volume change behaviour of

Fig. 20 Relationship between liquid limit (cup method) and equilib-

rium sediment volume (Data Source: [32])

Table 8 Expansive soil classification based on FSR [33]

Free swell ratio Clay type Degree of soil expansivity Dominant clay mineral type

B1.0 Non-swelling Negligible Kaolinitic

1.0–1.5 Mixture of swelling and non-swelling Low Mixture of kaolinitic and montmorillonitic

1.5–2.0 Swelling Moderate Montmorillonitic

2.0–4.0 Swelling High Montmorillonitic

[4.0 Swelling Very high Montmorillonitic

Fig. 19 Variation of the equilibrium sediment volume of montmo-

rillonitic and kaolinitic soils with electrolyte concentration (Data

Source: [53])

386 Indian Geotech J (October–December 2014) 44(4):371–399

123

kaolinitic soils and mechanism 2, the behaviour of mont-

morillonitic soils, even though both mechanisms operate

simultaneously [62].

With an increase in the dielectric constant or decrease in

the electrolyte concentration of the pore fluid, the attractive

forces decrease and the repulsive forces increase. This

reduces the modified effective stress (Eq. 10), which in turn

is responsible for the reduction in the shearing resistance at

the particle level, void ratio remaining the same. For kao-

linitic soils, since the volume change behaviour is governed

by the shearing resistance at the particle level, the shear stress

brought about by the self weight of the settling soil particles

is resisted by the shearing resistance at the particle level, at

reduced void ratio. A decrease in the dielectric constant or

increase in the electrolyte concentration increases in the

shearing resistance at the particle level as a consequence of

increased modified effective stress. This results in higher

volume with increased flocculation resisting the shear stress

at higher void ratio. Thus, the equilibrium volume is more

when the net attraction (A–R) is more and the same is less

when (A–R) is less. In the case of montmorillonitic soils, an

increase in the dielectric constant or decrease in the elec-

trolyte concentration of the pore medium favours an increase

in the double layer repulsive force. The resulting reduced

modified effective stress is responsible for the montmoril-

lonitic soils to equilibrate at higher volume with a dispersed

fabric. This is because the volume change behaviour of

montmorillonitic soils is not controlled by the shearing

resistance at the particle level, but rather by the double layer

repulsive force. For soils exhibiting high expansivity, the

diffuse double layer repulsion can be so high that the net

electrical force is repulsive. This situation leads to no contact

between the soil particles, forcing the modified effective

stress to be zero. In such cases, the net repulsion (R–A)

balances the self weight of the sediment and controls the

equilibrium sediment volume. Any decrease in the dielectric

constant or increase in the electrolyte concentration sup-

presses the double layer thickness and hence, the modified

effective stress increases, causing more volume change.

Sridharan and Jayadeva [50] presented an extensive

discussion on Gouy–Chapman theory of electrical double

layer and showed that the compressibility of clays depen-

ded primarily on the surface area of clay mineral, the

externally applied pressure and the characteristics of the

pore fluid.

Figure 22 shows the void ratio–pressure relationship for

montmorillonite clay for two different fluids of different

dielectric constants. It may be seen that the curves are

placed in the order of the dielectric constant of the pore

fluid used in the test, the one with water being at the top

and the one with carbon tetra chloride at the bottom. This

indicates the control of the mechanism 2 over the behav-

iour of montmorillonite clay.

Figure 23 shows the void ratio–pressure relationship for

kaolinite clay for two different fluids of different dielectric

constants. It may be seen that the curves are placed with the

one with water as the pore fluid at the bottom and the one with

carbon tetra chloride at the top. This indicated the control of

the mechanism 1 over the behaviour of kaolinite clay.

Figure 24 shows similar one dimensional consolidation

test results for Ariake clays (dominated by kaolinite clay

mineral).

Figure 25 shows the effect of replacement of moulding

pore fluid by another fluid of different dielectric constant at

a constant applied pressure in the oedometer test on black

cotton soil. It is seen that when water is replaced by carbon

tetra chloride, significant compression occurs because of

the reduction in repulsive pressure. When carbon tetra

0 10 20 30 400

10

20

30

40

50

60

70

80

III C III BIII A

II

I

1

V d c

c

Vk, cc

1

1.51

2

1

4

1

Soil Group Soil Type

I Kaolinitic

II (Kaolinitic + Montmorillonitic)

III AModerately Swelling Montmorillonitic

III BHighly Swelling Montmorillonitic

III CVery Highly Swelling Montmorillonitic

,

Fig. 21 Classification of soils as

montmorillonitic and kaolinitic

types [33]

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123

chloride is replaced by water at constant external pressure,

swelling occurs as a consequence of increased double layer

repulsion.

