STABILIZATION OF BLACK COTTON SOIL …civil.uonbi.ac.ke/sites/default/files/cae/engineering...STABILIZATION OF BLACK COTTON SOIL F16/29240/2009 1 UNIVERSITY OF NAIROBI COLLEGE OF ARCHITECTURE

STABILIZATION OF BLACK COTTON SOIL F16/29240/2009

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UNIVERSITY OF NAIROBI

COLLEGE OF ARCHITECTURE AND ENGINEERING

DEPARTMENT OF CIVIL AND CONSTRUCTION ENGINEERING

FCE 590 FINAL YEAR CIVIL ENGINEERING PROJECT

CEMENT STABILIZED BLACK COTTON SOIL FOR PAVEMENT SUBGRADE

CONSTRUCTION

BY

GITHAIGA ESTHER NYAKARURA

F16/29240/2009

SUPERVISOR:

DR SIMPSON N OSANO

PROJECT SUBMITTED AS PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE

AWARD OF THE DEGREE OF BARCHELOR OF SCIENCE IN CIVIL ENGINEERING


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CONTENTS

Acknowledgement____________________________________________________________5

Dedication__________________________________________________________________6

Abstract___________________________________________________________________7

List of figures_______________________________________________________________8

List of tables________________________________________________________________9

CHAPTER ONE ....................................................................................................................................... 10

1. INTRODUCTION ................................................................................................................................. 10

1.1 GENERAL INTRODUCTION .......................................................................................................... 10

1.2 PROBLEM STATEMENT ................................................................................................................ 10

1.3 PURPOSE AND SCOPE OF THE STUDY ..................................................................................... 11

1.4 OBJECTIVE OF THE STUDY ......................................................................................................... 11

1.5 METHODOLOGY ............................................................................................................................. 12

1.6 SOIL STABILIZATION .................................................................................................................... 12

1.7 CLAY SOIL ......................................................................................................................................... 13

1.8 CEMENT ............................................................................................................................................. 13

CHAPTER TWO ...................................................................................................................................... 15

2. LITERATURE REVIEW .................................................................................................................... 15

2.1 GENERAL ........................................................................................................................................... 15

2.2 SOIL IDENTIFICATION AND DESCRIPTION............................................................................ 16

2.2.1 Mass characteristics ......................................................................................................................... 17

2.2.2 Material characteristics ................................................................................................................... 17

2.3 SOIL CLASSIFICATION .................................................................................................................. 18

2.3.1 Unified Soil Classification System. ................................................................................................. 18

2.3.2 British soil classification .................................................................................................................. 18

2.4 CLAY MINERALS AND STRUCTURE OF CLAY ....................................................................... 19

2.4.1 BEHAVIOUR OF CLAY MINERALS .......................................................................................... 23

2.5 CONSISTENCY AND PLASTICITY OF CLAY SOILS ............................................................... 24

2.6 SWELLING AND SHRINKAGE OF CLAYS ................................................................................. 25

2.7 CONSOLIDATION OF CLAYS ....................................................................................................... 27

2.8 SOIL STABILIZATION .................................................................................................................... 28

2.8.1 SURVEY ON THE CONCEPT OF SOIL STABILIZATION .................................................... 28

2.8.2 REASONS FOR STABILIZING SOILS ....................................................................................... 30

2.8.3 CHOICE OF SOIL STABILIZATION METHOD ...................................................................... 30


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2.8.4 SOIL STABILIZATION TECHNIQUES ..................................................................................... 30

2.8.4.1Compaction ..................................................................................................................................... 31

2.8.4.2 Deep foundation techniques ......................................................................................................... 31

2.8.4.3 Stabilization by industrial waste .................................................................................................. 31

2.8.4.4 Stabilization by reinforcement ..................................................................................................... 32

2.8.4.5 Chemical stabilization .................................................................................................................. 32

2.9 Soil-cement stabilization ..................................................................................................................... 33

2.9.1 CEMENT .......................................................................................................................................... 34

2.9.2 CHEMICAL COMPOSITION OF CEMENT .............................................................................. 34

2.9.2.1 FUNCTIONS OF THE COMPOUNDS IN CEMENT .............................................................. 34

2.9.3 TYPES OF CEMENT ...................................................................................................................... 35

2.9.3.1 Portland cement ............................................................................................................................ 35

2.9.3.2 Other Varieties. ............................................................................................................................. 42

2.9.4. Action involved in cement-soil stabilization ................................................................................. 43

2.9.5 Constructional practice in soil-stabilized roads ............................................................................ 44

2.9.6. Quality control in soil-cement stabilization .................................................................................. 54

2.9.6.1 Factors affecting strength of soil-cement mixes. ........................................................................ 55

2.9.7 Uses of cement stabilized soil in road construction ....................................................................... 57

2.9.7.1 Subgrade stabilization for pavement ........................................................................................... 58

2.9.7.2 Correcting unstable subgrade areas. ........................................................................................... 58

2.10 ARRESTING THE SWELL AND SHRINK BEHAVIOUR OF EXPANSIVE SOILS. ............ 59

2.10.1 Methods for arresting the swelling of expansive soil. ................................................................. 59

2.10.1.1Under-reamed pile foundations .................................................................................................. 59

2.10.1.2 Granular pile-anchor .................................................................................................................. 59

2.10.1.3 Sub excavating and replacement of the expansive soil by cushions ....................................... 60

CHAPTER 3 .............................................................................................................................................. 61

3.0 PRELIMINARY TESTS .................................................................................................................... 61

3.1 INVESTIGATION OF SOIL PROPERTIES .................................................................................. 61

3.2 CLASSIFICATION ............................................................................................................................ 61

3.3 PROCTOR COMPACTION TEST .................................................................................................. 61

3.4 ATTERBERG LIMITS ...................................................................................................................... 61

3.5 CALIFORNIA BEARING RATIO TEST (CBR) . .......................................................................... 62

3.5.1 CBR ................................................................................................................................................... 62


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3.5.2 Swell .................................................................................................................................................. 63

3.6 SWELLL SHRINKAGE TEST ........................................................................................................ 64

3.6.1 Swell Test .......................................................................................................................................... 64

3.6.2 Shrinkage test .................................................................................................................................. 66

CHAPTER 4 .............................................................................................................................................. 67

4.0 RESULTS, DATA ANALYSIS AND DISCUSSION ....................................................................... 67

4.1 RESULTS AND DATA ANALYSIS ................................................................................................. 67

4.1.1 PARTICLE SIZE DISTRIBUTION .............................................................................................. 67

4.1.2 PROCTOR COMPACTION TEST ............................................................................................... 68

4.1.3.1 ATTERBERG LIMITS FOR NEAT SOIL ................................................................................ 69

4.1.3.2 ATTERBERG LIMITS FOR SOIL WITH 6% CEMENT CONTENT .................................. 69

4.1.3.3 ATTERBERG LIMITS FOR SOIL WITH 8% CEMENT CONTENT .................................. 70

4.1.3.4 ATTERBERG LIMITS FOR SOIL WITH 10% CEMENT CONTENT ................................ 70

4.1.4.1CALIFORNIA BEARING RATIO VALUES FOR NEAT SOIL ............................................. 71

4.1.4.2 CBR VALUES FOR SOIL WITH 6% CEMENT CONTENT ............................................... 72

4.1.4.3 CBR VALUES FOR SOIL WITH 8% CEMENT CONTENT ................................................ 73

4.1.4.4 CBR VALUES FOR SOIL WITH 10% CEMENT CONTENT............................................... 74

4.1.5 SWELL SHRINKAGE TEST RESULTS ...................................................................................... 75

4.1.5.1 Neat soil with fly ash cushion of varying depth .......................................................................... 75

4.1.5.2 Soil-6% Cement mix with fly ash cushion of varying depth ..................................................... 77

4.1.5.3 Soil-8% Cement mix with fly ash cushion of varying depth ..................................................... 79

4.1.5.4 Soil-10% Cement mix with fly ash cushion of varying depth ................................................... 81

4.2 DISCUSSION ...................................................................................................................................... 83

4.2.1 MAXIMUM DRY DENSITY .......................................................................................................... 83

4.2.2 SOIL CLASSIFICATION ............................................................................................................... 83

4.2.3 ATTERBERG LIMITS ................................................................................................................... 84

4.2.4. CALIFORNIA BEARING RATIO .............................................................................................. 87

4.2.5 SWELL AND SHRINKAGE .......................................................................................................... 88

CHAPTER 5 .............................................................................................................................................. 90

5.0 CONCLUSION AND RECOMMENDATIONS .............................................................................. 90

5.1 CONCLUSION ................................................................................................................................... 90

5.2RECOMMENDATIONS ..................................................................................................................... 92

REFERENCES .......................................................................................................................................... 93


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Acknowledgement

Writing this project allowed me to look back at the work, the days, months of studying and

researching and remembered everyone who was by my side throughout that period. This report

was only possible thanks to the invaluable contribution from a range of people and organizations

with whom this work could not have been completed.

I acknowledge with thanks Dr Simpson N Osano, my supervisor for his endless commitment in

providing me with the much needed guidance, constructive criticism, advice and encouragement

throughout the entire study and report writing period. He remains my inspirational focal point.

Am indebted to the University of Nairobi highway laboratory staff members especially Mr.

Mathew and Mr. Martin and the Blue Pyramid contractors’ laboratory staff Mr. Charles Ouko,

Mr. Francis Kyalo and Miss Grace for their immense support during the soil tests.

My Heartfelt gratitude goes to my entire family for their moral support. Special thanks to my

dad; G.K Wambugu for his guidance, motivation, diligent and financial support throughout the

project period

I wish to also register gratitude to my class mates for the unlimited directions and assistance in

my pursuance for information on issues related to this study. I also extend the same to my friends

for their great role in continuous moral support, encouragement and humility throughout my

study.

To all those whom I have not mentioned but supported me in one way or another, I say thank you

and may God bless you all.

Last but not least a humble thanks to almighty God for the strength, intelligence and sound mind

He has granted me throughout the entire course.

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Dedication

To my dear parents Mr. & Mrs. Wambugu for their immense encouragement, guidance,

financial, moral and spiritual support all through my life.

To my siblings Jane, Tamara, Faith and Carol who have cheered me on and for always being

there.


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Abstract

The quality and life of pavement is greatly affected by the type of sub-grade, sub base

and base course materials. The most important of these are the type and quality of sub-grade soil.

But in Kenya most of the flexible pavements are need to be constructed over weak and

problematic sub-grade. The California bearing ratio (CBR) of these sub-grades are very low and

therefore more thickness of pavement. Decrease in the availability of suitable sub base and base

materials for pavement construction have leads to a search for economic methods of

improvement of locally available problematic soil to suitable construction materials. The

improvement of soil properties is one of the main branches of geotechnical science that has been

considered by researchers in different countries. In developing country like Kenya due to the

remarkable development in road infrastructure, soil stabilization has become the major issue in

construction activity. Stabilization is an unavoidable for the purpose of highway and runway

construction, stabilization denotes improvement in both strength and durability which are related

to performance. Stabilization is a method of processing available materials for the production of

low-cost road design and construction. Fine clayey soils properties due to high swelling

necessitate the need to improve its geotechnical properties. Black cotton soils when used as a

subgrade for pavements has risks of substantial settlements, heave and low bearing capacity.

This project is an investigation carried out to study the effect of cement on engineering

and strength properties of the Black Cotton Soils. The properties of stabilized soil such as

compaction characteristics, consistency limits, California bearing ratio and swell potential were

evaluated and their variations with cement content evaluated. Ways of curbing the cyclic swell

shrinkage behavior of black cotton soil were looked in to by provision of fly ash cushion.

Available literature on the subject of soil stabilization is surveyed. Various types of

cement are looked into with a view of establishing their viability of use in soil stabilization.

Laboratory test to examine the engineering properties of black cotton soil and soil-cement

mixtures are presented based on existing procedures for testing materials.

The interpretation of test results leading to various conclusion and recommendation on

the use of cement in soil stabilization to counter the difficulty posed by black cotton soil for

subgrade material is discussed.


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LIST OF FIGURES

Fig 1 Black cotton soil_______________________________________________________15

Fig 2 Typical cracks in black cotton soil in dried state_______________________________15

Fig 2.4.1 Silica tetrahedron unit_______________________________________________20

Fig 2.4.2 alumina octahedron unit______________________________________________20

Fig 2.4.3 a rock with Montmorillonite mineral____________________________________21

Fig 2.4.4 structure of Montmorillonite__________________________________________21

Fig 2.4.5 a rock with kaolinite mineral__________________________________________22

Fig 2.4.6 a rock with illite mineral_____________________________________________22

Fig 2.4.7 A rock with vermiculite mineral_______________________________________23

Fig 2.8.1 the first scientifically controlled soil cement project________________________29

Fig 2.9.5.2(a) Mix in place___________________________________________________47

Fig 2.10.5.2(b) Mix in place__________________________________________________48

Fig 2.9.5.2(s) Mix in place___________________________________________________49

Fig 2.9.5.2(d) Stationary plant________________________________________________50

Fig 2.9.5.2(e) Spreading_____________________________________________________51

Fig 2.9.2(a) Kenya practice on pavement layers______________________________________57

Fig 2.9.2(a) American practice on pavement layers___________________________________57

Fig 2.9.2(a) British practice on pavement layers_____________________________________58

Fig 3.5.1 CBR machine______________________________________________________63

Fig 3.6.1 Schematic Diagram of the swell test Set-up______________________________65


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LIST OF TABLES

Table 2.2.1Field identification test for clay soil__________________________________17

Table 2.5 Atterberg classification of soils based on Plasticity Index___________________25

Table 2.6Relationship of swelling potential and plasticity by Chen (1988)_____________26

Table 2.9.2 chemical composition of cement____________________________________34

Table 2.9.3.1 General features of the main types of Portland cement_________________36

Table2.9.5.2Advantages and Disadvantages of Equipment Stabilization

Techniques_______________________________________________________________51

Table 3.6.1 Chemical composition of fly ash_____________________________________65

Table 4.2.3.1 Atterberg limits for the test soil_____________________________________84

Table 4.2.3.2 Classification from PI____________________________________________85

Table 4.2.4.CBR and MC values_______________________________________________87

Table 4.2.5 Variation of linear shrinkage and swell________________________________88


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CHAPTER ONE

1. INTRODUCTION

1.1 GENERAL INTRODUCTION

Expansive soil also known as Black cotton soil because of their color & suitability for growing

cotton swells when the moisture content is increased and shrinks massively when dry.

