BLACK COTTON SOILS OF INDIA
A review of engineering properties
and Construction Techniques
U.G. Project report submitted by
A.M. Patankar, D.M. Mukewar and S.L. Khankhoje
Final Year B.E .Students of
Vishveshvarayya Regional College of Engineering Nagpur
Under the guidance of
Dr. A.S. Nene
A Civil Engineer has often to face some problematic soil such as expansive
soils. Expansive soils of Central India, commonly known as Black Cotton
soils, cover approximately one-sixth of the total area of our country. Such
soils exhibit extreme stages of consistency from very hard to very soft when
Literature on Black Cotton soils dates back to thousands of years ago. Sage
Bhrugu in his scripture “Bhrugu Samhita” has classified all soils into four
groups based on their color, taste, odor, sound and their performance.
Six senses of perception: A site is to be selcted by using five senses of
perception for its color,smell, shape, sound and touch.
Soil Classification based on Color: The soil has four basic colors, white,red,
yellow or black. The site with black soil should be rejected for construction.
Classification based on Smell: The soil having smell of rotten fish should be
rejected for construction.
Classification based on Shape: shape of plot can be square, rectangular,
hexagonal, octagonal or circular, but a square plot is most suitable.
Classification based on Taste: The taste of soil can be sweet, sour, bitter.
The site with soil of sweet taste is most suitable.
Classification based on Sound: The ground when tamped with wooden
rammer produces different sounds such as that produced by horse, flute,
veena or drum. The ground which produces ringing sound should be
Classification based on Touch: The ideal site is one which is cold in summer and
warm in winter.
According to Sage Bhrugu, Soils, white in color, smelling like that of clarified
butter and of good taste is the best. Soils black in color, smelling like blood
and of sour taste is the worst.
World’s First Reference describing expansive soils: Bhrugu also mentioned that
marshy land, cracking when exposed to sun rays, made porous by wind or
insects, devoid of water, full of poisonous or thorny trees, used as cemetery,
sloping towards south or land of saline soil was worst for construction
purposes. In other words the sage has described the properties of expansive
Around 1950 the subject of expansive soils attracted attention of scientists
and engineers. Since then innumerable of technical papers are published.
This subject is also attaining more and more importance in our country.
Many institutes of higher education have introduced this subject in their
Though the references on this subject are many, there is no single text book
which presents update information on this subject. With this background it
was thought of compiling the vast information and presenting in a report
Mr. A.M. Patankar, D.M. Mukewar and S.L. Khankhoje have made an attempt
to review the technical literature and append with information from bulletins
and Indian standards.
Apart from partial fulfillment of the requirements for the degree of Bachelor
of Civil Engineering of Nagpur University, if this report can arose some
interest in the subject of expansive soils, the purpose of this edited review
report, will be more than fulfilled.
May 1975 (Dr. A.S. Nene)
About this E-Book of 2015
“Diabetics cannot be cured, it can be only controlled”. Similarly
problems posed by expansive soils can only be controlled by proper design
This project report was compiled in 1975 when no single reference book
was available for undergraduate students on the subject of swelling soils. No
computer or Internet facilities were available to student. Illustrations were
prepared on tracing sheets and project report was typed using manual
typewriter. But after 1980 the subject of “Expansive soils” was introduced in
the postgraduate curriculum. Now hundreds of reference papers are
available on Net and many text books are available on the subject of
Though the report was compiled 40 years ago, part of the information may
be still useful for undergraduate students of Civil engineering. With this hope
this project report is uploaded on Web.
1st May 2015 (Dr. A.S. Nene)
Chapter Title Page No
Prologue by the guide
1 Introduction 1
2 Identification and Classification 6
3 Engineering Properties of Expansive Soils 22
4 Construction Techniques 34
5 Under-reamed Pile foundations 44
6 Stabilization of Expansive Soils 47
7 Conclusions and Suggestions 62
LIST OF TABLES
No. Particulars Page
1.1 Morphology of a typical medium black soil
2.1 Swelling potential of soil 09
2.2 Identification criteria by U.S.B.R. 09
2.3 Characteristics of the B.C. soils 13
2.4 classification of swelling soils based on S.P. 17
2.5 Classification based on Shrinkage Index 19
2.6 Swelling Index Vs Plasticity Number 20
3.1 Locations of 16 soil samples 24
3.2 Notations used in tables 25
3.3 Properties of Black cotton soils S1-S8 26
3.4 Properties of Black cotton soils S9-S16 26
3.5 Ad.Properties of Black cotton soils S1-S8 27
3.6 Ad.Properties of Black cotton soils S9-S16 27
6.1 Permeability studies on stabilized soils
6.2 Permeability studies on stabilized soils (Nasik) 59
6.3 C .B. R. Test Value @ 5 mm Penetration 61
LIST OF FIGURES
No. Particulars Page
1.1 Extent of Swelling soils of India 01
1.2 failure of canal lining 02
1.3 Toe failure due to swelling soil 02
1.4 Cracking due to lifting of floor slab or
1.5 Damages to light weight building 03
2.1 Differential free swell test (DFS test) 08
2.2 Load expansion Curve 11
2.3 Typical dehydration curve for B.C. soil 12
2.4 Thermographs of clay minerals 13
2.5 Parameter for different n and CF 19
2.6 Shrinkage index Vs clay fraction 20
3.1 Site map of samples tested 24
3.2 Constant Pressure Method 28
3.3 Constant Volume method 29
3.4 Pressure Vs Volume Change curve 30
4.1 The pier and belled footing 37
4.2 Structural floor system 38
4.3 Flexible waterproof apron 42
5.1 Construction Stages 45
5.2 Measurement of bulb 45
5.3 Details of under-reamed pile 45
5.4 Boring in progress 46
5.5 Pullout of hand auger 46
5.6 Reinforcement details 46
5.7 Standard dimensions 46
In India the expansive soils cover approximately 20 percent of the total land
area. These expansive soils are known by various local names such as Black
cotton soils or Regur.
An attempt has been made to compile information from various text books,
technical papers, bulletins and codes of practices.
Chapter II describes identification and classification of expansive soils. In
addition to simple tests some specialized tests such as Differential thermal
analysis (DTA) are discussed. Classification systems suggested by various
agencies are also included in this chapter.
Chapter III describes the physical and engineering properties of expansive
soils. Various theories of swelling, measurement techniques and factors
affecting swelling -shrinkage of soils are also described briefly.
Chapter IV describes various construction techniques for sub-structures in
expansive soils. Remedial measures for damaged structures are also
Chapter V deals with under-reamed pile foundations in details.
Various stabilization methods for pavements on expansive soils are
discussed in chapter VI, Inorganic additives such as Lime, Cement fly-ash
and also organic additives for sub-grade stabilization are discussed in this
Based on the limited review of the available literature on expansive soils,
suggestions for further studies are made.
1.0 The definition of expansive soil may be stated as follows. “Expansive
soils are those soils which swell considerably on absorption of water and
shrink on removal of water. The expansive soil has considerable strength in
dry state, but the strength goes on reducing on absorption of water. The soil
exerts considerable pressure on foundations during swelling.
1.1 Expansive soils are found in some regions of India and many other
countries. These soils pose major foundation problems, causing damage to
the super structure if proper precautions have not been taken.
Fig.1.1-Extent of Swelling soils of India
The expansive soils, with their expanding lattice structure and resulting
capacity for wide ranges in water contents, can be particularly troublesome.
Settlement due to shrinkage and heave due to swelling causes structural
instability. This problem is magnified in hydraulic structures.
The amount of volume change in expansive soil is related to initial dry
density and water content, amount of clay fraction and type of clay minerals.
Fig.1.2 shows failure of concrete canal lining due to swelling of soil.
Fig.1.2 -failure of canal lining due to swelling of soil
Fig.1.3 shows a typical bank failure caused by deep shrinkage cracks at the
top of the slope and loss of the strength at the slope toe from expansion
under light loading with resulting increased water content.
Fig.1.3- Toe failure due to swelling soil
Such heave and stability failures are not limited to hydraulic structures
alone. For instance highway pavements and building footings may displace
by seasonal or other moisture changes due to desiccation by tree roots.
Radhakrishna, S. (41) has suggested that the presence of tree adjacent to a
foundation located in clay soil subjects the foundation to undue stresses due
absorption of subsoil moisture, resulting in shrinkage of the soil underneath
the foundation. Many houses and other lightly buildings have been literally
torn apart by sub soil volume changes. Cracking of a wall by uplift of the
expanding clay is shown in Fig.1.4.
Fig.1.4-Cracking due to lifting of floor slab or partition wall
Fig. 1.5 –Damages to light weight building
A type of damage common to light weight buildings on shallow continuous
foundation is caused by tilting of footings and walls. The tilting is caused by
the clay under the inside edge of the footing gaining moisture and expands
while the clay under the exterior edge remains dry and compressed. This
tilting is sometimes aided, and sometimes caused by lateral swelling of
compartmented clay fill. This tilting of the footing is shown in figure 1.5.
1.2 Soils are originated from rock due physical and chemical disintegration
processes and deposited due to wind, ice, gravity and water.