Figure 26 shows the effect of replacement of carbon

tetra chloride by water at a constant applied pressure in the

oedometer test on kaolinite. It is seen that when the carbon

tetra chloride is replaced by water, which reduces the net

attraction, significant compression takes place (as against

what has been seen in Fig. 25). As dielectric constant

increases attractive force decreases in kaolinitic soil as

brought out earlier.

Figure 27 shows typical results indicating the effect of

electrolyte concentration of pore fluid on the void ratio–

pressure curves for Na montmorillonite [21] where the

dominant effect of double layer repulsion can be seen. As

the electrolyte concentration increases, double layer

repulsion decreases and hence, lesser compression results.

As has been pointed out earlier, the Gouy–Chapman

theory does have a limitation in that it does not consider the

effect of ionic size. Figure 28 shows typical results

obtained by Sridharan et al. [67], which indicates the effect

of cationic size on the compressibility behaviour of ben-

tonite clay. Although Li?, Na?, K?, NH?4 are monovalent

ions, their void ratio–pressure relationships are not unique,

the clay type being one and the same. Similarly, the void

ratio–pressure relationships for the soils with divalent ions

also show variations in their behaviour.

The strong influence of pore fluid composition on the

volume change behaviour of clays has also been well brought

out by Di Maio [3]. She studied the effect of exposing the

bentonite to NaCl, KCl and CaCl2 solutions in consolidation

tests and showed that the depression of diffuse double layer

had brought down the compression in the case of potassium

and calcium solutions. Further, the results also showed that

NaCl effects were reversible when the specimens were re-

exposed to water, while KCl and CaCl2 effects persisted even

after some months of continuous testing. These results sup-

port the irreversible effect of divalent ions on the diffuse

double layer thickness (Fig. 29).

Results published by Olsen [30] are available regarding

the effect of pH on the consolidation behaviour of sedi-

mented specimens of sodium kaolinite (Fig. 30). They

clearly showed that the water content-log p curves posi-

tioned one below the other in the increasing order of pH of

the pore fluid. This shows that when pH 5 (acidic), the

edges are positively charged (Fig. 1) leading to flocculent

(edge–face) fabric resisting the external load at a higher

void ratio. When pH 9, the edges are negative (Fig. 1)

leading to relatively more dispersed fabric resulting in

lower void ratio at any stress level.

Fig. 22 Effect of dielectric constant (e) on e-log r0 curves for

montmorillonite (Data Source: [63])

Fig. 23 Effect of dielectric constant (e) on e-log r0 curves for

kaolinite (Data Source: [63])

Fig. 24 Effect of dielectric constant (e) on e-log r0 curves for Ariake

clay [48]

388 Indian Geotech J (October–December 2014) 44(4):371–399

123

Secondary Compression Coefficient

Sridharan and Rao [59] made a detailed study on the

mechanisms controlling the secondary compression of

clays. For their study, they used a black cotton soil com-

prised of mainly montmorillonite clay mineral and sodium

kaolinite. Further, they used seven different organic pore

fluids with dielectric constant varying from 2.2 (carbon

tetra chloride) to 37.7 (glycol) and to 80.4 (water). They

concluded that, irrespective of the clay mineralogy of the

soils, the non-dimensional secondary compression coeffi-

cient Cs (defined as the ratio of secondary compression per

unit of log time to the thickness of the sample) was related

to the strength at the particle level, which happened to be a

function of the modified effective stress (Eq. 10). As the

strength of the particle skeleton increases, a decrease in the

value of Cs can be observed (Fig. 31). As the strength at

the particle level decreases, Cs increases as the void ratio

increases. Void ratio level has a significant influence on Cs.

As the load increment increases, Cs increases for the same

void ratio level; but, the load increment ratio has no defi-

nite relationship with Cs.

A decrease in the dielectric constant of the pore medium

brings about an increase in attractive forces and a decrease

Fig. 25 Effect of replacement of moulding pore fluid on e-log r0 curves of black cotton soil (1 kg/cm2 = 98.1 kN/m2) (Data Source: [63])

Fig. 26 Effect of replacement

of moulding pore fluid on e-log

r0 curves of kaolinite

(1 kg/cm2 = 98.1 kN/m2) (Data

Source: [63])

Indian Geotech J (October–December 2014) 44(4):371–399 389

123

in repulsive forces, and hence, a net increase in modified

effective stress and a consequent increase in shearing

resistance irrespective of the soil type. This causes a

decrease in Cs for both black cotton soil and kaolinite.

The viscosity and dipole moment of the fluid may have a

marginal influence on Cs; but, the dielectric constant,

which primarily controls the bonding and disruptive forces,

dominates in controlling the behaviour. Use of a very

viscous fluid like glycol did not significantly affect the

secondary compression behaviour.

