EXPERIMENTAL AND NUMERICAL INVESTIGATION INTO BEHAVIOUR OF FLY ASH

EXPERIMENTAL AND NUMERICAL INVESTIGATION INTO BEHAVIOUR OF FLY ASH
EXPERIMENTAL AND NUMERICAL
INVESTIGATION INTO BEHAVIOUR OF FLY ASH
COMPOSITE MATERIAL IN THE SUBBASE OF
SURFACE COAL MINE HAUL ROAD
Banita Behera
~i~
EXPERIMENTAL AND NUMERICAL INVESTIGATION
INTO BEHAVIOUR OF FLY ASH COMPOSITE MATERIAL
IN THE SUBBASE OF SURFACE COAL MINE HAUL ROAD
A thesis submitted in partial fulfillment of the requirements
for the degree of
Doctor of Philosophy
in
Engineering
by
Banita Behera
Department of Mining Engineering
National Institute of Technology
Rourkela - 769 008, India
January, 2013
Dedicated to
My Parents
National Institute of Technology
Rourkela
CERTIFICATE
This is to certify that the thesis entitled “EXPERIMENTAL AND NUMERICAL
INVESTIGATION INTO BEHAVIOUR OF FLY ASH COMPOSITE MATERIAL IN
THE SUBBASE OF SURFACE COAL MINE HAUL ROAD” submitted by Banita
Behera to National Institute of Technology, Rourkela for the award of the degree of Doctor
of Philosophy in Engineering, is a record of bonafide research work under my supervision and
guidance. The candidate has fulfilled all prescribed requirements for the thesis, which is based
on candidate’s own work and the thesis in my opinion, is worthy of consideration for the
award of the degree of Doctor of Philosophy of the Institute.
The results embodied in this thesis have not been submitted to any other University/Institute
for the award of any other degree or diploma.
Prof. M. K. Mishra
Dept. of Mining Engineering
National Institute of Technology
Rourkela – 769008
~i~
ACKNOWLEDGEMENT
I would first like to express deep sense of respect and gratitude towards my supervisor Prof.
M.K. Mishra, for his inspiration, motivation, guidance, and generous support throughout this
research. I am greatly indebted for his constant encouragement and valuable advice at every
phase of the doctoral programme. The dissertation work would not have been possible
without his elaborate guidance and full encouragement. What I learned from him will be an
invaluable benefit for the rest of my life.
I would like to express my respect to Prof. N.R. Mohanty, Prof. M.R. Barik, Prof. K. Dey,
Prof. S.C. Mishra and Prof. S. Mula for teaching me in research related courses. They have
been great sources of inspiration to me and I really thank them from the bottom of my heart.
I would like to extend my special thanks to the Chief General Manager and other staff
members of Bharatpur Opencast Mine, Talcher, Odisha for their assistance in collecting the
necessary data and overburden materials used for this research and also to the Head, Captive
power plant II of Rourkela Steel Plant for providing fly ash used in this research.
My special thanks to Prof. H.K. Naik, Head of Mining Engineering Department, all faculty
and staff members of the department for their timely help in completion of this work.
I also express my thanks to HODs and staff members of Civil Engineering, Metallurgical and
Materials Engineering and Ceramic Engineering for their help and cooperation in sample
testing and instrumental analysis in their department.
I want to extend my sincere gratitude to all the members of my doctoral scrutiny committee,
Prof. P. Rath and Prof. S. Jayanthu for their comments and suggestions throughout this work.
I am also thankful to Fly Ash Unit, Department of Science and Technology, Govt. of India
which has provided fellowship to work in the project title “Bulk use of fly ash composite
material in the subbase of Surface Coal Mine Haul Road to reduce strain” under investigation
of my supervisor.
I am also grateful to my friends for their assistance and constant encouragement throughout
my dissertation work.
Finally, I would like to express my deepest gratitude to my beloved parents, my brother and
two sisters and also my husband who made all of this possible, for their endless
encouragement, support, love and patience throughout the research period.
(Banita Behera)
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ABSTRACTS
Surface mining will continue to play major role in meeting the demand of fossil fuel. India’s
power generation will be about 1, 65,000 MW this 11th five year plan out of which the share
of coal would be 75%. Majority of the coal demand is met from surface mining due to its
speed and ease of operations. The current coal production from surface mines in India is about
390 MT (85%) that will have to be increased substantially to meet the demand. Opencast
mine economy depends on the cost of haul road design, construction and its maintenance in
addition to other factors. These roads are used by heavy earth moving equipments. With a
poorly laid, constructed and maintained haul road, production suffers, accident and
breakdown occurs. The haul road has received inadequate attention although production,
usage of heavy machineries have increased manifold. The surface of the haul road depends on
the behaviour of material beneath it. Strengthening of the base and subbase layers beneath the
surface of the surface coal mine haul road are of vital importance to improve upon its
performance. The materials that are used in haul road construction, typically sourced locally.
It is envisioned that suitable material would address this issue.
Solid wastes from the surface mining as well as combustion of coal pose serious
environmental problems of vital concern to the producers and users of coal as well as the
general public. Opencast mining involves displacement of large amount of overburden dumps
materials as mine waste to excavate coal from the earth. The overburden dumps formed
outside the open pits not only occupy huge land area but also alter the surface topography and
contribute to the environmental degradation. Fly ash is at present an unavoidable coal
combustion byproduct. The major challenge with this production of the fly ash is in its huge
~ iii ~
land coverage, adverse impact on environment, etc. The problem with safe disposal of fly ash
is a major issue as India is poised to generate huge volume of fly ash due to the high ash (40%
to 50%) content of the coal. In most of the surface mines, the material used in the haul road is
not adequate to support the wheel loads. Fly ash posses many attributes to be used as an
engineering material in those sections/layers. The prospects of utilizing fly ash that would
have been dumped as waste is explored, investigated, experimented and evaluated in the
investigation. Different compositions of fly ash, mine overburden and lime have been
prepared. The geotechnical properties such as compaction characteristics, California bearing
ratio, unconfined compressive strength, Brazilian tensile strength, ultrasonic pulse velocity,
morphology, phase characteristics, chemical compositions and leaching behaviour have been
determined. The effects of lime content and curing period on the geotechnical characteristics
of the fly ash composites are highlighted. Numerical investigation have been carried out to
evaluate the behaviour of fly ash composite materials in reducing strain in the haul road of
surface coal mine. There were significant improvements in the stress-strain behaviour of
developed composites. All the composites resulted in enough strength values to be used as
subbase material. Curing periods and lime content have varying influence in the strength
development of the composites. The composites with 30% fly ash and 9% lime gave the best
performance in reducing the stress-strain values at different section of haul road pavement.
There were no traces of toxic elements in the developed composites.
Keywords: Brazilian tensile strength, CBR, Fly ash, Lime, Maximum dry density, Mine
overburden material, Pulse velocity, Surface mine haul road, Unconfined compressive
strength.
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CONTENTS
CERTIFICATES
i
ACKNOWLEDGMENT
ii
ABSTRACTS
iii
LIST OF FIGURES
ix
LIST OF TABLES
xv
CHAPTER 1: INTRODUCTION
1.1
Background
1
1.2
Statement of the problem
2
1.3
Research Objectives
4
1.4
Scope and Methodology
5
1.5
Parametric variations
8
1.6
Organization of Thesis
9
CHAPTER 2: LITERATURE REVIEW
2.1
Introduction
11
2.2
Mine haul roads and haul trucks
15
2.2.1
Classification of haul roads
16
2.2.2
Design of haul road pavement
17
2.2.2.1
Haul Road Pavement design based on CBR
20
2.2.2.1.1
21
2.2.2.2
Design Procedure
Haul Road Pavement design based on resilient modulus
23
2.2.2.2.1
24
Design Procedure
2.2.2.3
Critical strains and typical mode of failure in a haul road
25
2.2.2.4
Critical strain limit
26
2.2.3
Symptoms and causes of haul road deterioration
27
2.2.4
Characteristics of Base/Subbase course materials of the haul road
28
~v~
2.2.5
2.3
Haul Trucks
28
2.2.5.1
Haul trucks tires
29
2.2.5.1.1
30
Tire foot print area and pressure
Geotechnical properties of fly ash
35
2.3.1
Physical Properties
36
2.3.2
Chemical Properties
37
2.3.3
Engineering Properties
41
2.3.3.1
Compaction characteristics
42
2.3.3.2
Permeability characteristics
43
2.3.3.3
Strength characteristics
44
2.3.3.4
California bearing ratio (CBR) behavior
45
2.3.3.5
Ultrasonic velocity
46
2.4
Uses and strength behavior of fly ash
46
2.5
Environmental aspects of fly ash utilization
53
2.6
Fly ash – lime or fly ash – soil – lime interaction
56
CHAPTER 3: METHODOLOGY
3.1
General
60
3.2
Materials and Methods
60
3.2.1
60
3.2.2
Materials
3.2.1.1
Fly ash
60
3.2.1.2
Overburden Materials
61
3.2.1.3
Lime
65
Methods
3.2.2.1
66
Sample preparation
66
3.2.2.1.1
Sample preparation for CBR test
68
3.2.2.1.2
Sample preparation for UCS test
68
3.2.2.1.3
Sample preparation for tensile strength test
69
3.2.2.1.4
Sample preparation for Ultrasonic pulse
velocity test
~ vi ~
69
3.2.2.1.5
Sample preparation for SEM, EDX and XRD
Analyses
3.2.2.1.6
3.2.2.2
3.3
Sample preparation for leaching study
70
70
Experimental methods
71
3.2.2.2.1
Specific Gravity
71
3.2.2.2.2
Grain size distribution
71
3.2.2.2.3
Specific surface area
72
3.2.2.2.4
Consistency limits
72
3.2.2.2.5
Free swell index
72
3.2.2.2.6
X-ray diffraction (XRD) analysis
73
3.2.2.2.7
SEM and EDX studies
73
3.2.2.2.8
Loss on ignition (LOI)
74
3.2.2.2.9
pH test
74
3.2.2.2.10 Compaction test
74
3.2.2.2.11 Triaxial compression test
74
3.2.2.2.12 Permeability test
75
3.2.2.2.13 California bearing ratio test
75
3.2.2.2.14 Unconfined Compressive strength test
77
3.2.2.2.15 Brazilian tensile strength test
79
3.2.2.2.16 Ultrasonic Pulse velocity test
80
3.2.2.2.17 Leaching study
83
Experimental Size
83
CHAPTER 4: RESULTS AND DISCUSSION
4.1
Introduction
87
4.2
Results of Geotechnical properties of ingredients
88
4.2.1
Physical Properties
88
4.2.2
Chemical Properties
91
4.2.3
Engineering Properties
94
4.3
Geotechnical properties of developed composite materials
~ vii ~
97
4.3.1
Compaction characteristics
97
4.3.2
California Bearing Ratio behavior
101
4.3.2.1
Effect of curing on the CBR of untreated composites
101
4.3.2.2
CBR behaviour of lime stabilized fly ash composites
102
4.3.3
4.4
Unconfined compressive strength characteristics
107
4.3.3.1
Unconfined compressive strength of untreated composites
108
4.3.3.2
Unconfined compressive strength of treated composites
108
4.3.4
Brazilian Tensile strength characteristics
113
4.3.5
Ultrasonic pulse velocity
117
4.3.6
Microscopy analysis
123
4.3.7
Energy dispersive X-ray analysis
125
4.3.8
X-ray diffraction analysis
128
4.3.9
Leachate characteristics
129
Development of empirical models
131
CHAPTER 5: NUMERICAL INVESTIGATION
5.1
General
138
5.2
Modeling and Boundary condition
139
CHAPTER 6: SUMMARY AND CONCLUSIONS
6.1
Untreated materials
157
6.2
Treated materials
158
6.3
Scope for Further Research
160
REFERENCES
162
APPENDIX
185
LIST OF PUBLICATIONS
CURRICULUM VITAE
~ viii ~
LIST OF FIGURES
Title
Fig. No.
Page No.
1.1
Flowchart of the methodology
7
2.1
A typical permanent haul road
15
2.2
A typical system of haul road classification in an opencast
mine
17
2.3
Typical haul road cross-section
19
2.4
CBR design chart (recommended by Indian Roads Congress)
22
2.5
Method to obtain resilient modulus (after Bowles 1984)
23
2.6
Critical strains and failure mode in pavement structures
26
2.7
Bias ply and radial tires (Good Year, 2008)
30
2.8
Deflection factors for ESWL determination (after Foster and
Ahlvin, 1954)
34
2.9
Critical points for a fully loaded truck (after Thompson,
1996)
34
2.10
Load distribution beneath a tire
35
3.1
Map of Talcher Coalfield, Odisha
63
3.2
Sketch of Bharatpur Opencast coal mine
64
3.3
Collection of mine overburden
64
3.4
Undulations and potholes are marked in the haul roads
65
3.5
CBR mould
69
3.6
UCS mould, spacer discs and mixed ingredients
69
3.7
Set up for collection of leaching effluent
71
3.8
Prepared CBR samples inside the moulds
76
~ ix ~
3.9
Soaking of CBR samples
76
3.10
An experimental setup for CBR test
77
3.11
Sample of UCS specimens prepared (undergoing curing)
78
3.12
An experimental setup for UCS test
79
3.13
Schematic representation of ultrasonic velocity measurement
81
3.14
An experimental setup for Ultrasonic velocity measurement
82
4.1
Grain size distribution curves of fly ash and mine overburden
90
4.2
Scanning electron micrograph of (a) mine overburden
material and (b) fly ash
92
4.3
X-ray diffractogram of (a) mine overburden and (b) fly ash
93
4.4
Compaction curves of fly ash and overburden
97
4.5
Compaction curves of untreated composites
98
4.6
Variation of maximum dry density with fly ash content
98
4.7
Variation of optimum moisture content with fly ash content
98
4.8
Compaction curves of the composites containing 15, 20 and
25% fly ash
99
4.9
Compaction curves of the composites containing 30, 35 and
40% fly ash
100
4.10
Compaction curves of
50% fly ash
the composites containing 45 and
100
4.11
Variation of CBR with the addition of fly ash to mine
overburden
102
4.12
Effect of curing on the CBR of fly ash and mine overburden
composites
102
4.13
Effect of lime on CBR behavior of composites in soaked
condition
104
~x~
4.14
Effect of lime on CBR behavior of composites at 7 days
curing
104
4.15
Effect of lime on CBR behavior of composites at 28 days
curing
105
4.16
Influence of Lime in CBR Gain for all composites at soaked
condition
106
4.17
Influence of Lime in CBR Gain for all composites at 7 days
curing
107
4.18
Influence of Lime in CBR Gain for all composites at 28 days
curing
107
4.19
Effect of lime on compressive strength of composites at 7
days curing
109
4.20
Effect of lime on compressive strength of composites at 14
days curing
109
4.21
Effect of lime on compressive strength of composites at 28
days
110
4.22
Effect of lime on compressive strength of composites at 56
days
110
4.23
Post failure profiles of a few UCS specimens
111
4.24
Stress- strain behaviour of a sample
112
4.25
Post failure profiles of few Brazilian tensile test specimens
114
4.26
Effect of lime on tensile strength of composites at 28 days
curing
115
4.27
Effect of lime on tensile strength of composites at 56 days
curing
115
4.28
Effect of curing period on tensile strength as percentage of
unconfined compressive strength of the composites
containing 15, 20, 25 and 30% fly ash
116
4.29
Effect of curing period on tensile strength as percentage of
unconfined compressive strength of the composites
containing 35, 40, 45 and 50% fly ash
117
~ xi ~
4.30
Effect of lime on pulse wave velocity of fly ash composites
at 7 days curing
118
4.31
Effect of lime on pulse wave velocity of fly ash composites
at 14 days curing
119
4.32
Effect of lime on pulse wave velocity of fly ash composites
at 28 days curing
119
4.33
Effect of lime on pulse wave velocity of fly ash composites
at 56 days curing
120
4.34
A typical P wave velocity signal plot of fly ash composite
120
4.35(a)
SEM photograph of (15FA+85O/B) +2L
124
4.35(b)
SEM photograph of (30FA+70O/B) +6L
125
4.35(c)
SEM photograph of (30FA+70O/B) +9L
125
4.36
XRD patterns of (30FA+70O/B) stabilised with 2, 3, 6 and
9% lime at 28 days
129
4.37(a)
Relationship between Brazilian tensile strength and
unconfined compressive strength for all samples at 28 days
of curing
132
4.37(b)
Relationship between Brazilian tensile strength and
unconfined compressive strength for all samples at 56 days
of curing
133
4.38(a)
Relationship between bearing ratio and unconfined
compressive strength for all samples at 7 days of curing
133
4.38(b)
Relationship between bearing ratio and unconfined
compressive strength for all samples at 28 days of curing
134
4.39(a)
Effect CaO content, CaO/SiO2 and CaO/(SiO2 + Al2O3)
ratios on CBR and compressive strength values
136
4.39(b)
Effect CaO content, CaO/SiO2 and CaO/(SiO2 + Al2O3)
ratios on tensile strength and ultrasonic velocity values
137
4.40
Variation of maximum dry density with lime content
(Appendix)
186
~ xii ~
4.41
Variation of optimum moisture content with lime content
(Appendix)
186
4.42
Load vs penetration curves of untreated composites in
unsoaked condition (Appendix)
187
4.43
Load vs penetration curves of untreated composites in
soaked condition (Appendix)
187
4.44
SEM photographs of fly ash composites (Appendix)
188-189
4.45
XRD patterns of fly ash composites at 28 days curing
(Appendix)
190-193
5.1
A typical haul road pavement under wheel load
140
5.2
Schematic layout of FEM modelling of haul road
141
5.3
Haul road cross-section under axisymmetry loading
143
5.4
Maximum strain of haul road pavement with conventional
materials
145
5.5
Haul road pavement model with various positions in layers
145
5.6
Strain values at different depth of the pavement
146
5.7 (a)
Strain values at different depth of the pavement with varying
subbase thickness
147
5.7 (b)
Stress values at different depth of the pavement with varying
subbase thickness
147
5.8 (a)
Strain values at different depth of the pavement with
1.5subbase thickness with (30PA+70OB)+9L composite.