Montmorillonite mineral is mainly responsible for the swell-shrink characteristic of the black

cotton soil. The expansive nature decreases the bearing capacity of the soil. The black color in

Black cotton soil is due to the presence of titanium oxide in small concentration.

Black cotton soil is a highly clayey soil. It is so hard that the clods cannot be easily pulverized

for treatment for its use in road construction. This poses serious problems as regards to

subsequent performance of the road. The softened sub grade has a tendency to up heave into the

upper layers of the pavement, especially when the sub-base consists of stone soling with lot of

voids. Gradual intrusion of wet Black cotton soil invariably leads to failure of the road. The

roads laid on Black cotton soil bases develop undulations at the road surface due to loss of

strength of the sub grade through softening during the wet season. The damage will be apparent

usually several years after construction. The stability and performance of the pavements are

greatly influenced by the sub grade and embankment as they serve as foundations for pavements.

There is therefore need to stabilize black cotton soil in order for it to provide a good foundation

material. Stabilization denotes improvement in both strength and durability of the material which

are related to performance.

1.2 PROBLEM STATEMENT

Black cotton soil is an undesirable foundation material. An engineer may choose to remove the

undesirable material and replace it with a more desirable material in terms of strength and

durability. This may however turn out to be expensive considering pavements cover several

kilometers and therefore the need to find out other alternative methods of modifying the

properties of the soil in place. These undesirable properties are that:


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In rainy season, Black cotton soils become very soft by filling up of water in the cracks and

fissures. These soft soils reduce the bearing capacity of the soil hence the decrease of the

strength of foundation.

In saturated conditions these soils have high consolidation settlements which are uneven hence

the beam deflects which in turn affect the plastering to the wall. Black cotton soil has high

swelling nature which causes damages to the pavement.

There is therefore need to increase the bearing capacity and the unconfined compressive strength

of the soil, reduce both elastic and inelastic consolidation settlement, prevent weathering and

deteriorations of the black cotton soil.

1.3 PURPOSE AND SCOPE OF THE STUDY

There are ways of keeping expansive soils from either expanding or shrinking too much when

used as a subgrade. These help minimize the problems associated with black cotton soil. The

challenges of construction on clay were carried out and their effect on the pavement looked into.

Cement will be used for its suitability to improve the properties of black cotton soil and use this

soil as a foundation material for pavement subgrade construction. The data is analyzed so as to

obtain the best design for the local conditions.

1.4 OBJECTIVE OF THE STUDY

To establish the problems associated with black cotton in pavement construction.

To establish the use of cement as an effective method of stabilizing black cotton soil for use in

sub-grades or sub-base layers by improvement of soil parameters such as:

Plasticity and volume change characteristics of black cotton soils.

Bearing strength so as to provide a stable working platform on which pavement layers

may be constructed.


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To study the swelling behavior of black cotton soil and cement-stabilized black cotton soil when

fly ash cushion of varying depth is placed over it.

1.5 METHODOLOGY

Study is carried out on the challenges of construction on clays and their effect on the engineering

structure in areas where these materials exist looked into. Cement will be used to stabilize the

clay. This will entail collection of data from experimentation and use of any available literature.

It is hoped that the outcome of this research will be useful to the pavement construction,

construction industry and general research in the use of cement in expansive clays.

1.6 SOIL STABILIZATION

In most cases it is expensive to remove large volumes of unsatisfactory soils and replace them

with suitable material. This brings about the need to improve the soil in place so that it serves as

a good engineering construction material. The improvement of the stability or bearing power of a

poor soil and durability which are related to performance of the soil through mechanical, physio-

mechanical and chemical methods is referred to as soil stabilization. This is achieved by use of

controlled compaction, proportioning and addition of suitable admixture or stabilizer. The

stabilization process involves excavation of the in-situ soil, treatment of the in-situ soil and

compaction of the treated soil. Increase in strength is expressed quantitatively in terms of:

Adsorption, softening and reduction in strength

Direct resistance to freezing and thawing

Compressive strength, shearing strength or measure of load deflection to indicate the load

bearing quality

Stabilization process is ideal for improvement of soils in shallow depth such as pavements and

light weight structures as the process essentially involve excavation of the in-situ soil.


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1.7 CLAY SOIL

Clay soil is a type of soil that is formed by hydrothermal activity. It is composed of very fine

particles usually silicate and /or iron and magnesium. Clay soils impede the flow of water

meaning it absorbs water slowly and then retains it for a long time. It is smooth to touch and has

a sticky feel on the fingers when moist. Clay soils consolidate when compressed by weight of the

material above them.

1.8 CEMENT

Cement is a standard material that sets and hardens independently and can bind other materials

together. Its quality is tested and assured. Because of its very high flexural strength, it has a very

high load spreading property. Cement reduces liquid limit, plasticity index and the potential of

volume change, it increases the shrinkage limit and shear strength. It’s not effective for highly

plastic clay.

Portland cement

Types of Portland cement

1. Ordinary Portland cement (OPC): OPC is broadly classified into five categories

i. General purpose Ordinary Portland cement (Type I)

ii. Moderate (Type II) and high (Type V) Sulphate Resistant Portland cement

iii. Rapid Hardening or High Early Strength Cement (Type III)

iv. Low Heat Cement (Type IV)

v. White Cement

2. Colored Cement

3. Modified Portland cement

4. Quick Setting Cement

5. Water Repellent Portland cement

6. Water Proof Portland cement

7. High Alumina Cement

8. Portland Slag Cement

9. Air Entraining Cement

10. Portland Pozzolana Cement

11. Supersulphated cement

12. Masonry cement

13. Expansive cement


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Other Varieties.

1. Natural cements:

2. Jet set cement:

3. Hydrophobic cement:

4. Oil well cement


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CHAPTER TWO

2. LITERATURE REVIEW

2.1 GENERAL

This study is done to evaluate the nature and engineering properties of black cotton soils, their

behavior under various seasonal conditions and finally use cement to stabilize the soil as a

solution to the problems created by these soils in pavement subgrade construction. The physical

properties of clays and most soils are often poorer than may be required for a particular project

since shear strength is too low, their compressibility, water content and permeability too high.

Expansive soils, when associated with an engineering structure, will show a tendency to swell or

shrink causing the structure to experience movements which are unrelated to the direct loading of

the structure. Because of its high swelling and shrinkage characteristics, Black cotton soils (BC

soils) have been a challenge to the highway engineers.

The Black cotton soil is very hard when dry, but loses its strength completely when in wet

condition. It is observed that on drying, the black cotton soil develops cracks of varying depth.

Figure 1 shows black cotton soil and Figure 2 shows the typical cracks in Black cotton soils (BC

soils) in a dried state. As a result of wetting and drying process, vertical movement takes place in

the soil mass. All these movements lead to failure of pavement, in the form of settlement, heavy

depression, cracking and unevenness.

Problems Arising out of Water Saturation: Water penetrates into the road pavement from three

sides’ viz. top surface, side berms and from sub grade due to capillary action. It has been found


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during handling of various road investigation project assignments for assessing causes of road

failures that water has got easy access into the pavement. It saturates the sub grade soil and thus

lowers its bearing capacity, ultimately resulting in heavy depressions and settlement. In the base

course layers comprising of Water Bound Macadam (WBM), water lubricates the binding

material and makes the mechanical interlock unstable. In the top bituminous surfacing, ravelling,

stripping and cracking develop due to water stagnation and its seepage into these layers.

Design Problems in Black cotton soils: In Kenya, CBR method developed in USA is generally

used for the design of crust thickness. This method stipulates that while determining the CBR

values in the laboratory and in the field, a surcharge weight of 4.536 kg should be used to

counteract the swelling pressure of Black cotton soils. BC soils produce swelling pressure in the

range of 20-80 tons/m2 and swelling in the range of 10-20%.Therefore, CBR values obtained are

not rational and scientific modification is required for determining CBR values of expansive soil.

The problem become important if the movements are sufficiently large to distort the structure.

The engineers’ first problem is therefore to recognize the signs that indicate the soil at hand is an

expansive soil. This will be achieved by identifying and describing the soil in the field.

2.2 SOIL IDENTIFICATION AND DESCRIPTION

Soil is the relatively loose mass of mineral and organic materials and sediments produced by the

physical or chemical disintegration of rock. It consists of layers of mineral constituents of

variable thickness, which differ from the parent material in physical, morphological, chemical

and mineral characteristics.

According to the detailed description of the method of describing soils contained in BS 5930, the

basic soils are boulders, cobbles, gravels, sand, silt, and clay. Soil identification and description

includes the details of both mass and material characteristics.


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2.2.1 Mass characteristics

Mass characteristics are best determined in the field. They can also be determined in the

laboratory when undisturbed samples are available. Mass characteristics include firmness, details

of bending, strength, weathering and discontinuity.

FIELD IDENTIFICATION TEST FOR CLAY

TERM FIELD TEST

Very soft Exudes between fingers when squeezed in the

hand

Soft Molded by finger pressure

Firm Can be molded by strong finger pressure

Stiff Cannot be molded by fingers

Very stiff Cannot be indented by thumb nails

Table 2.2.1. Field identification test for clay soil

Macro fabric are thin layers of fine sand and silt in clay strata, silt filled fissures in clay, small

lenses of clay in sand, organic intrusion and root holes. They can influence the engineering

behavior of insitu soil considerably

According to BS 5930 a soil is of basic silt or clay when over 35% of the soil is in the silt clay

range.

2.2.2 Material characteristics

Material characteristics can be determined from samples having the same particle size

distribution as the insitu soil but whose insitu structure has been altered. The principal material

characteristics are particle size distribution and plasticity. Secondary material characteristics are

color of the soil, shape, texture and composition of the particles.


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2.3 SOIL CLASSIFICATION

Soil classification is the arrangement of various types of soils into specific group based on

physical properties e.g. Particle size distribution and plasticity and engineering behavior of soil

such as settlement upon loading and bearing capacity.

Particle size distribution is determined by performing particle size analysis. This analysis

includes sieve analysis and sedimentation analysis. Depending on the type of soil and extent of

particle size distribution required the analysis may involve both sieve analysis and sedimentation

analysis or it may be restricted to either. The distribution gravel and sand particles sieve analysis

will suffice but if silt and clay are present sedimentation analysis has to be performed.

In silts and clay soil sedimentation analysis will suffice

2.3.1 Unified Soil Classification System.

The system is based on both grain size and plasticity characteristics of the soil. In this system

soils are broadly divided into three divisions.

Coarse grained soils: If more than 50% by weight is retained on No 200 ASTM sieves.

Fine grained soils: If more than 50% by weight passes through No 200 ASTM sieve.

Organic soils: No specific grain size.

2.3.2 British soil classification

It is based on the particle size distribution and plasticity as plotted on a plasticity chart. Any

cobbles and boulders retained on 63mm BS sieve size are removed from the soil before

classification. The percentage of this very coarse portion is determined and mentioned in the

report. The soil groups are noted by the group symbols composed of main and qualifying

descriptive letters e.g. SW describes well graded sand. The fine grained soils are represented by a

point on the plasticity chart.

From both the UCS and British soil classification soil can be classified into three soil types

namely:


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1. Organic soils: They are typically in the upper 80cm of the soil profile. They are formed

by accumulation of partially decomposed organic matter. They are dark in color, light

weight and have extremely high water holding capacity. Sometimes may have distinctive

odor of decaying vegetation. They are of less consequence to the engineer and are always

removed when works are constructed.

2. Coarse grained soils: If more than 50% by weight is retained on No 200 ASTM sieves.

In accordance to BS 5930 a soil is classified as coarse if after removal of boulders and

cobbles, over 65% of the material is in the sand and gravel range. Mixtures containing

50% boulders and cobbles are referred to as very coarse soils.

Coarse grained soils are composed of rock fragments varying from boulders to gravel and

sand. Quartz is usually the predominant mineral in the composition of many gravels and

sand particularly when particles are well rounded.

3. Fine grained soils: If more than 50% by weight passes through No 200 ASTM sieve.

In accordance to BS 5930 a soil is classified as fine when over 35% of the soil is in the

silt clay range. Therefore they include silt and clay.

Silt is a type of soil intermediate between fine sand and clay. It is created by a variety of

physical processes capable of splitting the generally sand sized quartz crystals of primary

rocks by exploiting deficiencies in their lattice. Its mineral composition is more viable

than that of fine sands. Mineralogically it is composed of quartz and feldspar.

Clay is a type of soil that posses plasticity especially when moist. Clay particles are

composed mostly of hydrous layer silicates of aluminium and occasionally containing

magnesium and iron.

2.4 CLAY MINERALS AND STRUCTURE OF CLAY

Clay minerals are plate like particles with high specific surface which plays dominant role in

particle arrangement during sedimentation. Different clay minerals have demonstrated that they

are constructed from two basic building blocks i.e. Silica tetrahedron and Alumina octahedron.


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o Silica tetrahedron: Comprise of a central silicon ion surrounded by four oxygen ions

Fig 2.4.1 Silica tetrahedron unit

o Alumina octahedron: Comprise of a central silicon ion of either aluminium or magnesium

surrounded by six hydroxyl ions.

Fig 2.4.2 alumina octahedron unit

These basic units combine to form sheet structures. The clay minerals are therefore a

combination of these sheet structures with different bonds between them. Depending on the

combination of these sheets and type of ions, four main groups of clay have been identified.

(i) Montmorillonite: The mineral in this group occur as the chief constituents of

bentonite and tropical black cotton soils. The structure mainly consists of three layer

arrangement in which the middle layer is mainly gibbsite but with some substitution


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of aluminium and magnesium. Water molecules are easily admitted between sheets as

a result of weaker linkage. This results in a high shrinkage/swelling potential.

