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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.

  • 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)

  • 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 -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. 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.

  • 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

  • d= portion of numerical Plasticity Index greater than 10% and not exceeding

    30, expressed as positive number (0

  • 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,

  • 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.

  • Fig.2.5-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

    .

  • Fig.2.6-Shrinkage index Vs clay fraction

    2.3.4. E.A. Sorochan (1970) experimentally proved that swelling process is

    anisotropic. It is a result of textural and structural features as well as of the

    character of stratification of soils. So a new term swelling index ().

    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.)

  • 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

    ***.***

  • 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.

  • The suction potential theory of Schcefield, also does not account for the

    entire swelling as it is observed that there is residual swelling even if soil

    suction is nil.

    There is further intake of moisture until the hydration of ions and soil

    particles is complete and particles of soil have reoriented with respect to

    forces which keep them together, viz the confining pressures and the

    attraction between clay particles. Both these concepts viz the theory of

    double layer depending entirely on physical chemical properties and suction

    potential based on capillary only, do not take into consideration the effect of

    elastic properties in relation to external forces.

    Terzaghi, K. has advanced hid concept of swelling based on elastic properties

    of soils. According to him, the swelling is wholly due to elastic properties of

    soils, the physic-chemical properties of soil do not play any role in the

    swelling phenomenon. This is true for two reasons. Firstly, the surface

    behavior of charged particles leading to Base Exchange and absorption of

    water molecules as dipoles, have profound influence on swelling. Secondly

    the interlayer spaces in which water molecules are retained influence

    swelling. The application of pressure brings the particles closer expelling

    pore water. Increase of pressure expels more water that has been absorbed.

    The process goes on till the inter particle spacing has been reduced to a

    distance of approximately 20A. At this stage all the water between particles

    is tightly held and the extraction of inter particle water by inter granular

    pressure alone is thus impossible though there might be isolated areas of

    mineral to mineral contact where water has been completely eliminated. Also

    the inter layer water which is responsible for swelling to a large degree is not

    removed by mechanical means.

    It is thus evident that for any theory to explain swelling phenomenon in soils

    completely, it should take into account the physic-chemical affects due

  • hydration of exchangeable ions and that of clay particles, the soil suction

    and elastic behavior of soils in relation to external forces. Further research of

    the subject should aim at combining the three concepts to obtain a more

    rational theory of swelling phenomenon.

    3.3 Physical and engineering properties of black cotton soils varies from

    place to place. Out of various research papers available on this subject few

    papers contains properties of local soil. A compilation of various properties of

    black cotton soils, if made, will be very useful to engineers and research

    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

  • 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

  • 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

  • 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 - - -

  • 3.3.1 Measurement of swelling pressures: When an expansive soil attracts

    and accumulates water, a pressure known as swelling or expansion pressure

    builds up in the soil and it is exerted on the overlying material and structure

    if there are any.

    Swelling pressure is defined as If a swelling substance is tightly enclosed in

    a vessel with a wall permeable to a swelling solvent and latter is allowed to

    diffuse into the vessel, the dilation tendency of the soil solvent gel give rise

    to a pressure called Swelling pressure.

    The two commonly used methods for measurement of swelling pressure are,

    1Constant Volume method or Constant Pressure method

    3.3.2 Constant Volume method: In this the soil is mixed with appropriate

    quantity of water. After maturing period the soil is placed in a mould. The

    bulk density and water content of the specimen is determined by standard

    methods. The specimen is covered with porous stones and filter paper. The

    entire mould in placed in a water trough under loading machine with proving

    ring and dial gauge to measure force and swelling of soil. The expansion of

    soil specimen is nullified by applying force gradually and proving ring reading

    is recorded at different time intervals till there is no further swelling of soil.

  • Fig.3.2 -Constant Pressure Method

    Pressure intensity is calculated from proving ring reading and specimen

    area. A pressure Vs time graph is plotted. The maximum pressure intensity

    gives the swelling pressure of soil for a specific dry density and water

    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.

  • 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.

