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STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
1
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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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.
.
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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.
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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.
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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:
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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.
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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.
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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.
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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.
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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
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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
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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
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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
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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:
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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.
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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.
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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
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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
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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.
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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.
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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
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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.
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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
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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.
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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
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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.
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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.
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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.
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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
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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.
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(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.
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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
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(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
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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
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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.
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Type Advantages Disadvantages
(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.
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Type Advantages Disadvantages
(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.
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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
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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
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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.
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(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
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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.
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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
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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.
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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
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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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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.
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
67
CHAPTER 4
4.0 RESULTS, DATA ANALYSIS AND DISCUSSION
4.1 RESULTS AND DATA ANALYSIS
4.1.1 PARTICLE SIZE DISTRIBUTION
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
69
4.1.3.1 ATTERBERG LIMITS FOR NEAT SOIL
4.1.3.2 ATTERBERG LIMITS FOR SOIL WITH 6% CEMENT CONTENT
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
70
4.1.3.3 ATTERBERG LIMITS FOR SOIL WITH 8% CEMENT CONTENT
4.1.3.4 ATTERBERG LIMITS FOR SOIL WITH 10% CEMENT CONTENT
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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4.1.4.1 CALIFORNIA BEARING RATIO VALUES FOR NEAT SOIL
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
72
4.1.4.2 CALIFORNIA BEARING RATIO VALUES FOR SOIL WITH 6% CEMENT
CONTENT
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
73
4.1.4.3 CALIFORNIA BEARING RATIO VALUES FOR SOIL WITH 8% CEMENT
CONTENT
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
74
4.1.4.4 CALIFORNIA BEARING RATIO VALUES FOR SOIL WITH 10% CEMENT
CONTENT
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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)
Dial gauge reading (mm)
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)
Dial gauge reading (mm)
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)
Dial gauge reading (mm)
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)
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 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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
77
4.1.5.2 Soil-6% Cement mix with fly ash cushion of varying depth
Soil-6% cement mix Cycle 1
Time (hours)
Dial gauge reading (mm)
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
Soil-6% cement mix Cycle 2
Time (hours)
Dial gauge reading (mm)
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
Soil-6% cement mix Cycle 3
Time (hours)
Dial gauge reading (mm)
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
Soil-6% cement mix Cycle 1
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 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
Soil-6% cement mix Cycle 2
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
Soil-6% cement mix Cycle 3
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 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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
78
Cyclic swell-shrink behavior of soil-6%Cement
mix provided with fly ash cushion of varying
depth
Soil-6% Cement Mix
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 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
Cyclic Swell shrink behaviour
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
79
4.1.5.3 Soil-8% Cement mix with fly ash cushion of varying depth
Soil-8% cement mix Cycle 1
Time (hours)
Dial gauge reading (mm)
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)
Dial gauge reading (mm)
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
Soil-8%cement mix cycle 3
Time (hours)
Dial gauge reading (mm)
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
Soil-8% cement mix Cycle 1
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 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
Soil-8%cement mix cycle 2
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 3.6 3.5 3.2 3.0
Swell 3.455 2.667 2.188 1.730 1.429
Soil-8%cement mix cycle 3
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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
80
Cyclic swell-shrink behavior of soil-8%Cement
mix provided with fly ash cushion of varying
depth
Soil-8% Cement Mix
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 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
Cyclic Swell shrink behaviour
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
81
4.1.11.4 Soil-10% Cement mix with fly ash cushion of varying depth
Soil-10%cement mix cycle 1
Time (hours)
Dial gauge reading (mm)
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
Soil-10%cement mix cycle 2
Time (hours)
Dial gauge reading (mm)
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
Soil-10%cement mix cycle 3
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
Soil-10%cement mix cycle 1
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 4.0 3.9 3.9 3.5 3.4
Swell 3.636 2.889 2.438 1.892 1.619
Soil-10%cement mix cycle 2
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
Soil-10%cement mix cycle 3
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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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
%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 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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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.
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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).
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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.
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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
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linear shrinkage
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WE
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SWELL
STABILIZATION OF BLACK COTTON SOIL F16/29240/2009
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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.
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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
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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.
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REFERENCES
1) Civil engineering hand book 2nd
edition by W.F Chen and J.Y. Richard Liew
2) Basic soil mechanics 4th
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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
Recommended