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Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Clay particle-water interaction
&
Index properties
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Electrical nature of clay particles
a) Electrical charges
- - - - - -- - - i) The two faces of all platy particles
+
- - - have a negative charge.
+
+ - - +
 Resulting due to isomorphous
+ +
substitution that is not neutralized
by interlayer cation bonding
ii) The edge of clay particles usually have a
positive charge at low to moderate pH;
Increasing pH lead to a negative charge.
iii) The Net charge of clay particles is
always negative.
Eg.
Formation
of
bentonite
cake
around peripheries of
borehole
(electrophoresis)
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Electrical nature of clay particles
b) Exchangeable Ions
Since the soil must be electrically neutral
The negative faces attract
cations (Na+, Ca++, Mg++, etc.)
exchangeable
Postive edges attract exchangeable anions (or
cations if negatively charged)
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Electrical nature of clay particles
Surface charge density (σo) – Number of
charges per unit area
σo = CEC/SSA
Clay
CEC
SSA(m2/g) σo (C/m2)
Mineral
(meg/100g)
Kaolinite
5
15
0.32
Illite
28
84
0.29
Na Mont.
100
800
0.18
Meg = milligramequivalents
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Structure of clay soils (Fine-grained soils)
Forces between clay mineral particles
-
If two particles (platelet shape) approach each
other in a suspension, the forces acting on them
are:
a) The Van der Waals forces
of attraction, and
a) The repulsion between the two
+vely charged ionized absorbed
Adsorbed
water layers
water layer
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Structure of clay soils (Fine-grained soils)
At very small separations, the Van der Waals
forces are always larger, and particles which
approach sufficiently closely will adhere.
However, the Van der Waals forces decrease
rapidly with increasing separation.
 If the adsorbed layer is thick,
the repulsion will be large at
distances from the surface at
which the Van der Waals forces
Net Repulsion
are small.
Particles will remain Dispersed
(settle independently)
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Structure of clay soils (Fine-grained soils)
 Contact between dispersed particles will only be
established if an external force is applied which is
large enough to overcome net repulsive force.
 If the adsorbed layer is thin,
there will be little or no net
repulsion at any distance, and
random movements of particles
will be enough to bring them into
contact. This process is called
Attraction (net)
Flocculation.
Groups of particles settle together
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Net force between two particles in a suspension
Low ion
concentration in the
soil water
Net
repulsion
Net
attraction
High ion
concentration in the
soil water
Distance
between crystal
faces
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Dispersed Structure of clay soils
-The net forces of repulsion are greatest in the
case of particles approaching face to face.
Lacustrine clays
(deposited in fresh
water lakes) generally
have a dispersed
structure. In this case,
few of the particles
are in direct contact,
most being separated
by the adsorbed
water layers.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Flocculated Structure of clay soils
-Marine clays (deposited in sea water in which ion
concentration is high, so that the adsorbed water
layers are thin) generally have flocculated
structure.
Void space
Flocculated structure
has open structure
with large void
spaces with particles
attached to each
other with edge to
edge and edge to
face contacts.

Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Structure of clay soils (Fine-grained soils)
Typical arrangement of platelet particles
a) Dispersed
Low ion concentration
b) Flocculated
High ion concentration
(PH < 7)
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Structure of clay soils (Fine-grained soils)
A clay with an undisturbed flocculated structure
will possess larger void openings.
Silt/sand size
particle
Un-disturbed flocculent structure of Marine clay
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Structure of clay soils (Fine-grained soils)
When platelet particles are carried into fresh
water lake, they do not flocculate and settle
along with silt particles as they do in salt water.