The black cotton soils are grouped under tropical black earths of the great
soil group of the generic classification. The heavier black soils are called
black cotton soils because of their suitability to grow cotton. The black color
is variously assigned to the presence of humus, organic iron and aluminum
compounds etc. Locally these soils are also known as Ragur soils. These soils
cover the Deccan plateau covering entire Maharashtra state, South Gujarat,
central and western Madhya Pradesh, Southern part of Andhra and Orissa
states. Black soils also occur in a smaller area of Rajasthan, Uttar Pradesh
and Tamilnadu. In western half of the Deccan plateau the black soils rests on
trap or Basalt rock, while in the eastern part these soils rest on granite of
The Deccan Plateau is an undulating country with hills and dales. Accordingly
depending upon the situation along the slopes, the black soils are shallow,
medium or deep. They are brown chestnut and black in color, light, medium
or heavy in texture respectively. Along the slopes of Ghats , the soils are
coarse and gravelly. In the bases of hills and along the river valleys, the
black soils are often 20 ft deep.
The shallow black soils are light black in color, coarse in texture and often
eroded. These are usually of low fertility. The deep and heavy black soils are
highly clayey and unworkable during rainy season. The clayey soils in the
lower layer do not admit any drainage and hence the very deep black soils
are unfit for irrigation. They are workable during monsoon are therefore,
mostly used for rabbi crops only. The medium black soils are only 1.5 to 3
feet deep and are rich in lime and lime nodules. The subsoil and partially
disintegrated rock below, allow easy drainage because these medium black
soils are highly retentive of moisture and swell during rainy season. In hot
weather these shrink heavily and develop numerous cracks which may be
several feet deep. With advent of rains, the loose top soil fills up these
Black soils are usually deficient in nitrogen, organic matter and in many
places, of phosphoric acid also. These are rich in lime while potash content
varies widely. Their clay mineral consists of Montmorillonite type. In general
black soils are considered more fertile than any other Indian soils.
Owing to the undulating nature of undulating nature of Deccan plateau, the
black soils show considerable variation in morphology of their profiles.
Topography, rain fall and drainage seem to play an important role in soil
formation. In general, black soil profiles possesses approximately all the
three horizons, A, B and C. The A horizon can be divided into the darker A-1,
rich in organic matter and A-2 which is lighter in color. The deeper black
soils are highly clayey and top layer may extend to several feet. The
transition from A to B is gradual. The B horizon is alluvial horizon rich in
lime. Both calcium carbonate and calcium sulphate are found. The
morphology of a typical medium black soil is given below.
Table -1.1- Morphology of a typical medium black soil
No Depth Description
A1 0-30 cm Black, homogeneous, granular, porous, clay
loam, low in lime, plenty of cracks in
A2 15 - 50 cm Lighter black, homogeneous, granular, less
porous, clayey, few lime nodules, cracks
extend to this layer.
B 30 - 100 cm Grey black , gradual transition,
heterogeneous, slightly cloddy and
compact, clayey with plenty of lime nodules
C 50 - 100 cm Brownish, sharp transition, heterogeneous,
mottled, porous, partially disintegrated
In the heavier black soils called Regur, the A and B horizons may extend up
to 2-3 m. These are highly clayey and difficult to work.
1.3 The existence of expansive soils and the problems associated with such
soils present worldwide is discussed in the next chapter.
2-IDENTIFICATION & CLASSIFICATION
2.0 The expansivity or the capacity of a soil to swell depends upon the type,
amount of clay minerals and exchangeable bases. There are three major
mineral groups viz, Montmorillonite, Illite and Kaolinite. For the identification
of expansive soil different field and laboratory method are available. The
expansive soils in field can be identified by the cracking pattern of the soil in
summer. The laboratory identification tests can be grouped under a) simple
tests and b0 specialized tests. The test procedures of these tests are
2.1 Simple Laboratory Tests
2.1.1 Free swell test: This test is performed by slowly pouring 10 c.c. of
oven dry soil passing 425 micron sieve, in a graduated 100 ml cylinder filled
with distilled water. The volume of settled and swelled soil is read after 24
hours from the graduations of the cylinder. The percentage of free swell Sf is
Sf = (Vf-Vi) x 100/Vi %
Where Vf and Vi are final and initial volumes respectively.
2.1.2 Shukla, K.P.(ref.1) suggested an alternative method for determining
free swell value, which eliminates the probable errors due to initial
placement of dry soil in the graduated cylinder. In this method an oven dried
soil passing 425 micron sieve is weighed and placed in the sintered funnel.
The soil is first allowed to absorb Benzene from the micro pipette attached to
the lower end of the funnel. Next it is allowed to absorb distilled water in
place of benzene. The difference between the respective volumes are water
and benzene absorbed represents the swelling which may be expressed as a
percentage of the initial weight of soil. The results obtained are independent
of pore volume because the absorbed benzene measures pore volume and
the water measures absorption required to fill the pore volume and cause
2.1.3 Indian standard code of practice (I.S.2911-Part III, 1973 Appendix A)
has modified the free swell test and the modified test is known as
Differential free swell test (DFS test). In this method two samples of oven
dried soil passing 425 micron sieve and weighing 10 gm each are used. One
sample is poured slowly in 50 ml graduated glass cylinder filled with
kerosene ( a non-polar liquid). The other sample is poured in another 50 ml
graduated cylinder filled with distilled water. Both the cylinders are left for
24 hours and the respective volumes are noted. The DFS is calculated as
Fig.2.1-Differential free swell test (DFS test)
Sf = (Vw-Vk) x 100/Vk %
where Vw and Vk are final volumes of
Soil in water and kerosene respectively.
The degree of expansiveness of soil and consequent damage to the structure
with light loading may be qualitatively judged as described below.
Table 2.1- Swelling potential of soil
D.F.S. value Degree of expansiveness
< 20 % low
20-35 % Moderate
35-50 % High
>50 % Very high
However the above test cannot be considered realistic as drying may change
the soil characteristics considerably.
2.1.4 Colloid content, plasticity index and shrinkage limit
The colloid content of soil is fraction finer than 0.001 mm to be determined
from sedimentation analysis (Hydrometer or pipette method), and is the
most active part of any soil, causing swelling. The expansiveness is
proportional to colloid content present in soil. The high plasticity index (PI) is
indicative of the capacity of soil to absorb higher amount of water when
changing from plastic to liquid state. A low value of shrinkage limit (SL)
indicates the soil will start swelling at low water content. Thus all the three
Index properties are indicative of potential volume change. United States
Bureau of Reclamation (USBR) has proposed identification criteria as
mentioned in table 1.3 below.
Table 2.2- Identification criteria by U.S.B.R.
Index (PI) %
<15 <18 <15 <10 Low
15 -23 10-16 10-16 10-20 Medium
20- 31 25-41 7-12 20-30 High
>28 >35 >11 >30 Very high
# Probable expansion represents the percentage of total volume
change of soil from dry to saturated condition under a surcharge of
0.07 kg/sq.cm. (1 psi).
Recent studies indicate that the plasticity index of a soil alone can be used to
have an assessment of the capability of the soil for swelling accurate enough
for practical purposes.
2.1.5. Load Expansion Test
The purpose of this test is to measure total volume change from natural or
remolded condition to the air dried and saturated conditions respectively.
Two identical specimens (undisturbed or remolded) at desired density and
water content, are taken in the ring of “fixed ring type consolidometer”. The
specimen are allowed to dry in air to at least the shrinkage limit. Volume of
one specimen is measured by immersion in mercury. The other specimen is
loaded in consolidometer to a pressure intensity equivalent to that due to
the anticipated structural load and the specimen is saturated. The change in
volume is recorded.
2.1.6 Dehydration Test (Ref. 31)
The test consists of recording the percentage loss in weight of clay upon
heating to higher and higher temperatures and plotting volume vs
temperature. Heating is continued till there is no loss in weight occurs. The
position of the flexural point in temperature vs loss of weight curve gives an
indication of the type of mineral percent. Ref. fig.2.1.
Fig.2.2-Load expansion Curve
2.2. Specialized Tests
2.2.1 Differential Thermal analysis (DTA): Since the presence of certain clay
minerals is important to the engineering analysis of clayey soils,
identification of such minerals is necessary to facilitate the engineering test
When a material, such as soil, is heated chemical reaction take place at
different temperatures depending upon characteristics of mineral present.
These reactions may be due to structural or phase change or loss of water
content during heating process. The chemical reactions may be endothermic
2.2.2 X -Ray Diffraction
The absorption, reflection and scattering of electromagnetic radiation may be
employed to yield information on the size of particles whose smallest size or
spacing is greater than the wave length of radiation. The light rays whose
wave length is in the range of 0.3 to 0.9 micron can be used to measure the
size of and spacing of suspended particles with sizes varying from 1 to 10
Fig.2.3-Typical dehydration curve for B.C. soil
Since the spacing of atoms in crystalline structure is of the order of 10
diffraction of x-rays with wave length 1
is employed to determine the inter-
atomic distances and rearrangements of atoms in a crystal. The interference
patterns which result from the X rays passing through a crystal are
photographed, and distances between the resulting lines measured.
Calculations based on these distances and angle of incident radiation yield
the spacing between successive atomic layers in crystal. With crystalline
powders, the various angles already occur in the different orientations of the
grains so rotation of the specimen is necessary but may be carried out to
When an X ray diffraction pattern is obtained from a powdered mixture of
unknown minerals, the constituents of the mixture can be determined from
the comparison of the measure distances to various diffraction lines with
tables of diffraction data on known minerals. The intensity of lines, while
also indicative of the minerals present give a rough indication of the quantity
of each constituent in the sample. Information may also be obtained on the
thickness of molecular water layers on the particle surfaces.