149
5.8 (b)
Stress values at different depth of the pavement with
1.5subbase thickness with (30FA+70OB)+9L composite.
149
5.9 (a)
Strain at different depth of the pavement using dynamic
elastic parameters.
151
5.9 (b)
Stress at different depth of the pavement using dynamic
elastic parameters.
151
~ xiii ~
5.10 (a)
Strain values at different depth of the pavement with
composites containing 15 and 20% fly ash as subbase
material
153
5.10 (b)
Strain values at different depth of the pavement with
composites containing 25 and 30% fly ash as subbase
material
153
5.10 (c)
Strain values at different depth of the pavement with
composites containing 35 and 40% fly ash as subbase
material
154
5.11 (a)
Stress values at different depth of the pavement with
composites containing 15 and 20% fly ash as subbase
material
154
5.11 (b)
Stress values at different depth of the pavement with
composites containing 25 and 30% fly ash as subbase
material
155
5.11 (c)
Stress values at different depth of the pavement with
composites containing 35 and 40% fly ash as subbase
material
155
5.12
Total strain and stress at various layers of haul road
pavement with fly ash composites as subbase material
(Appendix)
193-200
~ xiv ~
LIST OF TABLES
Table. No.
Title
Page No.
1.1
Parametric variations considered for the study
8
2.1
Haul road cross-section based on the CBR chart for a
wheel load of 80mt
22
2.2
Range of chemical composition of Indian coal ashes
and soils
40
3.1
Various proportions of flyash and overburden
66
3.2
Compositions (%) of (FA+O/B)+L
67
3.3
Relationship between CBR and quality of subgrade
soil
76
3.4
Relationship between UCS and quality of subgrade
soil
78
3.5
Experimental Design Chart
84
4.1
Physical properties of fly ash and mine overburden
90
4.2
Chemical compositions of fly ash, mine overburden
and lime (wt. %)
94
4.3
Engineering properties of overburden and fly ash
96
4.4
Young’s modulus values of the fly ash composites
for 7, 14, 28 and 56 days curing
112
4.5
Poisson’s ratios of the fly ash composites for 7, 14,
28 and 56 days curing
121
4.6
Young’s (dynamic) modulus values of the fly ash
composites at 7, 14, 28 and 56 days
122
4.7
Chemical compositions of the composites, cured for
28 days
127
~ xv ~
4.8
Leachate concentration (ppm) at 7 days curing period
130
4.9
Best fit regression models between California bearing
ratio values, unconfined compressive strength and
tensile strength at different curing period
134
4.10
Best fit of regression models at 28 days curing period
135
5.1
Young’s modulus, E (MPa) and Thickness, t (m) of
the pavement layers for different cases
144
5.2
Dynamic Elastic parameters and Thickness of the
pavement layers
151
5.3
Young’s modulus, E (MPa) of fly ash composites
152
~ xvi ~
Chapter 1: Introduction
CHAPTER 1
INTRODUCTION
1.1
Background
The overall development of a nation primarily depends on the power or energy
produced as well as consumed as it is directly related to the industrialization of nation. India
needs huge power resources to meet the expectation of its denizen of as well as in its aim to
be a developed nation by 2020. Fossil fuel continues to enjoy the dominant statue in meeting
the demand for power generation and the trend will continue for next two to three decades.
Coal is the world’s most abundant and widely distributed fossil fuel. An estimate reflects that
75% of India’s total installed power is thermal of which the share of coal is about 90%.
Mining of the coal will remain a major activity.
With the recent advances in mining
technology, majority of the coal demand is met from surface mining due to its speed and ease
of operations. The current coal production from surface mines in India is about 390 MT
(85%) that will have to be increased substantially to meet the demand for power. Haul roads
are the life line of any surface mine. Opencast mine economy depends on the cost of haul
road design, construction as well as its maintenance in addition to other factors. These roads
are used by heavy earth moving equipments. Production suffers, accident and breakdown
occurs if they are not properly laid, constructed and maintained. Traditionally least attention
is extended to its design, construction and maintenance. As a result mine economics gets
adversely affected in terms of loss of production, dumper breakdown, poor working
1
Chapter 1: Introduction
conditions etc. The surface of the haul road depends on the behaviour of material beneath it.
Strengthening of the base and sub-base layers beneath the surface of the surface coal mine
haul road are of vital importance to improve upon mine economics. The materials used in
haul road construction are typically sourced locally. It is envisioned that suitable material
would address this issue. India produced huge quantity of fly ash due to high ash content in
its coal reserves and its disposal is a major challenge to power plant operators. However due
to technological advances fly ash has found multiple gainful usages in many applications. But
those approaches do not address the huge generation completely.
1.2
Statement of the problem
A stable road base is one of the most important components of road design. Haul road
is a multi-layered structure which consists of four layers as surface, base, subbase and
subgrade. A typical surface coal mine has about 3 to 5 kms of permanent haul road, larger
ones having longer lengths and various other branch roads that are constructed either with
overburden material or from locally available material found near to the mine property.
Common construction material for haul road as sand, gravels, clay, etc. result only in filling
the spaces instead of offering total solution to ground stability. The behaviour of the surface
course of haul road depends on the bearing capacity of the materials that are lying beneath it.
It has been observed that surface course exhibits excessive rutting, potholes, settlement,
sinking and overall deterioration. There has been exponential rise in carrying capacity of
dumpers.
But the construction of haul road has not been appropriately addressed to
accommodate these changes. Typically truck haulage cost is nearly 50% of the total operating
cost incurred by a surface mine (Thompson and Visser, 2003). The cost increases as the
tonnage increases and large capacity dumpers are employed. Poor construction materials
2
Chapter 1: Introduction
result in haul road accidents, high maintenance cost of road as well as the machines with
reduced profit. Surface mine operators spend a significant amount of money on haul road
construction and its maintenance. In the past 30 years the carrying capacity of hauling
equipments e.g. dumpers/trucks in India has grown from a 12 tons to 170 tons, 220 to 300
tons being envisioned at places, requiring better haul roads to carry heavy loads. However,
there is a need to reduce vehicle operating cost and maintenance cost by well constructed
good haul roads. Strengthening of the base and sub-base of the surface coal mine haul road is
of vital importance to improve upon mine economics. It is desired that the base and sub-base
of the haul road should exhibit reduced strain so as to achieve a strong and smooth road
surface course.
Solid wastes from the mining and combustion of coal are serious environmental
problems of vital concern to the producers and users of coal as well as the general public.
Opencast mining involves displacement of large amount of overburden dump materials as
mine waste to excavate coal from the earth. Overburden is the waste material which lies
above as well as in between the coal seams. With the rising demand for coal, often surface
mine operation go deeper and deeper. It creates dump site with huge excavated wastes. The
overburden dumps formed outside the open pits besides occupying the lands alter the surface
topography and contribute to the environmental degradation.
Fly ash is a waste by-product from thermal power plants, which use coal as fuel.
Typically thermal plants are located near to surface coal mines that produce huge amount of
fly ashes. The current annual production of coal ash is estimated around 600 million tons
worldwide, with fly ash constituting about 500 million tons at 75-80% of the total ash
produced (Ahmaruzzaman, 2010). Thus, the amount of fly ash generated from thermal power
3
Chapter 1: Introduction
plants has been increasing throughout the world, and the disposal of the large amount of fly
ash has become a serious environmental problem as well as ecological imbalance. The
problem with safe disposal of fly ash is a major issue as India is poised to burn 1800 million
tones generating about 600 million tones of fly ash by 2031-32 due to the high ash (30% to
40%) content of the coal. Present generation of fly ash in India is 160 MT/year and it is
expected to increase upto 300 MT/year by 2016-17 (Ram et al., 2011). Currently there exist
about 160 opencast coal mines in India of various capacities. In most of the mines, the
material used in the haul road is not adequate for supporting the wheel loads. Fly ash has
potential to meet this criterion. The prospects of utilizing about 20 to 25 million ton of fly ash
that would have been dumped as waste needs to be investigated, experimented and
documented.
It is expected to result (in the save) in cost to the nation in terms of reduced
extraction of top soil and other materials for road construction purposes. The research
undertaken focused on development of fly ash based composite materials using mine
overburden and evaluated its performance to support heavy truck loads or dumpers in both
dry as well as wet climatic conditions in the haul road.
1.3
Research Objectives
The aim of the investigation was to improve the performance of haul road so as to
have smooth and better riding conditions, least maintenance and operator fatigue. It was
proposed to be achieved with a strong surface course devoid of potholes, undulations and
exhibit sufficient elasticity. The behaviour of surface course depends on that of the subbase
course. The goal has been achieved by addressing the following specific objectives.
4
Chapter 1: Introduction
1.
Detail study of the design of haul road particularly the base/subbase course in a typical
surface coal mine.
2.
Study of the haul road construction materials particularly that used in the subbase layer.
3.
Determination of geotechnical properties of other available waste materials, as fly ash and
mine overburden.
4.
Investigation on available strength enhancing material.
5.
Preparation and development of alternate haul road construction materials with fly ash,
mine overburden and lime.
6.
Determination of different geotechnical properties of the developed composite materials.
7.
Leaching studies to determine the presence of heavy metals in the developed composite
materials.
8.
Numerical modeling to evaluate the performance of the developed material in haul road
construction.
9.
Prediction of quantum of fly ash usage.
1.4
Scope and Methodology
Coal extraction through opencast coal mines will continue to be a major source of
power. Opencast mine economy also depends on the cost of haul road design, construction
and its maintenance. Surface mine operators bear significant amount of expense on haul road
construction and its maintenance. The subbase material for the haul road is either sourced
from far off places or from the local soft clay, overburden material is used. Typically thermal
power plants are located near to surface coal mines that produce huge amount of fly ashes. Its
disposal is a major problem. Fly ash has many attributes for geotechnical applications. But
its effectiveness in the use of haul road has not yet been completely explored and established.
5
Chapter 1: Introduction
The present research focuses on the use of the fly ash based composite materials for haul road
construction and evaluate its performance to support heavy truck loads in both dry as well as
wet climatic conditions in the haul road. The outcome of the research would be useful in
improving the performance of haul road as well as increasing the prospects of utilization of
fly ash by the industry. In addition to improve mine economics, saving due to gainful
utilization of fly ash disposal would be enormous.
This investigation was an attempt to utilise coal mine overburden material and fly ash
in different compositions along with lime, a popular strength enhancing media to improve the
behaviour of haul road. The overall approach adopted to achieve the various objectives to
reach the goal is outlined below (Figure 1.1).

Review of literature on design and construction of haul road, impact of varying capacities
of dumpers/ trucks, specifications of larger trucks and their tires, geotechnical properties
of fly ash and its potential benefits for haul road construction.

Development of experimental setup and characterization of ingredients.

Development of fly ash and mine overburden mixed composite materials stabilized with
additives and optimization of parametric variations.

Determination of geotechnical properties of the developed composites by performing the
tests and analyses as moisture density relationship, unconfined compressive strength,
California bearing ratio, Brazilian tensile strength, Ultrasonic pulse velocity,
morphological behaviour, X-ray diffraction analysis, energy dispersive X-ray analysis etc.

Evaluation of potential of leaching heavy metals to ground water.

Simulation of stress-strain behaviour to predict the thickness of the subbase layer as well
as potential of fly ash usage.
6
Chapter 1: Introduction
The following methodology would be adopted to achieve the objectives and goal (Figure 1.1).
Review of available literature & field visit to mine
Collection of Fly Ash and Overburden (O/B)
Characterization of the fly ash and overburden
y ash
Selection and optimization of additives
Fly Ash & O/B
Development of Fly Ash Composites
Additives
Characterization of developed composites
Determination of
Geotechnical Properties
Major Laboratory Tests
Leaching Studies
Numerical Modeling
Figure 1.1: Flowchart of the methodology
7
Microstructural
parameters
Chapter 1: Introduction
1.5
Parametric variations
The objectives have been achieved by following a well designed methodology as well
as considering the following parametric variations.