 Remolding of
flocculated
structure results
in dispersed
structure
Remolded or dispersed structure of Fresh water deposit
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Clay structures
Natural clay
a)Dispersed
b) Flocculated
c) Bookhouse
d ) Turbostratic
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Methods to identify soil structure
a) Ordinary microscope
(valid for coarse grained soils only)
Scanning Electron Microscopy (SEM)
This is ideally suited for clayey soils, as the resolution
is sufficiently high and hence it is possible to go for
higher magnifications ( = 1 x 105 times)
A SEM is a type of electron microscope that images a sample by
scanning it with a beam of electrons in a raster scan pattern. The
electrons interact with the atoms that make up the sample producing
signals that contain information about the sample's surface
topography, composition, and other properties such as electrical
conductivity.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
SEM of coal ash blended with Na Bentonite
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
SEM of locally available silt
a)
b) 500x
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Identification of clay minerals
 No one method is satisfactory for identification. This
is partly because (i) Interference of minerals in a
mixture and (ii) range of composition and crystal
structure of clays from different sources.
Three methods:
i) X-Ray Diffraction (XRD)
ii) Differential Thermal Analysis (DTA)
iii) Casagrande’s Plasticity Chart
 Casagradne’s Plasticity chart method will be
discussed later.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
XRD Method
The most widely used method of
identification of clay minerals is from an
X-Ray Diffraction pattern of a powdered
sample of the clay size fractions of a
soil. Minerals can usually be identified
from diffraction lines.
Principle: Minerals with regular or
repeating patterns of crystal structure
diffract x-rays.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Schematic diagram of X-ray diffraction unit
for crystal identification
Geiger-MÜller
counting tube
2θ = Angle of
the counter
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Typical XRD pattern of Kaolinite,
Montmorillonite and Illite
Degrees, 2θ 
Intensity of
reflection 
 Different minerals with
different crystalline structures
will have different x-ray
diffraction patterns, and in
fact these different patterns
help to identify different
minerals.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Typical XRD pattern of locally available expansive soil
Q
Mt-Montmorillonite(48-50%)
Mt
Q-Quartz(30-32%)
C
Mt
C-Calcite(15-16%)
A-Anatase(1-2%)
Q
A
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
X-Ray Diffraction spectra for bentonite: Cu-kα radiation
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
XRD Method
Demerits:
(i) Not suitable for soils with mixtures of
clay minerals, organic matter and nonclay mineral constituents and
(ii) Inability to specify proportions of
each mineral in a mixture.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
DTA Method
 Differential Thermal Analysis determines the
temperature at which changes occur in a mineral
when it is heated continuously to a higher
temperature.
-The intensity of change is proportional to the amount
of the mineral present.
-Clays lose water or go through phase changes at
specific temperatures.
-The temperatures at which these reactions occur are
characteristic of the mineral and can, therefore, be
used for identification.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
DTA apparatus with associated
recording and control mechanisms
For the DTA measurement,
a sample of clay and a
sample of inert material are
slowly heated in a furnace.
 Calcined Aluminium
Oxide or Ceramic are used
as the reference material.
 When a temp. is reached at which the clay looses
water by vaporization, the sample temp. will drop
below that of inert material.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Review
1. Particulate arrangement in coarsegrained soils and fine-grained soils
2. Loose and Dense sand deposits –
Relative Density
3. Forces between clay mineral particles
4. Dispersed and Flocculent structures
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Index Properties
Index Properties refers to those properties
of a soil that indicate the type and condition
of the soil, and provide a relationship to
structural properties such as the strength
and the compressibility or tendency for
swelling, and permeability.
Can be divided into two categories, namely
-Soil grain properties
-Soil aggregate properties
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Index Properties
The development of the ability to think of
soils in terms of numerical values of their
index properties should be one of the
foremost aims of every engineer who deals
with Soil Mechanics.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Soil Grain Properties
The soil grain properties are the properties
of the individual particles of which the soil is
composed, without reference to the
manner in which these particles are
arranged in a soil mass.
For e.g.,
Mineralogical Composition, Specific Gravity
of Solids, Size and Shape of Grains.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Soil Aggregate Properties
Which are dependent on the soil mass as
a whole and thus, represent the collective
behaviour of soil.
