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 1974-1975
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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
1974-1975
PROLOGUE
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
saturated.
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
selected.
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
soils.
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
curriculum.
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
form.
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.
14th 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
of foundations.
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
Expansive soils.
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)
CONTENTS
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
Bibliography 63
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
(Wadgaon)
58
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
partition wall
03
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
**.**
SYNOPSIS
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
discussed.
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
chapter.
Based on the limited review of the available literature on expansive soils,
suggestions for further studies are made.
***.***
1- INTRODUCTION
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
gneisses.
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
cracks.
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
summer.
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
rock.
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
explained below.
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
calculated as,
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
swelling.
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
below.
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.
1-Colliod
content
2. Plasticity
Index (PI) %
3.Shrinkage
Limit (SL)%
4-Probable
expansion
#
5-Degree of
Expansion
<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
results.
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
or exothermic.
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
microns.
Fig.2.3-Typical dehydration curve for B.C. soil
Since the spacing of atoms in crystalline structure is of the order of 10A, the
diffraction of x-rays with wave length 1A 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
improve lines.
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. Classification
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
pH 8-9
CaCO3 5-15%
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
group Index.
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
soil.
6. Burmister classification (Based on grain size classification and
distribution.
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.
Where
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
20-30 A-7a
30-40 A-7b
40-50 A-7c
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,