Table 1.1: Parametric variations considered for the study
A. Laboratory
Parameters
Variations
1. Characterisation
Fly ash and
Physical properties:
of constituents
overburden
specific gravity, grain size distribution,
Investigation
consistency limits, plasticity index, free swell
index
Chemical properties:
chemical composition, morphology, mineralogy,
pH
Engineering properties:
compaction characteristics, permeability, shear
strength parameters, angle of repose, California
Bearing Ratio, Unconfined Compressive Strength
Lime
Chemical composition
Fly ash (%)
15, 20, 25, 30, 35, 40, 45 and 50
2. Development of
composite material
Overburden (%) 85, 80, 75, 70, 65, 60, 55 and 50
Lime (%)
2, 3, 6 and 9
Curing period
7, 14, 28, 56
(days)
3. Geotechnical characterization
Compaction characteristics, California Bearing
Ratio (CBR), Unconfined Compressive Strength
(UCS), Brazilian Tensile Strength, Pulse velocity,
Young’s modulus, Poisson’s ratio
4. Environmental parameters
Leaching study
8
Chapter 1: Introduction
B. Numerical modeling
1. Finite element analysis
ANSYS
software
2. Modeling
2D (Axisymmetry), Quadrilateral 4-noded
3. Model haul road pavement
4 layers: surface, base, subbase and subgrade
4. Layer thickness (s)
Surface (m): 0.2, 0.5
Base (m): 0.3, 1
Subbase (m): 0.8, 1, 1.5
Subgrade: semi-infinite
5. Tire pressure (kPa)*
700(max.)
* Tannant and Regensburg, 2001; Lav et al., 2006; Caterpillar Performance Handbook, 2010 (for new profile
heavier dumper tires)
1.6
Organization of Thesis
The thesis is covered in six chapters. The first chapter gives an introduction which
includes background of the research, statement of problem, objectives, scope and
methodology of research work as well as parametric variations. Second chapter includes a
detailed review of literature on mine haul road, haul trucks and geotechnical properties as well
as applications of fly ash. Besides these it also covers environmental aspects of fly ash
utilization and its interaction with soil, lime as well. The materials and methods of the
investigation come under chapter 3 which includes collection of ingredients, sample
preparation and testing techniques used for characterisation of materials as well as
development of composite materials. Chapter 4 deals with results, discussion and analysis
that include the results of geotechnical properties of ingredients and developed composite
materials, results of microstructural analyses and leaching studies and finally development of
model relationship between geotechnical parameters. It also includes the best fit models
among various parameters.
Numerical investigation to study the effectiveness of the
9
Chapter 1: Introduction
developed composite materials on the stress-strain behavior of haul road pavement is
described in Chapter 5.
Chapter 6 focused on summary and conclusion part of the
investigation. At the end the reference and the detail experimental and numerical results are
included in Annexure.
10
Chapter 2: Literature Review
CHAPTER 2
LITERATURE REVIEW
2.1
Introduction
Mining provides us with essential resources. Historically, mining has evolved from
small and simple operations to large and complex mining and processing systems that employ
the latest in engineering technology. The total numbers of working mines at present are 2628
in 2010-11 out of which 574 mines deal in coal and lignite, 608 mines deal in metallic
minerals and rest in non-metallic minerals. Presently, India produces around 90 minerals out
of which 4 are fuel minerals, 10 are metallic minerals, 50 are non-metallic minerals, 3 are
atomic minerals and 23 are minor minerals (Jha, 2011).
Coal is the world’s most abundant and widely distributed fossil fuel. India is the third
largest producer of coal in the world and has the fourth largest reserves of coal in the world
(approx 197 billion tonnes) (Rai et al., 2011). An estimated 55% of India’s installed capacity
of 124,287 MW of power generation is through coal based thermal power plants. As per XI
Plan, coal production would be raised to 680 million tonnes by the end of 2011-12 to meet the
energy demand of the country (Ministry of Coal, 2007). Coal is mined by two main methods
- surface or opencast mining and underground mining. Though underground mining is the
oldest method of excavation, surface mining have been in force in recent years for its
manifold advantages to meet the increasing demand of coal. In 1974-75 the share of total coal
production from opencast mines was only 11%, whereas in 2009-10 and 2010-11 this has
risen to 72.61% and 85% respectively.
11
Chapter 2: Literature Review
In most of surface coal mines, explosives are first used in order to break through the
surface or overburden of the mining area. The overburden is then removed either by draglines
or by shovel and truck. Once the coal seam is exposed, it is drilled, fractured and thoroughly
mined in strips. The coal is then loaded on to large trucks or conveyors for transport to either
the coal preparation plant or directly to where it will be used. The overburden originates from
the consolidated and unconsolidated materials overlying the minerals and coal seams, and is
required to be removed. The average stripping ratio (overburden to coal) during the last three
decades in India was 1.97m3/t (Chaulya et al., 2000). Though there are attempts to reclaim
the mined out area with filling by the waste dumps, the measures do not often accommodate
all the displaced overburden. One of the major environmental challenges is to manage the
huge volume of overburden generated in these opencast mines which is associated with the
problems as aesthetics, visual impacts and landslides, loss of topsoil, soil erosion, water and
air pollution, ecological disruption, social problems, safety, risk and health etc.
overburden is highly heterogeneous.
The
These consists of alluvium, laterite, sandstone,
carbonaceous shale, coal bands, clays, between coarse to medium grained highly ferruginous
sandstone, thythmide, turbidite, etc. Gradation results suggest that fines and coarse grains are
approximately equally represented in the soil (Ulusay et al., 1995).
A typical surface coal mine has about 3 to 5 kms of permanent haul road, larger ones
having longer lengths and various other lumpy roads that are constructed either with
overburden material or from locally available material found near to the mine property. Some
of those materials are asphaltic concrete, mudstone, sandstone, etc. Crushed gravel is often
placed on top surface of the road. Asphaltic concrete needs base layer with CBR value more
than 80 and is very costly. Common construction material for haul road as sand, gravels,
12
Chapter 2: Literature Review
clay, etc. result only in filling the spaces instead of offering total solution to ground stability.
Often it is observed that the operating and maintenance cost of dumpers are significantly high
in addition to haul road maintenance cost.
It results in reduced production, frequent
breakdown, accidents, death hazards, low worker motivation, etc. These days’ opencast
mines are planned to significant depths, often beyond industry’s current experience, expertise
and knowledge base. In the past 30 years the carrying capacity of hauling equipments e.g.
dumpers/trucks has grown from a tiny 10 tons to 170 tons, 350 tons being envisioned at
places, requiring better haul roads to carry heavy loads. So, better haul road construction
material would address the increase loading due to higher capacities.
Typically thermal power plants are located near to surface coal mines that produce
huge amount of fly ash as a waste byproduct. The combustion of powdered coal in thermal
power plants produces ash, which contains 80% fly ash and 20% bottom ash. The ash
collected in electrostatic precipitators is called fly ash. Coal based thermal power plants all
over the world face a serious problem of handling and disposal of the fly ash. The current
annual production of coal ash is estimated around 600 million tons worldwide, which
constitutes about 500 million tons of fly ash at 75-80% of the total ash produced
(Ahmaruzzaman, 2010). Hence this huge amount of fly ash generated from thermal power
plants and its disposal has become a threat to environment and even creating ecological
problem by occupying large tracts of scarce cultivated lands. The high ash content (40-50%)
of the coal in India makes this problem more complex.
In India, the current level of
generation of fly ash is 160 million tonnes per year and is projected to increase about 300
million tonnes by 2017 and 1000 million tonnes per year by 2032 (Kumar, 2010). Safe
disposal of the ash without adversely affecting the environment and the large storage area
13
Chapter 2: Literature Review
required are major issues and challenges for safe and sustainable development of the country.
Hence efforts are being made continuously by making stringent regulations by the
Government to fully utilize the fly ash. At present about 50% of the fly ash is being gainfully
utilized in India (Sahay, 2010). But a conservative estimate puts the unutilized fly ash
occupying about 65000 acres of land (Das and Yudhbir, 2006) which demands increase in the
utilization percentage. The disposal of fly ash would require 1000km 2 which in turn shall
necessitate new disposal areas, sites involving displacement and hence rehabilitation problems
by 2015 (Kumar, 2010). Thus it is very essential to find new avenues for its effective
utilization in bulk. Bulk utilization of fly ash can be accomplished only in geotechnical
engineering applications such as construction of embankments, as a base/subbase material in
roads, structural fills and dykes etc. Utilization of fly ash in such applications minimizes the
disposal problem of fly ash and also reduces the construction cost of the projects. Surface
mine haul road construction is one such avenue for fly ash use in bulk.
Fly ash, being very finer, is more reactive and consequently more suitable for haul
road construction material as compared to other materials. Potential application of fly ash
alone or soil stabilized with fly ash or fly ash and admixtures for road construction has been
reported by a number of researchers (Consoli et al., 2001; Kumar, 2005; Mohanty and Chugh,
2006; Mackos et al., 2009). The enhancement of mechanical strength of fly ash with addition
of lime has been reported elsewhere (Sivapullaiah, 2000; Beeghly, 2003; Mishra and Rao,
2006; Ghosh and Subbarao, 2007). There have been many successful instances of fly ash
being used as road construction material. Yet its effectiveness in the surface coal mine haul
road has not been evaluated so as to establish it commercially.
14
Chapter 2: Literature Review
Surface coal mine haul road undergoes more stress/strain due to multiple reasons such
as poor surface course, inadequate construction process, poor construction materials, varying
load on the surface, improper drainage system, etc. An attempt has been made in this research
to evaluate the potential of overburden and fly ash mixes in addressing the same. Literature
related to surface coal mine haul road construction and fly ash as construction material is
reviewed in this chapter.
2.2
Mine haul roads and haul trucks
In open cast coal mines, haul roads are basically required for the transportation of coal
from the various coal faces to the coal receiving pits, overburden materials to the dump yard
and also for the movement of vehicles to the workshops or parking places (Figure 2.1).
Construction of haul road is a very important part in controlling sediment-laden runoff from a
mine site.
Figure 2.1: A typical permanent haul road
15
Chapter 2: Literature Review
2.2.1 Classification of haul roads
Haul roads are classified into following categories depending upon the traffic and the
nature of operations on the various haul roads.
Permanent haul roads: These are the initial constructed roads, often include the
approach to property and extend to the end of the dumping yard. The characteristics of this
type of roads are long life, made of maximum thickness, high quality construction materials,
expensive to build, etc. These roads are generally made outside the quarry area which is the
first access. They have to be maintained for the whole life of the open cast project. A typical
system of haul road classification in an open cast mine is shown in Figure 2.2
Semi-permanent haul roads: The characteristics of these roads are medium life period,
engineered to desired thickness, high quality construction materials, relatively expensive to
build, used as main haul roads in pits and dumping yards. These types of roads which have a
lifespan of 3 to 5 years are often clubbed with permanent haul roads (Vittal and Mathur,
2010). These roads are also made of similar materials used in the permanent haul road with
lesser thicknesses.
Temporary haul roads: These roads are characterized as short life period, minimum
pavement thickness, low quality construction materials, inexpensive to build and used mainly
for shovel or dumping yard access. They change considerably with the advancement of the
quarry face. Typical construction materials consist of those found in the mine property in the
vicinity.
16
Chapter 2: Literature Review
Figure 2.2: A typical system of haul road classification in an opencast mine
2.2.2 Design of haul road pavement
Road surface should not only be stable, non-yielding but also even along the
longitudinal profile to enable the vehicles move faster safely and comfortably. The pavement
carries the wheel load and transfers the stresses through a wide area on the soil sub grade
below. It results in the stress being transferred to the sub grade soil through pavement layers
considerably lower than the contact pressure or compressive stresses under the wheel load on
the surface. This reduction in wheel stresses depends on factors as thickness of layer as well
as characteristics of the pavement layers. So though an effective pavement construction
should ensure distribution of wheel load stress to a larger area per unit depth of the layer, yet a
small amount of temporary deformation is always associated. Design and construction should
ensure to keep this elastic deformation of pavement within limits. Between the two pavement
structures- rigid and flexible type, the later is followed in the construction of haul road.
Pavement structure deflects or flexes, under loading in flexible pavement. Flexible pavement
structure is typically composed of several layers of materials. Each layer receives loads from
17
Chapter 2: Literature Review
the above layer, spreads them out and passes on these loads to the next layer below. Thus the
stresses are reduced from top layer to the bottom layer. The layers are usually arranged in the
order of descending load bearing capacity with the highest load bearing capacity material (and
most expensive) on the top and the lowest load bearing capacity material (and least
expensive) on the bottom. It transmits the vertical or compressive stresses to the lower layer
by grain to grain transfer through the points of contact in the granular structure. It needs a
well compacted granular structures consisting of strong materials to transfer the compressive
stresses. The stress or compressive loading is maximum on the pavement surface directly
under the wheel load and is equal to the contact pressure under the wheel. These stresses get
distributed to a larger area in the shape of truncated cone and hence decrease at lower layers.
Hence multilayer construction of road is desirable. Haul road pavement consists of four
distinct layers namely, surface course, base course, sub-base and sub-grade as shown in
Figure 2.3.
 The surface course is the layer of a haul road with which the wheels of vehicles are in
actual contact. The characteristics of the surface course should be of high adhesion, low
rolling resistance coefficient, no penetration under load. It is generally made of bitumen,
asphalt or compacted gravel to provide a smooth riding surface and will resist pressure
exerted by the tires.
 The base course is the layer of material which lies immediately below the surface course.
It consists of granular material like stone fragments or slag that can be stabilized with
binding materials like cement, natural pozzolans etc. The base course is the main source
of the structural strength of the road.
18
Chapter 2: Literature Review
 Subbase is the layer of a haul road pavement, which lies between base course and
subgrade. The base course and sub- base courses are primarily used to improve load
supporting capacity by distributing the load. It usually consists of same type of materials
used in base course like laterite, crushed stone, gravel, moorum, natural sand either
cemented or untreated. Apart from providing structural strength to the road, it serves
many other purposes such as preventing intrusion of sub-grade soil into the base course,
accumulation of water in the road structure, and providing working platform for the
construction equipment. The subbase distributes vehicle load over an area large enough
that the stresses can be borne by the natural, subgrade material (Khanna and Justo, 2001).
 The sub-grade is the naturally occurring surface on which the haul road pavement is
constructed. It may be leveled by excavation or back-filled to provide a suitable surface.
The performance of the haul road is affected by the characteristics of the sub-grade. The
loads on the pavement are ultimately received by the sub-grade to be transferred to the
earth mass. It should not be overstresses at anytime i.e. the pressure on top of it should
be within permissible limit.
Figure 2.3: Typical haul road cross-section
19
Chapter 2: Literature Review
There exist two road design methods that calculate the appropriate thickness of each
layer in the haul road by considering material properties such as CBR (California Bearing
Ratio) and resilient modulus (Yoder and Witczak, 1975; Kaufman and Ault, 1977; Thompson
and Visser, 1996; Mohammad et al., 1998). One of the most popular method that uses the
CBR values of the construction materials as a design criteria.
The CBR method was
developed by the California Division of Highways, USA during 1928–1929 for design of road
pavements. In resilient modulus based method, the road cross-section is designed using
predicted stresses, strains and each layer’s resilient modulus.