In the case of over consolidated soils, Cs decreases with

an increase in the over consolidation ratio.

Since secondary compression takes place almost at

constant modified effective stress under fully drained

condition, the secondary compression coefficient is a

function of strength at particle level, which is a function of

the modified effective stress as given in Eq. 10.

Shear Strength

Drained Tests

As has been noted earlier, the engineering behaviour of fine-

grained soils is influenced by many physico-chemical

parameters. For better understanding, the physico-chemical

mechanisms controlling the shear strength behaviour of

clays, Sridharan and Rao [64] conducted drained test on

compacted kaolinite and montmorillonite saturated with

various organic fluids with variations in their dielectric

constants in a shear box.

Figures 32 and 33 show the strength envelops. The

strength envelops are one above the other as the dielectric

constant reduces for both kaolinite and montmorillonite.

The effect is more pronounced for kaolinite than for

montmorillonite. Although the tests were conducted on a

saturated system, the cohesion intercept was obtained. This

is because the normal stress in the drained box shear test is

taken as the conventional effective stress and represented

on the X-axis for obtaining the strength parameters. The

true effective stress i.e. the contact stress as given by

Eq. (10) has, in addition, the net attractive force component

(A–R). It has been brought out earlier that the net attraction

increases as dielectric constant reduces. If this is consid-

ered, then the true effective stress will be higher by the

amount equal to the intrinsic effective stress, and the

cohesion intercept would become almost zero. The effect

of dielectric constant is more for kaolinite as the kaolinite

particles tend to become flocculated as the dielectric con-

stant becomes lesser. Repulsion being negligible for kao-

linite, the net attraction becomes more. Further, the net

attractive force is lesser for montmorillonite because of

high repulsion. The above results bring out the dominant

influence of the physico-chemical factors on shear strength

parameters. It may be seen that the mechanism controlling

the drained shear strength is one and the same for both

kaolinite and montmorillonite and the modified effective

stress concept as given in Eq. 10 governs the drained

strength behaviour.

Figure 34 shows the shear strength behaviour of homo-

ionised bentonite and kaolinite [6]. It is seen that, for ben-

tonite, the effect of valence on the friction angle is

significant. As valence increases, the effective angle of

friction increases significantly because of significant reduc-

tion in the repulsive forces. For kaolinite, repulsion being

Fig. 27 Effect of electrolyte concentration on e-log r0 curves for Na-

montmorillonite at pH 7 (1 kg/cm2 = 98.1 kN/m2) (Data source:

[21])

Fig. 28 Effect of cationic size e-log r0 curves for homoionised

bentonites (1 kg/cm2 = 98.1 kN/m2) (Data Source: [67])

390 Indian Geotech J (October–December 2014) 44(4):371–399

123

much less significant, the effective fiction angle remains

almost the same.

The strong influence of fluid composition on the mechan-

ical behaviour of clays has been well brought out by Di Maio

[3]. Ponza bentonite exposed to saturated NaCl, KCl or CaCl2solutions caused deformation due to depression of diffused

double layer and a large increase in the effective residual shear

strength. For KCl and CaCl2 treated clays, the increase in

residual strength is permanent and irreversible because of

higher values of valency of calcium and ionic size of mono-

valent potassium. Treatment with higher concentration of

CaCl2 is reversible when concentration is reduced.