  • 3.4.3 Time of saturation: The process of swelling is gradual because soil

    takes time for the water to penetrate into soil layers and cause expansion

    cumulatively. Therefore time allowed for expansion is an important factor.

    The affinity for absorption being great in soils with low moisture content,

    initial rate increases the swell pressure in those soils is greater than those

    soils with higher water content. It can thus be anticipated that soils with

    lower moisture will have a very percentage of swell even during initial

    contact with water. Initial rate of increase of swell pressure is lesser in soils

    with higher densities. This may be the effect of lower permeability of the soil

    and is also of great significance in practice.

    3.4.4 Free expansion permitted: Swell pressure is a consequence of the

    restraint on the free swelling. Any expansion allowed result in a reduction of

    swelling pressure. An expansion of 0.025 mm is said to reduce the swell

    pressure by as much as 5 KN/sqm.

    Two identical samples were tested using proving rings of different stiffness.

    A proving ring with lesser stiffness undergoes large deformation. A soil has

    thus a definite free expansion before developing the full swell pressure.

    The swell pressure under any building foundation will be equal to the

    foundation pressure. The difference between the possible maximum swell

    pressure and foundation pressure results in an expansion and consequent

    vertical movement of the structure.

    3.4.5 Sample height: Some tests were conducted by Uppal and Palit

    (Ref.32) to study the effect of height of sample on swelling pressure. The

    process of swelling is result of building absorbed water films. Given sufficient

    time such action will take place over the entire depth of clay stratum. The

    quantitative swell and swelling pressure should be a cumulative effect. The

    swelling pressure is observed to vary directly with the height and inversely

    with the diameter of the specimen. However if the skin friction is eliminated

  • the swelling pressure is found to be independent of the size of the test

    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.

  • 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.

    ***.***

  • 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.

  • 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.

  • 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

  • of the clay will be the governing factor. The uplift pressure is greatest near

    the top of the pier where the clay expands most. In some cases, uplift has

    been sufficient to pull the pier in two at the top of bell. Ref. Fig.4.1

    Fig.4.1 - The pier and belled footing

    It is believed that the following criteria can be used for the design of

    successful foundations of cast in place pier and belled footing units.

    (a) Use as high contact pressure as is consistent with carrying capacity

    of the soil.

    (b) Use bell 3 times diameter of pier for maximum anchor.

    (c) Use smallest pier compatible with load and bell size in order to keep

    surface area minimum.

    (d) Extend reinforcement into bell to within 4 of bottom in order to

    anchor pier to bell.

    Sometime the oversize hole is drilled to the entire depth and the bell is

    formed at the bottom of the oversize hole. The bell is filled with concrete to

    extend a slight distance in to the pier above the bell and the casing for the

    pier is pushed a short distance into the fresh concrete in order to prevent

    concrete from rising into the space around the outside of the casing. When

    using this procedure, care should be exercised to see that the casing is not

  • let into the hole before the concrete has been placed in the bell otherwise a

    shaft may be cast with no footing.

    4.3.9 The grade beams or plinth beams cast in contact with desiccated clay

    are sometimes broken be uplift pressure of expanding clay. Even if the grade

    beams were reinforced to resist this pressure, the uplift on the supports may

    cause as much damage as if the beam were allowed to break Provision

    should be made for a void under grade beams into witch the clay can expand

    without exerting uplift pressure.

    The use of collapsible card board beam boxes is much more practical and

    sure method of preventing uplift under grade beams. These cardboard boxes

    are shipped flat and are folded to form a hollow box of the proper

    dimensions for the purpose. The cardboard is treated to prevent immediate

    disintegration and to remain strong enough to support runways for concrete

    buggies long enough, for concrete to be placed and harden. These cardboard

    beam boxes are produced commercially in Kansas and Texas.

    4.3.10 Several methods have been devised for casting the structural floor

    system on forms that lie directly on the clay and disintegrate after a short

    period leaving a space for expansion of the clay.

