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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])
Indian Geotech J (October–December 2014) 44(4):371–399 373
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])
Indian Geotech J (October–December 2014) 44(4):371–399 375
123
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])
376 Indian Geotech J (October–December 2014) 44(4):371–399
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)
Indian Geotech J (October–December 2014) 44(4):371–399 381
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]
Indian Geotech J (October–December 2014) 44(4):371–399 387
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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