Fig 2.4.3. A rock with Montmorillonite mineral

Fig 2.4.4 structure of Montmorillonite.

(ii) Kaolinite group: Named for it locality in Kaoling, Jianxi, China is composed of

silicate sheets bounded to aluminium oxide/hydroxide layers called gibbsite layers.

The gibbsite and silicate layer are tightly bonded together with only weak layer

existing between these silicate/gibbsite layers. The weak bonds cause the cleavage

and softness of this mineral. Kaolinite shares the same chemistry as the minerals


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halloysite, nacrite, and dickite. They however have different structures. Kaolinite

forms from weathering of aluminium rich silicate minerals such as feldspars.

Fig 2.4.5 a rock with kaolinite mineral

(iii) Illite group: Illite is the most common clay mineral. They are characteristic of

weathering in temperate climates or in high altitudes in the tropics and typically reach

the ocean via rives and wind transport. The structure consists of three layers gibbsite

sheet with K+ ion providing a bond between adjacent silica layers. The linkage is

weaker than Kaolinite resulting in thinner and smaller particles.

Fig 2.4.6. A rock with illite mineral

(iv) Vermiculite: This is hydrated magnesium aluminium silicate mineral. It is formed by

natural weathering, hydrothermal action, percolating ground water or a combination

of all these processes. The mineral is composed of a silicate sheet composed of two

flat layers of silica and alumina tetrahedrons which are joined together in a layer


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composed of oxygen atoms, magnesium, and iron and hydroxyl molecules. Swelling

and shrinkage potential is similar to that of montmorillonite.

Fig 2.4.7 a rock with vermiculite mineral

2.4.1 BEHAVIOUR OF CLAY MINERALS

Clay minerals are plate like particles with high specific surface. For Montmorillonite the specific

surface can be up to 800m²/g, for Kaolinite it ranges from 10-20m²/g, for illite it ranges from 65-

200m²/g.

The clay surface has negative charges therefore the cations in water are attracted to the particles.

These also tend to move away because of their thermal energy, so that the resultant effect is that

the cations form a dispersed layer adjacent to the particles. The surface of the particle with

negative charge and the dispersed layer of cations form the double layer. Because water

molecules are dipolar, they are held round a clay particle by hydrogen bonding, so that a layer of

adsorbed water surrounds the particle. These water molecules are restricted from moving

perpendicular to the surface and their behavior is completely different from the free water of

soils. The double layer charges produce repulsion forces between particles whereas the van der

Waal forces are attractive. The structural arrangement of clay depends on the net interactive

forces.

There are two types of structures found in clay deposit.

Dispersed structure: This formation is as a result of the net adjacent particles at the time of

deposition being repulsive in nature. Structure is common in fresh water deposits. The particles

have face to face contact.

The flocculated structure: This formation is as a result of the net adjacent particles at the time


24

of deposition being attractive in nature. E.g. when clays are deposited in an electrolyte like

seawater. The attraction is either edge to edge or edge to face. Clays with this soil structure have

relatively high void ratio. Pressure application through compaction leads to slippage of particles

resulting in dispersed structure.

2.5 CONSISTENCY AND PLASTICITY OF CLAY SOILS

Consistency describes the relative ease with which a soil mass can be deformed. It is used to

describe the degree of firmness in fine grained soils like clay therefore to a large extent

consistency relates to the water content. Atterberg (1900) suggested four states of consistency

with water content defining the boundary between this states. Four states of consistency as

suggested by Atterberg are:

1) Solid state: In this state there is no change in volume with change in water content.

2) Semi-solid state: Increase in water content leads to an increase in volume of the soil and

vice versa. The soil mass cannot be deformed without cracking.

3) Plastic state: Change in water content is accompanied by change in volume of the soil

mass. Soil mass can be deformed without cracking.

4) Liquid state: Change in water content is accompanied by change in volume of the soil

mass. Soil mass behaves like a liquid of very low shear strength.

The water content defining the boundary between this states are referred to as consistency limits

or Atterberg limits. This are:

Liquid limit (LL): This is the boundary between plastic and liquid state. It is the minimum

water content at which the soil mass flows like a liquid. LL is determined in the laboratory by the

Casagrande apparatus test and the Cone penetration apparatus test. From the Casagrande test LL

is the water content at which the groove cut by a standard grooving tool close for a distance of

13mm when the cup containing the soil mass is imparted 25 blows. The cone penetration

method gives a more consistent estimate than Casagrande apparatus because the after the cone

has been lowered to just touch the surface for a period of 5 seconds and penetration recorded the

process is repeated over four different moisture content.

Plastic limit (PL): is the boundary between semi solid and plastic states of consistency. It is the

minimum water content at which the soil mass can still be deformed without cracking. A soil


25

element starts to crumble when rolled into a pencil shape of 3mm diameter.

Shrinkage limit: It is the maximum water content at which there is no reduction in volume of

soil mass accompanying reduction in water content.

Atterberg Indices.

Plasticity index (PI): Liquid limit (LL) minus plastic limit (PL) i.e. PI=LL-PL

Liquidity Index (LI) =Ratio of natural water content (W) minus Plastic limit (PL) to plasticity

index. LI= (W-PL)/PI

Quick clays have liquidity Index greater than one, soft clays near to unity and stiff clays near

zero.

Atterberg therefore classified soils as follows based on Plasticity Index.

Plasticity Index Plasticity

0 Non plastic

<7 Low

7-17 Medium

>17 High

Table 2.5 Atterberg classification of soils based on Plasticity Index

The plasticity index of the African soil is commonly about 50% but may vary.

2.6 SWELLING AND SHRINKAGE OF CLAYS

Haines (1923) recognized the widespread implications of the shrinking and swelling of clay

soils. He made the valuable start of defining shrinkage process as that stage where volume

change of the soil is less than the volume of water withdrawn and referred to this as the residual

shrinkage. Keen (1931) in his discussions of Haines’s experiment concluded that volume

change of the soil is equal to the water content change. He referred to this as Normal shrinkage.

Shrinkage is mainly due to clay swelling properties (Stirk 1945). According to Boivin et al 2006,

soil shrinkage can be measured in most soils with more than 10% clay content. This process is

reversible with changes in water content. The degree of shrinkage depends on the initial water


26

content, amount of clay and environment of geological deposition. Shrinkage not only occurs

horizontally but also vertically causing vertical shrinkage cracks. In highly compressible clays,

the crack may be as high as 0.5m and 5m deep. The reverse to shrinkage is swelling.

Free swell of soil is the increase in the volume of a soil without any external constraints on

submerged water (IS: 2720. 1977).Free swell ceases when the moisture reaches the plastic limit,

Swelling is caused mainly by repulsive forces which separate the clay particles causing volume

increase. Black cotton soils not only shrink but also swell when they come in contact with water.

Factors contributing to swelling include:

1) Clay mineral’s affinity for water.

2) Elastic rebound of soil grain.

3) The cation exchange capacity and electrical repulsive forces.

4) Expansion of entrapped air.

In addition to visual identification, the expansive soils can be identified by assessing the swell

potential of the soil. These is done by conducting an odometer test which measures the free swell

and swell pressure attained in an odometer when a sample held in an odometer ring is kept at the

same volume as swelling is induced by allowing the soil sample to take in water. Some of the

Nairobi black cotton soils have been found to have a swell pressure of up to 350 KN/m². Chen

(1988) related the swell potential to plasticity index.

Swelling potential Plasticity index (PI)

Low 1-15

Medium 10-35

High 20-55

Very high Over 55

Table 2.6. Relationship of swelling potential and plasticity by Chen (1988)

The mechanism of volume changes in clay soil is associated with the physio-chemical properties

of the clay particles and capillary water movement within the clay mass. (Soil mechanics 11,

section 3). The resulting forces of volume change are very strong and have exceeded 1000t/m in

the laboratory test. The process of cyclic swelling and shrinkage may be subject to fatigue. It has


27

come to attention through observation and laboratory test that structures e.g. pavements founded

on expansive clays with seasonal moisture changes have a tendency to reach a point of

stabilization after a number of years.

2.7 CONSOLIDATION OF CLAYS

Consolidation is the Process of reduction of bulk soil volume of a fully saturated soil of low

permeability due to flow of pore water under loading (Karl von terzaghi). During construction,

surface load from foundations or earth structures are transmitted to the underlying soil profile

and as a result, stresses increase within the soil mass and the structure undergoes a time

dependent vertical settlement. The total settlement is a sum of three components namely:

Immediate settlement (Elastic settlement): This is a as a result of shear strains that occur at

constant volume as the load is applied to the soil. Water and air in the voids is compressed. Soil

and rock grains are also deformed.

Primary consolidation settlement: In fine soils (silts and clays) with low permeability the soil

is undrained when the load is applied. Slow seepage occurs and the excess pore water dissipates

slowly. The rate of volume change diminishes with time.

Secondary consolidation settlement: This is the compression of soil that takes place because of

the plastic readjustment of the soil fabric at slow rate after the reduction of hydrostatic

pressure during the primary consolidation settlement phase.

Piezometers record the change in pore water pressure with time and are generally used to

monitor the process of consolidation in structures. Taking levels of the structure with

reference to a benchmark whose level is not affected by the consolidating soil is an

alternative method of monitoring consolidation.


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2.8 SOIL STABILIZATION

Expansive soils are one of the major deposits in Africa. A study of black clays from all over

Africa have shown that clay content in the African soil varies from 15% to 100% but mostly

more than 50%. The classification is normally CL and CH in the united system of classification.

Expansive soils are problematic soils because of their potential to undergo volume changes with

change in moisture content. The swell and shrink property causes structures found on them to be

damaged. The annual cost of damage is estimated at £150 million in the UK, $1000 million in

the USA and billions of pounds worldwide. (Gourley et al. 1993). There is therefore need to

stabilize expansive soils. Soil stabilization is the process of changing soil properties to improve

strength and durability

According to the MOTC material branch Report No 239, the typical black cotton soils in Kenya

has the following properties:

Grading: 60% clay, 30% silt, 10% sand sizes.

Plasticity: Liquid limit (LL) = 85%, Plastic limit (PL) =35%, Plasticity Index (PI)=50%

Linear shrinkage (Ls) =18%

Maximum dry density (MDD) =1300kg/m³

Optimum moisture content (OMC) =33%

CBR, soaked, 100% MDD=3%

Swelling pressure=300-500KN/m² PH=7.5 shear strength is dependent on moisture content.

2.8.1 SURVEY ON THE CONCEPT OF SOIL STABILIZATION

The concept of soil stabilization is not new and can be dated back to 5th

BC. Clay was admixed

with tamarisk branches during the construction of the Great Wall of China. During early

civilization sun dried soil bricks were commonly used as a building material. With continued

experience, the practice of mixing the soil with straw or other fibers available to them to improve

properties of clay became accepted (Dean 1986). Dry clay bricks were stabilized with reed and

straw during the building of Agar-Qut Ziggwarat of Baghdad. Various materials have since been

used to stabilize soil and vary greatly in terms of:


29

o Make - strips, grids, sheets bars or fibers.

o Texture-rough or smooth

o Relative stiffness- High such as steel or low such as polymeric fabrics

Amos and Wright (1972) studied the effect of mixing fly ash with black cotton soil and found

that it can be used to improve the geotechnical properties of black cotton soils. Haas (1985)

showed that flexible pavements could be effectively reinforced with polymer geogrids.

Due to interaction between the soil and the reinforcement surface, greater frictional resistance is

provided or increased in the angle of the internal friction of the soil. The resultant interaction

transmits stress from the soil particles to the reinforcement and these results into stress in both

the soil and the reinforcement provided. The material behaves as a composite material having

improved properties.


30

Fig 2.8.1: The first scientifically controlled soil cement project.

2.8.2 REASONS FOR STABILIZING SOILS

Reduction of plasticity or the swelling characteristics of clays due to moisture change

Reduce permeability

Improve soil gradation i.e. particle size distribution

Improve the bearing capacity hence increase durability and strength

In wet weather, stabilization may provide a working platform for construction operations.

2.8.3 CHOICE OF SOIL STABILIZATION METHOD

Choice of soil stabilization is influenced by:

Soil type: This primarily refers to the particle size distribution and chemical composition.

Compaction is not recommended for fine grained soils as they are easily powdered and could be

blown off. Treatment of some soils that has a lot of sulfate with calcium base stabilizers such as

lime and cement can cause extreme swelling of soil.

Moisture content: In very dry soils, dust may form when the soil is compacted while high

moisture content could cause soil particles segregation hence loss of soil stability which may

result the soil to become plastic.

Site conditions: Physical conditions such as space have to be considered. Stationary continuous

method, which requires space where a central unit is to be installed, will not be applicable where

there is space limitation.

Cost: The method of stabilization chosen must be cheaper than other available techniques

2.8.4 SOIL STABILIZATION TECHNIQUES

Soil stabilization techniques may be grouper under two main types

Improvement of soil property of the existing soil without using any admixture. E.g.

Compaction and drainage which improve the inherent shear strength of soil.


31

Improvements of soil property with the help of admixtures such as cement, lime, fly ash,

bitumen and chemicals.

The several practices on expansive soils include:

2.8.4.1Compaction

This is the process of increasing the density of soil by packing the particles together with

reduction in volume but does not involve removal of water. The reduction of air content results

in the reduction of pores which act as conduits of water and consequently reduces permeability

of the soil. The process primarily results in the increase of soil unit weight. In addition

compaction reduces the liquefaction and increase the erosion resistance of the soil. The result is

increased shear strength and less compressibility of the soil. The purpose of compaction is to

produce a soil having the physical properties appropriate to the particular project.

2.8.4.2 Deep foundation techniques

The foundation is made to rest at a depth below the zone within which volume changes in the

soil occur due to seasonal moisture changes. This includes the installation of piles, piers and

caisson.