    Fig.4.2 - Structural floor system

    One method for forming the slab which has been sued experimentally is to

    loosen the clay to a depth of 30 to 50 cm. and to form the loose soil in

  • windrows to make a form for Joists. In order for this method to be

    successful, the depth of the loosened clay must be adjusted to existing

    conditions. The volume decrease of the loosened soil must be equal to or

    greater that the volume increase of the undisturbed clay below the loosened

    material. This method cannot be considered reliable, as during construction

    of the loose fill, the soil may be compacted unfit is will itself swell as much

    as or more than, the undisturbed soil.

    This method consists of excavation deeply enough to form the area solid

    with baled hay or straw laid end to end and side by side. These bales are

    covered with roofing felt or sisal craft. The depressions between the bales

    are forms for joists. The hay or straw is sprayed with ammonium nitrate to

    accelerate disintegration of the straw. But the hay increases the fire hazard

    and makes the construction site look like a feed lot. The aesthetic value of

    rotting hay under the floor is questionable.

    An effective method of providing void spaces under slab and beams into

    which the clay can expand without producing uplift pressure is by the use of

    water proof cardboard forms of sufficient strength to support the fresh

    concrete and which later disintegrates. The cardboard forms are shipped flat

    and are folder into shape during installation. But when the basement floor is

    formed and cast before the basement walls are erected, the collapsible

    forms are exposed to the weather during construction of floor, and the banks

    of the excavation are susceptible to sloughing or sliding into the excavation,

    which weakens the exposed cardboard forms during rainy season and

    collapse. Sometimes a card board form is placed under the basement wall.

    Under a heavy load, this method is ineffective because the beam box may be

    crushed by the weight of fresh concrete.

    Another arrangement known as slab on sonotube forms may be used. In

    this method split sonotube are laid side by side to provide forms for joist

  • below the bottom of the concrete joists. The bottom of space between the

    two halves is filled with sand about 7 to 8 cm deep. The joist steel and

    concrete are placed to form a reinforced concrete floor slab supported on

    grade or plinth beams After a short time, the sonotubes disintegrate, the

    sand runs out from under the joists, and a void is formed into which the clay

    can swell without exerting pressure on the bottom of the slab.

    4.3.11 The most common and best suited of all is the under-reamed pile

    foundation. This method is discussed in detail in the next chapter.

    4.4.0 There are problems posed to the old buildings which are standing.

    The techniques or the remedial measures used for the prevention and

    further developments of cracks are discussed below.

    4.4.1 A. K. (9) and Subash Chandra suggests a simple method for the

    prevention recurrent in small buildings founded on Black Cotton Soil,

    directed at keeping the moisture content in soil immediately under and

    around the building as constant as possible so as to minimize the ground

    movement. Vertical sand drains connected by channels are placed about 2m.

    on centers all around the effected building. Waste water from the building

    was allowed to flow into them. A line concrete apron laid on polythene

    membrane may be added between the walls of the building and the sand

    drains to retard loss of moisture by evaporation as much as possible.

    4.4.2 Ward, W.H. (40) studied the effect of fast growing trees and shrubs on

    shallow foundation. According to him, in summer the trees absorb large

    quantities of water from the clay under footing which then shrinks

    appreciably and lets down the structure which is incapable of resisting the

    settlement. The shrinkage one reaches as far as the most remote root which

    generally extends distance greater than the height of the tree.

    (1) So the fast growing trees should not be planted near the foundation.

  • (2) The footing is placed sufficiently deep in a zone not affected by soil

    moisture movements and

    (3) The structure may have shallow foundation but be made strong

    enough to resist cracking.

    4.4.3 Rao N.V.R.L.N., and Krishnamurthy (29) suggested a method on the

    same principle that, the moisture content under the foundation and around

    the building should remain constant as far as possible. They put forward the

    idea of soak way pits, at proper spacing so that water drains quickly and the

    soil surrounding the building remains dry. The soak way pits are filled with

    materials like sand and gravel, and a concrete apron around the building is

    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

  • 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.

  • It has been observe that the underground flexible aprons around buildings

    arrest further cracking. After two cycles of seasons the cracks becomes

    stable and no further damage is generally noticed.