Fig.2.4 –Thermographs of clay minerals
2.3.1. The classification given by U.S.B.R. (1942) and U.S. Highway
research board (1948) is not suitable for Black cotton soils of India. This soil
is used for construction purposes also. Research was done in 1953 (Ref.15)
on various soil samples from Deccan plateau. The characteristics of the soils
are shown in a table 2.3 below.
Table 2.3- characteristics of the B.C. soils
Fine sand 3 -10 %
Fraction smaller than 200 microns 70-100%
Colloid content 40-50%
Liquid Limit 40-100%
Plasticity Index 20-60%
Shrinkage limit 9-14%
Volumetric shrinkage (wet basis) 40-50%
Hygroscopic moisture 12-13%
Exchangeable Calcium 40-80 m.e./10gm
Exchangeable Sodium+ Potassium 2-5 m.e./10gm
Base exchange capacity 40-50 m.e./10gm
SiO3 50-56 %
Fe2O3 8-12 %
SiO2 / Al2O3 3 to 5%
In all 210 soil samples were investigated, out of which some were subjected
to chemical tests also. The chemical test results did not show any specific
tendency for classification purpose.
Systems of classification based on the physical properties were developed.
Some of these are given below.
1. Textural classification-Grain size analysis and distribution.
2. Cassagrande‟s classification- Suitability for load carrying capacity.
3. U.S.P.R.A. classification-Based on L.L, P.I., mechanical analysis and
4. Civil Aeronautics Administration classification-Based of mechanical
analysis, P.I., expansivity, C.B.R. and general description of soil based
on field examination.
5. Compaction classification (Based on maximum compaction attained by
6. Burmister classification (Based on grain size classification and
Out of the above six classification systems the U.S.P.R.A. was approved in
1952 by Indian Road Congress. Initially in this system all the different soils
were divided in eight groups, ranging from A1 (well graded gravels or sands)
to A8 (Peat).It was based on six properties.
1. Particle size distribution.(P.S.D.)
2. Liquid Limit.(L.L.)
3. Plasticity Index.(P.I.)
4. Shrinkage Limit.(S.L.)
5. Field moisture equivalent.
6. Centrifuge moisture equivalent.
This system was revised in 1955. The number of groups was reduced from
eight to seven, by considering only first three properties i.e. PSD, LL and PI.
All black cotton soils of India fall under A-7 group of USPRA classification
system. The subgroups are given by group index method.
Group Index (GI) = 0.2 a+0.005 ac+ 0.01 bd.
a= than portion of percentage passing 200 B.S. Sieve (I.S.8), greater than
35 and not exceeding 75 expressed as number (0<a<40).
b= than portion of percentage passing 200 B.S. Sieve (I.S.8), greater than
15 and not exceeding 55 expressed as number (0<b<40).
c=portion of numerical liquid limit greater than 40% and not exceeding 60,
expressed as positive number (0<c<20)
d= portion of numerical Plasticity Index greater than 10% and not exceeding
30, expressed as positive number (0<d<20).
Thus Group Index varies between 0 and 28.
The soils collected from various states of India were found to have a Group
Index of more than 20 which is the upper limit of A-7 group. So the
extension of GI is done by fixing higher values of the fraction passing ASTM
200 sieve, L.L. and P.I. This was done by raising the values of a, b, c and d
from the following expressions.
a= than portion of percentage passing 200 B.S. Sieve (I.S.8), greater than
35 and not exceeding 100 expressed as number (0<a<65).
b= than portion of percentage passing 200 B.S. Sieve (I.S.8), greater than
15 and not exceeding 80 expressed as number (0<b<65).
c=portion of numerical liquid limit greater than 40% and not exceeding 85,
expressed as positive number (0<c<45)
d= portion of numerical Plasticity Index greater than 10% and not exceeding
44, expressed as positive number (0<d<34).
“a”, “b”, “c”, “d” have the same meaning and thus the new maximum value
of GI is 50.The group A-7 was subdivided as below.
Group Index GI New Sub-Group
Less than 20 A-7
2.3.2. Bolton Seed et al (1962) tried to classify the soil depending on the
swelling potential. Because they found that if the three properties i.e.
Plasticity Index (PI), Shrinkage Limit (SL) and clay content are considered at
a time, it leads to a contradictory results. So they found a clear out relation
between swelling potential and clay content. They arrived at an equation,
S = (3.6 x 10-5
Where S=Swelling potential
A= Swell activity= (Plasticity Index)/(Clay fraction)
c= % of clay fraction.
A set of curves were given for computing S for different values of PI and c.
A Table 2.4 gives the classification of swelling soils based on S.P.
Table 2.4- classification of
swelling soils based on S.P.
Low 0 to 1.5
Medium 1.5 to 5
High 5 to 25
Very high greater than 25
2.3.3 Ranganathan B.V. and Sally N.B. (1965) suggested a rational method
for the prediction of swelling potential. Swelling potential was defined as “the
percentage of swell under a surcharge load of 1 psi. of a soil compacted at
its optimum moisture content (OMC) to a dry density in standard AASHO
compaction test. They also defined swell activity as ratio of (LL-SL)/clay
Swell activity = (S.I. %) / (Clay fraction %)
With the help of swell activity they finally found out the relationship between
swelling potential and Shrinkage Index, which is as follows,
S.P. = (4.57x 10-5
) (SI) 2.57
S.P. = swelling potential
S.I. = Shrinkage Index (rational index for volume change of clays)
N = c3.44
Where c= clay fraction
n=Intercept on the curve (SI Vs Clay fraction) Ref. Fig.4) it varies from 4 to
Values of N can be readily computed for different values of c and n. A set of
curves are prepared for c, n and N, from which N could be read out
Fig.2.5-Parameter for different n and clay fraction
The authors have given another classification system as shown in Table 2.5
Table 2.5 -Classification based on S.I.
Classification Shrinkage Index
Low 0 -20
Medium 20 -30
High 30 -60
Very High >60
Fig.2.6-Shrinkage index Vs clay fraction
2.3.4. E.A. Sorochan (1970) experimentally proved that swelling process is
anisotropic. It is a result of textural and structural features as well as of the
character of stratification of soils. So a new term “swelling index (π).
Swelling index of soil is a ratio of porosities of soil in saturated and natural
Swelling index (π) = E/E0 where E is porosity of swollen soil and E0 is
porosity of natural soil. The swelling index (π) does not depend upon the
type of structure, method of testing, kind of wetting liquid etc. It is, on the
other hand a liner relationship with magnitude of relative expansion of soil.
Table 2.6 -Swelling Index and P.I.
Plasticity Number (P.I.)
3 ENGINEERING PROPERTIES OF EXPANSIVE SOILS
3.1 Introduction: Experimental and theoretical studies on swelling soils have
been going on since last century, in different parts of the world as the
damages caused by these soils were catastrophic. In these studies it was
found that swelling pressure plays an important role. There are number of
properties of swelling soil which are responsible for swelling. A degree of
expansion is more or less related to shrinkage index, plasticity index colloid
content. The available literature on properties of expansive soils is presented
3.2 Theories of swelling: It is common observation that when swelling soil
comes in contact with water, the volume of soil increases. This phenomenon
is swelling. Many theories on swelling of expansive soils have been proposed
by various research workers. Gupta et al (Ref.10) in his report “Physico-
chemical properties of expansive soils” has summarized various theories.
According to Canoy Chapmon‟s theory of double layer, the swelling should
completely at large concentration of electrolytes. It has however observed
from laboratory experiments that there is always a residual swelling;
however large concentration of electrolytes is used. The theory of double
layer as applied to behavior of soils is derived from the analogy colloid taken
in membrane surrounded by an electrolyte. In this case mid-plane between
soil particles is imagined to function as membrane. Such an assumption is
not fully justified as soil is the mass of gel in which particles are in contact
with each other having their double layers overlapping in a complicated
manner and thus mid-plane cannot be precisely defined. Further there is
hydration of ions as well as clay particles on account of which the hydrostatic
repulsive forces are not wholly balanced by attractive forces as a result of
introduction of electrolytes.
The suction potential theory of Schcefield, also does not account for the
entire swelling as it is observed that there is residual swelling even if soil
suction is nil.
There is further intake of moisture until the hydration of ions and soil
particles is complete and particles of soil have reoriented with respect to
forces which keep them together, viz the confining pressures and the
attraction between clay particles. Both these concepts viz the theory of
double layer depending entirely on physical chemical properties and suction
potential based on capillary only, do not take into consideration the effect of
elastic properties in relation to external forces.
Terzaghi, K. has advanced hid concept of swelling based on elastic properties
of soils. According to him, the swelling is wholly due to elastic properties of
soils, the physic-chemical properties of soil do not play any role in the
swelling phenomenon. This is true for two reasons. Firstly, the surface
behavior of charged particles leading to Base Exchange and absorption of
water molecules as dipoles, have profound influence on swelling. Secondly
the interlayer spaces in which water molecules are retained influence
swelling. The application of pressure brings the particles closer expelling
pore water. Increase of pressure expels more water that has been absorbed.