2.2.2.1 Haul Road Pavement design based on CBR
The load bearing capacity of a soil is directly related to its Shear strength defined by
Mohr-Coulomb relation. Tire loadings of haul roads often exceed the bearing capacity of
most road base materials at their normal insitu moisture content and hence strong material
construction is needed for stable design. CBR test is a laboratory penetration test of a soaked
sample of pavement construction materials as an inference of its shear strength. CBR value is
a relationship between the force necessary to drive a piston into the material and
corresponding value to likewise drive the piston into a standard gravel sample upto a known
depth and the result are reported as a percentage of standard (gravel) tests. California bearing
ratio (CBR) method is one of the most popular and widely used empirical methods for road
construction. It was observed that failure or poor pavement performance of road occur due to
inadequate compaction of materials forming the road layers and insufficient cover thickness
over weak in situ material. Porter (1949) developed the cover thickness requirements over in
situ materials of specific CBR (%) values that were applicable for airfield pavement design.
The use of CBR method for the design of haul roads in surface mines was first recommended
20
Chapter 2: Literature Review
by Kaufman and Ault (1977). CBR value for a specific material was developed from a
laboratory penetration test of a soaked samples of pavement material from which its shear
strength could be inferred.
2.2.2.1.1 Design Procedure
The CBR method estimates the bearing capacity of a construction material by
measuring the resistance offered by it to the penetration of a standard cylindrical plunger.
The detail procedure for conducting the test is described in the Indian Standard (IS): 2720 Part
16. Design charts have been developed that relate pavement, base and sub base thickness to
vehicle wheel load and CBR values (Figure 2.4). Cover thickness requirements for various
wheel loads corresponding to a wide range of CBR values of the construction materials are
also illustrated (Figure 2.4). The CBR method assumes that failure will occur when the cover
thickness above a certain material is less that required, according to standard CBR chart. The
maximum wheel load is determined by dividing the loaded vehicle weight over each axle by
the number of tires on that axle. The highest wheel load of a loaded vehicle is used in the
CBR design chart.
A relation between CBR, layer thickness, layer type and total fill cover has been
suggested (Table 2.1). The layer thickness can be determined from the cover thickness
required by one possible layer from the cover thickness required for the immediate lower
layer. The CBR method of haul road design has been very popular and is being followed
(Kaufman and Ault, 1977; Atkinson, 1992; Thompson, 1996; CMPDIL, 2000). The method
is simple, well understood and gives good design guidelines for haul roads. In India, CBR
method is used for haul road construction in surface mines (CMPDIL, 2000).
21
Chapter 2: Literature Review
Table 2.1: Haul road cross-section based on the CBR chart for a wheel load of 80mt (Tannant
and Regensburg, 2001)
Layer
Typical material
CBR (%)
Total fill cover
Layer thickness (m)
(m)
Surface
Crushed rock
95
-
0.30
Base
Pitrun sand & gravel
60
0.30
0.30
Sub-base
Till, mine spoil
25
0.60
1.60
Sub-grade
Firm clay
4
2.20
-
Figure 2.4: CBR design chart (recommended by Indian Roads Congress, 1970)
22
Chapter 2: Literature Review
2.2.2.2 Haul Road Pavement design based on resilient modulus
It is a non-linear approach to measure the pavement roadbed soil strength under
dynamic loading. The determination of resilient modulus is a complicated process (Ping,
2001).
AASHTO (1993) T294 is the most commonly used laboratory test method to
determine the resilient modulus of an unbound soil by repetitive loading of a soil sample in a
triaxial chamber. Alternatively, other methods are available to estimate the resilient modulus.
Thompson (1996) estimated the resilient modulus by the falling weight deflectometer test.
This test is easier to conduct and can provide in-situ layer moduli at a lower cost and with a
minimum amount of disturbance, although the values may not always be accurate.
A
material’s resilient modulus is actually an estimate of its modulus of elasticity (E). While the
modulus is stress divided by strain for a slowly applied load, resilient modulus is stress
divided by strain for rapidly applied loads.
The stiffness of a material increases with
repetition of loading and thus the initial Young's modulus is lower than the resilient modulus
(Figure 2.5). Conventionally, determination of the Young's modulus gives a reasonable
estimate of the resilient modulus, even if on the conservative side as there is no confining
pressure and stiffening of soil due to repeated loading.
Figure 2.5: Method to obtain resilient modulus (after Bowles, 1984)
23
Chapter 2: Literature Review
Mohammad et al. (1998) and Rahim et al. (2002) described yet another method for
calculation of resilient modulus using a cone penetration test with continuous measurement of
tip resistance and sleeve friction. Kim et al. (2001) proposed an alternative testing method to
determine resilient modulus of soils using a static triaxial compression test. The resilient
modulus test provides a relationship between deformation and stresses in pavement materials,
including subgrade soils, subjected to moving vehicular wheels. It also provides a means of
analyzing different materials and soil conditions, such as moisture and density and stress
states that simulate the loading of actual wheels. Determination of the subgrade resilient
modulus is important for designing pavement thickness.
If the selected design resilient
modulus value is much higher than actual field, or in situ resilient modulus, then thickness of
the pavement will be insufficient. If the design value is too low, the design will be too
conservative and uneconomical (Kasaibati et al., 1995). The magnitude of resilient modulus
is greatly affected when low values of specimen deflection or strain, occur because of
physical difficulties and limitations in measuring very small deflection values. Generally,
values of resilient modulus tend to be more accurate as specimen deflections increase and fall
within the accuracy range of equipment used to measure deflections (Hopkins et al., 2001).
The resilient modulus method is based on the strain caused in different layers of the haul road
provided by Morgan et al. (1994) and Thompson and Visser (1997). The induced strain is a
function of the modulus of the material for a given stress in a layer.
2.2.2.2.1 Design Procedure
The resilient modulus method is a mechanistic-empirical based method in which
pavement structure and load configuration is assumed. The structure is then simplified to four
distinct layers (AASHTO, 1962; Lav et al., 2006). Initially, the thickness of each layer is
24
Chapter 2: Literature Review
estimated based on past experience or designs at mines with similar conditions.
After
simplifying the structure, the stress induced by specified wheel loading is calculated in order
to identify the critical strains in the structure (pavement analysis) by means of purpose
developed computer programs or software. These are usually based on linear elastic theory or
finite element methods. The layers in the pavement structure are generally considered to be
homogenous and isotropic.
Fundamental properties of layers are expressed by elastic
modulus and poisson’s ratio. The layer thickness depends on the resilient (Young’s) modulus
of the haul road construction material. Strain modeling is performed to ensure that the
vertical strain at all points is less than the critical strain limit.
Mines using ultra-large trucks/ dumpers with gross vehicular weight more than 400T
use the results of resilient modulus of the construction materials (Tannant and Regensburg,
2001).
2.2.2.3 Critical strains and typical mode of failure in a haul road
The critical strains usually occur under the wheel paths (Lav et al., 2006). These are
horizontal tensile strains developed at the bottom of the surface layer and base/sub base layer
due to axial load which control fatigue cracking, while the vertical compressive strains at the
top of the subgrade layer control the permanent deformation (Figure 2.6).
The failure of a flexible pavement structure supported on a subgrade soil and subjected
to repeated traffic loading can occur through two primary mechanisms - collapse of the
pavement structure or cracking of the surface of the pavement. A collapse of the pavement
structure occurs due to large plastic (permanent) deformations in the subgrade soils. At times,
even when the loads on the pavement are not excessive but nominal, the pavement surface
crack due to fatigue, caused by the reversal of elastic strains at any location in the pavement
25
Chapter 2: Literature Review
system. As a result of repeated (cyclic) loads such as those caused by moving traffic,
cohesive soils in the subgrade incur repeated elastic deformations. When these deformations
exceed a threshold value, premature fatigue failure of the flexible pavement through cracking
of the pavement surface occurs.
Figure 2.6: Critical strains and failure mode in pavement structures
2.2.2.4 Critical strain limit
The important criterion for haul road design is a critical strain limit for each layer. A
road cannot adequately support haul trucks when vertical strain exceeds a critical strain limit
(Tannant and Regensburg, 2001). Critical strain limit was about 1500 micro-strains at the top
of the subgrade found by Morgan et al. (1994). Thompson and Visser (1997) noted that the
critical strain limit was around 2000 micro-strains at the road surface. The critical strain limit
is determined for a particular road depending on the number of loaded trucks expected to
travel over it during the designed life of the road. The number of loads passing a particular
section of a road depends on the designed life of the road as well as the traffic density. The
26
Chapter 2: Literature Review
maximum critical strain limits have been established to be 1500-2000 micro-strains for typical
haul roads (Thompson and Visser, 1999; Tannant and Regensburg, 2001).
2.2.3 Symptoms and causes of haul road deterioration
Haul road exhibits excessive rutting, potholes, settlement, sinking and overall
deterioration. The precipitation/runoff, heavy traffic volume, spring breakup and vehicle
spillage and poor compaction are the major causes of the surface course of haul road
deterioration. The base course of the haul road deteriorates due to precipitation/runoff, heavy
traffic volume, spring breakup and poor compaction. Poor compaction, high ground water
level and precipitation are major causes of deterioration to subbase and existing layer (Mining
officials, 2008). Lack of sufficient rigid bearing material beneath the surface course exhibits
excessive rutting, potholes, settlement, sinking and overall deterioration of the travel way
(Kaufman and Ault, 1977; Wade, 1989; Collins et al., 1986; Thompson and Visser, 1996;
Tannant and Kumar, 2000). Potholes are those depressions in the haul road surface that occur
in the wheel path mostly due to traffic movement. One of the reasons is local structure failure
that arises from poor compaction and/or shear in the subgrade. Excessive roughness on haul
roads also causes corrugations. Though it is a surface phenomenon, its origin may be linked
to low plasticity materials on the base and subbase courses, especially those with high sand
and gravel fraction (Heath and Robinson, 1980).
Rutting is the formation of progressive longitudinal depression in the wheel tracks. It
primarily originates on mine haul road either due to deformation of wearing course materials
as due to sub-grade materials. Poor construction materials result in haul road accidents, high
maintenance cost of road as well as the machines with reduced profit.
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Chapter 2: Literature Review
2.2.4 Characteristics of Base/Subbase course materials of the haul road
Base and sub-base layers are constructed either with interburden/overburden material
or from locally available material found near to the mine property. The stabilization of the
above materials is required when the design with current materials yields unacceptable
thickness of layers and/or the suitable construction materials are uneconomic to use due to
distance or depth limitations or environment restrictions. Generally, pit run gravel is used for
the base layer. The sub-base is often constructed from interburden/overburden, sand, silty or
sandy till, or other suitable materials. Usually the materials used in base and sub-base layers
are not crushed thus a particular particle size distribution is difficult to enforce. Some
materials have high plasticity index and cohesive in nature. Common construction materials
for haul road base/ subbase result only in filling the spaces instead of offering total solution to
ground stability (Chironis, 1978; Fung, 1981; Atkinson, 1992; Thompson and Visser, 1997).
2.2.5 Haul Trucks
In the past 30 years the carrying capacity of hauling equipments e.g. dumpers/trucks
has grown from a tiny 10 tons to 170 tons, 350 tons being envisioned at places, requiring
better haul roads to carry heavy loads. Larger haul trucks are being designed, produced, and
accepted by the industry due to economy of scale. Haul trucks used in surface mines have
grown significantly in terms of size and capacity. The larger haul trucks have an impact on
road design. The haul road width depends upon the width as well as turning radius of the
larger haul trucks. The maximum width of the haul truck has gone up from 9m in 1999 to
15m in 2010. The turning radius of the haul trucks has increased by 12% over that of a
generation earlier. Hence, larger turning radius and width of road is required to accommodate
the largest trucks.
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Chapter 2: Literature Review
2.2.5.1 Haul trucks tires
Haul truck tires have grown with the size and capacity of trucks.
The major
component materials of a tire are: rubber (both synthetic and natural), carbon black, sulphur,
steel cable, polyester, nylon, and other chemical agents. A common ratio of rubber to other
materials is 50:50 for a radial car tire and about 80:20 for an off-road haul truck tire. For
large haul truck tires, about 80% of the rubber comes from natural sources.
A higher
proportion of natural rubber means a greater capacity to dissipate heat, but lower wear
resistance. A higher proportion of carbon black leads to greater wear resistance of tires, but
carbon tends to retain heat, thus the tire gets heated more easily. If the haul road has an
abrasive surface, a tire with a greater percentage of carbon black would be desired. But, if the
haul road is smooth and free of abrasive materials, a tire with higher percentage of natural
rubber would give better service in terms of tonne-km/hr (Tannant and Regensburg, 2001).
There are two major types of tire: bias ply and radial (Figure 2.7). Bias ply tires use
rubber-cushioned nylon to form the carcass and steel wire bundles for beads. Radial tires
consist of a ply of steel cables laid radially about the tire as carcass. The bead of the radial
tire is formed by a single bundle of steel cables or steel strip. Radial tires have longer tread
life, greater stability, more uniform ground pressure, less rolling resistance and less heat
buildup from internal friction when the tire is in motion as compared to bias ply tires
(Michelin, 2005). Large haul trucks tend to use radial tires due to these reasons. Werniuk
(2000) reported that the 95% of the tires used on large surface haul trucks are radial tires. The
information on tires is described in detail in the Tire Maintenance Manual (Good Year, 2008)
and Caterpillar Performance Handbook (2010).
29
Chapter 2: Literature Review
Figure 2.7: Bias ply and radial tires (Good Year, 2008)
2.2.5.1.1
Tire foot print area and pressure
Two important elements of tires that affect haul road design are foot print area and tire
pressure. The inflation pressure of new low profile truck tires vary between 586 kPa to 703
kPa (Good Year, 2008; Caterpillar Performance Handbook, 2010). The bearing capacity of
the haul road construction materials should be greater than the tire pressure. Thus the bearing
capacity of materials should be more than 1MPa (equivalent to compressive strength of soft
rock) used for the surface course (Tannant and Regensburg, 2001). A well designed subbase
and base layers with sufficient bearing capacities and stiffness is very much important
because the stress bulb below a tire can extend quite deep due to the large tire footprint areas.
The shape of tire footprint is approximated as either a circular or rounded rectangle. The
pressure distribution beneath a tire is non-uniform, especially for bias ply tires. However, an
assumption of uniform pressure distribution across the tire foot print area for the purpose of
stress analysis in haul road layers is suggested (Kumar, 2000).
The different wheel loading conditions typically considered are based on (i) maximum
wheel load, (ii) contact pressure, (iii) multiple wheel loads or its equivalent, and (iv) repetition
of loads.
30
Chapter 2: Literature Review
The vertical stresses due to wheel loading on the haul road have strong influence on
pavement strain prediction. There are many approaches to predict the stresses at any point in
the pavement mass; one among those is elastic theory. According to it there exists constant
ratio between stresses and strain even though the material is not elastic. Initially the thickness
of the pavement is determined using traffic volume, single wheel load and increased wheel
load based on permissible maximum allowable shear stress for specific materials. Now the
dual wheel cases have been considered as equivalent single wheel load (ESWL) where a load
is determined that generates the same tyre contact area and maximum deflection as would the
group of wheels. The concept to consider equivalent single wheel load to multiple wheels
give rise to corresponding equivalent deflection. Turnbull and Ahlvin (1957) suggested
mathematical approach to determine thickness of pavement using CBR method.
The contact pressure and its distribution between any tire and the pavement depends
on tire pressure, wheel load and tire construction. Usually the maximum weight of the
haulage machine is considered in designing the road section. Though the true contact area
between tire and road surface is elliptical, it is considered circular in shape for case of
calculation (Husrulid and Kuchta, 2006). The contact area is usually circular for a low ratio
between applied load and maximum rated load (Marshek, 1978). Typically the ratio varies
between 0.7 to 0.9 for fully loaded mine trucks (Good Year, 1990) and the contact area is
almost rectangle. The governing mathematical relations adopted from Thompson (1996) are
given below.