Undrained Shear Strength

In general, the shear strength of a soil can be considered to

have three components viz; cohesion, friction and dilat-

ancy. Cohesion, in general, is considered as a part of the

shear strength that can be mobilised due to forces arising at

particle level and is independent of the effective stress [14,

39] and hence, is regarded as a physic-chemical component

of the shear strength. Yong and Warkentin [81] states that

the cohesion of clays is so dependent on the interaction

characteristics of the clay–water system that a definite

description as to what constitutes cohesion becomes vir-

tually impossible. Despite of this, two general concepts

regarding the development of cohesion in clays could be

identified in the literature. According to the first concept,

cohesion is due to the layer of adsorbed water surrounding

the clay particles, which can be considered as the inner

layer of the diffused double layer. Langmuir [16] presented

the evidence to show that the water held directly on the

surface of colloidal particles was in a physical state dif-

ferent from that of free water. Leonards [17] summarising

the properties of water pertinent to the clay–water system,

suggested that the force fields due to oriented water mol-

ecules in the vicinity of clay particles had a counter part in

the macroscopic clay behaviour. The works by Low,

Pickett and Lemcoe as quoted by Seed et al. [40] showed

that the viscosity of adsorbed water was somewhat higher

than that of free water. According to Rosenqvist [36], the

cohesion could be due to some kind of welding between the

quasi-crystalline water surrounding the soil particles. Ter-

zaghi as referred by Rosenqvist [37] suggested that the clay

Voi

d ra

tio

Voi

d ra

tio

NaCl Water

Replacement of pore fluid

NaCl

Water

8

7

6

5

4

3

2

1

0

8

7

6

5

4

3

2

1

0 101 102 103 104101 102 103 104

Effective Consolidation pressure, kN/m2 Effective Consolidation pressure, kN/m2

(a) (b) - - - - - Water

– – – – NaCl

Replacement of pore fluid

Fig. 29 Comparisons of consolidation and swelling for a specimen in water, a specimen in saturated NaCl solution and two specimens with

replacement of the pore fluid (Data source: [3])

Wat

er c

onte

nt, %

Effective Consolidation pressure, kN/m2

102 103

pH = 5 pH = 7pH = 9

55

50

45

40

35

30

Fig. 30 Effect pH on virgin consolidation curves for sedimented

specimens of sodium kaolinite (Data source: [30])

Indian Geotech J (October–December 2014) 44(4):371–399 391

123

properties were due to flaky particles surrounded by

adsorbed water and that the water molecules stuck to each

other and to the minerals because of their dipole moment.

Grim [5, 6] and Haefeli [7] also supported the concept of

attributing the cohesion to the water molecules stuck to

each other and to the minerals because of their dipole

moment. It is important to note Hvorsleve [10] who states,

while studying the component of shear strength of satu-

rated clays, that most cohesive soils possess an apparent

structural viscosity and that the corresponding strength

component may be called the ‘viscous component’.

CS

0.5 – 1.0 1.0 – 2.0 2.0 – 4.0 4.0 – 8.0

Pressure incrementkg/cm2

Dielectric constant Dielectric constant

(a) (b)

0.5 – 1.0 1.0 – 2.0 2.0 – 4.0 4.0 – 8.0

Pressure incrementkg/cm2

10-2

10-3

10-3

10-24.0-8.0 kg/cm2

2.0-4.0 kg/cm2

1.0-2.0 kg/cm2

0.5-1.0 kg/cm2 4.0-8.0 kg/cm2

1.0-2.0 kg/cm2

0.5-1.0 kg/cm2CS

2.0-4.0 kg/cm2

0 20 40 20 80 0 20 40 20 80

10-3

10-3

Fig. 31 Effect of dielectric constant on secondary compression coefficient Cs for: a black cotton soil; b sodium kaolinite (1 kg/cm2 = 98.1 kN/

m2) (Data Source: [59])

Shea

r st

ress

, kN

/m2

Normal stress, kN/m2

Kaolinite (sta�cally compacted and saturated)

AirHexaneHeptane Carbon tetrachlorideBenzeneEthyl acetateAcetone Ethyl alcohol Methyl alcohol Water

100

80

60

40

20

0 0 20 40 60 80 100

Fig. 32 Strength envelops for statically compacted and saturated

kaolinite (Data Source: [64])

AirHexaneHeptane Carbon tetrachlorideBenzeneEthyl acetateAcetone Ethyl alcohol Methyl alcohol Water

Montmorillonite(sta�cally compacted and saturated)

Shea

r st

ress

, kN

/m2

Normal stress, kN/m20 20 40 60 80 100

100

80

60

40

20

0

Fig. 33 Strength envelops for statically compacted and saturated

montmorillonite (Data Source: [64])

392 Indian Geotech J (October–December 2014) 44(4):371–399

123

The second concept is that the cohesion is due to the

manifestation of the net inter particle attractive forces in

the clay–electrolyte system. There is an over whelming

support to this concept in the literature ([14, 25, 36, 37, 81];

to name a few). Michaels [22] and Rosenqvist [37]

expressed their opinion that van der walls’ forces of

attraction were of a magnitude more than adequate to

account for cohesion in clays and that any contribution to

shear strength resulting from water viscosity was negligible

in comparison with the contribution of inter-particle

attractive forces. Many researchers have observed and

opined that the cohesion is due only to intrinsic forces (i.e.

net inter-particle attraction) and that it is purely frictional

in nature as given by Eq. 10 ([14, 25, 31, 36, 71, 73]; to

name a few). In summary, two concepts exist to explain as

to what constitutes the soil cohesion, one attributing the

cohesion to the viscosity of the double layer water a part of

which is the adsorbed water and the other, to the net inter-

particle attraction [54]. This difference of opinion can be

owed to generalising the complex soil behaviour without

considering the effect of clay mineralogy on soil properties

and behaviour. In the following, the validity of the above

concepts as applied to the undrained shear strength

behaviour of clays is examined in conjunction with the clay

mineralogical aspects.