Soil aggregate properties = f (Stress history,
mode of soil formation and the soil
structure)
Aggregate refers to soil itself- It may differ in
Porosity, Relative Density, Water and Air Content
and Consistency.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Soil Aggregate Properties
Although soil grain properties are
commonly used for identification purposes,
the engineer should realize that the soil
aggregate properties have a greater
influence on the engineering behaviour of a
soil
Because: Engineering structures are
founded on natural deposits or undisturbed
soil mass.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
INDEX PROPERTIES
Grain Size Distribution
Mechanical
Analysis
(Coarse Grained Soil)
-Dry Method
- Wet Method
Consistency Limits
Hydrometer
Analysis
(Fine Grained Soil)
-Liquid Limit
-Plastic Limit
-Plasticity Index
-Shrinkage Limit
(Fine Grained Soil)
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Grain Size Distribution (GSD)
-In Soil Mechanics, it is virtually always
useful to quantify the size of grains in a type
of soil.
-Since a given soil will often be made up of
grains of many different sizes, sizes are
measured in terms of grain size
distributions.
- GSD assists in providing rough estimates of soil
engineering properties.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Grain Size Distribution (GSD)
 A subject of active research interest
today is the accurate prediction of soil
properties based largely on GSDs, void
ratio, and soil particle characteristics.
When measuring GSDs for soils, two methods are
generally used:
For grains larger than 0.075 mm sieving is used.
For grains in the range of 0.075 mm > d > 0.5µm,
the hydrometer test is used.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Sieve Analysis (For coarse-grained soil with d > 0.075 mm)
opening size
Large Sieves –
by their
opening size
#8 per inch – 64
per sq. inch
Size < 1/8 inch
(width of the wire)
Particle
(grain) size d
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Typical Grain Size Distribution Curves
Idealized Fuller Packing
Fuller Theoretical
Curve
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Sedimentation Analysis –
(for fine-grained soils: 0.5 µm < d < 75µm)
-It is assumed, as a first approximation, that finegrained soil particles can be idealized as small
spheres.
-Spherical particle falling in a liquid of infinite extent
and all particles have the same unit weight.
-Particles reach constant terminal velocity within a
few seconds after it is allowed to fall.
Although clay particles are far from spherical, the
application of Stroke’s law based on equivalent diameters
provide a basis for arriving at GSD of fine-grained soils
(Sufficiently Realistic).
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Sedimentation Analysis
According to Stroke’s Law, the viscous drag force
FD on a spherical body moving through a laminar
fluid at a steady velocity v is given by
FD = 3πµvd
Where µ is the viscosity of the fluid
v is the steady velocity of the body
d is the diameter of the sphere
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Sedimentation Analysis
If we drop a grain of soil into a viscous fluid, it
eventually achieves a terminal velocity v where
there is a balance of forces between viscous
drag forces, gravity weight forces, and buoyant
forces, as shown below:
Fg-Fb = (1/6) (Gs-1)γwπd3
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Sedimentation Analysis
For equilibrium of the soil grain: FD = Fg – Fb
From this equation, we solve for the equilibrium or
terminal velocity v of the soil grain as:
Observe: v ∝ d2
 (Gs − 1)γ w  2
v=
d

 18µ 
 Strokes law
After Sir George
Strokes (1891)
Thus, the larger a soil grain is, faster it settles in
water. This critical fact is used in the hydrometer
analysis to obtain GSDs for fine-grained soil.
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Process of Sedimentation of
Dispersed Specimen

Theory of sedimentation is based on the fact
that the large particles in suspension in a liquid
settle more quickly than small particles, assuming
that all particles have similar densities and shapes.
 If all the particles were of a single size, with
effective diameter d, by knowing terminal
velocity v, we can calculate td:
The velocity which a
falling particle reaches is
known as terminal
velocity


18µz
td = 
2
 (Gs − 1)γ w d 
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay
Limitations of Stroke’s law
-Soil particles are not truly spherical and
sedimentation is done in a jar (For d > 0.2 mm
causes turbulence in water and d < 0.0002 mm
Brownian movement occurs (too small velocities of
settlement) --- Can be eliminated with less
concentrations.
-Floc formation due to inadequate dispersion
-Unequal Sp.Gr of all particles (insignificant for soil
particles with fine fraction)
Prof. B V S Viswanadham, Department of Civil Engineering, IIT Bombay