Contact Area (m2),
The contact radius (m), r
……………………………(1)
…………………………………(2)
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Chapter 2: Literature Review
The thickness (m), t
……………………………….(3)
The above equation is applicable for CBR less than 12 with multiple wheel groups the
corresponding thickness for multiple wheel groups is
……(4)
where Pe = equivalent tyre pressure at depth t
=
…………..…………………….. (5)
Incorporating the repetition factor, the equation to calculate CBR is
CBR (%) =
……………………. (6)
where α = repetition factor as determined elsewhere (Ahlvin et al., 1971)
or
……(7)
The deflection under a single wheel load (Ws) is given by
Ws =
……………………………………….. (8)
and deflection for a group of wheels (Wd) is
Wd =
where,
………………………………………(9)
re = contact radius for single wheel (m)
E = Elastic modulus of pavement (MPa)
bs = tyre pressure for single wheel (MPa)
Fs = deflection factor for single wheel.
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Chapter 2: Literature Review
rd = contact radius for multiple wheels (m)
bd = tyre pressure for multiple wheel (MPa)
Fd = deflection factor for group of wheels
Rearranging eq. (6) with input from eq. (1) and eq. (2)
We have Ps = π rs2 bs and Pd = π rd2 bd ………………………………..(10)
where, Ps,d represent tyre pressure for single wheel load and multiple wheel loads
respectively.
Equating Ws = Wd and rs = rd we have
=
or
=
…………………. (11)
The equation (11) shows the relationship between tire load and the deflection factor,
which can be obtained from the established curve (Figure 2.8). It was reported that four
critical points for stress level exists under a haul truck (Yoder and Witczak, 1975). The
ESWL represents dual assembly and the critical points occur either under the center of one
rear load (D) or at the center of the rear axle (C) (Figure 2.9). Two additional critical points
(A and B) are also analyzed considering the front axle interaction in proportion to the fully
laden axle weight distribution.
The influence of the each wheel at depth increment is
calculated and the maximum ESWL at that depth is determined using equation (7 and 11).
The load distribution beneath a tire in the road pavement is shown in Figure 2.10.
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Chapter 2: Literature Review
Figure 2.8: Deflection factors for ESWL determination (after Foster and Ahlvin, 1954)
Figure 2.9: Critical points for a fully loaded truck (after Thompson, 1996)
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Chapter 2: Literature Review
Figure 2.10: Load distribution beneath a tire
2.3
Geotechnical properties of fly ash
Fly ash is a byproduct of burning of pulverized coal in thermal power plants. The
pulverized coal is burnt at 13000 to 15000C. Typically the fine residue composed of unburned
particles that solidified while suspended in exhaust gases is called fly ash. It is carried off in
stack gases from boiler and is collected either by mechanical methods or by electrostatic
precipitator. Typically it constitutes 80% of ash and rest being termed as bottom ash. The
physical and chemical properties of ash vary depending on origin of coal, type of plant,
burning process, inorganic chemical composition of coal, degree of pulverization, types of
emission control systems, handling and collection systems etc. Typically two types class C
and class F exists. Class F is a non-self cementing type and is produced from burning of
anthracite and bituminous coal. It contains very small amount of lime (CaO) unlike class C
which possess higher quantity to produce cementitious product. Class F fly ash (pozzolans)
has silicon and aluminum material that itself possess little or no cementitious value level. It
reacts chemically with lime and cement at room temperature to form cementitious compounds
35
Chapter 2: Literature Review
(Chu et al., 1993). The following features of various properties of fly ash as well as reports
the many application of it elsewhere.
2.3.1 Physical Properties
Physical properties of fly ash help in classifying the fly ashes for engineering
purposes. Rehsi and Garg (1988) have made a critical assessment of the different physical
properties of Indian fly ashes. The particles of the fly ashes were angular as well as irregular.
The shape of the particles were affected the different physical properties of fly ash. The
specific gravity of fly ash is generally low compared to that of soil (Ghosh, 1996; Sridharan
and Prakash, 2007). Pandian et al. (1998) reported that the range of specific gravity varies
from 1.46 to 2.66 for Indian fly ashes. Specific gravity of Indian fly ashes varies in the range
of 1.66 to 2.55 reported by Sridharan and Prakash (2007). Gray and Lin (1972) reported that
the variation of specific gravity of the fly ash is the result of a combination of many factors
such as gradation, particle shape and chemical composition. The low specific gravity of the
fly ash is mainly attributed due to the presence of large number of hollow cenospheres from
which the entrapped air cannot be removed, or the variation in the chemical composition, in
particular iron content, or both (Ghosh, 1996; Pandian et al.,1998; Pandian, 2004; Sridharan
and Prakash, 2007).
Grain size distribution indicates if a material is well graded, poorly graded, fine or
coarse, etc. Pandian et al. (1998) carried out experimental investigation on Indian coal ashes
and reported the fly ashes are fine grained substances consisting of mainly silt-size particles
with some clay-size particles of uniform gradation. Consistency limits namely liquid limit,
plastic limit and shrinkage limit are extensively used in the field of geotechnical engineering.
Pandian (2004) reported that fly ashes have liquid limit ranging from 26 to 51%. He observed
36
Chapter 2: Literature Review
from the experimental study that fly ashes are non-plastic and hence plastic limit could not be
determined. It was also not possible to carry out shrinkage limit tests since the ash pats
crumbled upon drying. Free swell index has been developed in the field of geotechnical
engineering to differentiate between the swelling and non-swelling soils (Sridharan and
Prakash, 2007). Nearly 70% of Indian coal ashes exhibit negative free swell index which is
due to flocculation low specific gravity and less quantity of clay size particles (Pandian, 2004;
Sridharan and Prakash, 2007).
The classification of coal ashes from geotechnical engineering point of view is
important for an effective and efficient use in geotechnical engineering practice (Pandian,
2004). The fly ashes are classified as fined grained ashes as they comprise of predominantly
silt sized particles. They can belong to one of the five subgroups, namely MLN, MLN-MIN,
MIN, MIN-MHN and MHN reported by Sridharan and Prakash (2007). If more than 50% of
fines belongs to either the coarse silt size (20 μm < particle size ≤ 75 μm) category, or the
medium silt size (7.5 μm < particle size ≤ 20 μm) category or fine silt plus clay size (particle
size ≤ 7.5 μm) category, then the ash is accordingly represented as MLN, MIN and MHN.
The coarse and medium silt size fractions comprise more than 50% fines and more than the
percentage of combined medium silt and fine silt plus clay size fractions, then the ash is
designated as MLN-MIN group. If the medium and fine silt plus clay silt size fractions
comprise more than 50% fines and also more than the percentage of combined coarse silt and
medium silt size fractions, then the ash is designated as MIN-MHN group.
2.3.2 Chemical Properties
Sridharan and Prakash (2007) stated that the chemical properties of fly ash depends
upon many factors such as geological factors related with coal deposits deciding its quality,
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Chapter 2: Literature Review
the composition of the parent coal, the combustion conditions like the method of burning and
control of combustion process, the additives used for flame stabilization, corrosion control
additives used, hopper position, flow dynamics of the precipitators and the removal efficiency
of pollution control devices. In particular, fly ashes that are produced from the same source
with similar chemical composition can have significantly different mineralogy depending
upon the coal combustion technology used, which in turn affect the ash hydration properties.
The mineral groups present in coal such as hydrated silicates, carbonates, silicates, sulphates,
sulphides, phosphates and their varying proportions normally play a dominant role in deciding
the chemical composition of the ash. When the pulverized coal is subjected to combustion,
the clay minerals undergo complex thermo-chemical transformations. During this process,
sillimanite and Mullite are crystallized as slender needless along with glass formation. Pyrites
and other iron bearing minerals form iron oxides and calcite gets transformed into CaO. The
glassy phase formed renders pozzolanity to the fly ash. In almost all geotechnical engineering
applications, the pozzolanic property of fly ash plays an important role (Krishna, 2001). He
stated that the term pozzolana is employed to designate a siliceous or a siliceous and
aluminous material which by itself, possesses no cementitious value but in the presence of
water, chemically reacts with calcium hydroxide to form the compounds possessing
cementitious properties. Based on pozzolanic property, fly ashes can be classified as self
pozzolanic, pozzolanic and non-pozzolanic. Fly ashes can also be classified as reactive and
non reactive fly ashes. Reactive fly ashes are those, which react with lime to give sufficient
amount of strength. Non-reactive fly ashes are those, which do not give sufficient even on
addition of lime. Self pozzolanic and pozzolanic fly ashes are reactive fly ashes where as
non-pozzolanic fly ashes are nothing but non reactive fly ashes.
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Chapter 2: Literature Review
According to the American Society for Testing Materials (ASTM C618 – 08a) the ash
containing more than 70 wt% SiO2 + Al2O3+ Fe2O3 and being low in lime are defined as class
F, while those with a SiO2 + Al2O3+ Fe2O3 content between 50 and 70 wt% and high in lime
are defined as class C. The low-calcium Class F fly ash is commonly produced from the
burning of higher-rank coals (bituminous coals or anthracites) that are pozzolanic in nature.
The high-calcium Class C fly ash is normally produced from the burning of low-rank coals
(lignites or sub-bituminous coals) and is self pozzolanic in nature. Ahmaruzzaman (2010)
stated that the chief difference between Class F and Class C fly ash is in the amount of
calcium and the silica, alumina, and iron content in the ash. In Class F fly ash, total calcium
typically ranges from 1 to 12%, mostly in the form of calcium hydroxide, calcium sulphate,
and glassy components, in combination with silica and alumina. In contrast, Class C fly ash
may have reported calcium oxide contents as high as 30 - 40%. Another difference between
Class F and Class C is that the amount of alkalis (combined sodium and potassium), and
sulphates (SO4), are generally higher in the Class C fly ash than in the Class F fly ash. The
range of chemical composition of Indian coal ashes together with that for soil (for comparison
purposes) is reported in Table 2.2.
Roode (1987) reported that loss on ignition is generally equal to the carbon content.
Throne and Watt (1965) observed that the amount of SiO2 or SiO2+Al2O3 present in fly ash
influences the pozzolonic activity for a longer period of time. Minnick (1959) has reported
that a relatively high percentage of carbon decreases the pozzolonic activity. Torrey (1978)
reports that fly ash collected by electrostatic precipitators (ESP) has 38% more CaO and 58%
less carbon than ash collected by mechanical collectors. Moreover the former is finer than the
latter. Davis (1949) has stated that finer the fly ash, higher is the pozzolanic reactivity.
39
Chapter 2: Literature Review
Tannant and Kumar (2000) reported that the fly ash collected by ESP is more reactive and
consequentially, more suitable as haul road construction material than fly ash collected by
mechanical collectors.
Table 2.2: Range of chemical composition (%) of Indian coal ashes and soils (Pandian, 2004)
Compounds
Fly ash
Soils
SiO2
38–63
43–61
Al2O3
27–44
12–39
TiO2
0.4–1.8
0.2–2
Fe2O3
3.3–6.4
1–14
MnO
0–0.5
0–0.1
MgO
0.01–0.5
0.2–3.0
CaO
0.2–8
0–7
K2O
0.04–0.9
0.3–2
Na2O
0.07–0.43
0.2–3
LOI
0.2–3.4
5–16
The investigations carried out on Indian fly ashes show that all the fly ashes contain
silica, alumina, iron oxide and calcium oxide (Pandian and Balasubramonian, 2000). The
mineralogical composition of fly ash, which depends on the geological factors related to the
formation and deposition of coal, its combustion conditions, can be established by X-ray
diffraction (XRD) analysis. Quartz and mullite are the major crystalline constituents of lowcalcium fly ash (CaO < 5%), whereas high-calcium fly ash (CaO > 15%) consists of quartz,
C3A, CS and C4AS (Sen Gupta, 1991; Erol et al., 2000; Singh and Kolay, 2002; Pandian,
2004; Sridharan and Prakash, 2007; Ahmaruzzaman, 2010).
The morphological studies
through Scanning Electron Microscope (SEM) indicate that the coal ash contains glassy solid
spheres (plerospheres), hollow spheres called cenospheres, sub rounded porous grains,
40
Chapter 2: Literature Review
irregular agglomerates and irregular porous grains of unburned carbon (Erol et al., 2000;
Sridharan and Prakash, 2007).
When water or any aqueous medium comes in contact with fly ash, iron, aluminum
and manganese oxides sink determine the release of the trace elements associated with them
into the aqueous medium. The degree of solubility of these oxides in turn depends upon the
pH of the aqueous medium (Sridharan and Prakash, 2007). The fly ash with higher free lime
and alkaline oxides exhibits higher pH values (Pandian, 2004; Sridharan and Prakash, 2007).
Sridharan and Prakash (2007) reported that about 50% of Indian fly ashes are alkaline in
nature.
Formation of cementitious materials by the reaction of lime with the pozzolans in
presence of water is called hydration. The hydrated calcium silicate or calcium aluminate,
join the inert materials together. The pozzolanic reactions for stabilisation are given by the
following equation.
CaO + H2O → Ca(OH)2 + Heat
Ca(OH)2 → Ca++ + 2(OH)
Ca++ + 2(OH) + SiO2 → CSH (Calcium Silicate Hydrate)
(Gel)
Ca++ + 2(OH) + Al2O3 → CAH (Calcium Aluminate Hydrate)
(Fibrous)
Class C type fly ash does not need addition of lime where class F does (Senol et al., 2002).
2.3.3 Engineering Properties
A thorough understanding of the engineering behaviour of fly ash is very much
essential for the bulk use of fly ash in geotechnical applications. The density of fly ash is an
important parameter since it controls the strength, compressibility and permeability.
41
Chapter 2: Literature Review
Densification of ash improves the engineering properties. The compacted unit weight of the
material depends on the amount and method of energy application, grain size distribution,
plasticity characteristics and moisture content at compaction (Krishna, 2001; Pandian, 2004).
Compaction is a process of densification of the material by packing the particles closer
together with reduction in the volume of air voids.
2.3.3.1 Compaction characteristics
Moulton (1978) as well as Sridharan et al. (2000) observed the variation of dry density
with moisture content for fly ashes is less compared to that for a well-graded soil, both having
the same median grain size. The tendency for fly ash to be less sensitive to variation in
moisture content than for soils could be explained by the higher air void content of fly ash.
