-
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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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.
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ii
Worlds 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)
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iii
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)
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iv
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
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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
***.***
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vi
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
**.**
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vii
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.
***.***
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1
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.
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2
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
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3
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
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4
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,
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5
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
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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.
***.***
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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
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8
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.
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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
-
10
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.
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11
Fig.2.1-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
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12
size of and spacing of suspended particles with sizes varying
from 1 to 10
microns.
Fig.2.2-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.
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13
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.2.3 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%
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14
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. Cassagrandes 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.
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15
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
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16
d= portion of numerical Plasticity Index greater than 10% and
not exceeding
30, expressed as positive number (0
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17
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)x A2.44 x c3.44
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.
Degree of
expansion
Swelling
potential %
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
content. Thus,
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18
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 x N
Where
S.P. = swelling potential
S.I. = Shrinkage Index (rational index for volume change of
clays)
N = c3.44/(c-n) 2.67
Where c= clay fraction
n=Intercept on the curve (SI Vs Clay fraction) Ref. Fig.4) it
varies from 4 to
22.
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
Ref.Fig.2.4.
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19
Fig.2.4-Parameter for different n and clay fraction
The authors have given another classification system as shown in
Table 2.5
below.
Table 2.5 -Classification based on S.I.
Classification Shrinkage Index
Low 0 -20
Medium 20 -30
High 30 -60
Very High >60
.
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20
Fig.2.5-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 ().
Swelling index of soil is a ratio of porosities of soil in
saturated and natural
conditions.
Swelling index () = E/E0 where E is porosity of swollen soil and
E0 is
porosity of natural soil. The swelling index () 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.)
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21
15-19.9 20-24.9 25-29.9 30-34.9 35-39.9
Type of soil swelling index ()
Non-Swelling 1.12 1.11 1.09 1.08 1.07
Slightly Swelling 1.12-
1.23
1.11-
1.21
1.09-1.19 1.08-1.17 1.07-1.15
Medium Swelling 1.23-
1.39
1.21-
1.30
1.19-1.28 1.17-1.25 1.15-1.22
Highly Swelling 1.39 1.30 1.28 1.25 1.22
***.***
-
22
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
in brief.
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 Chapmons 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.
-
23
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
-
24
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
workers.
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
map.
Table 3.1-Locations of 16 soil samples
S1-Solapur 2 S2-Poona1
Fig. 3.1 Site map of samples tested
S3-
Siddheshwar
S4-Nasik
S5-Nagpur S6-Solapur 1
S7-Yeldhari S8-Amraoti
S9-Baroda S10-Bezwada
S11-
Wadgaon1
S12-Wadgaon2
S13- Poona2 S14-Calcium
Bentonite
S15-Sodium
Bentonite
S16-Powai
Mumbai
-
25
Table 3.2- Notations used in tables
L.L % Liquid Limit S.G. Specific Gravity
P.I.% Plasticity Index Clay -5 Fraction < 5
S.L. % Shrinkage Limit Clay -1 Fraction < 1
S.R. Shrinkage ratio
..
Density -SP Max. dry density as per light compaction
OMC-SP Optimum moisture content as per light
compaction
Density-MP Max. dry density as per heavy compaction
OMC-MP Optimum moisture content as per heavy
compaction
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
B.E.C. -2 Base Exchange capacity for particles smaller
than 2
SiO2 % Silica Content
Al2O3 % Alumina content
CaO % Calcium hydroxide
MgO % Magnesium hydroxide
FeO3 % Ferric Oxide
TiO3 % Titanium Oxide
SO3 % Sulphur oxide
-
26
LOI % Loss on ignition
Table 3.3- Properties of Black cotton soils
Property
Sample No. (See legend in Fig. 11
S1 S2 S3 S4 S5 S6 S7 S8
L.L % 69.2 67.2 70.3 72.3 59.2 65.7 68.0 81.0
P.I.% 27.3 18.3 28.4 24.6 15.9 25.0 21.8 34.0
S.L. % 12.4 8.2 13.5 7.4 10.3 11.9 14.1 10.0
S.R. 2.07 2.1 2.0 2.0 2.1 2.0 1.9 2.1
S.G. 2.74 2.72 2.71 2.7 2.7 2.67 2.72 2.72
Gravel
%
21.0 0.0 3.0 2.4 8.5 3.0 3.5 0.0
Sand% 18.0 17.5 21.0 16.6 12.5 18.0 10.0 13.5
Silt % 28.2 48.5 34.5 32.5 28.2 26.5 32.5 32.5
Clay -5 32.8 39.0 41.5 48.5 50.8 52.5 54.0 54.0
Clay -1 - - - - - - - -
IS
classi-fication
M.H. M.H. M.H. M.H. M.H. M.H. M.H. M.H.
..
Table 3.4- Properties of Black cotton soils
Property
Sample No. (See legend in Fig. 11
S9 S10 S11 S12 S12 S14 S15 S16
L.L % 56.5 91.8 52.9 73.3 67.0 300.0 325.0 65.0
P.I.% 30.5 53.5 21.3 31.6 18.0 250.0 265.0 44.0
S.L. % 8.2 9.8 17.8 12.7 8.0 - - 20.0
S.R. 2.2 2.2 1.8 1.9 2.1 - - -
S.G. 2.73 2.81 2.79 2.76 2.8 - - 2.9
Gravel
%
0.0 1.5 4.0 0.0 0.0 0.0 0.0 0.0
Sand% 17.0 20.5 26.0 12.0 15.2 0.0 0.0 28.0
Silt % 27.0 17.2 18.0 25.0 15.8 0.0 0.0 27.1
Clay -5 56.0 60.8 62.0 68.0 69.5 0.0 0.0 11.2
Clay -1 - - - - 42.5 100.0 100.0 27.2
IS
classi-fication
MH MH MH MH MH - - CH
..