2.8.4.3 Stabilization by industrial waste

Industrial waste is the waste produced by industrial activity. Stalin et al suggested that utilization

of industrial waste in the geotechnical engineering field can solve the problem of disposal of

industrial waste such as:

Copper slag: This the byproduct created during the copper smelting and refining process. As

refineries draw metal out of copper ore, they produce a large volume of non metallic dust, soot

and rock. Collectively these materials make up the slag. When mixed with calcium-base

compound like lime, the silica and alumina present in copper slag may react chemically on

hydration and may be used for improvement of expansive soils.

Ground granulated blast furnace slag (GGBFS): GGBFS is a byproduct of iron and steel

making and is obtained by quenching the molten slag from blast furnace in water to produce a

glassy granular product that is then dried and ground into a fine powder. To attain strength, when


32

GGBFS is added to the mixture it reacts with water and produces calcium silicate hydrates from

its available supply of calcium oxide and silica. A pozzolanic reaction also takes place which

uses the excess SiO from the slag source, Ca(OH) produced by the hydration of the silicates, and

water to produce more of the desirable silicate hydrates making slag a beneficial mineral

admixture to attain soil stabilization with GGBFS

Quarry dust: Is the rock particles generated from the process of breaking and handling rocks.

Quarry dust can be used as an admixture in soil stabilization

Coal ash: Coal ash refers to the distinct materials produced when coal is combusted to produce

electricity. Studies by Sharma (2004) on free swell index, swell potential, plasticity, compaction,

strength characteristics of expansive soil showed that fly ash improves the plasticity, compaction

and strength characteristic of black cotton soil.

2.8.4.4 Stabilization by reinforcement

Using fibers like rubber tire chips, waste plastics, synthetic fibers can successfully stabilize the

expansive soils. Geosynthetics (sheet polymeric material) have been used since 1970s in

geotechnical structures for functions such as separation, reinforcement, drainage, filtration and

liquid containment and as gas barriers. Raid R and Faris J (1991) reported from swelling test

conducted using cylindrical geogrids of varying stiffness values embedded in clays of different

plasticity indices that the reduction in swell increased with increasing geogrids stiffness This is

due to a strong interference bond restricting the relative movement between clay and the grid.

2.8.4.6 Chemical stabilization

Chemical stabilization of expansive soils consists of changing the physio-chemical around and

inside of clay particles where by the clay requires less water to satisfy the static imbalance and

making it difficult for water that moves into and out of the system. The most common chemical

admixtures used in soil stabilization are lime and cement.

Lime stabilization has been widely used for modification of expansive soils. Lime is sparingly

soluble in exchange reactions. Generally 3 to 8% by weight hydrated lime is added to the top


33

several inches of the soil (John et al). Lime diffusion into the soil either from lime piles or lime

slurry pressure injection is hardly 38-50mm in 1 to 4 years unless extensive fissures and cracks

are present. Field experiences has shown that treatment of some soils that has so much sulfate

with calcium base stabilizers such as lime can cause extreme swelling of soil (Kota et al 1996).

This failure happens when sulfate and free alumina in natural soils react with calcium in the

stabilizer and this causes crystalline minerals which are highly expansive. (Rauch et al 2002)

Chemicals like calcium chloride, calcium sulfate, potassium chloride, aluminium chloride e.t.c

have also been used by some investigators and succeeded in minimizing the swelling of

expansive soils.

Another chemical stabilizer is the Ionic solution (ISS). ISS are suitable for improvement of

expansive soils. The absence of calcium and their ability to not cause extreme expansion makes

them suitable for use in soils containing sulfates. The stabilization process involves excavation of

the in-situ soil, treatment of the in-situ soil and compaction of the treated soil.

2.9 Soil-cement stabilization

The principal advantages with soil-cement are that almost all soils are amenable to this

technique. It is a scientifically designed engineering material and cement itself is a standard

material whose quality is tested and assured. Because of its very high flexural strength, it has a

very high load spreading property. Thus soil cement is able to spread the load over a wider area

and bridge over locally weak spots of the underlying sub-grade or sub-base. In view of its high

flexural rigidity, it is often classed as a semi-rigid pavement, something which is intermediate

between a flexible pavement and a rigid pavement. The durability of soil cement is of a high

order and its strength is known to increase with age.

The main disadvantages are the higher cost than lime-soil and the need for a high degree of

quality control. Because of volumetric changes that take place when cement hydrates, early

shrinkage cracks are formed in soil-cement layers.


34

2.9.1 CEMENT

Cement is a mixture of various chemical compounds. All compounds in cement have their own

specific roles to play and impart different properties to cement. Ratio of all compounds is

required to be maintained to get the desired quality.

2.9.2 CHEMICAL COMPOSITION OF CEMENT

Compounds %age Effect

Lime (CaO) 60-65 Controls strength and

soundness

Sillica (SiO₂ ) 20-25 Gives strength, excess

quantity causes slow setting

Alumina (Al₂ O₃ ) 4-8 Quick setting, excess lowers

strength

Iron Oxide (Fe₂ O₃ ) 2-4 Imparts color, helps in fusion

of ingredients

Magnesium Oxide (MgO) 1-3 Color and hardness, excess

causes cracking

Na₂ O 0.1-0.5 Controls residues, excess

causes cracking

Sulphur Trioxide (SO₃ ) 1-2 Makes cement sound

Table 2.9.2 chemical composition of cement

2.9.2.1 FUNCTIONS OF THE COMPOUNDS IN CEMENT

Lime: It is the major constituent of cement. The right proportion makes cement sound and

strong. Its excess makes the cement unsound and causes the cement to expand and disintegrate.

In case of its deficiency, the strength of cement is decreased and cement sets quickly.


35

Silica: It imparts strength to the cement due to formation of di-calcium silicate (2CaOSiO₂ or

C₂S) and tri-calcium silicate (3CaOSiO₂ or C₃S). In excess silica provides greater strength to the

cement but at the same time it prolongs its setting time.

Alumina: It imparts quick setting quality to the cement, acts as a flux (rate of flow of energy)

and lowers the clinkering temperature. Alumina in excess reduces strength of cement.

Iron oxide: It provides color, hardness and strength. It also helps the fusion of raw materials

during manufacture of cement.

Harmful compounds:

Alkali oxides (K₂O & Na₂O): if the amount of alkali oxides exceeds 1%, it leads to the

failure of concrete made from that cement

Magnesium oxide (MgO): If the content of MgO exceeds 5%, it causes cracks after

mortar or concrete hardness.

2.9.3 TYPES OF CEMENT

2.9.3.1 Portland cement

Typical Portland cement contain 5-9% Alumina (Al₂O₃), 19-25% Silica(SiO₂), 60-64% Calcium

oxide(CaO), 2-4% Ferric Oxide (FeO). Mineral present include Tri-calcium silicate (C₃S), Di-

calcium silicate (C₂S), Tri-calcium silicate (C₃S), Tetra-calcium aluminates (4CaO.Al₂O₃.FeO).

The reaction is solution, re-crystallization and precipitation of silicate structure.

1. Ordinary Portland cement (OPC)

The ASTM has designated five types of Portland cement, designated Types I-V. Physically

and chemically, these cement types differ primarily in their content of C3A and in their fineness.

In terms of performance, they differ primarily in the rate of early hydration and in their ability to

resist sulfate attack. The general characteristics of these types are listed in Table 2.9.3.1


36

Table 2.9.3.1 General features of the main types of Portland cement.

Classification Characteristics Applications

Type I General purpose Fairly high C3S content for

good early strength

development

General construction (most

buildings, bridges,

pavements, precast units,

etc)

Type II Moderate sulfate

resistance

Low C3A content (<8%) Structures exposed to soil or

water containing sulfate ions

Type III High early strength Ground more finely, may

have slightly more C3S

Rapid construction, cold

weather concreting

Type IV Low heat of hydration

(slow reacting)

Low content of C3S (<50%)

and C3A

Massive structures such as

dams. Now rare.

Type V High sulfate resistance Very low C3A content (<5%) Structures exposed to high

levels of sulfate ions

White White color No C4AF, low MgO Decorative (otherwise has

properties similar to Type I)

The differences between these cement types are rather subtle. All five types contain about 75

wt% calcium silicate minerals, and the properties of mature concretes made with all five are

quite similar. Thus these five types are often described by the term “ordinary Portland cement”,

or OPC.

I. General purpose Ordinary Portland cement (Type I)

It is used in general construction works. All other varieties of Cement are derived from this

Cement.


37

II. Moderate (Type II) and high (Type V) Sulphate Resistant Portland cement

Types II and V OPC are designed to be resistant to sulfate attack. Sulfate attack is an

important phenomenon that can cause severe damage to concrete structures. It is a

chemical reaction between the hydration products of C3A and sulfate ions that enter the

concrete from the outside environment. The products generated by this reaction have a

larger volume than the reactants, and this creates stresses which force the concrete to

expand and crack. Although hydration products of C4AF are similar to those of C3A,

they are less vulnerable to expansion, so the designations for Type II and Type V cement

focus on keeping the C3A content low. There is actually little difference between a Type

I and Type II cement, and it is common to see cements meeting both designations labeled

as “Type I/II”. The most effective way to prevent sulfate attack is to keep the sulfate ions

from entering the concrete in the first place. This can be done by using mix designs that

give a low permeability (mainly by keeping the w/c ratio low) and, if practical, by putting

physical barriers such as sheets of plastic between the concrete and the soil

Percentage of tri-calcium Aluminates (C3A) is kept below 5% resulting in increase in

resisting power against sulphates.

Heat developed is almost same as Low Heat Cement.

Theoretically ideal cement. Costly manufacturing because of stringent composition

requirements.

Used for structures likely to be damaged by severe alkaline conditions like bridges,

culverts, canal lining, siphons, etc.

III. Rapid Hardening or High Early Strength Cement (Type III)

Type III cement is designed to develop early strength more quickly than a Type I

cement. Gains strength faster than type I OPC. In 3 days develops 7 days strength of type

I OPC with same water cement ratio. After 24 hours – not less than 160 kg/cm2 .After 72

hours – not less than 275 kg/cm2 . This is useful for maintaining a rapid pace of

construction, since it allows cast-in-place concrete to bear loads sooner and it reduces the

time that precast concrete elements must remain in their forms. These advantages are


38

particularly important in cold weather, which significantly reduces the rate of hydration

(and thus strength gain) of all Portland cements.

The downsides of rapid-reacting cements are a shorter period of workability, greater heat

of hydration, and a slightly lower ultimate strength

Initial and final setting times are same as type I.

Contains more tri-calcium silicate (C3S) and finely ground.

Emits more heat during setting, therefore unsuitable for mass concreting.

Lighter and costlier than type I. Short curing period makes it economical.

Used for structures where immediate loading is required e.g. repair works

IV. Low Heat Cement (Type IV)

Type IV cement is designed to release heat more slowly than a Type I cement, meaning of

course that it also gains strength more slowly. A slower rate of heat release limits the increase in

the core temperature of a concrete element. The maximum temperature scales with the size of

the structure, and Type III concrete was developed because of the problem of excessive

temperature rise in the interior of very large concrete structures such as dams. Type IV cement is

rarely used today, because similar properties can be obtained by using a blended cement

Low percentage (5%) of tri-calcium aluminates (C3A) and silicate (C3S) and high (46%)

of di-calcium silicate (C2S) to keep heat generation low.

It has low lime content and less compressive strength.

Initial and final setting times nearly same as type I.

Very slow rate of developing strength.

Not suitable for ordinary structures.

o Shuttering required for long duration so cost will increase.

o Prolonged curing is required.


39

o Structure utilization will be delayed

V. White Cement

White Portland cement (WPC) is made with raw ingredients that are low in iron and

magnesium; the elements that give cement its grey color. These elements contribute

essentially nothing to the properties of cement paste, so white Portland cement actually

has quite good properties. It tends to be significantly more expensive than OPC;

however, it is typically confined to architectural applications. WPC is sometimes used

for basic cements research because the lack of iron improves the resolution of nuclear

magnetic resonance (NMR) measurements

2. Colored Cement

Suitable pigments used to impart desired color. Strong pigments can be added to type I cement

in quantities up to 10%. For the lighter colors, white Portland cement should be used as a basis.

Pigments used should be chemically inert and durable under light, sun or weather.

3. Modified Portland cement

This cement on setting develops less heat of generation than OPC.

It is best suited in hot climate for civil works construction.

4. Quick Setting Cement

Sets faster than OPC.

Initial setting time is 5 minutes.

Final setting time is 30 minutes.

Used for concreting underwater and in running water.

Mixing and placing has to be faster to avoid initial setting prior to laying.

5. Water Repellent Portland cement


40

It contains a small percentage of water-proofing material with the cement and is

manufactured under the name “Aqua-crete”.

The cement is prepared with ordinary or rapid hardening cement and white cement.

It is used in to check moisture penetration in basements etc.

6. Water Proof Portland cement

It is prepared by mixing ordinary or rapid hardening cement and some percentage of

some metal stearate (Ca, Al etc).

It is resistant to water and oil penetration.

It is also resistant to acids, alkaline and salt discharged by industrial water.

It is used for water retaining structure like tanks, reservoir, retaining walls, pool, dam etc

7. High Alumina Cement

Black chocolate color cement produced by fusing bauxite and limestone in correct

proportion, at high temperature.

Resists attack of chemicals, Sulphates, seawater, frost action and also fire. Useful in

chemical plants and furnaces.

Ultimate strength is much higher than OPC.

Initial setting time is 2 hours, followed soon by final setting.

Most of the heat is emitted in first 10 hrs. Good for freezing temperatures in cold regions

(below 18°C).

Develops strength rapidly, useful during wartime emergency.

Unsuitable for mass concrete as it emits large heat on setting.

8. Portland Slag Cement

Produced by mixing Portland cement clinker, gypsum and granulated blast furnace slag.


41

Cheaper than OPC, blackish grey in color.

Lesser heat of hydration. Initial setting in 1 hr and final setting 10 hrs.

Better resistance to soil agents, sulphates of alkali metals, alumina, iron and acidic

waters.

Suitable for marine works, mass concreting.

9. Air Entraining Cement

OPC with small quantity of air entraining materials (resins, oils, fats, fatty acids) ground

together.