The process goes on till the inter particle spacing has been reduced to a
distance of approximately 20A. At this stage all the water between particles
is tightly held and the extraction of inter particle water by inter granular
pressure alone is thus impossible though there might be isolated areas of
mineral to mineral contact where water has been completely eliminated. Also
the inter layer water which is responsible for swelling to a large degree is not
removed by mechanical means.
It is thus evident that for any theory to explain swelling phenomenon in soils
completely, it should take into account the physic-chemical affects due
hydration of exchangeable ions and that of clay particles, the soil suction
and elastic behavior of soils in relation to external forces. Further research of
the subject should aim at combining the three concepts to obtain a more
rational theory of swelling phenomenon.
3.3 Physical and engineering properties of black cotton soils varies from
place to place. Out of various research papers available on this subject few
papers contains properties of local soil. A compilation of various properties of
black cotton soils, if made, will be very useful to engineers and research
Katti, R.K. and others (ref.21) collected soil samples from 16 different
locations and conducted detailed laboratory investigations and have given
physical and engineering properties of Black cotton soils a tabular form. The
same table is reproduced here. The various locations are indicated in the soil
Table 3.1-Locations of 16 soil samples
S1-Solapur 2 S2-Poona1
Fig. 3.1 –Site map of samples
S5-Nagpur S6-Solapur 1
S13- Poona2 S14-Calcium
Table 3.2- Notations used in tables
L.L % Liquid Limit S.G. Specific Gravity
P.I.% Plasticity Index Clay -5 Fraction < 5 μ
S.L. % Shrinkage Limit Clay -1 Fraction < 1 μ
S.R. Shrinkage ratio
Density -SP Max. dry density as per light compaction
OMC-SP Optimum moisture content as per light
Density-MP Max. dry density as per heavy compaction
OMC-MP Optimum moisture content as per heavy
Sw.Pr. Swelling pressure
pH Acidity/ Alkalinity
Org. Mat. Organic material
CO3 Carbonate contents
B.E.C. -400 Base Exchange capacity for particles smaller
than 400 μ
B.E.C. -2 Base Exchange capacity for particles smaller
than 2 μ
SiO2 % Silica Content
Al2O3 % Alumina content
CaO % Calcium hydroxide
MgO % Magnesium hydroxide
FeO3 % Ferric Oxide
TiO3 % Titanium Oxide
SO3 % Sulphur oxide
3.3.1 Measurement of swelling pressures: When an expansive soil attracts
and accumulates water, a pressure known as swelling or expansion pressure
builds up in the soil and it is exerted on the overlying material and structure
if there are any.
Swelling pressure is defined as “If a swelling substance is tightly enclosed in
a vessel with a wall permeable to a swelling solvent and latter is allowed to
diffuse into the vessel, the dilation tendency of the soil solvent gel give rise
to a pressure called “Swelling pressure”.
The two commonly used methods for measurement of swelling pressure are,
1Constant Volume method or Constant Pressure method
3.3.2 Constant Volume method: In this the soil is mixed with appropriate
quantity of water. After maturing period the soil is placed in a mould. The
bulk density and water content of the specimen is determined by standard
methods. The specimen is covered with porous stones and filter paper. The
entire mould in placed in a water trough under loading machine with proving
ring and dial gauge to measure force and swelling of soil. The expansion of
soil specimen is nullified by applying force gradually and proving ring reading
is recorded at different time intervals till there is no further swelling of soil.
Fig.3.2 -Constant Pressure Method
Pressure intensity is calculated from proving ring reading and specimen
area. A pressure Vs time graph is plotted. The maximum pressure intensity
gives the swelling pressure of soil for a specific dry density and water
Fig.3.3 -Constant Volume method
3.3. Constant Pressure Method: In this method minimum three identical soil
specimen are subjected to three different load intensities and allowed to
saturate and swell or consolidate. The load intensities are so selected that
soil swells under lowest load intensity and consolidate under maximum load
intensity. After the equilibrium is achieved the changes in the volume of
specimen are recorded. A graph between load intensity as abscissa and
volume change as ordinate. The load intensity at which volume change is
zero is called swelling pressure.
Fig.3.4 –Pressure Vs Volume Change curve
3.4 Factors affecting the magnitude of swelling pressure: The swelling
pressure of an expansive soil is not unique but it is influenced by number of
factors such as initial density and water content, method of compaction,
confining pressure and specimen size etc.
Murthy, VNS and Chari R. (Ref. 22) studied these factors affecting the
swelling pressure of expansive soil.
3.4.1 Initial water content: Swelling being basically processes of absorption
of water, the initial water content represents the state of initial swelling. A
soil with lower water content is expected to swell more than soil with higher
water content. The lowest water content at site during a dry season may be
taken as datum for the purpose of field computation.
3.4.2 Density of soil sample: For constant moisture content, the soil density
has a definite effect on swell pressure. This is mainly due to the grater scope
for building up of absorbed film around each of clay particles. Uppal and Palit
(ref 38) have shown that as dry density increases the swell pressure also
increases. The have found that at low density up to 15 kN/m3
pressure is very small but as the degree of compaction increased beyond
this value there is abrupt rise in swelling pressure.
3.4.3 Time of saturation: The process of swelling is gradual because soil
takes time for the water to penetrate into soil layers and cause expansion
cumulatively. Therefore time allowed for expansion is an important factor.
The affinity for absorption being great in soils with low moisture content,
initial rate increases the swell pressure in those soils is greater than those
soils with higher water content. It can thus be anticipated that soils with
lower moisture will have a very percentage of swell even during initial
contact with water. Initial rate of increase of swell pressure is lesser in soils
with higher densities. This may be the effect of lower permeability of the soil
and is also of great significance in practice.
3.4.4 Free expansion permitted: Swell pressure is a consequence of the
restraint on the free swelling. Any expansion allowed result in a reduction of
swelling pressure. An expansion of 0.025 mm is said to reduce the swell
pressure by as much as 5 KN/sqm.
Two identical samples were tested using proving rings of different stiffness.
A proving ring with lesser stiffness undergoes large deformation. A soil has
thus a definite free expansion before developing the full swell pressure.
The swell pressure under any building foundation will be equal to the
foundation pressure. The difference between the possible maximum swell
pressure and foundation pressure results in an expansion and consequent
vertical movement of the structure.
3.4.5 Sample height: Some tests were conducted by Uppal and Palit
(Ref.32) to study the effect of height of sample on swelling pressure. The
process of swelling is result of building absorbed water films. Given sufficient
time such action will take place over the entire depth of clay stratum. The
quantitative swell and swelling pressure should be a cumulative effect. The
swelling pressure is observed to vary directly with the height and inversely
with the diameter of the specimen. However if the skin friction is eliminated
the swelling pressure is found to be independent of the size of the test
3.5 Field measurement of swelling pressure: The problem of safe and
economic design of foundations in expansive soil has been engaging the
attention of geotechnical engineers all over the world. The problem which
has proved most difficult is that of a single storied building on heaving clay
because of light foundation pressures. In India many housing schemes are
located in areas made up of expansive clay. Therefore the problem needs to
be studied in detail. Results of laboratory measurement of swelling pressure
of black cotton soils and failures of few buildings made it clear that it would
be useful to conduct some field measurement of swelling pressure and
compare it with laboratory investigations.
3.5.1 Swelling pressure determination in field: The general soil profile in the
chosen area consists of 2.2 to 2.5 m. of B.C. soil as top layer underlain by
2.5 m brownish yellow sticky clay resting on soft morrum which extend
below to a fairly great depth.
Field Set-up for swelling pressure measurement: At test site bore holes 15
cm diameter and 5 to 6 m depths were sunk with the help of power augers.
In each bore hole a reinforcement cage was lowered and concreting was
done. The concrete piles protruded 1 m above ground level. The threaded
portion of reinforcement was 15 cm above the pile head. A steel plate was
attached to the pile for uniform load distribution. Steel I section was fixed to
a pair of piles which were free from vertical movements due to swelling of
soil. Plates 75 cm to 25 cm diameter were placed at a depth 30 cm to
measure swelling force exerted by soil, using proving ring attached to I
3.6. Lateral swelling pressure: The phenomenon of lateral swelling of
expansive soil is well known. Many structures crack due to lateral swelling
Kassif at el (ref.15) measured lateral swelling pressures on two
instrumented underground conduits buried in swelling soil. The strain gauges
were fixed along the longitudinal direction of conduits. The field data was
compared with theoretical data.
Komornik et el (Ref.20) developed a special device for laboratory evaluation
of lateral swelling pressure by modifying the mould of consolidometer to
which strain gauges were attached. The modified apparatus was also useful
to measure earth pressure at rest.
4 CONSTRUCTION TECHNIQUES
4.1 Expansive soils always pose various problems to foundation engineers.
Almost all cohesive soils have expansive property from insignificant to highly
significant. Expansive soils are found in various parts of the world such as
USA, South Africa, Australia, Spain, Israel, Myanmar and India. In India
these expansive soils are known by local names such as Black Cotton soils
(BC) in central India, Bentonite in Rajasthan and Kashmir, Mar or Kabar in
Uttar Pradesh. These soils occupy about 30 to 40 % of the land area of
4.2 The problems posed by expansive soils of India can be summarized as
4.2.1 Deep excavation for foundation: BC soils are residual soils resulting
from weathering of Igneous rock (Basalt). The thickness of soil stratum can
be high as 3 to 10 m. laying the foundation on a firm non-swelling stratum
involves deep excavation in stiff clay and increases the cost of construction.