    ***.***

  • 5-UNDER-REAMED PILE FOUNDATIONS

    5.1 Introduction: The best method of foundations in expansive soils is

    foundation which is anchored in the stable zone of the ground, in which the

    moisture variations are negligible. This was observed from the performance

    of cast-in-situ piles with enlarged bases. Such piles were successfully

    installed in South Africa and Israel. CBRI Roorkee realized the importance of

    such piles and undertook a research project to develop a simple procedure

    for manually operated hand augured piles. More than 5000 piles were

    constructed and tested in various parts of India and based on the practical

    experience CBRI Roorkee published and published a manual on under-

    reamed piles and gave design tables for various diameters of augured piles.

    Subsequently Bureau of Indian Standards published a code of practice

    I.S.2911 part 3. The code describes the various parts such as pile, grade

    beams and reinforcement details. The code also includes a design formula

    for working out load carrying capacity of a single on multi-reamed piles. The

    code also includes the equipment required for such construction. A method

    of load test on piles is also included.

    5.2 Limitations of UR piles: There are many limitations to construction of

    under-reamed piles and are discussed below;

    Needs strict supervision: Unless there is strict supervision by expert, the

    whole purpose of this technique is lost. The check points as listed below.

    Exact location-Insist use of guide on ground for proper location and

    inclination of pile.

    Proper length of pile- The top bulb must be in the stable.

    Checking of bulb diameter- Use L bar to check the bulb diameter

    Spacing between two bulbs- Adequate spacing is must to avoid

    collapse of side wall of bore.

  • Concreting Use PVC pipe during poring of concrete of desired slump.

    No vibrator is to be used. Use heavy tamping rods.

    Piles should be randomly selected for load test.

    5.3 The different design and construction steps are illustrated through

    Fig. 5,1 to 5.7 below

    Fig.5.1 - Construction Stages Fig. 5.2 Measurement of bulb

    .

    Fig.5.3- Details of under-reamed pile

  • Fig.5.4 Boring in progress Fig.5.5 Pullout of hand auger

    Fig.5.6 Reinforcement details Fig.5.7 Standard dimensions

    ***.***

  • 6 STABILISATION OF EXPANSIVE SOILS

    6.0 Introduction: Stabilization in a broad sense incorporates the various methods

    employed for modifying the properties of a soil to improve its engineering

    performance. Stabilization is being used for a variety of engineering works, the

    most common application being in the construction roads and foundation purposes,

    where the main objective is to increase the strength, improve the stability of soil

    mass and to reduce the construction cost.

    With this in mind studies were conducted by Katti (21) and others to evaluate the

    effect of inorganic chemical on various properties of black cotton soils.

    6.1 Effect of inorganic chemicals on the consistency properties.

    For this study they selected soils S-2, S-4, S-5, S-6, S-9, S-9, S-10 and S-11 i.e.

    from Poona, Nasik, Nagpur, Sholapur, Baroda, Bezawada, Wadagaon sites. The

    chemicals used for treating some or all the soils were hydroxides of Na, K, Ca, Mg,

    Ba and Fe, carbonates of Na, Mg and Ba; cement, sodium silicate, Di-ammonium

    phosphate, suplhates of Na and Cu, phosphates of Mg and Ca and potassium

    dichromate. The percentage of chemicals used varied between 0 to 10 percent

    based on the over dry weight of the soil.

    6.1.1 Hydroxides :The variation in the consistency properties of the soils treated

    with hydroxides, of potassium, sodium and calcium is represented in fig. In case of

    all soils other than S-4, the addition of KOH varying from 1.5 to 7 percent has

    made the soil non-plastic. S-4 shows disruptive effect. KOH goes on reducing the

    liquid limit and plasticity index. 0.75 to 3 percent, the shrinkage limit value

    significantly increased indication that volume change tendency has been

    considerably decreased. The shrinkage limits go as high as 40 in some cases from

    initial value of around 8 to 10. The increase in Plasticity Index at small percentage

    may be due to the dispersion effect.