Soils normally have air void content ranging between 1 and 5% at maximum dry density,
whereas fly ash contains 5 to 15%. The higher void content could tend to limit the buildup of
pore pressures during compaction, thus allowing the fly ash to be compacted over a larger
range of water content reported by Toth et al. (1988). Gatti and Tripiciano (1981) carried out
compaction tests on coal ashes collected from Vado Ligure Power Plant, Italy and obtained
maximum dry density varied between 11.4kN/m3 and 45kN/m3 with the corresponding
optimum moisture contents ranging between 28% and 36%. DiGioia et al. (1986) provided
typical standard Proctor compaction curves for Western Pennsylvania Class F fly ash. They
found that the maximum dry density ranged from 11.9 to 18.7 kN/m 3 and optimum water
content ranged from 13 to 32%. Das and Prakashi (1990) reported optimum moisture content
(OMC) of 40% and maximum dry density (MDD) of 10.1kN/m3 for Titagarh fly ash by
performing standard Proctor compaction test. There is a very little difference between the
results obtained from standard Proctor’s (light) and modified Proctor’s (heavy) compaction
42
Chapter 2: Literature Review
tests on fly ashes (Mclaren and Digioia, 1987). Indian fly ashes have been observed to have
maximum dry density in the range 8.9 to 13.8 kN/m 3 and optimum moisture content in the
range 17.9 to 62.3% from standard Proctor compaction test (Sridharan et al., 2001b; Das and
Yudhbir, 2006; Sridharan and Prakash, 2007). Fly ash has low MDD and high OMC due to
presence of cenospheres and plerospheres compared to soil with organic content reported by
Das and Yudhbir (2006). Fly ashes originating from different sources themselves show large
variations in OMC and MDD due to their specific gravity depends on iron content and carbon
contents (Singh, 1996; Krishna, 2001).
2.3.3.2Permeability characteristics
Shenbaga and Gayathri (2004) reported that the values of coefficient of permeability
were in the same range as those of non-plastic silts. They reported the permeability of fly
ashes produced from bituminous coals is in the range of 1×10-5 to 3×10-6 cm/s.
The
compacted fly ash deposits, therefore, would be moderately permeable. The permeability of
ash depends on its grain size distribution, degree of compaction and pozzolanic activity
reported elsewhere (Pandian and Vesperman et al., 1985; Balasubramonian, 1999; Sridharan
and Prakash, 2007). Porbaha et al. (2000) carried out permeability tests on two class F
Japanese fly ashes and reported the permeability values are in the range of 10 –4 –10–5 cm/s.
Glogowski et al. (1992) reported the average value of coefficient of permeability of eastern
U.S. fly ashes as 1.9×10–5 cm/s, with standard deviation and coefficient of variation as
2.8×10–5 cm/s and 147%, respectively. The corresponding values for the western U.S. fly
ashes are 3.1×10–5 cm/s, 6×10–5 cm/s and 194%, respectively. The permeability of Indian fly
ashes is in the range of 8×10–6 cm/s to 1.87×10–4 cm/s (Pandian, 2004).
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Chapter 2: Literature Review
2.3.3.3Strength characteristics
An important engineering property that is necessary for using fly ash in many
geotechnical applications is its strength. The unconfined compressive strengths for fine ash
are higher than those for the coarser ash specimens reported by Leonards and Bailey (1982).
Gray and Lin (1972) observed that the unconfined compressive strength (UCS) increased
from 390 to 900 kPa at 7 days curing and 400 to 1200 kPa at 90 days curing of British fly
ashes compacted at Proctor’s maximum dry densities. Sherwood and Ryley (1966) reported
that the fraction of lime, present as free lime in the form of calcium oxide or calcium
hydroxide, controls self-hardening characteristics of fly ashes. Singh (1996) studied the
unconfined compressive strength of fly ashes as a function of free lime present in them.
Yudhbir and Honjo (1991) reported that the UCS of fly ash increased exponentially with the
free lime content. They also reported the carbon content in fly ashes reduced the strength.
This could be attributed to the lower frictional resistance of carbon particles at the inter
particle level. The class-F fly ash achieved unconfined compressive strength of 126 kPa at 7
days, 137 kPa at 28 days and 172 at 90 days curing investigated by Ghosh and Subbarao
(2006).
The major advantage of fly ashes with regard to shear strength in the compacted and
saturated condition is that the variation of effective friction angle is negligibly small,
irrespective of whether it is obtained from consolidated drained test or consolidated undrained
test (Sridharan and Prakash, 2007). Mclaren and Digioia (1987) reported that the shear
strength of class F fly ash is primarily depend on cohesion component when it is in partially
saturated (compacted with OMC) state. When the sample is fully saturated or dried, it loses
its cohesive part of the strength. Its frictional component depends on the density of the
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Chapter 2: Literature Review
sample. When density increases its friction also increases (Capco, 1990). Indraratna et al.
(1991) compared cohesion intercept and angle of shearing resistance of saturated and
unsaturated fresh fly ash specimens and reported complete loss of cohesion owing to full
saturation and no change in the angle of shearing resistance. The shear strength parameters of
typical Indian fly ashes obtained by drained test under compacted condition were in the range
of 33o to 43o (frictional angle) and 16 to 93 kPa (cohesion) and by undrained test under
compacted condition were in the range of 27o to 39o (frictional angle) and 16 to 96 kPa
(cohesion) reported by elsewhere (Sridharan et al., 2001a; Pandian, 2004; Sridharan and
Prakash, 2007).
A simple time-saving experimental procedure has been developed to determine the
angle of repose which can be used in the field to determine the angle of internal friction under
loose and dry conditions (Pandian et al., 2000; Pandian, 2004). As the fly ash exhibits
pseudo-cohesion, even in its dry state by virtue of its fineness, it is difficult to determine the
angle of repose for fly ashes (Sridharan and Prakash, 2007).
2.3.3.4California bearing ratio (CBR) behavior
A major application area for fly ash utilization is its use as a sub-base material in the
construction of pavements. The CBR value of fly ashes varies between 6.8 and 13.5% in
soaked condition and 10.8 and 15.4% in unsoaked condition reported by Toth et al. (1988).
Das and Prakashi (1990) reported a CBR value of 11% for fly ash under compacted unsoaked
condition and this increased to 19% when soaked for 15 days. Indraratna et al. (1991)
conducted investigation on the engineering behavior of low carbon, pozzolanic fly ash and
reported that CBR values at soaked condition are less than those at unsoaked condition and
also concluded that properly compacted and cured fly ash samples give excellent CBR values.
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Chapter 2: Literature Review
The CBR values of typical Indian fly ashes vary from 11.3 to 20.6% for unsoaked condition
and 0.2 in soaked condition as reported in literature (Sridharan et al., 2001c; Pandian, 2004;
Sridharan and Prakash 2007).
2.3.3.5 Ultrasonic velocity
Ultrasonic nondestructive testing is a versatile technique which is applied to a wide
variety of material analysis applications. Leslie and Cheesman (1949) in Canada and Jones
(1949) in England developed this method at the same time. The ultrasonic method has been
applied to evaluate the properties of concrete mixes (Malhotra, 1976; Kewalramani and
Gupta, 2006; Solis-Carcano and Moren, 2007).
Mishra et al. (2003) reported that the
ultrasonic velocity of fly ash – cement composites increased over time. The ultrasonic
velocity increased with increase in curing period of the masonry composite material made of
limestone powder and fly ash (Turgut, 2010). He reported that the ultrasonic pulse velocity
increased from 1150 to 1800 m/s in the composite containing 10% fly ash at 7 days curing.
Dimter et al. (2011) reported that the increase in fly ash percentage (from 0% to 75%) causes
a decrease in ultrasonic velocity. The ultrasonic velocity of the mix containing 25% fly ash
was 2.06 km/s and 75% fly ash was 1.36 km/s. He also reported that the ultrasonic velocity
increases with increase in density of the material.
2.4
Uses and strength behaviour of fly ash
Research work and studies have been carried out for utilizing fly ash for various
purposes to minimize the environmental and disposal problems. Fly ash has been proposed
for various applications, including soil stabilization, wastewater treatment, land reclamation,
recovery of metals, cement replacement, fill material, manufacture of brick, road and
embankment construction etc.
It has been found that fly ash can be used for various
46
Chapter 2: Literature Review
applications due to its beneficial properties. There are many reports on potential use of fly
ash in road constructions (Laguros and Zenieris, 1987; Moulder, 1996; Marsh, 1996; Barstis
and Crawley, 2000; Vittal and Mathur, 2005; Mohanty and Chugh, 2006; Jackson et al.,
2009). The addition of fly ash reduced the volume change characteristics and improved the
subgrade strength. Fly ash provides silica and alumina needed for cementitious reaction with
lime to increase the strength, stiffness, and durability of the stabilized base layer (Butalia,
2007; Mackos et al., 2009).
Gray and Lin (1972) have reported the engineering properties of compacted fly ash
and opined that properly compacted and stabilized fly ash had the requisite properties for use
in load-bearing fills or highway sub-bases. Joshi et al. (1975) observed that the plasticity
index of clay was decreased with the addition of lime and fly ash.
The use of fly ash to stabilize soils should have multiple benefits to their use in haul
roads improving mining productivity (Hobeda, 1984). There exist a couple of road projects,
more specifically in rural sectors where cementitious fly ash has been used as a sub-base
material (Laguros and Zenieris, 1987). The suitability of fly ash as construction material in
road base and subbase construction on stabilizing the ash with 5 and 10% lime or cement was
investigated by Poran and Ahtchi-Ali (1989). The addition of fly ash to clay-based building
materials improves the quality of products (Temimi et al., 1995). But the effectiveness of fly
ash for surface coal mine haul road is still in its infancy (Mulder, 1996; Tannant and Kumar,
2000).
Incorporation of fly ash in the clay material improves the mechanical properties of the
clay (Queralt et al., 1997; Temimi et al., 1998). The gain in strength and modulus is
dependent on the fly ash and cement contents in the fly-soil mixture (Kaniraj and Havanagi,
47
Chapter 2: Literature Review
1999). In India, a regulation issued by the Ministry of Environment and Forests (MOEF) in
1999 stipulated that all new coal thermal power plants should be able to use 100% of the fly
ash they produce within the first nine years of operation. For existing power plants, MOEF
has set a time period of 15 years for 100% fly ash utilisation from the date of issue of the
regulation (MOEF, 1999).
Tannant and Kumar (2000) mixed fly ash, kiln dust and mine spoil at 25:5:70 ratios
and found the composite suitable for use in constructing coal mine haul road base and subbase
layers. They observed that the unconfined compressive strength increased from 0.4 to 0.6
MPa after 7 days curing and 0.6 to 1.1 MPa at 28 days curing with elastic modulus ranged
between 150 and 350 after 14 to 28 days. The composite showed Young’s moduli high
enough to meet the strain requirement for haul road construction. The reclaimed fly ash fill
was inexpensive compared to typical pavement base material and it gained strength over time
(Bergeson and Mahrt, 2000). Bulk utilization of fly ash is possible only through geotechnical
applications (Sridharan, 2000).
In more recent years, for economic and environmental reasons, renewed attention has
been given to the use of 'waste' materials in lieu of conventional aggregates in pavements
(Brennan and O'Flaherty, 2001). Sahu (2001) confirmed that higher amount of fly ash is
needed for effective pozzolanic activity in case of poorly graded material. Consoli et al.
(2001) reported unconfined compressive strength and Brazilian tensile strength characteristics
of fly ash mixed with varying percentages of soil and carbide lime. Unconfined compressive
strengths and Brazilian tensile strengths were obtained from 410 to 1924kPa and 17 to 200kPa
at 7 to 90 days of curing for the mixes containing soil and 25% fly ash stabilized with 4%, 7%
48
Chapter 2: Literature Review
and 10% of lime. They also reported that the compressive strength of fly ash-lime-soil were
410 - 634 KPa in 7 days, 822 - 1,243 KPa at 28 days and 6.9 - 9.4 MPa in 180 days.
Edil et al. (2002) reported that the use of fly ash in the subgrade stabilization achieved
higher stiffness (10-18 MN/m) than other construction materials like excavated rock (9-12
MN/m) from the soil stiffness gage (SSG) test in the field. They also reported the CBR value
and unconfined compressive strength of fly ash (10%) stabilized subbase to be 32% and
540kPa. Sobhan and Mashnad (2002) reported that recycled plastic strip mixed with soilcement-fly ash can develop a sufficient bond which delay the propagation of tensile cracks in
the base course of the road structure that can be suitable for haul road construction. Mingkai
et al. (2002) reported strength of fly ash mixed with varying percentage of phosphogypsum
(20% to 80%) and 8% of lime. Unconfined compressive strengths increased from 3.04MPa to
3.13MPa at 7 days curing for the fly ash contents from 12% to 52% in the mixes and then
increase in fly ash content from 62% to 72%, the strength value decreased from 2.76 to
2.56MPa.
Acosta et al. (2003) reported that the soil - fly ash mixtures prepared with 18% fly ash
content and compacted at 7% wet of optimum water content showed significant improvement
compared to the untreated soils, with CBR ranging from 15% to 31%. The resilient modulus
was approximately 30% higher as compared to the resilient modulus of untreated soil. The
resilient modulus also increased from 20% to 50% with increasing curing time from 7 days to
28 days and unconfined compressive strength of the soil-fly ash mixture was 4 times higher
than that of untreated soil. It was found out that a reduction of 33% in base layer thickness
obtained using developed soil – fly ash mixtures.
49
Chapter 2: Literature Review
Siswosoebrotho et al. (2003) reported an investigation on examining the use of
pulverized fuel ash (PFA) mixed with granite as a suitable material for constructing access
roads into the oil field explorations taking place in Riau Province, Sumatera, Indonesia.
According to their investigation, percentages of PFA in the PFA and Granite mixes were 5,
10, 15, 15 and 50% by weight of the total mixtures used. The strongest mix, in term of CBR
strength, was obtained when the PFA content was 15% with a minimum curing period of 14
days and the weakest when the mixture contained a 50% PFA content. The maximum CBR
attained was about 235% in the mix containing 15% PFA after 14 days curing.
According to Pandian (2004), fly ash has good potential use in geotechnical
applications.
Its low specific gravity, freely draining nature, ease of compaction,
insensitiveness to changes in moisture content, good frictional properties etc. can be gainfully
exploited in the construction of roads, embankments etc. Prabakar et al. (2004) studied fly
ash plus soil mixes and concluded that the addition of fly ash reduces the dry density of the
soil due to low specific gravity and unit weight. They also concluded that the CBR values of
pure soils are 4.7%, 2.03% and 3.53%; by adding fly ash up to 46%, the CBR value is
increased up to 11.41%. Thus, fly ash effectively utilized in the soil to get improvement in
shear strength, cohesion and the bearing capacity. Fly ash addition in soil can also be
effectively used as the base materials for the roads, back filling, and improvement of the soil
bearing capacity of any structure.
Arora and Aydilek (2005) evaluated the engineering properties of Class F fly ash
amended soils as highway base materials. They mixed fly ash (40%) with sandy soils with
plastic fines contents and activated the mix with 7% cement and obtained California bearing
ratio (CBR) and unconfined compressive strength of 140% and 3.2 MPa respectively. Similar
50
Chapter 2: Literature Review
observations were made by Vishwanathan et al. (1997) when silty and sandy soils were
stabilized with lime-activated Class F fly ash for highway bases.
Sahu (2005) reported that the plasticity index and linear shrinkage of all types of soil
decreased from 15% to 1% and 7% to 1% respectively. The CBR value increased from 36%
to 162% which enhanced the suitability of the soils for construction of base and subbase in
road works by conducting a study on four types of soil samples as clayey sand, gneiss with
calcrete, quartz and schist and friable calcrete, each mixed with 4%, 8%, 16% and 20% fly
ash. There are many attempts to use fly ash in raw stage in road construction, particularly in
rural road construction (Vittal and Mathur, 2005; Singh and Kumar, 2005). Chugh and
Mohanty (2005) reported successful use of 60,000 m3 of unstabilised fly ash in the
construction of a 3.4 km long and 7.3 m wide road.