Figure 35a, b represent the undrained shear strength–

equivalent water content–void ratio relationship for kao-

linite and Kundara clay, which is a kaolinitic soil, with

different pore fluids. The equivalent water content is

defined as the ratio of the percentage of fluid content by

weight to the specific gravity of the fluid [54]. A decrease

in the dielectric constant or an increase in the electrolyte

concentration of the pore medium causes a decrease in the

electrical repulsive force and an increase in the electrical

attractive force at the particle level. This in turn increases

the shear strength at the particle level. This is clear from

the results shown in Fig. 35a, b. At a given void ratio or

equivalent water content, the undrained shear strength of

Kundara clay with carbon tetra chloride and 0.5 N sodium

chloride are higher than that when the water is the pore

fluid. A similar trend has been observed with kaolin, which

exhibits higher undrained shear strengths with carbon tetra

chloride than with water at any given void ratio.

However, the montmorillonitic soils namely, bentonite

and black cotton soil exhibit quite opposite undrained shear

strength behaviour (Fig. 36a, b). With a decrease in the

dielectric constant and an increase in the electrolyte con-

centration, they show a decrease in the undrained shear

strengths.

Following mechanisms are proposed to explain the

contradictory undrained shear strength behaviour of kao-

linitic and montmorillonitic soils:

1. The undrained shear strength of kaolinitic soils is

mainly dependent on the net attractive force and the

mode of particle arrangement as determined by the

inter-particle forces. A decrease in the dielectric

constant or an increase in the electrolyte concentration

of the pore fluid or an increase in the valency of the

exchangeable cation increases the inter-particle attrac-

tive forces while reducing the repulsive forces. This

leads to an increase in the net attractive force in the

system [i.e. net (A–R)] and in turn in an increase in the

shear strength at the particle level, which favours the

development of more flocculent fabric. This gets

manifested in an increase in the undrained shear

strength. Modified effective stress concept (Eq. 10)

supports this behaviour.

2. The undrained shear strength of montmorillonitic soils

mainly arises from the viscous resistance generated by

the viscous diffuse double layer water to the shear

deformation. A decrease in the dielectric constant or an

increase in the electrolyte concentration of the pore

fluid or an increase in the valency of the exchangeable

cation suppresses the viscous diffuse double layer

thickness. Hence, the viscous shear resistance needed

to resist the shear deformation gets reduced and hence,

a reduction in the undrained shear strength. On the

other hand, an increase in the dielectric constant or a

decrease in the electrolyte concentration of the pore

fluid or a decrease in the valency of the exchangeable

cations promotes an increase in the diffuse double

layer thickness. Relatively higher viscosity of the

diffuse double layer water significantly contributes to

the viscous shear resistance and hence, an increase in

Shea

r st

ress

, (…

. x 6

.895

) kN

/m2

Normal stress, (…. x 6.895) kN/m2

Wyoming Bentonite (montmorillonite)

Kaolinite

20

15

10

5

0

20

15

10

5

0 0 10 20 30 40 50 60

Al

Ca

Na

Al

Ca

Na

Fig. 34 Shear resistance versus pressure relationships for homoion-

ised montmorillonites and kaolinites (Data Source: [6])

Indian Geotech J (October–December 2014) 44(4):371–399 393

123

the undrained shear strength. The undrained strength

behaviour of montmorillonitic soils cannot be

explained by the modified effective stress concept.

The validity of the proposed mechanisms is further

examined by analysing the undrained shear strength

behaviour of naturally available montmorillonitic and ka-

olinitic soils subjected to different chemical treatments.

Figure 37a, b illustrate the variation of undrained shear

strength with the moulding water content for black cotton

soil (montmorillonitic soil) and red earth (kaolinitic soil)

subjected to different chemical treatment, obtained from

vane shear test. Important observations made from these

figures are indicated below.

• The black cotton soil, on homo ionisation with higher

valency ions, gives lower undrained shear strengths

(S) at all water contents (i.e. Srep [ SCa [ SAl). Any

increase in the valency of exchangeable cation reduces

the diffused double layer thickness. This results in a

Water

CCl4

Soil: KaolinCurve Fluid

Und

rain

ed sh

ear

stre

ngth

, kPa

(a) (b)Void ratio Void ratio

Water 0.5 N NaCl CCl4

Soil: Kundara clayCurve Fluid

7

6

5

4

3

2

1

0

4

3

2

1

0

0 0.5 1.0 1.5 2.0 0 0.5 1.0 1.5 2.0

10 20 30 40 50 60 70 80 90 20 40 60 80Equivalent water content, % Equivalent water content, %