4.2.2 Assumption of low bearing capacity: The correct estimation of
allowable bearing capacity of BC soils is complicated by various factors such
as swelling pressure, ground water table variations, site conditions etc. This
leads to assumption of lower bearing capacity. But if the probable swelling is
higher than the assumed bearing capacity, the foundations are subjected
differential settlements. Cracking of single storied buildings is very common
than that of double storied buildings.
4.2.3 Non uniform swelling or shrinkage: The equilibrium water content is
not same below the foundation. This leads to differential settlements and
diagonal cracking of masonry superstructure.
4.2.4 High cost and low reliability of rehabilitation: Remedial measures for
damaged structure are costly and not reliable in long term. Hence
prevention is better than cure.
4.3 Construction techniques for foundations in expansive soils:
4.3.1 Removal of entire expansive soil: The first and very simple method is
to remove the entire layer of expansive soil up to firm and non-expansive
4.3.2 Other practice is to provide a cushioning layer between bottom of
foundation and top of soil. The cushioning layer is granular soil to allow the
swelling of soil to penetrate in its voids. Laboratory tests have shown that if
an expansive soil is permitted to expand by slight amount, the swelling
pressure is reduced by considerable amount. This method is suitable if the
thickness of swelling soil stratum is less than 2 m.
Dawson (ref.7) conducted study of foundations on expansive soil, permitted
to swell laterally by providing honeycomb tiles.
Reiner (Ref.42) presented an economical type of foundation. As per his
method the foundation pit was covered by a thin layer of lean concrete
covered with a layer of bitumen. The lean concrete layer cracks and bitumen
enters into the cracks and provides a cushion.
Boardman (Ref.3, 4) proposed a method in which brick walls are reinforced
and building is divided into separate units allowing open joints. But this
method is suitable for sites at which seasonal changes in water content of
ground are not much.
Date (ref.18) adopted an inverted T beam and pile foundation system. It
was assumed that during dry season loads would be transferred to piles and
in wet season the swelling pressures would be resisted by inverted T beams.
4.3.6 A raft or mat is a combined footing that covers the entire area
beneath the structure and supports all the walls and columns. This type is
used when the allowable soil pressure is low and building loads are heavy.
The raft is also used when where soil mass contains compressible layers
which may lead differential settlements. The raft or mat tends to bridge over
the erratic deposits and eliminates the differential settlement. It is also used
to reduce settlement above highly compressible soils by making the weight
of the structure and raft approximately equal to the weight of the soil
4.3.7 Sorochan E.A. (35) Suggested the use of compensating sand
cushions in case of continuous footing for comparatively stiff structures. The
working principle of a compensating cushion consists in a controlled pressure
rise on the foundation role at the soil swelling location under the foundation.
This leads to the formation of compacted core in the cushion, which aids to
the flowing of sand from the foundation base. The possibility of such a
flowing depends on the different pressures produced by the foundation and
by the side backfill material and transmitted to the cushion surface.
Nevertheless, a rise of foundation cannot be excluded in this case. The
efficiency of cushion action can be evaluated by the magnitude of the
“Compensation coefficient” compensation coefficient K being the ratio of the
actual foundation rise to the possible magnitude of the soil swelling.
4.3.8 The pier and belled footing cast in a drilled and under-remed hole is in
reality a cast in place pile with an enlarged base. If the clay is dry or below
the shrinkage limit when the pier is cast, it will subsequently swell both
laterally and vertically and exert pressure against the sides of the pier and
uplift along the pier. This uplift force along the surface of the pier is limited
by friction along the pier surface, by the shear strength of the clay, and by
the expansive force of the clay. Without precautions for reducing the friction
between clay and concrete of the pier, it is probable that the shear strength
of the clay will be the governing factor. The uplift pressure is greatest near
the top of the pier where the clay expands most. In some cases, uplift has
been sufficient to pull the pier in two at the top of bell. Ref. Fig.4.1
Fig.4.1 - The pier and belled footing
It is believed that the following criteria can be used for the design of
successful foundations of cast in place pier and belled footing units.
(a) Use as high contact pressure as is consistent with carrying capacity
of the soil.
(b) Use bell 3 times diameter of pier for maximum anchor.
(c) Use smallest pier compatible with load and bell size in order to keep
surface area minimum.
(d) Extend reinforcement into bell to within 4” of bottom in order to
anchor pier to bell.
Sometime the oversize hole is drilled to the entire depth and the bell is
formed at the bottom of the oversize hole. The bell is filled with concrete to
extend a slight distance in to the pier above the bell and the casing for the
pier is pushed a short distance into the fresh concrete in order to prevent
concrete from rising into the space around the outside of the casing. When
using this procedure, care should be exercised to see that the casing is not
let into the hole before the concrete has been placed in the bell otherwise a
shaft may be cast with no footing.
4.3.9 The grade beams or plinth beams cast in contact with desiccated clay
are sometimes broken be uplift pressure of expanding clay. Even if the grade
beams were reinforced to resist this pressure, the uplift on the supports may
cause as much damage as if the beam were allowed to break Provision
should be made for a void under grade beams into witch the clay can expand
without exerting uplift pressure.
The use of collapsible card board beam boxes is much more practical and
sure method of preventing uplift under grade beams. These cardboard boxes
are shipped flat and are folded to form a hollow box of the proper
dimensions for the purpose. The cardboard is treated to prevent immediate
disintegration and to remain strong enough to support runways for concrete
buggies long enough, for concrete to be placed and harden. These cardboard
beam boxes are produced commercially in Kansas and Texas.
4.3.10 Several methods have been devised for casting the structural floor
system on forms that lie directly on the clay and disintegrate after a short
period leaving a space for expansion of the clay.
Fig.4.2 - Structural floor system
One method for forming the slab which has been sued experimentally is to
loosen the clay to a depth of 30 to 50 cm. and to form the loose soil in
windrows to make a form for Joists. In order for this method to be
successful, the depth of the loosened clay must be adjusted to existing
conditions. The volume decrease of the loosened soil must be equal to or
greater that the volume increase of the undisturbed clay below the loosened
material. This method cannot be considered reliable, as during construction
of the loose fill, the soil may be compacted unfit is will itself swell as much
as or more than, the undisturbed soil.
This method consists of excavation deeply enough to form the area solid
with baled hay or straw laid end to end and side by side. These bales are
covered with roofing felt or sisal craft. The depressions between the bales
are forms for joists. The hay or straw is sprayed with ammonium nitrate to
accelerate disintegration of the straw. But the hay increases the fire hazard
and makes the construction site look like a feed lot. The aesthetic value of
rotting hay under the floor is questionable.
An effective method of providing void spaces under slab and beams into
which the clay can expand without producing uplift pressure is by the use of
water proof cardboard forms of sufficient strength to support the fresh
concrete and which later disintegrates. The cardboard forms are shipped flat
and are folder into shape during installation. But when the basement floor is
formed and cast before the basement walls are erected, the collapsible
forms are exposed to the weather during construction of floor, and the banks
of the excavation are susceptible to sloughing or sliding into the excavation,
which weakens the exposed cardboard forms during rainy season and
collapse. Sometimes a card board form is placed under the basement wall.
Under a heavy load, this method is ineffective because the beam box may be
crushed by the weight of fresh concrete.
Another arrangement known as slab on sonotube forms may be used. In
this method split sonotube are laid side by side to provide forms for joist
below the bottom of the concrete joists. The bottom of space between the
two halves is filled with sand about 7 to 8 cm deep. The joist steel and
concrete are placed to form a reinforced concrete floor slab supported on
grade or plinth beams After a short time, the sonotubes disintegrate, the
sand runs out from under the joists, and a void is formed into which the clay
can swell without exerting pressure on the bottom of the slab.
4.3.11 The most common and best suited of all is the under-reamed pile
foundation. This method is discussed in detail in the next chapter.
4.4.0 There are problems posed to the old buildings which are standing.
The techniques or the remedial measures used for the prevention and
further developments of cracks are discussed below.
4.4.1 A. K. (9) and Subash Chandra suggests a simple method for the
prevention recurrent in small buildings founded on Black Cotton Soil,
directed at keeping the moisture content in soil immediately under and
around the building as constant as possible so as to minimize the ground
movement. Vertical sand drains connected by channels are placed about 2m.
on centers all around the effected building. Waste water from the building
was allowed to flow into them. A line concrete apron laid on polythene
membrane may be added between the walls of the building and the sand
drains to retard loss of moisture by evaporation as much as possible.
4.4.2 Ward, W.H. (40) studied the effect of fast growing trees and shrubs on
shallow foundation. According to him, in summer the trees absorb large
quantities of water from the clay under footing which then shrinks
appreciably and lets down the structure which is incapable of resisting the
settlement. The shrinkage one reaches as far as the most remote root which
generally extends distance greater than the height of the tree.
(1) So the fast growing trees should not be planted near the foundation.
(2) The footing is placed sufficiently deep in a zone not affected by soil
moisture movements and
(3) The structure may have shallow foundation but be made strong
enough to resist cracking.