    The dispersive action of NaOH with small addition is evident. The L.L. of nearly all

    the soils increases up to about 1 to 1.5% and in the same range the P.L. decrease

  • and P.I. increases. Larger addition invariably causes lowering of L.L., increase in

    P.L. and decrease in P.I. At small percentage of NaOH decrease in S.L. is observed.

    However beyond about 0.75% the S.L. value nearly always increase with increasing

    additive. These results indicate that while at low percentages of NaOH these is a

    tendency for dispersion to take place, further addition results in less of plasticity

    and increase in S.L.

    The addition of Ca(OH)2 beyond about 1% distinctly goes on reducing the L.L. and

    P.I. and increasing P.L. These results indicate that all the soils become non-plastic

    beyond 1.5%, except S-10 soil. The shrinkage limit value continuously increase

    with the addition Ca(OH)2.

    Mg(OH)2 does not seem to have appreciable effect on the consistency properties of

    any of the soils.

    6.1.2 Chlorides: CaCl2, BaCL2 and MgCl2, do not have much effect on the

    P.L. and S. L. of the soil. However, there is decrease in L.L. values and decrease in

    P.L. value. It may be noted that while in case of Ca(OH)2 there is an increase in

    P.L. and S. L. with the additive, these values more or less remains constant in ease

    of calcium chlorides. This effect may be due to the fact that the chlorides are more

    alkaline than the corresponding hydroxides.

    With the addition FeCl3, the L.L. value show a tendency to decrease and P.L. values

    more or less constant. It was possible to determine S.L. only in case of S-9, S-10,

    S-11 soils and these did not show significant change. In other soils, it was not

    possible to determine S.L. values. It was observed that the addition of FeCl3

    beyond L percent makes the soil mass porous like bread. This may be due to the

    formation of HCL which on reaction with the carbonates present on the soil evolves

    CO2. the escape of the gas gives rise to the porous structure. Chemical test

    confirmed that CO2 was liberated during the processes. It may be noted that S-9,

    S-10, and S-11 soils contain less than 0.5% carbonates which the other contain

    even up to 6.65%.

  • KCI and NaCL were tried only on S-2 soil, These chemicals increase the S.L. values

    to a great extent while L.L. and P. L. values decrease. KCI seems to be more

    effective than NaCL.

    6.1.3 Carbonates :MgCo3 increases the L.L. and P.L. values while BaCo3

    does not show any marked effect. The S. L. values tend to increase. Na2Co3 was

    used with S-2,S-4,S-5, and S-6 soils. All carbonates may be said to produce

    dispersion and cause increase in plasticity.

    6.1.4 Cement :It can be noted that cement has a similar effect as Ca(OH)2

    but to a lesser degree. This may be due to the lesser amount of free lime available

    from cement. It may be noted that even with 10 per cent of cement, the soils do

    not become non-plastic. The S.L. values however, considerably increase with the

    addition of cement.

    6.1.5 Na2Sio3 :Sodium silicate increase the L.L. and P.I. for all the soils and

    make them highly plastic. This may be attributed to the disperse effect. The S.L.

    values seem to increase with the additive.

    6.1.6 Di-ammonium Phosphate: This chemical was tried on soils S-2, S-4, S-

    5 and S-6 and its effect is found to be similar to that of FeCl3. the S.L. Values could

    not be determined since the soil turned porous due to the evolution of NH3.

    6.1.7 Other Chemicals: Na2SO4, CuSO4, K2Cr2O7, Ca3(PO4)2 and Mg3

    (PO4)2 were tried only on S-2 soil. In general Na2SO4 shows an increase in L.L.

    and P.I. due to the dispersion. Variation in P.L. and S. L. were not significant CaSO4

    and MgSO4 behave more like dispersing agent K2Cr2O7 decreases, L.L., P.L. and

    P.I. and S.L. is increased.

    6.2 Effect of aging on consistency: The amount of complex compound formed

    due to the reaction between soil and chemical is dependent upon (i) The amount of

    chemicals (ii) pH of the soil (iii) The amount of time allowed for the reaction. The

    chemicals used are hydroxides of Na, K, Mg and Ca, chlorides of Ba, Ca and Mg,

    Carbonates of Na, Ba, and Mg and cement. The chemical used in various

    percentages between 0 and 7.