Und

rain

ed sh

ear

stre

ngth

, kPa

Fig. 35 Undrained shear strength–equivalent water content–void ratio relationship for kaolinitic soils (Data Source: [54])

Und

rain

ed sh

ear

stre

ngth

, kPa

(b)(a)

Water 0.5 N NaCl CCl4

Soil: B.C. Soil Curve Fluid

Equivalent water content, %Equivalent water content, %

Void ratio

Water 0.5 N NaCl CCl4

Soil: BentoniteCurve Fluid

0 2 4 6 8 10 12 14

100 200 300 400 500

10

8

6

4

2

0 20 40 60 80 100

10

8

6

4

2

0

Void ratio0 0.5 1.0 1.5 2.0 2.5

Und

rain

ed sh

ear

stre

ngth

, kPa

Fig. 36 Undrained shear strength–equivalent water content–void ratio relationship for montmorillonitic soils (Data Source: [54])

394 Indian Geotech J (October–December 2014) 44(4):371–399

123

reduction in the viscous shear strength and hence, in the

undrained shear strength of the montmorillonitic soil.

• The red earth, on homo ionisation with higher valency

ions, gives higher undrained shear strengths (S) at all

water contents (i.e. Srep \ SCa \ SAl). Any increase in

the exchangeable cationic valency favours an increase

in the level of flocculation, which in turn results in

higher undrained shear strength of kaolinitic soils.

Mention has already been made about the two concepts

regarding the development of cohesion in the fine-grained

soils. In the light of the results discussed above, it can be

said that the cohesion is due to net inter-particle attraction

in the case of kaolinitic soils and that it is primarily due to

the viscous resistance of the diffuse double layer held water

in the case of montmorillonitic soils.

Michaels [22] concluded that the net consequence of the

presence of water around the soil particles was a general

reduction in the particle adhesion. Seed et al. [40] stated

that the role of the water in clays was that of a filler sep-

arating the particles and resisting close approach and that a

lesser adhesive bond was formed than would exist if water

was removed from the clay. The very observation that, at

any level of equivalent water content, the undrained shear

strength is less with water than with liquids of higher

electrolyte concentration and with lower dielectric constant

as pore fluids for Kundara clay and kaolinite (Fig. 35a,

35b) indicates that such observations are valid for kaolin-

itic soils and not for montmorillonitic soils. On the other

hand, the observation that the montmorillonitic soils

namely bentonite and black cotton soil exhibit higher

undrained shear strength with water than with liquids of

higher electrolyte concentration and with lower dielectric

constant at any level of equivalent water content (Fig. 36a,

b) rule out the possibility of considering the double layer

water as just a filler material responsible for a reduction in

the strength. In fact, the viscous nature of the double layer

water adds to the undrained strength of such soils.

All the above discussions essentially mean that the

undrained shear strength of montmorillonitic soils is pri-

marily due to viscous shear resistance of the diffuse double

layer water and that of kaolinitic soils is primarily due to

the net attractive force and mode of particle arrangement as

dictated by the inter-particle forces. The nature and the

surface properties of clay particles of kaolinitic soils are

such that the extent of double layer formation is very

minimum and of negligible consequence from the point of

view of providing any viscous resistance component.

Hence, it is justifiable in stating that the cohesion is due to

inter-particle attraction, which results in an increased

flocculation and higher shear strength at the particle level

in the case of kaolinitic soils and that it can be attributed to

the viscous resistance of double layer water in the case of

montmorillonitic soils.

Permeability

Determination of permeability of clays is required in many

situations in Geotechnical engineering practice like prob-

lems related to waste containments. It is well known that

highly plastic clays like montmorillonite is affected sig-

nificantly by the pore fluid characteristics. It is very diffi-

cult to assess the relative importance of factors affecting

5

4

3

2

1

30 40 50 60 70

4

3

2

1

Und

rain

ed sh

ear

stre

ngth

, kPa

Und

rain

ed sh

ear

stre

ngth

, kPa

Soil: Red earthRepresentative Calcium HomoionizedAluminum HomoionizedSesquioxide extracted

Soil: Black cotton soilRepresentative Calcium HomoionizedAluminum HomoionizedSesquioxide extracted

30 50 70 90 110

Water content, % Water content, %

(a) (b)

Fig. 37 Variation of undrained shear strength with the moulding water content for a black cotton soil (montmorillonitic soil) b red earth

(kaolinitic soil) (Data Source: [43])

Indian Geotech J (October–December 2014) 44(4):371–399 395

123

the permeability since many of them are interdependent.