Air is entrained in the form of tiny air bubbles during chemical reaction.

Concrete is more plastic, more workable, more resistant to freezing.

Strength of concrete reduces to some degree.

Quantity of air entrained should not be more than 5% to prevent excess strength loss.

10. Portland Pozzolana Cement

OPC clinker and Pozzolana (Calcined Clay, Surkhi and Fly ash) ground together.

Properties same as OPC.

Produces less heat of hydration and offers great resistance to attacks of Sulphates and

acidic waters.

Used in marine works and mass concreting.

Ultimate strength is more than OPC but setting timings are same as OPC.

11. Supersulphated cement

o Initially not less than 70% finely ground blast furnace slag, calcium sulphate and a small

quantity of ordinary Portland cement clinker.


42

o It is finer than ordinary Portland cement

o Its physical and other properties are almost same as those of OPC except the heat

hydration which is considerably lower

o It is a slag cement and is resistant to majority of chemicals found in construction industry.

It is also resistant to sulphate attack

o It is used in: Marine structure, mass concrete works subjected to aggressive waters,

Reinforced concrete pipes in ground water, Concrete construction in sulphate bearing

soil, underside of railway bridges.

o Can be used as a general purpose cement with adequate precautions.

12. Masonry cement

Unlike OPC it’s more plastic. It’s made by mixing hydrated lime, crushed stone, granulated slag

or highly colloidal clays mixed with it. These materials reduce the strength of cement

13. Expansive cement

There is an increase in volume when expansive cement settles. It is used to neutralize shrinkage

of concrete made from OPC so as to eliminate cracks. A small percentage of this cement will not

let it crack. It is specially made desirable for hydraulic structures. It is also used in repair works

where it is essential that the new concrete should be tight fitting in the old concrete.

2.9.3.2 Other Varieties.

Natural cements: These are produced from naturally occurring cement rocks which have

compositions similar to the artificial mix from which Portland cement is manufactured. The

properties of these cements depend largely on the composition of the natural rock. They are

burned at lower temperatures than those used for the production of Portland cement clinker.

Jet set cement: It is produced by mixing high alumina cement with OPC at the burning stage

during production. It sets rapidly.

Hydrophobic cement: Film forming substances such as oleic acid if ground with OPC during

manufacture has the capacity of forming a water repellant film around each other. This reduces

the deterioration and formation of lumps by cement during storage.


43

Oil well cement: They set slowly but harden quickly after setting. They are used in drilling of oil

wells to fill the space between the steel lining tube and the wall and to grout up porous strata to

prevent water or gas from gaining access to the oil bearing strata.

2.9.4. Action involved in cement-soil stabilization

Cement can be used to modify and improve the quality of the soil or to transform the soil into a

cemented mass with increased strength and durability. When water is added to cement, major

cementitious products like calcium silicate hydrates and calcium aluminium hydrates are

produced. In stabilization of granular materials with cement, these cementitious materials

provide the bond between the mineral particles. In the case of fine grained soils, the cementitious

bond provided by the calcium silicate hydrates and the calciumaluminate hydrates is further

helped by the secondary hydrous calcium silicates and aluminates formed by the reaction of free

lime to the cement paste and the clay mineral particles. When cement is added to a fine-grained

soil, the reaction phenomenon between the free lime and the clay minerals is that a number of

reactions take place. Some of them occur immediately while others are slow to occur. One of the

early reactions is base-exchange (ion- exchange). Clay particles are usually negatively charged,

with exchangeable ions of sodium, magnesium, potassium or hydrogen adsorbed on the surface.

The strong positively charged ions of calcium present in cement replace the weaker ions of

sodium, magnesium, potassium or hydrogen, resulting in a preponderance of positively charged

calcium ions on the surface of the clay particles. This in turns reduces the plasticity of the soil.

The clay particles tend to agglomerate into large sized particles (flocculation), imparting

friability to the mixture. After the above first stage reactions are complete, any additional

quantity of cement will react chemically with the clay minerals. The aluminous and siliceous

materials in the clayey soil will react with lime in the presence of water to form cementitious

gels, which increase the strength and durability of the mixture. These pozzolanic reactions are

slow and extend over a long period of time, several years in some instances. Another possible

source of strength is the formation of calcium carbonate due to the absorption of carbon dioxide

from air


44

2.9.5 Constructional practice in soil-stabilized roads

The constructional practice in soil-stabilization varies with the type of stabilization, but there are

certain steps and procedures which are common. It is, therefore, convenient to deal with the

construction practice for all types of stabilization together. The minor variations needed for each

type of stabilization will be indicated at the appropriate place. The construction technique to be

adopted for a given situation depends upon a number of factors:

(i) Type of stabilization

(ii) Type to binder, if any, to be added

(iii) Type of soils

(iv) Leads involved for the materials

(v) Magnitude of the project

(vi)Availability of equipment

(v)Availability of labour.

Broadly, the following three construction techniques can be identified:

1. Labour intensive methods

2. Machinery/ Equipment intensive methods

3. Intermediate or appropriate technology methods

2.9.5.1 Labor intensive methods

Labour intensive techniques are indicated by the following conditions:

(i) Availability of cheap labour, as in developing countries, making it more economical to use

labour-intensive techniques than equipment intensive techniques.

(ii) Small magnitude of the work, which is also scattered. This condition is prevalent in

developing countries where the construction of link roads to villages is given emphasis.


45

(iii) Equipment is not manufactured indigenously, and the level of skill needed for operation and

maintenance of imported equipment has not been developed

The various operations involved are:

(i)Collection of materials

(ii) Preparation of the subgrade

(iii) Pulverisation, where necessary

(iii) Mixing

(v) Spreading

(vi) Compaction.

The materials (soil, sand, gravel etc.) are collected on the sides of the sub-grade in requisite

proportions and stacked in the form of windrows. The sub-grade is well-compacted to the

required density and true to grades and the desired cross-profile.

If clay is one of the soil materials to be used, it is necessary to pulverise it. The clods are broken

with the help of pick-axes or rammers. Application of a country plough driven by a bullock can

also be tried. If a power roller is available, the same can be passed over the layer of clods a

number of times, with frequent raking of the crushed material.

If the materials to be mixed are soil, sand and gravel, they are mixed by dry labour using spades

or shovels. The required quantity of water is added and the materials are wet mixed by manual

labour. If an additive such as lime or cement is to be added, the soil is first spread to a uniform

thickness and the bags of lime or cement are spotted at the desired spacing. The bags are then

opened and the contents spread by manual means to cover the calculated area, which should be

marked by strings. Water to the required quantity is added in stages and the soil and lime are

mixed till the mixture has a uniform colour and the desired moisture content. If a bituminous

binder is to be added, the mixing should preferably be done in a paddle type mixer, for a period

of about 1 to 2 minutes.


46

Bitumen stabilised mixtures are spread to a uniform thickness in loose layers not exceeding 15

cm. Mechanically stabilised mixes, soil-lime mixes and soil-cement mixes are spread to a

thickness which will give a compacted thickness of not more than 150 mm. The thickness of any

stabilised laver should not be less than 100 mm.

The cement soil mix should be compacted within 2 hours of the mixing. When cut-back bitumen

is used as a binder, rolling should start only after the mix has cured. The curing time depends

upon the type of cut-back used and varies from 1 to 7 days. With penetration grade mixes, rolling

can start as soon as the mix is laid and spread. When emulsions are used, the rolling can start

after 3 hour.

Rolling of stabilised mixtures should be by 8- 10 tonne power rollers. When sand-bitumen and

soil bitumen stabilization is used, it is preferable to carry out initial rolling by means of a light

pneumatic tyred roller. Rolling is carried out till 100 per cent laboratory density is achieved.

Traffic is allowed on bitumen and sand-bitumen layers only after 24 hours. Only light pneumatic

vehicles are allowed initially. Normal traffic is allowed only after a month. Soil-lime and soil-

cement layers are moist cured for a period of 7 days. Curing is achieved by providing or covering

the surface with damp sand, straw or hessian.

2.9.5.2 Machinery/Equipment intensive methods

Machinery/Equipment intensive techniques are indicated for the following conditions:

(i) The equipment is produced indigenously.

(ii) Labour is scarce, and it becomes more economical to use equipments.

(iii) The work is of a large magnitude, fairly concentrated, and the time schedule for compaction

is tight

Three basic construction methods are available when machinery is employed:

(i) Mix-in-place

(ii) Travelling plant


47

(iii) Stationary plant.

Mix-in-place method

In this method, a train of machines is run over the soil to be processed. For breaking and

pulverising the soil, rippers, cultivators, rotary tillers, ploughs, scarifiers or disc harrows are

used. Water is then added to the loose soil from a water tanker. If the stabiliser is liquid, it is

distributed by a spraying tanker. Dry powder is either spread manually or from bulk spreaders.

Mixing is carried out by means of disc harrows or pulvi-mixers. Dry mixing is initially done in

two to three passes of the machines and is followed by wet mixing with the addition of water. A

single-pass stabiliser is also used, and it performs the various operations such as cutting the soil,

pulverising and mixing in one operation itself. Compacting is done by rollers which follow the

machines for laying the mix.

Fig 2.9.5.2(a) Mix in place


48

Fig 2.9.5.2(b) Mix in place


49

Fig 2.9.5.2(c) Mix in place


50

Travelling plant method

This method involves the use of a travelling plant which travels along the job site, picking up the

soil and stabiliser, mixing it in a mixer, discharging the mix on the ground. Compacting is done

separately by rollers which follow the travelling plant.

Stationary plant method

This method is based on the process of mixing the ingredients in a centrally located plant,

conveying the mix to the site, laying and compacting the same. The central mixing plant can be

of the batch type or continuous type.

Fig 2.9.5.2(d) Stationary plant


51

Fig 2.9.5.2(e) SPREADING

Table 2.9.5.2 summarises the advantages and Disadvantages of Stabilization Techniques using

Equipment

Type Advantages Disadvantages

1. Mix-in-place ( i ) Plant is simple, cheap and

easily transported.

(i) It is difficult to obtain a

uniform thickness of lift,

because of the difficulty of

setting the machines to a given

depth.


52


(i) The number of machines

can be adjusted to suit the

quantum of work. Flexibility is

available.

(ii) The mixing is not uniform as

with travelling plant or station-

ary plant.

(iii) The whole processed

section is ready for compaction

at the same time.

(iii) Heavy rain is likely to spoil

the whole section.

(iv) A large out-put may be

maintained.

(iv) In a dry climate, water lost

by evaporation is difficult to

replace.

(v) If excess moisture is to be

got rid of as, for example, in a

wet area, this is the only

suitable method.

2. Travelling

plant

(i) Accurate proportioning of

added water possible.

(i) The cost of plant initially is

high.

(ii) Uniform mixing obtained. (ii) Is suitable for concentrated

and large/ quantum of work.

(iii) Short mixing time is

involved.

(iii) Minor breakdowns can

cause considerable dislocation.

(iv) Uniform surface can be

obtained.

(v) Depth of lift can be

accurately controlled.


53


(vi) It has the highest output for

given expenditure of plant and

labor.

3. Stationary

Plant

(i) Accurate proportioning of

mixture and water is possible.

(i) Expensive if in-situ soil is to

be processed.

(ii) The depth can be easily and

accurately controlled.

(iii) Material must be compacted

as delivered and not as a

complete section.

(iii) Concrete mixers can be

used.

(iv) Losses of moisture during

mixing and transporting are

small.

( v ) Suitable for location where

formwork is needed, as in the

case of sandy layers where

vibrators are needed.

(vi) No additional haulage in

soil has to be taken from a

borrow pit.


54

2.9.5.3 Intermediate or appropriate technology for soil stabilization

Intermediate or appropriate technology is an intelligent blend of labour and machinery. It is

recognised that small implements and tools and simple mechanical equipment can raise the

productivity of labour and aid in obtaining good quality of work. Kenya is a country which has

already a good industrial base for manufacturing and servicing simple tools and equipment, and

at the same time has surplus labour. Intermediate technology can be applied with good benefit in

Kenya under the prevailing conditions

The use of simple tools, implements and equipment can be beneficial in soil-stabilization work in

many ways.

It lends itself to a reasonable control over the quality of the work, which is so essential

for the success of the specification.

It is suitable for a large quantum of work which is to be completed in a tight schedule.

It does not do away with labour totally, and hence is not inappropriate to labour-surplus

economies.

The implements that are frequently used are the agricultural attachments such as disc harrows,

disc ploughs, grader blades, rotillors etc. which can be conveniently towed by a small

agricultural tractor or even by animal power. Water tankers for adding water can be pneumatic-

wheeled and pulled by bullocks. The Central Road Research Institute has developed simple

equipment known as the Rotillors which is a versatile multi-purpose machine suitable for

agriculture as well as for road making. For road making, the machine scarifies the top soil up to

the required depth, pulverises the soil and mixes the soil and stabiliser. The equipment is towed

by an agricultural tractor.

2.9.6. Quality control in soil-cement stabilization

Quality control is essential to ensure that the final product will be adequate for its intended use. It

must also ensure that the contractor has performed in accordance with the plans and

specifications as this is a basis for payment. Cement content, moisture content, soil, degree of


55

pulverization in mixing, mixing, compaction and curing are the important factors for quality

control. They are also the factors affecting strength of soil-cement mixes.

2.9.6.1 Factors affecting strength of soil-cement mixes.

(i) Cement Content.

The cement content necessary for effective stabilization varies with the soil type. The strength of

soil-cement mix for a particular soil type varies with the cement content. As a rough guide, the

cement content, expressed as a percentage by weight of the dry soil, varies between 4 and 14. For

preliminary estimation purposes, a value of 10 per cent seems reasonable. The cement content is

generally selected to obtain the desired compressive strength. Ordinary Portland cement is used

for the majority of soil-stabilization work. Rapid-hardening cement can be used if high strengths

are desired initially.