In one specific study fly ash utilization resulted in maximum savings in the sub-base
course limited to about 60 to 90 km of lead for rigid and flexible pavements (Kumar and Patil,
2006). This is important as most of our power plants are situated within this range of most
coal mines. One conservative estimate puts the unutilized fly ash occupying about 65000
acres of land (Das and Yudhbir, 2006) which demands increase the utilization percentage.
Unconfined compressive strength (UCS) and California bearing ratio (CBR) values
increased due to increase in fly ash contents (10% - 20%) in the soil - fly ash mixtures as
observed by Senol et al. (2006).
They opined stabilized fly ash-soil mixtures offer an
alternative for soft sub-grade improvement of highway construction. The class-F fly ash after
stabilizing with only lime (10%) and lime (10%) with gypsum (1%) achieved compressive
strength of 552 kPa and 5902 kPa at 28 days curing and 4046 kPa and 6308 kPa at 90 days
curing reported by Ghosh and Subbarao (2006). They also reported that the CBR value of the
51
Chapter 2: Literature Review
specimens stabilized with only lime (10%) and lime (10%) with gypsum (1%) result 77% and
172% at 28 days curing period compared to 34% at 28 days for unstabilised fly ash.
Fly ash provides silica and alumina needed for cementitious reaction with lime to
increase strength, stiffness, and durability of the stabilized base layer (Butalia, 2007). He
reported that fly ash acted as a mineral filler to fill the voids in the granular pulverized
pavement mix, reducing the permeability of the Full Depth Reclamation (FDR) stabilized
base layer. Shen et al. (2007) experimented with lime, phosphogypsum and fly ash in ratio of
8:46:46 and observed continued increase in strength value of 2.4 MPa at 7 days to more than
16 MPa at 365 days. They concluded that the fly ash mixture was a better material for semirigid road base material.
Full-Depth Cold In-Place Recycling (CPR) with self-cementing fly ash was observed
to be an effective method of converting conglomerate pavement sections into durable roads
(Misra, 2008). CPR demonstration projects have been performed in several states, where it
has been shown to produce longer life at a major saving. This technology provides several
economic and environmental benefits. In addition to providing environmental benefits and
longer lasting pavements, it has been reported that this technology could result in saving of
upto 33% over conventional techniques.
Shen et al. (2009) reported that the fly ash and steel slag mixed with 2.5% of
phosphogypsum dosage results in highest strength. They studied the solidified material by
comparing with some typical road base materials and observed increase in strength value of 8
MPa at 28 days to 12 MPa at 360 days respectively has higher strength than lime-fly ash and
lime-soil road base material and opined the material as a road base application.
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Chapter 2: Literature Review
Jackson et al. (2009) successfully used bottom ash and fly ash for pavement
construction. With increase in height of compacted fly ash layer over soft soil layer, the
bearing ratio value of compacted fly ash increases as reported by Ghosh and Dey (2009). The
silica and alumina in the fly ash reacts with lime to form hydration products which increase
the strength, stiffness and durability of the stabilized base layer (Mackos et al., 2009).
Cetin et al. (2010) reported that the CBR value of the soil stabilized with fly ash (10%
and 20%) and lime kiln dust (2.5% and 5%) were found to be 69 to 142 after 7 days curing
and greater than 164 after 28 days curing.
2.5
Environmental aspects of fly ash utilization
It becomes increasingly important to assess the environmental risk involved in the
storage and utilization of waste materials. The toxic elements contained in the fly ash may
slowly and gradually get released from the ash and cause contamination of surface and
groundwater as well as soil degradation, there by posing health hazards to the neighboring
habitat. The physico-chemical properties of fly ashes combined with operational parameters
of power plants and disposal environments, in which ashes are placed, control the leaching
susceptibility of trace elements from ashes. The utilization of fly ash can be accepted only
when it meets the regulations for environmental safety. Stabilization of fly ash with proper
additives is one of the promising methods to mitigate the problems of leaching and dusting
(Canter and Knox, 1985). Gidley and Sack (1984) presented a general survey on waste
utilization in construction from environmental point of view and reported different
solidification techniques for waste disposal, among which stabilization with lime was one of
the most suitable methods.
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Chapter 2: Literature Review
Huang and Lovell (1990) conducted leaching tests through extraction procedure (EP)
toxicity test and studied the leaching behavior of bottom ash and its effect on ground water
quality. They reported bottom ashes were characterized as nonhazardous wastes and were
recommended for use in different civil engineering applications as construction material.
Edit et al. (1992) studied the feasibility of using compacted fly ash or fly ash mixed
with sand as a construction material for waste containment liners or impermeable covers.
Compacted specimens of fly ash and sand mixture of different permeabilities were subjected
to long term permeability tests. It was observed from their study that the permeability can be
reduced to below 10-7cm/sec. A synthetic inorganic solution simulating a fly ash landfill
leachate did not have any adverse effect on the permeability of the compacted fly ash.
Permeability affected the concentrations of certain elements leaching or desorbing out of, or
adsorbing into the fly ash. Calcium (Ca), sulphur (S) and boron (B) concentrations were
lower in low permeability specimens while cadmium (Cd) and zinc (Zn) concentrations were
higher. The high pH of the fly ash leachate helped in keeping the cadmium, zinc and boron in
the fly ash matrix.
Goh and Tay (1993) reported large scale leachate tests on municipal solid waste
incinerator fly ash stabilized with lime as well as cement to simulate field recycling
conditions as closely as possible. Sridharan et al. (1994) reported the use of fly ash as a prefilter media for clay liner used in the construction of landfill. They examined the feasibility of
fly ash as pre-filter media for Zn. It was observed that fly ash with more lime retained more
Zn. Pandian et al. (1995) observed that fly ash having more lime content was very effective
to retain lead (Pb) ions compared to low lime fly ash.
54
Chapter 2: Literature Review
The quantity of a metal in the leachate is predominantly influenced by the hydraulic
conductivity of the stabilized material and the concentration of a metal in the leachate (Ghosh,
1996; Ghosh and Subbarao, 1998). Ghosh and Subbarao (1998) reported the concentration of
heavy metals in the leachate effluent emanating from the hydraulic conductivity specimens of
stabilized fly ash with higher proportions of lime or lime with gypsum were below threshold
limits acceptable for contaminants flowing into ground water. Wang et al. (1999) carried out
comparative leaching experiments for trace elements in raw coal, fly ash and bottom ash and
identified lead (Pb) and Arsenic (As) as the potential toxic elements. The pH of the solution
and leaching time were also found to strongly influence the leaching behavior. They observed
that the leaching intensity of strontium (Sr), zinc (Zn), lead (Pb), nickel (Ni) and arsenic (As)
was found to increase with decreasing pH of the solution. The stronger the acidity of
solution, then larger is the leaching intensity of these elements.
Pandian and Balasubramonian (2000) conducted leaching studies on fly ashes by
Oedometer method using two types of fly ashes, one with low and other with higher free lime
content. In the, Oedometer method a known amount of fly ash was statically compacted and a
solution of known pH was allowed to pass through the compacted sample in the upflow mode.
The outflow was collected periodically and analyzed. They observed that the leaching of
trace elements from fly ash mainly depends upon pH of the leaching solution and duration of
reaction time. If the pH value is higher, the complete leaching of metal ions from fly ash is
possible.
Kim and Batchelor (2001) listed three major factors affecting leaching based on their
studies on dynamic leaching of metal contaminants from wastes subjected to solidification/
stabilization (s/s) process as acid/base reactions that determine the pH within the waste, pH
55
Chapter 2: Literature Review
dependent reactions that determine whether the contaminants are in mobile or immobile forms
and diffusion that transports mobile contaminants from the waste.
Pozzolanic-based stabilization/solidification (S/S) is an effective, yet economic
remediation technology to immobilize heavy metals in contaminated soils and sludges
(Dermatas and Meng, 2003). They reported that addition of quicklime and fly ash to the
contaminated soils effectively reduced heavy metal leachability well below the nonhazardous
regulatory limits. Overall, fly ash addition increases the immobilization pH region for all
heavy metals and significantly improves the stress-strain properties of the treated solids, thus
allowing their reuse as readily available construction materials.
Goswami and Mahanta (2007) found out that on the use of fly ash and lime for
stabilization of lateritic soil did not have significant impact on the environment, as most of the
toxic metals present in the fly ash were within the threshold limits. Most of the metals were
kept within the stabilised soil matrix by the high pH induced by lime treatment of the mixes.
Recent experiments showed that residual lateritic soils not only responded well to fly ash and
lime stabilisation, but also provided a tremendous opportunity for the bulk utilisation of fly
ash through geotechnical applications such as construction of embankments, roads and
various fill applications.
2.6
Fly ash – lime or fly ash – soil – lime interaction
The enhancement of mechanical strength and microstructural development of fly ash
with addition of lime has been reported elsewhere (Das and Prakashi 1990; Sivapullaiah et al.,
1995; Lav and Lav, 2000; Ghosh and Subbarao, 2001; Beeghly, 2003; Antiohos, 2004;
Mishra and Rao, 2006; Cicek and Tanrıverdi, 2007; Cetin et al., 2010). The lime stabilization
of soils or soil-fly ash mixtures have been extensively described in the literature which
56
Chapter 2: Literature Review
involves the physico-chemical mechanisms of both short- and long-term reactions (Consoli et
al. 2001; Faluyi and Amu, 2005; Goswami and Mahanta, 2007; Mackos et al., 2009). The
permeability of fly ash or soil–fly ash mixes stabilized with lime gradually decreased due to
the formation of hydration products, reduced the interconnectivity between the pores (Ghosh
and Subbarao, 2001; Sridharan and Prakash, 2007).
Chu et al. (1955) observed lime-fly ash interaction while studying stabilization of soil
with lime and fly ash mixture. They stated that the improvement in the properties of soillime-fly ash mixes was due to the formation of cementitious compounds such as calcium
silicate hydrate (CSH) which is formed as a result of the reaction between lime and fly ash in
the presence of water.
The interaction between fly ash and lime was complex and pozzolanic reaction was
very slow reported by Croft (1964). He observed from his investigation on lime-stabilized fly
ash that the appearance of new crystalline phases were considered as gel like varieties of
calcium silicate hydrate in the X-ray diffraction patterns after a curing period of 4 weeks and
also the presence of hydrated calcium aluminate (C4AH13) after a curing period of 8 week.
Luxa’n et al. (1989) identified the formation of calcium aluminum hydrate (C 4AH13),
carboaluminate (C4ACH11), monosulfoaluminate (C4ASH12), and calcium silicate hydrate
(CSH) as pozzolanic reaction products of fly ash and calcium hydroxide (hydrated lime).
Soliman (1990) observed that the lime-fly ash reaction (pozzolanic) was a slow process that
took place without any heat generation. When water and fly ash are mixed with lime, the
silica and alumina reacts with lime and the resulting product is tricalcium silicate and
tricalcium aluminate.
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Chapter 2: Literature Review
Goh and Tay (1993) observed from long-duration leaching experiments on municipal
solid waste incineration fly ash (MSWIF) that the concentration of all the elements in the
leachate of MSWIF samples stabilized with lime were significantly lower than that of the
unstabilized MSWIF samples. Malhotra (1994) observed that class F fly ashes hydrate with
the addition of lime. The reactive silica in the fly ash reacts with the calcium hydroxide
producing calcium silicate hydrate (CSH). The alumina in the fly ash also takes part in the
reaction and the reaction products formed include calcium aluminate hydrate (CAH),
ettringite, gehlenite and calcium monosulfoaluminate hydrate.
Sivapullaiah et al. (1995) reported that for every fly ash, there is an optimum lime
content for its maximum reactivity. They conducted a simple physico-chemical test to assess
the optimum lime content and compare it with the results obtained from strength test and
showed that a good relationship exists between the optimum lime content determined by pH,
liquid limit or free swell index with that obtained by an unconfined compressive strength test.
Ghosh and Subbarao (1998) observed that hydraulic conductivity of lime stabilized fly
ash reduced with increase in lime content, moulding water content, flow period and curing
period.
They also reported that the concentration of metals in the leachate effluents
emanating from stabilized fly ash show positive effects of lime stabilization and concluded
that the stabilized material has the potential for use in a broad spectrum of applications such
as structural fills, road subbase construction, covers, liners and cutoff trench walls, minimize
the potential for groundwater contamination.
Lav and Lav (2000) conducted microstructural, chemical, mineralogical and thermal
analysis as well as unconfined compressive strength tests on cement and lime stabilized fly
ash for use as pavement base material. They observed that the strength development is
58
Chapter 2: Literature Review
dependent on the amount of hydration products as well as their interlocking mechanisms.
Ghosh and Subbarao (2001) studied the physico-chemical and microstructural developments
of low calcium fly ash modified with 6 and 10% and 1% gypsum through X-ray diffraction,
differential thermal analysis, SEM and EDAX. They observed that the compact matrix was
formed due to the new cementitious compounds, developed within the pore spaces result in a
reduction of the radius of pore spaces that confirms the reduction of permeability and increase
in strength without any adverse effect on ground water quality.
Lav et al. (2006) observed from SEM analysis that when cement or lime and fly ash
mixed with water, the self-hardening mechanism of fly ash occurred due to reaction with
calcium ion in solution to form CSH (pozzolanic reaction) and due to presence of either
cement or lime produces hydration products resulting higher strength in the material. They
also observed that ultrasonic pulse velocity of the cement stabilized fly ash increased with
increasing curing period.
When lime kiln dust (LKD) is mixed with moist soil and fly ash, the hydration of
calcium oxide (CaO) causes the formation of Ca(OH)2 and dissociation of Ca(OH)2 favors
dissolution of silica and alumina in fly ash. This phenomenon gives rise to formation of CSH
and CASH around soil and fly ash particles (Cetin et al., 2010).
59
Chapter 3: Methodology
CHAPTER 3
METHODOLOGY
3.1
General
The aim of the investigation is to improve upon the condition of surface coal mine
haul road as well as evaluate the potential of fly ash to achieve this. This chapter reflects
methodology adopted and materials used to achieve the objectives. The main inputs are
overburden material, fly ash and lime. Fly ash collected from local power plant used in this
investigation.
Sample preparation and testing techniques used for characterisation of
materials as well as development of composite materials are also reported.
3.2
Materials and Methods
3.2.1 Materials
The details of ingredients used in this study are given below.
3.2.1.1 Fly ash
Fly ash, a by-product of thermal power units was collected from nearby places,
namely Rourkela Steel Plant (RSP), SAIL. The reasons for selecting the units are:
1. It is situated near the two main coalfields i.e.Talcher & IB Valley areas.
2. It uses coal from Talcher coalfields.
3. It produces huge quantities of fly ash and dumping is a problem. There are many
adverse environmental issues associated with the ash ponds
60
Chapter 3: Methodology
4. The ash sample if found suitable would help in addressing the utilization issue as well
as in improving the haul road economics.
Rourkela Steel Plant is an integrated steel plant of Steel Authority of India Limited
located in Rourkela set up in 1958. It has a captive power plant (CPP) consumes 2230 tonne
of coal a year generates 120 MW and produces about 600 tonne of ash. The ash so generated
is typically dumped in a nearby pond area but of late it has reached its capacity and hence
alternate methods are being actively explored.