Rao and Sridharan [35] have highlighted the role of various

factors, essentially brought out by changes in the chemical

environment in influencing the permeability of montmo-

rillonites. Table 9 brings out the influence of exchangeable

cation type and pore fluid characteristics on coefficient of

permeability of montmorillonites.

Replacement of exchangeable sodium by calcium ions

resulted in almost tenfold increase in the coefficient of

permeability at a given void ratio. Earlier investigations

have shown that the diffuse double layer associated with

the particle surfaces is the cause of the decrease in the

permeability of montmorillonite by constricting the flow

channels, thus reducing the amount of effective pore space

of water flow by mobilising a dispersed clay fabric, which

increases the tortuocity factor and bringing a decrease in

the permeability [47].

Exchange of Monovalent sodium ion by calcium ion

leads to a marked reduction in the diffuse double layer

thickness, to an increase in the effective pore space for

water flow resulting in high permeability values.

Replacement of monovalent sodium ion by monovalent

pottasium ion results in almost five fold increase in the

permeability coefficient at a given void ratio. This brings

out the effect of cation size also on the permeability

coefficient of montmorillonite. This is attributed by

Sridharan et al. [67] to the higher adsorption of monovalent

pottasium ions in the Stern layer and partial fixation of

cation in the hexagonal oxygen holes in the surface of the

silicate layer, which leads to a substantial reduction in the

thickness of the diffuse double layer, in comparison to

sodium ions. This results in an increase in the effective

void space for water flow leading to higher coefficient of

permeability.

In comparison to the effect of exchangeable cations,

increase of pore salt concentration from 0.001 to 0.1 N of

sodium chloride leads to a marginal increase of 1.25 times

in the permeability coefficient of montmorillonite. This is

primarily due to the suppression of diffuse double layer

thickness brought out by higher pore salt concentration.

When compared to the effect of exchangeable cations

and pore salt concentration, the effect of dielectric constant

of the pore medium is several folds pronounced. For

example, at a given void ratio, ethanol (dielectric con-

stant = 25.0) and carbon tetra chloride (dielectric con-

stant = 2.24) as pore fluids exhibit increase in the

permeability coefficient from 104 to 106 cm/s. A decrease

in the dielectric constant of the pore fluid acts to contract

the thickness of the diffuse double layer and to flocculate

the clay fabric, creating larger macro pores responsible for

the dramatic increase of the permeability coefficient.

Sridharan and Choudhury [47] brought out the concept

of effective void ratio, which is the void ratio devoid of

equivalent diffuse double layer thickness. They calculated

the equivalent diffuse double layer thicknesses due to

various pore medium system based on the diffuse double

layer theory and determined the equivalent void ratio.

Their analysis resulted in a unique relationship between the

effective void ratio and permeability coefficient irrespec-

tive of the type of pore medium chemistry.

Conclusions

The variations and complexities observed in the engineer-

ing behaviour of fine-grained soils can be mainly attributed

to the soil clay mineralogical composition and pore fluid

constituents. Clay particles are characterised by their spe-

cific surface and surface charges on them leading to diffuse

double layer induced repulsive forces and van der Waals’

as well as Coulombic attractive forces. While the Gouy–

Chapman theory of electrical diffuse double layer enables a

qualitative prediction of the variation of repulsive pressure

(quantitative under specific conditions), the factors

Table 9 Effect of cation, cation size and pore fluid characteristics on coefficient of permeability of montmorillonites [35]

Pore medium chemistry Soil Void ratio (e) Ratio of coefficients of permeability

Valency effect Montmorillonite 2.50kCa2þ

kNaþ¼ 28

Bentonite 2.00kCa2þ

kNaþ¼ 08

2.00kFe3þ

kNaþ¼ 33

Cation size effect Bentonite 2.00kKþ

kNaþ¼ 05

Electrolyte concentration Montmorillonite 2.00k0:1NNaCl

k0:001NNaCl

¼ 1:25

Organic Solvent Montmorillonite 2.00kEthylAlcohol

kWater

¼ 104

396 Indian Geotech J (October–December 2014) 44(4):371–399

123

affecting the electrical attractive forces in clay particles are

complex, and their individual effects cannot be readily

differentiated. The nature of the inter-particle contact is

also not well understood.

Quite a good number of experimental evidences are

documented in the geotechnical engineering literature to

show that the compressibility and undrained shear strength

behaviour of kaolinitic soils are mainly controlled by the

net attractive force and the mode of particle arrangement as

determined by the inter-particle electrical forces. Com-

pressibility and undrained shear strength behaviour of

montmorillonitic soils are primarily due to diffuse double

layer repulsion and the viscous shear resistance generated

by the viscous diffuse double-layer water to the shear

deformation.