  • Free water is essential for reaction to take place between soil and the chemical

    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.

  • 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.

  • The increase in C.B.R. values is an indication that the complex cementations gel

    which are formed have cementing property even under highly wet condition. This is

    an important factor with respect to the stability of the soil-chemical system under

    field condition. Further studies on expansive soils subjected to drying and rewetting

    is needed, because it is expected after drying the gel may attain a condition of

    insolubility. Na2CO3 does not seem to have much effect on C.B. R. Values.

    6.4 Permeability characteristics: Permeability being one of the important factor

    to be considered in the design and construction of Civil Engineering works, it is

    intended to study the effect of inorganic chemicals on the permeability

    characteristics of three black cotton soils, viz. S-2, S-4, and S-12 i.e., from Poona,

    Nasik and Wadagaon. The chemicals selected for the study are hydroxides of Na, K,

    and Ca chlorides of K, Na, Ca and Mg and carbonates of K, Na, Ca, Ba and Mg and

    were used in proportions of 0.1, 0.25, 0.50, 0.75, 1.0, 1.5, 2.0, 3.0, 5.0, 7.0 and

    10.0 percent on the basis of even dried weight of soil. The procedure for mixing

    was the same as in consistency studies. The mixtures were compacted to field

    densities of 1.330, 1.225, 1.253gm/cc. For S-2, S-4 and S-12 soils respectively in

    Jodhpur pattern moulds by static compaction. The samples were then saturated

    under vacuum for 36 hours prior to conduction the permeability test by the falling

    head method. The experiments were run in duplicate. The values obtained were

    erratic during the first few hours but attained fairly constant values at the end of 10

    hours and the values are recorded at the end of 12 hours.

    The data collected in the case of soils S-4 and S-12 is presented in tables.

    6.4.1 Hydroxides: NaOH when added up to about 2-3 percent in all the

    three soils bring down the permeability values to less than that obtained for the

    bank soil. Beyond this percentages, the permeability values increase continuously

    up to about 10 percent, the increase being very rapid beyond 5 percent. The values

    in units of 10-7 cm/sec. for S-2 soil at 0.1.3 and 10 percent additives at 3.2, 0.5,

    1.4, 869.4 respectively while corresponding values for S-4 and S-12 soils are 9.6

    and 4.8, 1.6 and 2.6,8.0 and 36.1 and 3140 and 1685 respectively. The decrease in

    permeability at the lower percentages may be due to the dispersion effect of NaOH.

  • At higher percentages, aggregation effect seems to set in, leading the higher value

    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

  • 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

  • percentages enough calcium ion released to cause not effect of aggregation

    resulting in higher values of permeability, inspire of the unassociated chemicals

    continuing to plug the pores. BaCo3 and MgCo3 did not show any consistent trend

    with the soils probably to the simultaneous action of both aggregation and plugging

    the pores process.

    It is evident from the previous investigation that certain inorganic chemical are

    effective in significantly changing the textural and permeability of black cotton soils.

    Some of these chemicals are soluble and some are insoluble.

    6.5 Use of Lime-Cement and Combination of Lime and Cement: The

    primary purpose of this study is to evaluate the unconfined compressive strength,

    bearing capacity, shear strength, flexural strength and durability characteristics of

    black cotton soil samples treated with lime and cement.

    Lime used is this investigation was a calcium hydroxide of technical grade and the

    cement was a normal Portland cement.

    6.5.1 The studies conducted by Katti on lime alone on soils. S-1 to S-12 i.e.

    from Sholapur, Poona, Sidheswar, Nasik, Nagpur, Sholapur, Veldhari, Amravati,

    Baroda, Wadagaon, sites. It may be seen that 7 day compressive strength in all

    cases, are less than 300 psi. However in most soils, 28 days compressive strength

    of over 300 psi can be obtained. These large increases in strength with time may be

    a

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