4.4.3 Rao N.V.R.L.N., and Krishnamurthy (29) suggested a method on the
same principle that, “the moisture content under the foundation and around
the building should remain constant as far as possible. They put forward the
idea of soak way pits, at proper spacing so that water drains quickly and the
soil surrounding the building remains dry. The soak way pits are filled with
materials like sand and gravel, and a concrete apron around the building is
4.4.4 Jaspar J.L. and Shetenko V. W. (18) suggested the foundation anchor
piles in clay shale. Earth dams and appurtenant structures in the Prairie
Provinces are often constructed on clay shale foundation. Concrete
structures such as spillways may be damaged due to swelling of foundation
or to differential movements. Various protective devices have been installed
to reduce and control the amount of swelling and differential heave beneath
structures. Hold-down piles have been used, which were mainly reinforced
concrete with bottom flared out. This type which could take little strain often
became ineffective either through breakage or slippage. To overcome this
problem, anchor piles were designed to stretch a certain extent without
failure of the shale. It was not intended that this type of pile would eliminate
swelling but that would reduce the rate and amount of swelling or differential
4.4.5 A flexible waterproof apron, of about 2m. width provided at a depth of
about 90 cm. forms a suitable remedial measure for cracked buildings. The
best time for providing an apron is at the end of monsoons. The soil should
be neither too dry nor too wet. It should be dug out around the building up
to a depth of about 50 cm. The surface is them dressed and given an
outward slope of 1 in 30. Over this surface a flexible apron which may
accommodate ground movement s without rupture is laid. It can be a 10 cm.
lime concrete layer over which a tar felt is laid. In place of tar-felt and
alkathene sheet 0.25 mm. thick can be used. Care should be taken that no
mechanical damage is caused to the water proof membrane. Alternatively a
bituminous concrete layer of about 75 mm. thickness can be adopted in
place of lime concrete and alkathene sheet. The apron should go about 75
mm. into the foundation wall by cutting a chase so that no room is left for
evaporation or saturation from the joint. The width of apron is kept 2m. A
typical section of the apron treatment is shown in figure 4.3
Fig.4.3- A flexible waterproof apron
After the apron is laid the soil should be back filled and properly dressed to
give an outward slope of 1 in 30. It will serve to protect the apron against
It has been observe that the underground flexible aprons around buildings
arrest further cracking. After two cycles of seasons the cracks becomes
stable and no further damage is generally noticed.
5-UNDER-REAMED PILE FOUNDATIONS
5.1 Introduction: The best method of foundations in expansive soils is
foundation which is anchored in the stable zone of the ground, in which the
moisture variations are negligible. This was observed from the performance
of cast-in-situ piles with enlarged bases. Such piles were successfully
installed in South Africa and Israel. CBRI Roorkee realized the importance of
such piles and undertook a research project to develop a simple procedure
for manually operated hand augured piles. More than 5000 piles were
constructed and tested in various parts of India and based on the practical
experience CBRI Roorkee published and published a manual on under-
reamed piles and gave design tables for various diameters of augured piles.
Subsequently Bureau of Indian Standards published a code of practice
I.S.2911 part 3. The code describes the various parts such as pile, grade
beams and reinforcement details. The code also includes a design formula
for working out load carrying capacity of a single on multi-reamed piles. The
code also includes the equipment required for such construction. A method
of load test on piles is also included.
5.2 Limitations of UR piles: There are many limitations to construction of
under-reamed piles and are discussed below;
Needs strict supervision: Unless there is strict supervision by expert, the
whole purpose of this technique is lost. The check points as listed below.
Exact location-Insist use of guide on ground for proper location and
inclination of pile.
Proper length of pile- The top bulb must be in the stable.
Checking of bulb diameter- Use L bar to check the bulb diameter
Spacing between two bulbs- Adequate spacing is must to avoid
collapse of side wall of bore.
Concreting – Use PVC pipe during poring of concrete of desired slump.
No vibrator is to be used. Use heavy tamping rods.
Piles should be randomly selected for load test.
5.3 The different design and construction steps are illustrated through
Fig. 5,1 to 5.7 below
Fig.5.1 - Construction Stages Fig. 5.2 –Measurement of bulb
Fig.5.3- Details of under-reamed pile
Fig.5.4 –Boring in progress Fig.5.5 –Pullout of hand auger
Fig.5.6 –Reinforcement details Fig.5.7 –Standard dimensions
6 STABILISATION OF EXPANSIVE SOILS
6.0 Introduction: Stabilization in a broad sense incorporates the various methods
employed for modifying the properties of a soil to improve its engineering
performance. Stabilization is being used for a variety of engineering works, the
most common application being in the construction roads and foundation purposes,
where the main objective is to increase the strength, improve the stability of soil
mass and to reduce the construction cost.
With this in mind studies were conducted by Katti (21) and others to evaluate the
effect of inorganic chemical on various properties of black cotton soils.
6.1 Effect of inorganic chemicals on the consistency properties.
For this study they selected soils S-2, S-4, S-5, S-6, S-9, S-9, S-10 and S-11 i.e.
from Poona, Nasik, Nagpur, Sholapur, Baroda, Bezawada, Wadagaon sites. The
chemicals used for treating some or all the soils were hydroxides of Na, K, Ca, Mg,
Ba and Fe, carbonates of Na, Mg and Ba; cement, sodium silicate, Di-ammonium
phosphate, suplhates of Na and Cu, phosphates of Mg and Ca and potassium
dichromate. The percentage of chemicals used varied between 0 to 10 percent
based on the over dry weight of the soil.
6.1.1 Hydroxides :The variation in the consistency properties of the soils treated
with hydroxides, of potassium, sodium and calcium is represented in fig. In case of
all soils other than S-4, the addition of KOH varying from 1.5 to 7 percent has
made the soil non-plastic. S-4 shows disruptive effect. KOH goes on reducing the
liquid limit and plasticity index. 0.75 to 3 percent, the shrinkage limit value
significantly increased indication that volume change tendency has been
considerably decreased. The shrinkage limits go as high as 40 in some cases from
initial value of around 8 to 10. The increase in Plasticity Index at small percentage
may be due to the dispersion effect.
The dispersive action of NaOH with small addition is evident. The L.L. of nearly all
the soils increases up to about 1 to 1.5% and in the same range the P.L. decrease
and P.I. increases. Larger addition invariably causes lowering of L.L., increase in
P.L. and decrease in P.I. At small percentage of NaOH decrease in S.L. is observed.
However beyond about 0.75% the S.L. value nearly always increase with increasing
additive. These results indicate that while at low percentages of NaOH these is a
tendency for dispersion to take place, further addition results in less of plasticity
and increase in S.L.
The addition of Ca(OH)2
beyond about 1% distinctly goes on reducing the L.L. and
P.I. and increasing P.L. These results indicate that all the soils become non-plastic
beyond 1.5%, except S-10 soil. The shrinkage limit value continuously increase
with the addition Ca(OH)2.
Mg(OH)2 does not seem to have appreciable effect on the consistency properties of
any of the soils.
6.1.2 Chlorides: CaCl2, BaCL2 and MgCl2, do not have much effect on the
P.L. and S. L. of the soil. However, there is decrease in L.L. values and decrease in
P.L. value. It may be noted that while in case of Ca(OH)2 there is an increase in
P.L. and S. L. with the additive, these values more or less remains constant in ease
of calcium chlorides. This effect may be due to the fact that the chlorides are more
alkaline than the corresponding hydroxides.
With the addition FeCl3, the L.L. value show a tendency to decrease and P.L. values
more or less constant. It was possible to determine S.L. only in case of S-9, S-10,
S-11 soils and these did not show significant change. In other soils, it was not
possible to determine S.L. values. It was observed that the addition of FeCl3
beyond L percent makes the soil mass porous like bread. This may be due to the
formation of HCL which on reaction with the carbonates present on the soil evolves
CO2. the escape of the gas gives rise to the porous structure. Chemical test
confirmed that CO2 was liberated during the processes. It may be noted that S-9,
S-10, and S-11 soils contain less than 0.5% carbonates which the other contain
even up to 6.65%.
KCI and NaCL were tried only on S-2 soil, These chemicals increase the S.L. values
to a great extent while L.L. and P. L. values decrease. KCI seems to be more
effective than NaCL.
6.1.3 Carbonates :MgCo3 increases the L.L. and P.L. values while BaCo3
does not show any marked effect. The S. L. values tend to increase. Na2Co3 was
used with S-2,S-4,S-5, and S-6 soils. All carbonates may be said to produce
dispersion and cause increase in plasticity.
6.1.4 Cement :It can be noted that cement has a similar effect as Ca(OH)2
but to a lesser degree. This may be due to the lesser amount of free lime available
from cement. It may be noted that even with 10 per cent of cement, the soils do
not become non-plastic. The S.L. values however, considerably increase with the
addition of cement.
6.1.5 Na2Sio3 :Sodium silicate increase the L.L. and P.I. for all the soils and
make them highly plastic. This may be attributed to the disperse effect. The S.L.
values seem to increase with the additive.
6.1.6 Di-ammonium Phosphate: This chemical was tried on soils S-2, S-4, S-
5 and S-6 and its effect is found to be similar to that of FeCl3. the S.L. Values could
not be determined since the soil turned porous due to the evolution of NH3.