(ii) Moisture Content

Hydration of cement takes place only in the presence of water. Water also improves the

workability of the soil and facilitates compaction. One important factor governing the exact

amount of water to be added is that the soil-cement mixtures exhibit the same type of moisture-

density relationship as an ordinary soil. Thus, for a given compaction, there is an "optimum

moisture content" at which the maximum density is obtained. The best moisture content for

maximum density may not necessarily be the optimum moisture content for maximum strength.

It is generally seen that highest compressive strength can be obtained with specimens compacted

slightly below the optimum for maximum density. Some of the water is taken up by the cement

for hydration. The moisture necessary for maximum compaction is sufficient to provide for this.

(iii) Soil

Soil type has a profound influence on the success of stabilization with cement. It is often claimed

that almost any type of soil can be stabilized with cement. Though this is true in a large measure,

certain soil types cannot be stabilized with cement at economical costs. Soils with a low organic

matter are generally preferred. A safe- upper limit is 2 per cent, though soils with 3 to 4 per cent

organic matter have also been successfully stabilized with cement. Presence of sulphates has a

harmful effect on the life of cement concrete. For the same reasons, the presence of sulphates in


56

the soil has to be viewed with suspicion. For cohesive soils, a maximum sulphate content of 0.25

per cent is usually specified, though for non-cohesive materials an upper limit of 10 per cent may

be all right. The presence of a small amount of clay in the soil is beneficial to cement

stabilization, but large clay content brings in problems of mixing and pulverizing. It is desirable

if the clay content is restricted to 5 per cent. A thumb rule often employed is that the practical

upper limit for stabilization with machinery is when the PI multiplied by the percentage finer

than 425 is greater than 3500. As the plasticity of the soil, increases, the amount of cement

needed to effectively react increases. Highly plastic soils cannot, therefore, be economically

stabilized with cement. An upper limit of 45 for LL (Liquid Limit) and 20 for PI (Plasticity

Index) is generally observed. More plastic soils can be treated with cement after being pre-

treated with lime. As regards the grading of the soils, it is recognized that a well-graded mixture

requires less of cement and is preferred.

(iv) Degree of pulverization in mixing

The presence of lumps of soil inhibits effective stabilization. Pulverization of soils, especially

clays, must be carried out before mixing

(v) Uniformity of mixing

For best results, cement should be uniformly distributed and mixed throughout the material. The

addition of water helps the cement to adhere to the particles of the soil and prevents segregation.

Uniformity must be checked across the width of the pavement and to the desired depth of

treatment. Trenches can be dug and visually inspected. A satisfactory mix will exhibit a uniform

color throughout whereas a streaked appearance indicates a non uniform mix. Special attention

should be given to the edges of the pavement.

(vi) Compaction

The hydration of cement starts as soon as water is added, and it therefore is desirable to compact

the material as soon as mixing is completed. Any delay is likely to result in the loss of the

cementing action of the additive and in the need for extra compactive effort to break down the

cement bonds that have already formed. A serious loss in strength can follow. For this purpose, it

is often stipulated that compaction should be completed within two hours of mixing.


57

(vii) Curing

As in the case of cement concrete, soil cement requires the presence of sufficient moisture to

meet the needs of chemical reactions. A seven days' moist curing is necessary

2.9.7 Uses of cement stabilized soil in road construction

Cement modified silty clay soils are used in pavement construction to increase bearing strength

and reduce volume changes and plasticity properties of fine grained subgrades and highway fill.

Cement is also added to wet unstable subgrades as a construction expedient. The cement dries

out the wet soil, improves the soil characteristics, and produces a firm foundation on which the

pavement layers can be placed. The pavement layers are subgrade, subbases, base and surfacing.

Fig 2.9.7 (a) Kenyan practice on pavement layers

Fig 2.9.7 (b) American practice on pavement layers


58

Fig 2.9.7 (c) British practice on pavement layers

2.9.7.1 Subgrade stabilization for pavement

Poor quality subgrade soils are improved by cement. The primary purpose is to provide

supporting power and a firm, stable working foundation for pavement construction. Cement

treated subgrades also provide an effective solution to the problem of fatigue failures caused by

repeated high deflection of asphalt surfaces where a weak subgrade exists in the pavement

structure. Field tests and experience in areas of resilient subgrades, micaceous soils for example,

show a marked decrease in deflection when subgrade are stabilized with cement. Performance

indicates the cost of subgrade stabilization is well worth the modest cost involved.

2.9.7.2 Correcting unstable subgrade areas.

Sometimes localized soft spots of very wet and unstable subgrades are encountered unexpectedly

during construction. In addition to difficulty of operating construction equipment, adequate

compaction of subbase and base layers placed on top of these soft areas may not be possible.

These areas may be corrected by cement modification. Cement is spread and mixed into the soil

to the best extent possible. If the material is too wet or cohesive to use a travelling mixer, several

passe of disc harrow or mortar patrol using its clarifier teeth may process it. The material is then

compacted to whatever density that can be achieved. The drying action of the cement and its

hydration for two or three days will stabilize the area sufficiently so that construction may

proceed.


59

2.10 ARRESTING THE SWELL AND SHRINK BEHAVIOUR OF EXPANSIVE SOILS.

Expansive soil deposits are problematic to engineering structures because of their swelling and

shrinkage property. Structures on these soils experience large-scale damage due to heaving

accompanied by loss of strength of these soils during rainy season and shrink during dry seasons.

This alternate swelling and shrinkage causes differential movements resulting in severe damage

to the structures founded in them. This problem is more severe in case of light load structures

such as single storeyed buildings, canal linings, roads, retaining structures. The apparent effect of

swelling is observed as considerable distress in the form of ground cracks, building cracks, canal

lining slides, beds of canal heave, heaving and rutting of pavements etc.

The soil engineer must choose the most efficient method considering the environment, type of

structure and most important of all, establish the degree of treatment needed for the structure to

survive under future moisture changes.

Recent research findings enabled engineers to put forth several remedial techniques to mitigate

these damages (Gourly et al. 1993). These techniques include use of belled piers, drilled piers,

friction piles and moisture barriers. Stabilizing expansive soils with admixtures like lime,

cement, chemicals etc. has been found to be effective but uniform blending of large quantities of

soils with admixtures is difficult.

2.10.1 Methods for arresting the swelling of expansive soil.

2.10.1.1Under-reamed pile foundations

Under-reamed piles are piles which are provided with enlarged bulbs near the bottom. The bulbs

provide larger resistance to the pile both in compression and uplift. However, when the piles are

to be anchored in sand underlying the expansive clay bed, this is not useful because formation of

bulb in sandy soils is difficult as sands cannot take negative slope.

2.10.1.2 Granular pile-anchor

Based on the investigations carried out on large-scale laboratory models (Srirama Rao et al.,

2007), it was found that heave of expansive clay beds can be reduced significantly by reinforcing

them with granular pile-anchors, which are granular columns with an anchor rod placed centrally


60

in it connecting the foundation at the top and an anchor plate at the bottom. The frictional

resistance at the interface between the granular pile and the soil is instrumental in inhibiting the

upward movement of the soil (heave) up to some distance around the granular pile

2.10.1.3 Sub excavating and replacement of the expansive soil by cushions

The expansive soil is replaced either in part or full with a material that doesn’t undergo swell.

The replacement materials include:

Sand cushion: Satyanarayana (1969) suggested that the entire depth or a part of the expansive

soil may be removed and replaced with a sand cushion, compacted to the desired density and

thickness. Swelling pressure varies inversely as the thickness of the sand layer and directly as its

density. The advantage of sand cushion is its ability to adapt itself to volume changes in the soil.

Limitations come in particularly when it’s adopted in deep strata.

Cohesive- non-swelling (CNS) soils method. Katti (1978) developed a technique whereby

about 1m of expansive soil is removed and replaced with CNS layer beneath foundations.

According to Katti cohesive forces of significant magnitude are developed with depth in an

expansive soil system during saturation which is responsible for reducing heave and

counteracting swelling pressure. This behavior is attributed to the influence of electrical charges

present on the surface of clay particles on the dipolar nature of water molecule, producing

absorbed water bonds that give rise to cohesion. Studies conducted later (Subba Rao et al, 1995)

indicated that CNS cushion was effective in arresting heave only during the first cycle of

seasonal moisture fluctuation and during subsequent cycles the heave may be more than that

recorded by a black cotton soil without cushion.

Fly Ash cushion: Studies carried using fly ash as a cushion have shown that developments of

cohesive bonds in lime stabilized fly ash cushion is expected to produce an environment similar

to the one obtained in the CNS material. It also solves the problem of fly ash utilization and

disposal to some extent.


61

CHAPTER 3

3.0 PRELIMINARY TESTS

3.1 INVESTIGATION OF SOIL PROPERTIES

Soil samples was collected from Donholm area and transported to the lab. A portion of the soil

was taken for the determination of natural moisture content in accordance to BS 1377-2: 1990.

3.2 CLASSIFICATION

The rest of the sample was air dried.500grams of the air dried sample was soaked in water for

two hours and washed over 0.075 sieve. The retained soil was oven dried after which it was

placed on an already prepared stack of sieve with different aperture sizes arranged in a way that

every upper sieve had a larger opening than the sieve below. The test sieves were agitated so that

soil samples roll over the test sieves and mass of retained soil in each sieve was determined. The

particle size distribution curve is as shown in the graph in section 4.1.1

3.3 PROCTOR COMPACTION TEST

A sample soil containing not less than about 90% passing the 19mm Bs sieve No 7 was

compacted while varying its moisture content so as to determine its maximum dry density

(MDD) and Optimum Moisture content (OMC) as described in BS 1377-4:1990. The

compaction was achieved by free fall of the 2.5 kg rammer through 300mm in three layer each

layer receiving 27 blows. The compaction test results are as shown in the graph in section 4.1.2

3.4 ATTERBERG LIMITS

An air dry sample passing the 425µm sieve No 36 was mixed with water and used to determine

the consistency limits of the soil.

Liquid limit was determined using the cone penetrometer apparatus where an air dry soil sample

passing the 425 micron sieve was mixed with water and the soil paste filled in the metal cup and

the surface struck of level. The cone was then lowered to just touch the surface of the soil paste

and then released for a period of 5 seconds and the penetration recorded. This test was repeated


62

over four different moisture content. The moisture content corresponding to a cone penetration of

20mm was taken as the Liquid limit.

Plastic limit was determined by dipping a small portion soil paste in the air dry sample passing

sieve No 36 to form a mould. The mould was rolled between fingers then on a smooth plate in to

a thread of 3mm diameter. This was being repeated until the thread first showed signs of

cracking. A portion of the mould was taken for water content determination.

Shrinkage was determined by placing a saturated sample in a trough of known length (14cm) and

left to air dry before being dried in the oven. The length of the oven dried soil in the tough was

measured using a Vanier caliper. This new length was then subtracted from the length of the

trough to get the shrinkage. The Atterberg limits for the neat soil are as shown in Graph 4.1.3.1

The Atterberg limit test was repeated over the different soil samples mixed with varying

percentages of cement content i.e. the soil from the CBR mould was air dried and Atterberg

limits determined. The Atterberg limits for the stabilized soil are as shown in the graphs in

sections 4.1.3.2, 4.1.3.3, and 4.1.3.4 for soil with 6%, 8% and 10% cement content respectively.

3.5 California Bearing Ratio (CBR) tests.

3.5.1 CBR

CBR tests were conducted on neat soil as well as stabilized soils. To stabilize the soil cement

was added in different percentages i.e. 6%, 8% and 10%. The dry weight required for filling the

mould was calculated based upon the maximum dry density (MDD) and corresponding optimum

moisture content was achieved from standard proctor test. The static method of compacting soil

specimen in the CBR mould was used. The correct mass of the wet soil was placed in the mould

in five layers and each layer gently compacted with the spacer disc. A filter paper was placed on

top of the soil followed by a 5cm displacer disc. The mould was compacted by pressing it in

between the platens of the compression testing machine until the top of the spacer disc came

flush with the top of the mould. The load was held for 30 seconds then released. The neat soil

was tested after soaking in water for four days. The stabilized soils were left to cure for 7 days

and then soaked in water for seven days. The load penetration curve was drawn for the neat soil

as well as the stabilized soils and the CBR values were calculated from these curves. The


63

variation of CBR (soaked condition) of the three black cotton soil samples with the addition of

cement in increasing percentages is shown in Graphs in section 4.1.4.1 Neat sample, 4.1.4.2 Soil

treated with 6% Cement content, 4.1.4.3 Soil treated with 8% Cement content 4.1.4.4 Soil treated

with 10% Cement content.

Fig 3.5.1 Testing of CBR specimen

3.5.2 Swell

For swell purpose the initial height (H) of specimen was determined in mm. A dial gauge was

mounted the on the edge of the mould and the initial dial gauge reading (L) recorded. The final

reading of the dial gauge at the end of soaking period (K) was also recorded.

Calculations for Swelling

(S) = (K- L)*F*100/ (H)

Where

S = swell expressed as a percentage of the height of the moulded material before soaking

K = dial gauge reading after soaking

L = dial gauge reading before soaking

F= dial gauge reading factor

H= Initial height of specimen


64

3.6 Swell shrink test

3.6.1 Swell Test

The experimental study was carried out in Galvanized Iron cylindrical test moulds. A 10 mm

thick sand layer, compacted to its maximum dry density and OMC, was laid at the bottom of the

mould. A cylindrical casing made of Galvanized Iron was greased and placed centrally in the test

mould.

The black cotton soil was compacted at its MDD (1.236 Kg/m3 ) and OMC (19.4%) in 3 layers in

the casing. Sand was poured in the gap between the cylindrical mould and the casing pipe and

compacted with a poking rod simultaneously. The process of compaction of the expansive clay

bed and the sand packing was continued till the clay bed and sand packing attained the same

height.

A hollow PVC pipe was placed on the top of the clay bed. The space inside the casing pipe was

filled with fly ash around the PVC pipe and compacted. Sand was poured in the annular space

between the casing pipe and the mould and the process of compaction was continued till the clay

bed and sand packing attained the same height.

When compaction of both the clay bed and the fly ash cushion was completed, the casing pipe

was withdrawn. The sand layers at the bottom and all around help quick saturation of the

expansive soil following inundation.