Hence, any factor responsible for an increase in the

attractive forces and in the extent of flocculation results in

higher un-drained shear strength and lesser compressibility

in kaolinitic soils. Any factor, which promotes the expan-

sion of the diffuse double layer thickness is responsible for

the higher un-drained shear strength and lesser compress-

ibility in montmorillonitic soils.

Similarly, the liquid limit and sediment volume behav-

iour of kaolinitic soils can be attributed mainly to inter-

particle attractive forces and those of montmorillonitic

soils to diffuse double layer related factors.

Thus, the liquid limit, sediment volume, undrained shear

strength and compressibility behavior of kaolinitic and

montmorillonitic clayey soils are quite opposite to changes

in pore medium chemistry.

The hydraulic conductivity of fine-grained soils is sig-

nificantly affected by pore medium chemistry, especially so

in montmorillonitic soils.

The drained strength and secondary compression coef-

ficient of both kaolinitic and montmorillonitic fine-grained

soils are primarily controlled by the modified effective

stress (i.e., net contact stress at particle level).

Natural clays consist of different clay minerals in dif-

ferent proportions. The pore medium constituents are also

not simple and well defined. However, the soil clay min-

eralogy and the dominant controlling mechanism can be

identified by simple laboratory tests such as Free Swell

Ratio test, and the behaviour of natural clays can be

qualitatively predicted.

In view of this discussion, the need to classify the fine-

grained soils as montmorillonitic or kaolinitic types

deserves serious consideration.

Acknowledgments The author started his academic career more

than five decades ago under the guidance of Prof. N.S. Govinda Rao,

the then chairman of the department of civil engineering, Indian

Institute of Science, Bengaluru, who could be considered as the father

of Civil Engineering Research in India. The author is highly grateful

to Prof. N.S. Govinda Rao for the motivation and whole hearted

timely support. The research atmosphere and the freedom of carrying

out independent research the author enjoyed at IISc cannot be

described in simple words. The author is thankful to all his former

Ph.D., M.Sc (Engg.), and M.E Students whose significant contribu-

tions to the field of geotechnical engineering have made this lecture

possible. The author is thankful to all his professional colleagues in

India and abroad for their contribution towards this work he received

during his research association with them. The author wishes to place

on record his appreciations to Prof. K. Prakash, Head of the depart-

ment of Civil Engineering, Sri Jayachamarajendra College of Engi-

neering, Mysore, for his continuous help during the preparation of this

paper, in offering critical comments wherever needed and in

reviewing the final version of the manuscript. His very useful com-

ments have helped a great deal in bringing out the paper in the present

form. The contribution of Mr. H.V. Naveen Kumar in formatting the

paper is highly appreciated.

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A. Sridharan Ph.D. (Purdue)

and D.Sc. (IISc) had a dis-

tinguished career at the Indian

Institute of Science for more

than four decades (as Professor

of Civil Engineering, Chairman

of department of Civil engi-

neering, Divisional Chairman of

Mechanical Sciences, Deputy

Director, and Advisor, con-

tributing significantly in all the

areas of activity viz., research,

teaching, consultancy and

administration. Presently he is

INSA Honorary Scientist. Prof

Sridharan is a Fellow of the top three academies, Indian National

Academy of Sciences, Indian National Science Academy and Indian

national Academy of Engineering (FASc, FNA & FNAE). He is also

Honorary Fellow of the Indian Geotechnical Society for which he was

also the President (1996–1998). He was also an Humboldt Fellow

during 1975–77 at University of Karlsruhe, Germany. He has been

visiting Professor in Japan, Germany, Turkey and Cyprus. As part of

his research activity, his contributions to the fundamental under-

standing of the Engineering behavior of clays have been well

acknowledged internationally. He has also contributed significantly in

the areas Soil Dynamics and Reinforced Earth Structures. He has

published more than 350 papers out of which 160 are in reputed

international journals. He has guided 35 candidates for their PhDs. He

has delivered IGS annual lecture in 1990. He was awarded the

Kuecklemann award of IGS in 1992. He was presented with The

Prasanta Chandra Mahalanobis Medal by the Indian National Science

Academy in 2006 and IGSFERROCO TERZAGHI ORATION in

2014. Prof Sridharan has presented several invited and key note

lectures in International Conferences and seminars. Prof Sridharan

has been an active Consultant in his specialisation having handled

more than 240 projects. (The Indian Consulting Civil Engineering

Association has honoured him with ACCE Gourav Award as best

Consultant in 2002). In view of his significant contributions to

research and Technical education Purdue University, Indiana, USA,

presented him the Distinguished Engineering Alumnus award in

1995. The Indian Institute of Science has conferred on him the Dis-

tinguished Alumni award in 2002.

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