6.1.7 Other Chemicals: Na2SO4, CuSO4, K2Cr2O7, Ca3(PO4)2 and Mg3
(PO4)2 were tried only on S-2 soil. In general Na2SO4 shows an increase in L.L.
and P.I. due to the dispersion. Variation in P.L. and S. L. were not significant CaSO4
and MgSO4 behave more like dispersing agent K2Cr2O7 decreases, L.L., P.L. and
P.I. and S.L. is increased.
6.2 Effect of aging on consistency: The amount of complex compound formed
due to the reaction between soil and chemical is dependent upon (i) The amount of
chemicals (ii) pH of the soil (iii) The amount of time allowed for the reaction. The
chemicals used are hydroxides of Na, K, Mg and Ca, chlorides of Ba, Ca and Mg,
Carbonates of Na, Ba, and Mg and cement. The chemical used in various
percentages between 0 and 7.
Free water is essential for reaction to take place between soil and the chemical
6.2.1 Hydroxides: Plasticity characteristics of the soil are arranged by the addition
of hydroxides at zero aging period. The L.L. value of the chemically treated soil
show an increasing trend upto 3% of NaOH, 0.5% of KOH, 0.1% of Ca(OH)2. The
L.L. values at the above percentages fro NaOH, KOH, Ca(OH)2. are 147,84.8, and
87.7% compared to the value of 81% for raw soil. The initial increase is more
predominant in case of NaOH, due to its highly dispersive nature. These effects are
also reflected in the variation of P.I. NaOH increases the P.I. from 35% to 85% at
3% additive and decreases to 15.5 percent at 7 per cent additive. The hydroxides in
general improve the shrinkage properties of soils at zero aging period.
With aging L.L. tend to decrease with all hydroxides while the P.L. remains constant
or show a tendency to decrease. For instance it may be noted that from fig. that
L.L. values with 0.5 percent of NaOH at 0, 48 and 96 hours aging are 101.5, 85.0,
and 80.3 per cent respectively while the P.L. values at the same percentages at the
corresponding curing period are 56.7, 47.0 and 47.0 This decrease in L.L. may be
due to the formation of complex cementing gel produced due to the reaction
between chemical and the soil constituents. The amount of this cementing gel
formed depends upon the amount of chemical added and time allowed for the
reaction and pH of the system. With more chemical and more time, more quantity
of the gel like cementation material would be formed.
The S.L. values increase with aging beyond 1.5% of NaOH, while the values reduce
with aging when Ca(OH)2 and Mg(OH)2 are added.
6.2.2 Chlorides: S-8 soil i.e. the soil from Amravati shows the same behavior with
chlorides at the aging period as other soils described earlier, showing decrease in
L.L. with the addition of chemicals and a negligible effect on P.I. and S.L. values.
With aging chlorides decrease the L.L. and the P.I. The P.L. values show a slightly
decreasing trend although in the case of CaCl2, there seems to be an increasing
value beyond 72 hours. This may be due to the gel formation.
Fig. shows the effect of aging of CaCl2
on consistency properties. It may be noted
that at 1% additive the L.L. values at 0, 48 and 96 hours are 76.8,72.6 and 71.0
respectively and the corresponding P.L. values are 34.7, 31.7, 31.7 and 34.9.
6.2.3 Carbonates: The zero hour L.L. and P.I. values of the soil sample
increase with the addition fo carbonates , the effect being more pronounced with
Na2Co3 , S.L. is unaffected by carbonates.
With aging there is a definite decreasing trend in L.L. and P.I., the change being
predominant at higher percentages. This may be attributed to the formation of gel
The values of L.L. at 0.5 percent Na2Co3 at 0,48 and 72 hours are 89.2, 79.0 and
77.0 percent and P.I. values are 48.9, 37.6 and 35.3 at the corresponding curing
6.2.4 Sodium Silicate: (Na2Sio3) – Sodium Silicate produces high dispersion
and increase in L.L. and P.I., S. L. remaining nearly constant.
With aging all consistency limits shows a tendency to decrease. At 0.5% additive
the L.L. reduces from 85 to 77.7% and P.I. from 50.4% to 41/8 when cured for 96
hours. This behavior is the same as for the other chemicals.
Cement: - The addition of cement brings about changes similar to those of
Ca(OH)2, both with aging and amount.
6.3. Bearing Characteristics: A study was conducted on S-2 soil i.e. from Poona
treated with KOH, NaOH, Ca(OH)2, cement and Na2Co3, to get an idea about the
bearing characteristics used for this study was 0.1, 0.25, 0.5, 0.75, 1.0, 3.0, and
7.0 percent of the oven dry weight of the soil. C.B.R. test at standard proctor
density with surcharge on soaked samples were conducted. The No. of days soaking
was 4 days.
The test results are presented in table. From the data it may be noted that beyond
1 percent KOH, NaOH, Ca(OH)2 and cement appreciably increase the C.B.R. values.
The increase in C.B.R. values is an indication that the complex cementations gel
which are formed have cementing property even under highly wet condition. This is
an important factor with respect to the stability of the soil-chemical system under
field condition. Further studies on expansive soils subjected to drying and rewetting
is needed, because it is expected after drying the gel may attain a condition of
insolubility. Na2CO3 does not seem to have much effect on C.B. R. Values.
6.4 Permeability characteristics: Permeability being one of the important factor
to be considered in the design and construction of Civil Engineering works, it is
intended to study the effect of inorganic chemicals on the permeability
characteristics of three black cotton soils, viz. S-2, S-4, and S-12 i.e., from Poona,
Nasik and Wadagaon. The chemicals selected for the study are hydroxides of Na, K,
and Ca chlorides of K, Na, Ca and Mg and carbonates of K, Na, Ca, Ba and Mg and
were used in proportions of 0.1, 0.25, 0.50, 0.75, 1.0, 1.5, 2.0, 3.0, 5.0, 7.0 and
10.0 percent on the basis of even dried weight of soil. The procedure for mixing
was the same as in consistency studies. The mixtures were compacted to field
densities of 1.330, 1.225, 1.253gm/cc. For S-2, S-4 and S-12 soils respectively in
Jodhpur pattern moulds by static compaction. The samples were then saturated
under vacuum for 36 hours prior to conduction the permeability test by the falling
head method. The experiments were run in duplicate. The values obtained were
erratic during the first few hours but attained fairly constant values at the end of 10
hours and the values are recorded at the end of 12 hours.
The data collected in the case of soils S-4 and S-12 is presented in tables.
6.4.1 Hydroxides: NaOH when added up to about 2-3 percent in all the
three soils bring down the permeability values to less than that obtained for the
bank soil. Beyond this percentages, the permeability values increase continuously
up to about 10 percent, the increase being very rapid beyond 5 percent. The values
in units of 10-7 cm/sec. for S-2 soil at 0.1.3 and 10 percent additives at 3.2, 0.5,
1.4, 869.4 respectively while corresponding values for S-4 and S-12 soils are 9.6
and 4.8, 1.6 and 2.6,8.0 and 36.1 and 3140 and 1685 respectively. The decrease in
permeability at the lower percentages may be due to the dispersion effect of NaOH.
At higher percentages, aggregation effect seems to set in, leading the higher value
KOH shows the similar trend to that of NaOH. However, the dispersive action is
noticed over a much smaller range (0.1 to 0.25 percent ) ain this care and the rate
of increase is much higher at larger percentages. The values show a decreasing
trend beyond 7 percent, in all the soils. This trend can be observed from the tables.
The permeability values increase as high as 15, 450 x 10-7, 21275 x 10-7 and
17,300 x 10-7 cm/sec at 7 percent in soils S-2 S-4 and S-12 which are about 2000
to 5000 times their original values.
The dispersion and aggregation effect due to K ion are similar to Na ion. It has
already been noted while discussion the consistency properties of the soils, that
KOH is more effective in causing aggregation effect due to the proper co-ordination
number and ionic radius of the K ion. Moreover KOH is stronger alkali than NaOH
and therefore the permeability values obtained much higher than NaOH. When the
percentage, however, is increased more than 7 percent, the mineral breaks up into
their constituents in the highly alkaline environment and complex compound that
are formed block the horse, thus causing decrease in the values of the
Ca(OH)2 was used with soils S-2 and S-4 Even at 0.1 percent level, there is
significant increase in the coefficient of permeability. The coefficient of permeability
goes on increasing with the addition of chemical and reaches a value of 304.6 x 10-
7 cm.sec. in case of soil S-2 at 7 percent and 711.5 x 10-7 cm/sec. in case of S-4
soil at 5 percent. Beyond these percentages the permeability values tend to
6.4.2 Chlorides: NaCL and KCL are not much effective on account of their
lower alkalinity, as the corresponding hydroxides in changing the permeability
characteristics. The values obtained up to 1.5 percent addition are erratic, beyond
which aggregation occurs and permeability increases. However, even at as high
percentage as 10, there is no evidence of the formation and subsequent removal of
the humates, possibly due to the pH not rising adequately to initiate the reaction
with the humus of the soil. With 10 percent of NaCL, the permeability value of the
S-2, S-4, and S-12 soils are 14.8, 45.2 and 30.1 x 10-7cm/sem., while with the
same amount of KCL, the values are 68.1, 1775.0 and 988 x 10-7cm/sec.