Table 3.5- Properties of Black cotton soils
-
27
Property
Sample No. (See legend in Fig. 11
S1 S2 S3 S4 S5 S6 S7 S8
Density -SP 1.40 1.33 1.46 1.42 1.57 1.43 1.46 1.33
OMC-SP 29.5 29.4 28.0 29.5 23.0 28.5 29.2 33.0
Density-MP 1.67 1.66 1.63 1.68 1.80 1.63 1.64 1.43
OMC-MP 23.0 24.0 24.5 20.0 17.0 20.0 22.0 24.5
Sw.Pr. - 3.9 - - 0.95 3.0 - -
pH 8.75 8.45 8.9 8.5 8.2 8.5 8.7 7.4
Org.Mat. 0.55 1.42 0.7 0.7 0.4 0.8 0.8 0.6
CO3 2.42 6.65 4.4 3.3 0.5 2.6 1.9 0.2
B.E.C. -400 57.6 60.0 57.9 65.3 51.1 59.1 58.5 72.4
B.E.C. -2 109.2 - 84.4 124.6 99.4 111.0 160.6 132.4
SiO2 % 49.3 50.3 45.6 47.1 58.1 48.6 47.7 53.2
Al2O3 % 13.7 21.9 14.5 16.7 15.6 13.8 15.5 15.7
CaO % 6.9 8.0 7.4 6.2 2.7 7.2 4.4 2.8
MgO % 4.8 4.4 4.1 3.2 2.5 5.0 3.7 2.7
FeO3 % 14.8 1.4 12.6 12.6 10.3 13.4 15.1 14.0
TiO3 % 1.9 0.3 2.0 1.5 1.3 2.2 2.4 2.0
SO3 % 1.6 - 1.1 1.9 1.8 2.0 1.4 1.2
LOI % 16.5 13.6 13.9 13.0 8.6 4.8 10.7 9.2
..
Table 3.6- Properties of Black cotton soils
Property
Sample No. (See legend in Fig. 11
S9 S10 S11 S12 S13 S14 S15 S16
Density -SP 1.57 1.41 1.52 1.40 1.33
0.0 0.0 1.29
OMC-SP 24.5 30.4 26.0 30.0 29.4 - - 36.0
Density-MP 1.84 1.63 - 1.59 - - - 1.61
OMC-MP 19.6 28.5 - 25.0 - - - 30.5
Sw.Pr. 0.95 - - - - - -
pH 8.5 8.8 6.7 7.5 8.5 - - -
Org.Mat. 0.6 0.4 3.6 1.0 1.4 - - -
CO3 0.2 0.4 - 0.3 6.7 - - -
B.E.C. -400 38.2 47.4 - 70.8 57.0 - - 44.5
B.E.C. -2 83.2 97.8 69.4 110.8 108.0 - 140.0 -
SiO2 % 61.3 57.0 48.3 47.5 50.3 - - 42.5
Al2O3 % 13.6 17.5 22.0 17.8 21.9 - - 21.2
CaO % 2.7 1.6 1.0 4.5 8.0 - - 0.62
MgO % 1.8 2.6 1.9 3.9 4.4 - - 1.51
FeO3 % 11.3 10.3 7.5 13.7 1.5 - - 9.45
TiO3 % 2.0 1.1 1.0 1.3 0.3 - - 0.52
SO3 % 0.9 1.3 0.01 1.2 - - - -
LOI % 9.4 8.2 - 8.8 13.7 - - -
-
28
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.
-
29
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
content.
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.
-
30
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 the
swell
pressure is very small but as the degree of compaction increased
beyond
this value there is abrupt rise in swelling pressure.
-
31
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
-
32
the swelling pressure is found to be independent of the size of
the test
specimen.
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
section.
-
33
3.6. Lateral swelling pressure: The phenomenon of lateral
swelling of
expansive soil is well known. Many structures crack due to
lateral swelling
pressures.
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.
***.***
-
34
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
India.
4.2 The problems posed by expansive soils of India can be
summarized as
below,
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.
-
35
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
stratum.
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.
-
36
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
excavated.
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
-
37
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
-
38
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
-
39
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
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40
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.
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41
(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
suggested.
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
movement.
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
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42
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
damage.
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43
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.
***.***
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44
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.
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45
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
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46
Fig.5.4 Boring in progress Fig.5.5 Pullout of hand auger
Fig.5.6 Reinforcement details Fig.5.7 Standard dimensions
***.***
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47
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48
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.
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49
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%.
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50
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.
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51
Free water is essential for reaction to take place between soil
and the chemical
added.
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.
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52
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
like cementing.
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
period respectively.
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.
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53
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.
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54
At higher percentages, aggregation effect seems to set in,
leading the higher value
of permeability.
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
permeability.
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
decrease.
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
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55
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.
respectively.
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
soils used.
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
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56
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 an