After the compaction of the fly ash cushion, a heave stake was placed through the PVC pipe on

the top of the clay bed. A dial gauge was mounted on the top of the heave stake and the initial

(L) dial gauge reading recorded.

A schematic diagram of the experimental set up is given in Figure 3.6.1


65

Fig.3.6.1: Schematic Diagram of the Experimental Set-up

Water was admitted into the moulds. The dial gauge readings were noted down every day at the

same time. This set up was kept in such undisturbed state while maintain a constant water level

throughout the period.

It was presumed that equilibrium was reached meaning that complete saturation of soil had

occurred after 120 hours. The ultimate heave was recorded i.e. the final reading (K) of the dial

gauge at equilibrium.

Experiments were conducted for different thickness ratios of soil (Ts) fly ash (Tf) given by

Tf/Ts = 0.25, 0.5, 0.75, 1.0.

Ts=100mm

The sand used was of MDD=1.8 Kg/m3 and OMC= 11%

The commercial fly ash was used had an MDD of 1.4 Kg/m3,

OMC of 24% and Liquid limit of

26%. Its chemical composition is as shown in Table 3.6.1

Name of the chemical Symbol Range Range [% by weight]

Silica SiO2 63.19

Alumina Al2O3 24.76

Ferri Oxide Fe2O3 3.5

Titanium Dioxide TiO2 1.5

Manganese oxide MnO 0.05

Dial gauge

Heave stake

Hollow PVC pipe

Test tank

Fly ash layer

100 mm thick soil bed

10 mm sand layer at

the bottom & Sand

drain all around


66

Calcium oxide CaO 2.56

Magnesium oxide MgO 1.43

Phosphorous P 0.1

Sulphur Trioxide SO3 0.07

Potassium Oxide K2O 0.37

Sodium oxide Na2O 0.6

Table 3.6.1 Chemical composition of fly ash

3.6.2 Shrinkage test

The ultimate heave (K) was taken as the initial gauge reading. The specimen was air dried and

the heave stake and PVC pipe removed. The specimen was then oven dried at a temperature of

45° C. The dial gauge readings were noted down every day at the same time by re-inserting the

heave stake in the hollow space and mounting a dial gauge on the top of the heave stake and the

dial gauge reading recorded. It was however difficult to keep the specimen in undisturbed state

throughout the period since the specimen had to be removed from the oven momentarily to take

the readings after every 24 hrs.

It was presumed that equilibrium was reached meaning that the soil was completely dry after

72hours. Dial gauge reading at the end of shrinkage process was recorded. After the shrinkage

process was completed, which marks the completion of one cycle, the next cycle of swelling and

shrinkage was started.


67

CHAPTER 4

4.0 RESULTS, DATA ANALYSIS AND DISCUSSION

4.1 RESULTS AND DATA ANALYSIS

4.1.1 PARTICLE SIZE DISTRIBUTION


68

4.1.2 PROCTOR COMPACTION TEST


69

4.1.3.1 ATTERBERG LIMITS FOR NEAT SOIL

4.1.3.2 ATTERBERG LIMITS FOR SOIL WITH 6% CEMENT CONTENT


70




71

4.1.4.1 CALIFORNIA BEARING RATIO VALUES FOR NEAT SOIL


72

4.1.4.2 CALIFORNIA BEARING RATIO VALUES FOR SOIL WITH 6% CEMENT

CONTENT


73


CONTENT


74


CONTENT


75

4.1.5 SWELL SHRINK TEST RESULTS

4.1.5.1 Neat soil with fly ash cushion of varying depth

Neat soil Cycle 1

Time (hours)

Dial gauge reading (mm)

0.00 0.25 0.50 0.75 1.00

0 14.9 11.6 11.0 10.1 9.7

24 14.5 11.3 10.8 10.0 9.6

48 14.0 11.1 10.6 9.9 9.5

72 13.0 10.7 10.4 9.7 9.4

% Shrinkage 1.727 0.667 0.375 0.216 0.143

Neat soil Cycle 2

Time (hours)


0.00 0.25 0.50 0.75 1.00

0 28.0 18.7 17.4 15.4 14.3

24 27.5 17.3 16.8 14.9 13.2

48 26.9 16.3 15.7 13.9 12.4

72 26.0 13.0 12.0 11.0 10.5

% Shrinkage 1.818 4.222 3.375 2.378 1.810

Neat soil Cycle 3

Time (hours)


0.00 0.25 0.50 0.75 1.00

0 14.5 7.0 6.9 5.6 5.0

24 13.6 4.6 4.3 3.7 2.6

48 13.0 3.7 3.1 2.5 2.0

72 12.0 2.0 1.8 1.7 1.4

% Shrinkage 2.273 3.704 3.188 2.108 1.714

Neat soil Cycle 1

Time

(hours)

Dial gauge reading for different

Tf/Ts

0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 14.0 11.0 10.7 9.8 8.9

48 14.4 11.3 10.9 9.9 9.2

72 14.9 11.6 11.0 10.1 9.7

Swell 13.545 8.593 6.875 5.459 4.619

Neat soil Cycle 2

Time

(hours)


0.00 0.25 0.50 0.75 1.00

0 13.0 10.7 9.6 9.0 8.4

24 19.3 15.5 12.6 10.4 10.0

48 24.9 17.1 16.6 14.0 13.2

72 28.0 18.7 17.4 15.4 14.3

Swell 13.636 5.926 4.875 3.459 2.810

Neat soil Cycle 3

Time

(hours)


0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 5.9 3.3 2.8 2.5 2.4

48 10.2 4.7 4.4 4.0 3.3

72 14.5 7.0 6.9 5.6 5.0

Swell 13.182 5.185 4.313 3.027 2.381


76

Cyclic swell-shrink behavior of neat soil

provided with fly ash cushion of varying depth

Neat soil

Cycle

%Swell shrink for varying Tf/Ts

0.00 0.25 0.50 0.75 1.00

0 0.00 0.00 0.00 0.00 0.00

0.5 13.545 8.593 6.875 5.459 4.619

1.0 1.727 0.667 0.375 0.216 0.143

1.5 13.636 5.926 4.875 3.459 2.810

2.0 1.818 4.222 3.375 2.378 1.810

2.5 13.182 5.185 4.313 3.027 2.381

3.0 2.273 3.704 3.188 2.108 1.711

Key

line Tf/Ts

0.00

0.25

0.50

0.75

1.00

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

% S

we

ll S

hri

nk

Cycle

Cyclic Swell shrink behaviour


77

4.1.5.2 Soil-6% Cement mix with fly ash cushion of varying depth

Soil-6% cement mix Cycle 1

Time (hours)


0.00 0.25 0.50 0.75 1.00

0 11.0 10.7 10.2 9.0 8.2

24 10.8 10.6 10.1 8.9 8.1

48 10.7 10.5 10.0 8.8 8.0

72 6.0 6.9 7.0 7.0 6.0

%Shrinkage 4.545 2.815 2.000 1.081 1.048


Time (hours)


0.00 0.25 0.50 0.75 1.00

0 6.6 4.3 3.8 3.6 3.3

24 5.6 4.0 3.4 3.0 2.7

48 4.2 3.5 3.0 2.5 2.0

72 2.0 1.0 0.5 0.4 0.3

%Shrinkage 4.182 2.444 2.063 1.730 1.429


Time (hours)


0.00 0.25 0.50 0.75 1.00

0 6.1 3.9 3.9 3.5 3.4

24 5.6 3.8 3.5 3.0 2.7

48 4.4 2.9 2.7 2.7 2.0

72 2.6 0.2 0.8 0.2 0.3

%Shrinkage 4.182 2.741 1.938 1.784 1.476


Time

(hours)


0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 10.2 10.0 9.5 8.6 7.7

48 10.5 10.2 9.7 8.8 7.9

72 11.0 10.7 10.2 9.0 8.2

Swell 10.000 7.926 6.375 4.865 3.905


Time

(hours)

Dial gauge (mm)

0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 2.6 1.2 1.2 1.0 0.8

48 4.3 2.4 2.0 1.8 1.4

72 6.6 4.3 3.8 3.6 3.3

Swell 6.000 3.185 2.375 1.946 1.571


Time

(hours)


0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 2.9 1.2 1.2 1.0 0.8

48 4.5 2.4 2.0 1.8 1.4

72 6.1 3.9 3.9 3.5 3.4

Swell 5.545 2.889 2.438 1.892 1.619


78

Cyclic swell-shrink behavior of soil-6%Cement

mix provided with fly ash cushion of varying

depth

Soil-6% Cement Mix

Cycle


0.00 0.25 0.50 0.75 1.00

0 0.00 0.00 0.00 0.00 0.00

0.5 10.000 7.926 6.375 4.865 3.905

1.0 4.545 2.815 2.000 1.081 1.048

1.5 6.000 3.185 2.375 1.946 1.571

2.0 4.182 2.444 2.063 1.730 1.429

2.5 5.545 2.889 2.438 1.892 1.619

3.0 4.182 2.741 1.938 1.784 1.476

Key

line Tf/Ts

0.00

0.25

0.50

0.75

1.00

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

% S

we

ll S

hri

nk

Cycle



79



Time (hours)


0.00 0.25 0.50 0.75 1.00

0 6.0 3.5 3.5 3.0 3.0

24 3.2 2.3 2.0 2.0 1.4

48 2.8 1.4 1.3 1.2 1.0

72 2.6 0.2 0.8 0.2 0.3

%Shrinkage 3.091 2.444 1.688 1.514 1.286

Soil-8%cement mix cycle 2

Time (hours)


0.00 0.25 0.50 0.75 1.00

0 3.8 3.6 3.5 3.2 3.0

24 3.0 2.9 2.6 2.0 1.4

48 2.2 2.0 1.4 1.2 1.0

72 0.5 0.2 0.8 0.2 0.3

%Shrinkage 3.000 2.519 1.688 1.622 1.286


Time (hours)


0.00 0.25 0.50 0.75 1.00

0 3.8 3.5 3.5 3.3 3.1

24 2.2 2.5 2.6 2.0 1.4

48 2.0 2.0 1.4 1.2 1.0

72 0.3 0.1 0.6 0.2 0.2

%Shrinkage 3.182 2.519 1.813 1.676 1.381


Time

(hours)


0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 2.9 1.2 1.2 1.0 0.8

48 4.5 2.4 2.0 1.8 1.4

72 5.0 3.5 3.5 3.0 3.0

Swell 4.545 2.593 2.188 1.622 1.429


Time

(hours)


0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 1.9 1.2 1.2 1.0 0.8

48 2.7 2.4 2.0 1.8 1.4

72 3.8 3.6 3.5 3.2 3.0

Swell 3.455 2.667 2.188 1.730 1.429


Time

(hours)

Dial gauge reading(mm)

0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 1.9 1.2 1.2 1.0 0.8

48 2.7 2.4 2.0 1.8 1.4

72 3.8 3.5 3.5 3.3 3.1

Swell 3.455 2.593 2.188 1.784 1.476


80

Cyclic swell-shrink behavior of soil-8%Cement

mix provided with fly ash cushion of varying

depth

Soil-8% Cement Mix

Cycle


0.00 0.25 0.50 0.75 1.00

0 0.00 0.00 0.00 0.00 0.00

0.5 4.545 2.593 2.188 1.622 1.429

1.0 3.091 2.444 1.688 1.514 1.286

1.5 3.455 2.667 2.188 1.730 1.429

2.0 3.000 2.519 1.688 1.622 1.286

2.5 3.455 2.593 2.188 1.784 1.476

3.0 3.182 2.519 1.813 1.676 1.676

Key

line Tf/Ts

0.00

0.25

0.50

0.75

1.00

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

% S

we

ll S

hri

nk

Cycle



81



Time (hours)


0.00 0.25 0.50 0.75 1.00

0 4.0 3.9 3.9 3.5 3.4

24 3.0 2.9 2.6 2.0 1.4

48 2.2 2.0 1.4 1.2 1.0

72 0.0 0.2 0.8 0.2 0.3

%Shrinkage 3.636 2.741 1.938 1.784 1.476


Time (hours)


0.00 0.25 0.50 0.75 1.00

0 4.1 3.8 3.7 3.5 3.3

24 3.0 2.9 2.6 2.0 1.4

48 2.2 2.0 1.4 1.2 1.0

72 0.2 0.1 0.6 0.2 0.2

%Shrinkage 3.545 2.741 1.938 1.784 1.476


Time (hours)

Dial gauge reading for different

Tf/Ts

0.00 0.25 0.50 0.75 1.00

0 4.1 3.8 3.7 3.5 3.3

24 3.0 2.9 2.6 2.0 1.4

48 2.2 2.0 1.4 1.2 1.0

72 0.2 0.1 0.6 0.2 0.2

%Shrinkage 3.545 2.741 1.938 1.784 1.476


Time

(hours)


0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 1.9 1.2 1.2 1.0 0.8

48 2.7 2.4 2.0 1.8 1.4

72 4.0 3.9 3.9 3.5 3.4

Swell 3.636 2.889 2.438 1.892 1.619


Time

(hours)

Dial gauge reading(mm)

0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 1.9 1.2 1.2 1.0 0.8

48 2.7 2.4 2.0 1.8 1.4

72 4.1 3.8 3.7 3.5 3.3

Swell 3.727 2.815 2.313 1.892 1.571


Time

(hours)


0.00 0.25 0.50 0.75 1.00

0 0.0 0.0 0.0 0.0 0.0

24 1.9 1.2 1.2 1.0 0.8

48 2.7 2.4 2.0 1.8 1.4

72 4.1 3.8 3.7 3.5 3.3

Swell 3.727 2.815 2.313 1.892 1.571


82

Cyclic swell-shrink behavior of soil with 10%

cement content & provided with fly ash cushion

of varying depth

Soil with 10% cement content

Cycle


0.00 0.25 0.50 0.75 1.00

0 0.00 0.00 0.00 0.00 0.00

0.5 3.636 2.889 2.438 1.892 1.619

1.0 3.636 2.741 1.938 1.784 1.476

1.5 3.727 2.815 2.313 1.892 1.571

2.0 3.545 2.741 1.938 1.784 1.476

2.5 3.727 2.815 2.313 1.892 1.571

3.0 3.545 2.741 1.938 1.784 1.476

Key

line Tf/Ts

0.00

0.25

0.50

0.75

1.00

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

% S

we

ll S

hri

nk

Cycle

Cyclic Swell shrink behaviour of soil with 10% cement


83

4.2 DISCUSSION

4.2.1 MAXIMUM DRY DENSITY

The Natural moisture content (NMC) of the soil was found to be 6.9%. It’s the ratio of weight of

water to weight of the solids in a mass of soil. It gives an idea of the state of soil in the field. The

knowledge of NMC was essential in determination of the bearing capacity.