The continuous increase of permeability up to 10 percent NaCL in the case of S-12
Soil, show that the aggregation continuous to occur even up to that percentage and
this may be due to the clay content of the soil being the highest of all the three
CaCL2 behaves in a very much similar way as Ca(OH)2 increasing the permeability
values at all percentage, permeability as high as 65.0 x 10-7cm/sec. at 7 percent in
the case of S-2 Soil, 1150 x 10-7 cm/sec Percent in the case of S-4 Soil, 988 x
10-7 cm/sec. at 10 percent in the case of S-12 Soil are obtained.
MgCl2 was tried on S-2 and S-4 soils and was found to be not much effective; the
permeability values obtained being less than those for blank soils. At higher
percentage, however, the values increase.
6.4.3 Carbonates: Na2Co3 being a highly dispersing agent, decreases the
value of the permeability even at low percentage. Further addition of additive does
not appreciably alter the values.
K2Co3 and CaCo3 were tried on S-2 and S-4 soils. K2Co3 being a comparatively
stronger alkali than Na2 Co3, permeability value decreases initially up to about 2 to
3 percent, due to the dispersion and beyond this the values increase due to the
removal of humus. The values obtained at higher percentage are in between those
of KCL and KOH.
CaCo3 reduces the permeability up to 1.5 percent, where after the values
continuously increase up to 10 percent. For instance in the case of S-4 soil, the
permeability at 1.5 per cent is 2.6 x 10-7cm/sec. which rises to 34.8 x 10-7cm/sec.
at 10 percent. The chemical has low order of solubility and dissociation and hence
at low percentages, the fine particle of the un-dissociated chemicals, plug the pores
into the soil sample, thereby lowering the permeability values. At higher
percentages enough calcium ion released to cause not effect of aggregation
resulting in higher values of permeability, inspire of the unassociated chemicals
continuing to plug the pores. BaCo3 and MgCo3 did not show any consistent trend
with the soils probably to the simultaneous action of both aggregation and plugging
the pores process.
It is evident from the previous investigation that certain inorganic chemical are
effective in significantly changing the textural and permeability of black cotton soils.
Some of these chemicals are soluble and some are insoluble.
6.5 Use of Lime-Cement and Combination of Lime and Cement: The
primary purpose of this study is to evaluate the unconfined compressive strength,
bearing capacity, shear strength, flexural strength and durability characteristics of
black cotton soil samples treated with lime and cement.
Lime used is this investigation was a calcium hydroxide of technical grade and the
cement was a normal Portland cement.
6.5.1 The studies conducted by Katti on lime alone on soils. S-1 to S-12 i.e.
from Sholapur, Poona, Sidheswar, Nasik, Nagpur, Sholapur, Veldhari, Amravati,
Baroda, Wadagaon, sites. It may be seen that 7 day compressive strength in all
cases, are less than 300 psi. However in most soils, 28 days compressive strength
of over 300 psi can be obtained. These large increases in strength with time may be
attributed to the long term pozzolanic reaction taking place in the soil mixes.
6.5.2 In general soil- Cement mixes show increase in strength with
increasing amount of cement. There are several mixes in all soil, containing less
than 15 per cent total admixture which give 7 day strengths as high as 500 to 600
psi. Normally 6 to 12% of total admixtures is found to be sufficient to give more
than 300 psi after 7 day curing, the amount of lime content in combination may
vary between 2 to 4 percent and cement 4 to 10 percent. These observations show
that the presence of lime has changed the texture of the soil giving rides to
reduction in surface area and making the mixes behaves like silty or sandy soils.
It may be noted that the strength of soil-lime-cement mixes is due to the combined
effect of aggregation, hydration and posssilanic reactions.
6.5.3 The soil samples used for the investigation of bearing characteristic
are S-2 and S-8 i.e. from Poona and Amravati.
The addition of increasing amounts, of lime cement or combination of lime and
cement show a variation of bearing characteristics almost similar to that observed
for unconfined compressive strength.
6.5.4 Effect of lime and Cement on Strength characteristics
Lime and cement added together generally resulted in comparatively large
improvement in strength characteristics. An optimum lime content varying between
6 to 8% gives for a given amount of cement .The maximum cohesion, internal
friction and initial tangent modulus values. Cohesion values of over 100 psi. Are
obtained for almost all the mixes containing more than 20 per cent total admixture,
while the values of angle of internal friction are of the order of 200 to 300 and
initial tangent modulus are over 30 kips per sq. in for most of the mixes.
6.5.5 Effect of lime and cement on flexural characteristics of soils : The soil sample
use in the investigation was S-2 soil. The raw soil did not have any flexural
strength, the addition of lime and of cement enhanced Mr and Est value, where Mr
is the modulus of rupture and Est is the modulus of elasticity of rupture. The
addition of 4 to 6 per cent of lime, the Mr and Est value have increased to 71 and
128000 psi respectively. Further addition of lime have not been useful Effect of
cement are however smaller combination of lime and cement have given Mr values
of up to even 100 psi. In general with increasing amounts of cement for constant
lime contents, there is increase in strength and the strengths of combinations of
admixtures are not a superposition of the strengths of two components added
6.5.6 As soil-lime and soil-lime-cement mixes are anticipate to be used for
base and sub-base construction with a wearing coat in top, the mixes are subjected
to wetting and drying tests, as per ASTM procedure, for volume change
6.5.7 Both increasing moisture content and aging period cause reduction in
density and increase in O.M.C. However with increasing amount oaf lime for zero
aging period, there is increase in density upto certain amount of lime added and
decrease thereafter. For other aging period with the addition of lime up to a certain
amount the density rapidly decrease. Further addition of lime would not bring about
any greater reduction in density.
The alteration in density due to the addition of various percentages of cement to
the different soil-lime mixtures does not show any definite trend.
6.5.8 Field Test on Soils with Lime and Cement: It can be observed from the
laboratory investigation that combination of lime and cement were promising in
improving the plasticity, unconfined compressive strength, shearing strength and
flexural properties. It was there-fore decided to try a few of these mixes in the field
as a base coarse material under the various weather and sugared moisture
condition to evaluate the performance of soil-lime- cement mix, and to compare the
strength obtained with similar mixes under laboratory controlled condition.
The unconfined compressive strength studies on laboratory and field cured
specimen indicate that about 48 to 60 percent of the laboratory strength can be
attained in the field. C.B.R. values for laboratory cured samples exceeded beyond
200 for the mixes tried even after 20 day mixing. The field studies indicate that
even under the severe weather conditions, the soil-lime cement base courses may
stand fairly well.
6.6 Stabilization of soils with Aniline and furfural: Early stabilization
studied indicated that artificial resins were a mean for accomplishing a practical
stabilization with satisfactory results. The most effective of these artificial resins is
the resin formed by the reacting on two parts of anticline to one part of the furfural.
Aniline and furfural are two liquid organic chemicals that polymerize on contact to
form a resign know as aniline furfural resin. Aniline reacts with baldheads to from a
group of compounds that are known as „Schiff‟ bases.
6.7 Use of Molasses for stabilization: Molasses which is a byproduct from
sugar factory in area close to the factory it is available cheap. Some of the
laboratory experiments have shown that the soil can be successfully stabilized with
molasses. Molasses is a mixture of Glucose (C6H12O6) and Sucrose (C12H22O12). Also
small quantities of inorganic salts and organic compounds are present.
Table No. 6.1- Variation of Permeability of S-12 soil, with the addition
of various Inorganic Chemicals.
Chemical in Percent of oven dry weight of soil
1.0 1.5 2 3 5 7 10
K expressed in unit of 10-7
6.0 1.4 1.7 2.6 4.2 4.9 36.1 379.
3.8 8.1 9.9 17.
8.0 8.4 8.2 8.7 9.5 13.4 16.9 20.8 24.7 30.1
60.5 80.5 159.
3.6 3.9 2.6 0.9 1.3 1.3 0.8 1.9 2.0 1.9
Permeability of blank soil was 4.8 x 10-7
Table No 6.3-C .B. R. Test Value @ 5 mm Penetration
0.0 0.1 0.25 0.50 0.75 1.0 3.0 7.0
KOH 3.84 3.72 4.66 4.22 5.19 3.90 16.46 26.18
NaOH 3.84 4.50 3.99 3.87 3.86 4.28 21.44 23.59
Ca(OH)2 3.84 4.54 3.84 4.86 5.90 8.38 47.20 83.18
Cement 3.84 4.23 4.13 4.59 4.67 5.64 11.39 42.33
Na2CO3 3.84 4.78 3.33 3.27 4.07 4.11 5.16 6.31
An attempt has been made to review and edit the vast literature available on the
subject of expansive soils, so as to elucidate the present status of knowledge on the
Though there are number of identification methods and techniques, classifications
systems of expansive soils are only few. None of the classification system takes into
account the percentage of clay minerals.
The literature on engineering properties is scanty and gives properties of some local
soils only. A systematic soil survey of expansive soils should be undertaken. The
procedure for measurement of swelling pressure needs standardization taking into
consideration of various factors affecting it. There is much scope for effect of
swelling on retaining structures.
Each of the construction technique described in the chapter 4, has its own
limitations. Under-reamed pile foundations described in detail in chapter 5 but the
other foundation techniques are becoming important. There is a scope to develop
some simple techniques for low cost houses.
Stabilization methods using inorganic and organic additives are described in chapter
6, but more practical methods need to be developed.
Our sincere efforts to include maximum available information had certain limitations
such as non-availability of references and time limit to complete the project work.
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