The maximum dry density (MDD) that was attained for black cotton soil with a standard amount

of compactive effort was found to be 2.236 Kg/m3 . For every addition of cement the maximum

dry density goes on decreasing. It reduces from 2.236 Kg/m3 for neat to 1.04 Kg/m

3 for B C+

10% Cement mixtures. Moisture content of the mixtures continuously decreases with addition of

cement. It reduces from 36.96% for neat to 18.97% for B C+ 10% Cement mixtures. Optimum

water content decreases from 34% to 23.3%. The decrease in maximum dry density is due to

domination of low specific gravity of ash. Further the soil gradation may be adversely affected

the dry density at higher content of cement in the mixture. However above factors decreases the

water holding capacity of the mixture and hence optimum moisture content decreases

continuously with every increment in cement

4.2.2 SOIL CLASSIFICATION

The soil was classified as CH i.e. Clay of high plasticity under the Unified Soil Classification

System (USCS). USCS is a soil classification system used in engineering, geology and soil

science disciplines to describe the texture and grain size of soil.


84

4.2.3 ATTERBERG LIMITS

Liquid Limit

%

Plastic Limit (PL) Plasticity Index (PI)

Neat soil 58.38 22.73 35.65

6%Cement 54.49 30.91 23.58

8%Cement 50.45 34.78 15.67

10% cement 49.33 34.37 14.96

Table 4.2.3.1 Atterberg limits for the test soil

Liquid limit: The liquid limit decreases with the addition of cement. The results show a

considerable decrease in the liquid limit up to 8% cement content increase and then after the

decrease is observed to be marginal for further increase of cement content to 10%. The liquid

limit of the black cotton soils is essentially controlled by the thickness of the diffused double

layer and the shearing resistance at particle level. The addition of cement results in the decrease

of liquid limit due to the effect of reduction in the diffused double layer thickness as well as due

to the effect of dilution of clay content of the mix. The decrease of liquid limit becomes very

marginal at 10% cement content of due to the increased dilution effect i.e. due to the increased

percentage of coarser size particles in the mix because of the increased percentage of Cement.

In some cases addition of cement may result in an increase in liquid limit. The increase in the

liquid limit of the soil may be attributed to prolonged equilibrium of the cement–soil mixture

which results in formation of more flocculated particle arrangement. Possibly, the water

entrapped in the large void spaces of the flocculated structure of the soil fabric, thereby increase

in liquid limit (prakash et al 1989)

Plastic Limit: The addition of cement results in a steady increase in the plastic limit of the soils.

Immediately on addition of 6% cement, the plastic limit of black cotton soil increases from

22.73% to 30.91%. The plastic limit increases to 38.67% for the soil with 8% cement content. A

further increase to 41.44% for the soil with 10% cements content.

The increase in plastic limit is due to decrease in diffused double layer thickness of clay particles

and flocculation owing to the presence of free lime in the cement. Decrease in diffused double

layer leads to increase in shearing resistance. The soil fabric varies with changes in exchangeable


85

cation and cement concentration. The pores vary depending upon the particles arrangement, size

and shape. Thus the flocculated structure will have higher plastic limit (Sivapullaiah et al 1995).

Plasticity Index

The addition of cement decreases the plasticity index of the soil.

Soil PI % USCS Plasticity

Neat 35.65 CH high

6% Cement 23.58 MH High

8% Cement 15.67 ML Medium

10% Cement 14.96 ML Medium

Table 4.2.3.2 Classification from PI

Plasticity index of untreated black cotton soil is 35.65% and can be classified using Plasticity

chart unified system as CH i.e. Inorganic clay of high plasticity and liquid limit greater than 50

(LL=58.38). Plasticity index of black cotton soil reduces with increase in cement proportions;

black cotton soil changes its classification to ML i.e. inorganic silts, silty or clayey fine sands of

medium plasticity with medium when treated with 8% cement. The decrease in plasticity index

when black cotton soil treated with 10% cement content is marginal and the soil is still classified

as ML. This indicate that there is slight variation in plasticity index which lies below the A-line

having high compressible material. The reduction in plasticity indices are indication for soil

improvement (Amu et al 2011).


86


87

4.2.4. CALIFORNIA BEARING RATIO

%CBR value % Moisture content

Neat soil 10 36.96

Soil+6% cement 15 30.72

Soil+ 8%cement 31 24.71

Soil+10%cement 20 18.97

Table 4.2.4.CBR and MC values

The Soaked CBR value of the mix with 6% cement content has been found to be 15%, which is

1.5 times the CBR value of soil alone. For soil admixed with 8% cement content, the proportion

which yielded maximum CBR value was found to be 31%, which is 3.1 times the CBR value of

soil alone. The Soaked CBR value of the mix with 10% cement content has been found to be

20% which 2 times the CBR value of soil alone. There is an observed decrease in the CBR value

from the soil cement mix with 8% cement content.

The low CBR of untreated black cotton soil as compared to the black cotton soil-cement mixes is

attributed to its inherent low strength which is due to the dominance of the clay fraction.

Addition of cement to the black cotton soil increases gradually the CBR of the mix up to a peak

value of addition of 8% of cement. This is due to the frictional resistance contributed from the

cement in addition to the cohesion from the black cotton soil. Further increase in the cement to

10% causes a reduction in the CBR due to the reduction in the cohesion because of the

decreasing black cotton soil content in spite of increase in strength due to increase in cement

content. It is hence observed that, a suitable mix proportion of 8% cement content optimizes the

frictional contribution of cement and the cohesive contribution from black cotton soils; leading

to the maximization (peak value) of the CBR.


88

4.2.5 LINEAR SHRINKAGE AND SWELL

%Linear shrinkage %Swell Swell potential from

PI(Chen 1988)

Neat soil 17.86 5.1 High

Soil+6% cement 11.8 4.3 Medium

Soil+ 8%cement 7.8 2.6 Medium

Soil+10%cement 7.5 1.4 Low

Table 4.2.5 Variation of linear shrinkage and swell

Variation of linear shrinkage and swell with cement content

The linear shrinkage and swell potential of the samples follow a steady decrease with the

addition of cement in increasing percentages. When cement is added to expansive clays in

presence of water, two important reactions take place: one is flocculation and the

other is cementation. The decrease is mainly due to the flocculation of clay particles caused by

the free lime present in cement resulting in the reduction of friction between the particles. The

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

17.0

18.0

0.0

0

2.0

0

4.0

0

6.0

0

8.0

0

10

.00

% L

ine

ar

sh

rin

ka

ge

% CEMENT CONTENT

linear shrinkage

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

0.0

0

2.0

0

4.0

0

6.0

0

8.0

0

10

.00

% S

WE

LL

% CEMENT CONTENT

SWELL


89

second reaction taking place upon the addition of cement is cementation. As an effect of this

reaction, cementitious products in the form of calcium alumino-silicates develop in the blend.

To further reduce the swell potential, fly ash cushions of varying depth were placed on top of the

various soils. Stabilizing agents like cement reacts with reactive silica present in the fly ash to

produce cementitious bonds that help in arresting swell.

The swell-shrink behavior was studied keeping in view the fact that CNS cushion was effective

only during the first cycle of wetting and drying and that its effect reduces in the subsequent

cycles (Subba Rao, 2000). The cyclic swell behavior of neat soil without fly ash cushion shows

the excessive swell and shrinkage that black cotton soil undergoes with change in seasons.

Reduction of swell with every cycle was observed for the soils stabilized with varying cement

contents in the absence of fly ash cushion. With addition and increase in the thickness of the

cushion, there was a corresponding reduction in swell. Unlike the swell-shrink behavior of CNS-

cushioned expansive soil, which shows that CNS becomes less effective with cycles of swelling

and shrinkage, the swell-shrink behavior of an expansive soil provided with fly ash cushion

improves with every successive cycle

It can be further seen that the band-widths corresponding to any given cycle decreases with

increase in cement content in soil and thickness of fly ash cushion. In a CNS-cushioned

expansive soil bed, the band-widths of swelling with a surcharge of 50 KN/m² are fairly

significant over five cycles of swelling and shrinkage (Subba Rao, 2000). As against this, in

cement-stabilized black cotton soil provided with fly ash cushions, has taken three cycles to

attain a bandwidth that is almost negligible.


90

CHAPTER 5

5.0 CONCLUSION AND RECOMMENDATIONS

5.1 CONCLUSION

The problems associated with black cotton soil in pavement construction have been identified to

arise out of water saturation as well as design based problems. The road surfacing must be

impervious, side berms paved and sub grade well treated to check capillary rise of water. Cement

stabilization reduces permeability, helps keep moisture out and maintains high level of strength

and stiffness even when saturated. An approach to the design problem can be in having semi

rigid sub-base or the CBR value of the BC soil being used as subgrade is improved by giving a

suitable treatment with the appropriate technology.

It was also establish that cement can be used as an effective stabilizer for improving the

geotechnical characteristics of black cotton soils for use in sub-grades or sub-base layers. The

BC soil parameters are seen to have improved i.e. decreased plasticity, volume change and

increased bearing strength of the BC soils.

Addition of cement significantly improves the index properties of soil. Plasticity index is one of

the important criteria for selection of soil as construction material. The relative decrease in the

plasticity index of the soils is a favorable change since it increases the workability of these soils.

The decrease in linear shrinkage of the soils with the addition of cement facilitates in checking

the volume change behavior of the soils over a large variation in the moisture content as the

season changes.

California bearing ratio of the study soil increases gradually with the addition of cement. The

improvement in the California bearing ratio value of the black cotton soil upon the addition of

cement suggests that cement can be effectively used in bulk as sub-grade material in combination

with the study soils, for the road construction works. The increase in CBR values indicates an

increase the bearing strength. Bearing strength provides a stable working platform on which

pavement layers may be constructed.

The decrease in linear shrinkage and swell of the black cotton soil with addition of cement shows

that cement reduces the heaving potential of the soil. Stabilizing expansive soils with admixtures


91

like cement has been found to be effective but uniform blending of large quantities of soils with

admixtures is difficult. Among the several methods for arresting the swelling of expansive soil,

providing a cushion on top is commonly adopted. Fly ash cushion was used in this study. Fly ash

cushion is effective in minimizing heave of expansive clays. With increase in cement content in

soil and the thickness of the fly ash cushion, heave decreases and the band width of swelling and

shrinkage over successive cycles decreases gradually and becomes almost negligible.The swell-

shrink behavior fly ash cushion improves with every successive cycle.

The study of variations of different parameters viz. liquid limit, plastic limit, plasticity index,

shrinkage limit, maximum dry density, optimum moisture content, swell and California bearing

ratio with the addition of cement suggest that the effects of cement treatment vary depending

upon the quantity of cement that is mixed with the black cotton soil and therefore for each

parameter of the study soil samples, there exists an optimum cement percentage for mixing with

the soil under consideration; at which the respective parameter attains its most desirable value

from geotechnical point of view.

Cement soil stabilization technology has been found useful, cost-effective and suited to manual

methods of construction.


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5.2RECOMMENDATIONS

The use of cement should be embraced in the construction industry as an alternative method of

stabilizing weak subgrades. The cement treatment can be utilized for the following purposes:

To overcome the susceptibility of foundations to volume change and to increase their

shearing resistance and bearing capacity

To consolidate sub grades and base courses for concrete pavement in order to make them

resistant to volume changes and displacement or erosion in the presence of moisture even

under the rocking action of curled slabs, if any.

To provide a pavement foundation of marginally weaker in strength than that of concrete

pavement, but much improved strength than natural Black cotton soil.

There is a possibility of using industrial wastes such as fly ash which pose problems when it

comes to their disposal in geotechnical applications such as arresting heave in black cotton soils

This study was limited in scope and hence further research should be done to establish the

additives to cement that could be used to lower the percentage of cement required without

compromising on the strength.


93

REFERENCES

1) Civil engineering hand book 2nd

edition by W.F Chen and J.Y. Richard Liew

2) Basic soil mechanics 4th

edition 2001. Roy Whitlow, Person Education LTD.

Edinburg Gate Harlow.

3) University of Nairobi lecture notes. Geotechnical engineering FCE 311 on soil

mechanics.

4) University of Nairobi lecture notes. Transportation Engineering FCE 546 and

Geotechnical engineering FCE 511 on soil stabilization.

5) Chen, F. H (1998): “Foundations Expansive Soils,” American Elsevier Science

Publication, New York.

6) Dunn I. S., Anderson, L. R. & Kiefer, F.W. (1980): “Fundamentals of

Geotechnical Analysis,” John Wiley & Sons, Inc. New York.

7) John Nelson, D & Debora Millar. J (1991): “Expansive Soils,” John Wiley &

Sons, Inc. New York

8) EBook: Soil stabilization method. www.gobookee.org/stabilized-soils.

9) Czernin, W. (1962) Cement Chemistry and Physics for Civil Engineers, Crosby

Lockwood, London

10) B.S. 1377 (1990) “Methods of testing soil for civil engineering purposes”. British

Standards Institute, London.

11) B.S. 1924 (1990) “Methods of Tests for stabilized Soils.” British Standards

Institute, London

12) AASHTO (1986) “Standard Specifications for Transport Materials and Methods

of Sampling and Testing.”14th Edition, American Association of State Highway

and Transport Officials (AASHTO), Washington, D.C

http://www.gobookee.org/stabilized-soils

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