Collection method
The fly ash used in the present study was collected in dry state from electrostatic
precipitators of CPP-II of RSP. During the combustion of pulverized coal in suspension-fired
furnaces of thermal power unit, the volatile matter is vaporized and the majority of the carbon
is burned off. The mineral matter associated with the coal, such as clay, quartz and feldspar
disintegrate or slag to varying degree. The finer particles that escape with flue gases are
collected as fly ash using electrostatic precipitators in hoppers and stored. The hoppers have
small outlets. Gunny bags made of strong poly-coated cotton with 50kg capacity each were
used to collect the dry fly ash. The chute of hoppers was slowly opened and the bags were
filled. The mouth of each bag was sealed immediately after collection and the same was
again inserted in another polypack to prevent atmospheric influences.
The bags were
transported with utmost care from the plant to laboratory and kept in a secure and controlled
environment. Samples of fly ash were taken out as per requirement of test.
3.2.1.2 Overburden Materials
Overburden from a surface coal mine is an important material for the investigation. It
was sourced from an active mine Bharatpur opencast coal mine, Talcher, Odisha. It is
61
Chapter 3: Methodology
situated at about 300 km from Rourkela (Figure 3.1).
The coal belongs to Gondwana
sequence. Major part of the area is covered by Barakar / Karharbari / Talcher exposure with
east west strike of beds. The typical overburden material consists of alluvium, laterite, fine to
medium grained sandstone, carbonaceous shale, pink clays, etc. The mine produces coal of
about 9 MT annually. The layout of the mine permanent haul roads and branch haul roads
which are connected with the coal transportation roads is shown (Figure 3.2). The length of
main haul road is 5.760km and branch haul roads are 930m, 1.150km, and 1.335km at
different locations of the mine. Haul roads are basically flexible pavement type which are
designed by California Bearing Ratio (CBR) method. The mine follows drilling, blasting,
loading and dumping operations. Shovel and dumper combinations are used to handle over
burden materials. 50ton and 85ton dumpers (make: BEML and Hindustan) are used to haul
overburden material.

Collection method of overburden material (Figure 3.3)
The overburden dump area is about 2 to 2.5 kms from the active benches. The over
burden area spreads about 300m long and 400m wide. Dozers/spreaders are used to spread
the material once dumpers dump those. Samples for testing were selected and collected that
those represent the average materials found in the mine. Sample collections were carried out
from all parts of the dump area. Gunny bags were used to collect the loose soils sample
leaving the gravels, boulders etc aside. The process followed for fly ash collection was also
repeated to collect overburden material. The material was explored in the laboratory sieved to
discard gravels, pebbles etc. The materials were thoroughly mixed, covered for 2 hours for
homogenization and then stored in poly pack for experimentations.
62
Chapter 3: Methodology
Figure 3.1: Map of Talcher Coalfield, Odisha
63
Chapter 3: Methodology
Figure 3.2: Sketch of Bharatpur Opencast coal mine
Figure 3.3: Collection of mine overburden
64
Chapter 3: Methodology

Observation from mine visit (Figure 3.4)
Frequent mine visits were made to Bharatpur opencast mine, Talcher not only to
collect overburden samples but also to obtain mine operation data, to inspect the haul road
condition and study the haul road construction process, to hold discussion with mine officials
as well as with dumper operators, to critically observe the various problems associated with
haul road, etc. A sample photograph (Figure 3.4) depicts the observation.
Pothole
Undulation
s
Figure 3.4: Undulations and potholes are marked in the haul roads
3.2.1.3 Lime
The additive selected was commercially available superior grade quick lime (make:
Rajasthan Lime, India). Quicklime is manufactured by chemically transforming calcium
carbonate (limestone – CaCO3) into calcium oxide (CaO).
65
Chapter 3: Methodology
3.2.2 Methods
3.2.2.1 Sample preparation
These are many published reports of evaluating the performance of soil stabilized with
fly ash (Kaniraj and Havanagi, 1999; Tannant and Kumar, 2000; Consoli et al., 2001;
Prabakar et al., 2004; Lav et al., 2006; Goktepe et al., 2008; Jackson et al., 2009; Cetin et al.,
2010). The soil chosen in those studies represent more or less uniform material. Overburden
is a highly heterogeneous formation.
The over burden material disposal is not only a
challenge to the mine operator but also has many drawbacks in its use as construction
materials of haul road and branch roads. At the same time all avenues are being explored to
increase the usage of fly ash. The aim of the study was to improve the behaviour of haul road
with fly ash replacing a part of the conventional subbase material i.e. the overburden. Its aim
was to develop and evaluate the performance of composite material with fly ash and mine
waste i.e. overburden materials. Hence experiments were carried out to maximum of 50% fly
ash addition in the overburden (Table 3.1).
Table 3.1: Various proportions of flyash and overburden
Fly ash (%)
Overburden (%)
15
85
20
80
25
75
30
70
35
65
40
60
45
55
50
50
66
Chapter 3: Methodology
Availability of free lime enhances the pozzolanic activity of materials.
In this
investigation a varying percentage of lime 2%, 3%, 6% and 9% were used in preparing the fly
ash-overburden composites (Table 3.2).
Table 3.2: Compositions (%) of (FA+O/B)+L
Sample ID
Compositions
Sample ID
(%)
Compositions
(%)
1(a)
(15FA+85O/B)+2L
5(a)
(35FA+65O/B)+2L
1(b)
(15FA+85O/B)+3L
5(b)
(35FA+65O/B)+3L
1(c)
(15FA+85O/B)+6L
5(c)
(35FA+65O/B)+6L
1(d)
(15FA+85O/B)+9L
5(d)
(35FA+65O/B)+9L
2(a)
(20FA+80O/B)+2L
6(a)
(40FA+60O/B)+2L
2(b)
(20FA+80O/B)+3L
6(b)
(40FA+60O/B)+3L
2(c)
(20FA+80O/B)+6L
6(c)
(40FA+60O/B)+6L
2(d)
(20FA+80O/B)+9L
6(d)
(40FA+60O/B)+9L
3(a)
(25FA+75O/B)+2L
7(a)
(45FA+55O/B)+2L
3(b)
(25FA+75O/B)+3L
7(b)
(45FA+55O/B)+3L
3(c)
(25FA+75O/B)+6L
7(c)
(45FA+55O/B)+6L
3(d)
(25FA+75O/B)+9L
7(d)
(45FA+55O/B)+9L
4(a)
(30FA+70O/B)+2L
8(a)
(50FA+50O/B)+2L
4(b)
(30FA+70O/B)+3L
8(b)
(50FA+50O/B)+3L
4(c)
(30FA+70O/B)+6L
8(c)
(50FA+50O/B)+6L
4(d)
(30FA+70O/B)+9L
8(d)
(50FA+50O/B)+9L
Note: FA= fly ash, O/B = Overburden, L = Lime
Heavy compaction (modified Proctor compaction) tests were performed as per IS:
2720-Part (1983) to determine the maximum dry density and optimum moisture content of all
67
Chapter 3: Methodology
the mixes for preparation of sample. The samples were prepared at their respective optimum
moisture content and maximum dry density.
The raw materials such as fly ash, mine
overburden and lime were blended in the required proportion in dry condition. The dry mixed
ingredients were put in a poly pack, covered and left for about an hour for homogenization.
Then the required amount of water was added to the mixture and mixed thoroughly. Then the
mixture was left in a closed container for uniform mixing and to prevent loss of moisture to
atmosphere. The wet mixture of amount corresponding to the required dry density was
compacted in the mould.
3.2.2.1.1 Sample preparation for CBR test
The California bearing ratio (CBR) test samples were prepared using standard CBR
mould of 150mm diameter and 175mm height (Figure 3.5) as per IS: 2720-Part 16 (1987).
The sample was statically compacted in the mould, such that the height was maintained at
127mm. A circular metal spacer disc of 148 mm diameter and 47.7mm height was used to
compact the sample.
3.2.2.1.2 Sample preparation for UCS test
Split mould of 38mm diameter and 86mm length was used for preparation of the
unconfined compressive strength (UCS) test samples as per IS: 2720-Part 10 (1991). Samples
were prepared with uniform tamping. Two circular metal spacer discs of height 5mm and
diameter 37.5mm each with base (7mm height, 50mm diameter) were used at top and bottom
ends of the mould to compact the sample such that the length of the specimen was maintained
at 76mm. Then the discs were removed and an another spacer disc of height 100mm and
diameter 37.5mm with a base (height 7mm, 50mm diameter) was used to remove the sample
68
Chapter 3: Methodology
from mould. The final prepared specimen has its length to diameter ratio of 2. Figure 3.6
shows the mould, spacer discs and prepared ingredients.
Figure 3.5: CBR mould
Figure 3.6: UCS mould, spacer discs and
mixed ingredients
3.2.2.1.3 Sample preparation for tensile strength test
The sample for Brazilian tensile strength test was prepared using the same mould of
UCS test samples. For this purpose, two circular metal spacer discs of 5mm and 62mm
heights and 37.5mm diameters with base (height 7mm, 50mm diameter) were used. The final
prepared specimen has its length to diameter ratio of 0.5.
3.2.2.1.4 Sample preparation for Ultrasonic Pulse velocity test
Split mould of 38mm diameter and 86mm length was used for preparation of the
Ultrasonic test. The procedure for the sample preparation of the above test is same as
unconfined compressive strength (UCS) test samples. The final prepared specimen has its
length to diameter ratio of 2.
69
Chapter 3: Methodology
3.2.2.1.5 Sample preparation for SEM, EDX and XRD analyses
The raw materials after oven dried were taken for scanning electron microscopy
(SEM), energy dispersive X-ray (EDX) and X-ray diffraction studies. Fractured pieces of
composites samples retrieved from CBR moulds after CBR tests at 28 days curing periods
were taken for the above analyses. For SEM and EDX analyses, fragments of the specimens
were collected and oven dried to constant mass. For, XRD analyses, fractures pieces of the
samples after oven dried were crushed and sieved the powders passing through 75µm sieve.
3.2.2.1.6 Sample preparation for leaching study
Leaching study was conducted by permeability method as per IS: 2720 (Part 17) for
the collection of leaching effluents. Samples for permeability test were prepared following
the same process as per CBR test. The wet mixtures of the samples were compacted in the
permeameter mould of 100mm internal diameter and 127mm height. The moulds were fitted
properly and kept for 7 days curing of the samples after compaction. Permeability tests were
carried out at the end of 7 days curing period. The moulds were connected through the top
inlet to the constant head water reservoirs after curing of the specimens. Air vents were kept
open for sufficient time till air bubbles were removed from the specimens to make the
samples fully saturated.
It took one to two days for saturation. The permeability was
measured by falling head method according to IS: 2720-Part 17. Water was allowed to flow
through the specimens continuously for seven days and readings were recorded daily. The
leaching effluent coming out from specimen through the outlet of the permeability mould was
collected in a measuring cylinder. The measuring cylinder was washed with dilute HCL acid
and distilled water to clean any impurities before collecting the effluent for leachate analysis.
The set up for collection of leaching effluent is shown in Figure 3.7. The leachate sample
70
Chapter 3: Methodology
obtained from the experiment was filtrate using Gelman filter paper and acidified with
concentrated nitric (HNO3) acid to maintain pH value between 2 to 2.5 (US EPA, 1985;
Dermatas and Meng, 2003). Then the leachate sample was stored in a sampling bottle and
kept at ~10oC before use to prevent it from evaporation and volume change.
Figure 3.7: Set up for collection of leaching effluent
3.2.2.2Experimental methods
3.2.2.2.1 Specific Gravity
The specific gravity of the mine overburden and fly ash were determined using
volumetric flask method as per IS: 2720-Part 3 (1980).
3.2.2.2.2 Grain size distribution
Grain size distribution was carried out through a standard set of sieves as per IS: 2720Part 4 (1985) by wet sieving using a sieve of 75μm size. The material retained on the 75μm
sieve was oven dried and sieved using IS standard sieves of 4.75, 2, 1, 0.6, 0.425, 0.212, 0.15,
71
Chapter 3: Methodology
0.075 mm sizes. The material passing through the 75μm size was collected carefully and
grain size distribution analysis was performed by using Hydrometer method. The overburden
and fly ash have been classified based on particle size (IS: 1498, 1970).
3.2.2.2.3 Specific surface area
The specific surface area of the mine overburden and fly ash were determined using Blain’s
air permeability method as per IS: 1727 (1967).
3.2.2.2.4 Consistency limits
The consistency limits of the mine overburden were determined as per IS: 2720-Part 5
(1985) and Part 6 (1972). The liquid limit is the minimum moisture content at which the soil
type material can flow under a specified small disturbing force, the disturbing force being
defined by the method of testing. The liquid limit of overburden was determined using
Casagrande liquid limit device. The liquid limit of fly ash was determined by the cone
penetration method as per BS: 1377-Part 2 (1990) due to difficulty in cutting a groove using
Casagrande device. The plastic limit is the minimum water content at which soil ceases to
behave as a plastic material. It was determined by rolling about 5 gm of wet soil pat into a
thread on the glass plate with tips of the fingers of one hand to 3-4 mm diameter. The
shrinkage limit is the maximum water content below which the soil ceases to decrease in
volume on further drying. It was determined using shrinkage limit dish.
3.2.2.2.5 Free swell index
Free swell index of the overburden and fly ash was determined as per IS: 2720-Part 40
(1977).
72
Chapter 3: Methodology
3.2.2.2.6 X-ray diffraction (XRD) analysis
X-ray diffraction provides a powerful tool to study the structure of the materials which
is a key requirement for understanding materials properties. X-ray diffraction is based on
constructive interference of monochromatic X-rays and a crystalline sample. X-ray powder
diffraction is most widely used for the identification of unknown crystalline materials (e.g.
minerals, inorganic compounds). It is a technique for analyzing structures unknown solids
which is critical to studies in geology, environmental science, material science, engineering
and biology. X-ray beam hits a crystal, scattering the beam in a manner characterized by the
atomic structure. Even complex structures can be analyzed by x-ray diffraction, such as DNA
and proteins.
In the present investigation, XRD analysis was performed on a Philips
diffractometer (PANalytical X-ray B.V., UK) employing Cu Kα radiation in the range 2θ = 00
to 900 at a goniometer rate of 2θ = 20/min to detect the mineral phases.
3.2.2.2.7 SEM and EDX studies
Scanning Electron Microscopy (SEM) is a powerful analytical technique for the
evaluation of particulate matter. Scanning electron microscope uses a beam of energetic
electrons to examine objects on a very fine scale. It is capable of performing analyses of
selected point locations on the sample and is especially useful for determining chemical
compositions. The SEM analyses were conducted in a JEOL JSM 6480 LV, (Japan) model
operated at 15 kV and linked with an energy dispersive X-ray (EDX) attachment.
Microstructure and chemical composition of the samples were examined by SEM and EDX
techniques.
73
Chapter 3: Methodology
3.2.2.2.8 Loss on ignition (LOI)
Loss on ignition of overburden and fly ash were determined as per IS: 1760-Part 1
(1991).
3.2.2.2.9 pH test
The pH value was determined as per IS: 2720-Part 26 (1987) to identify the acidic or
alkaline characteristic of overburden and fly ash. The measurement of pH was carried out
using pH meter (make: Systronics, India) with accuracy up to
0.02 units. The instrument
was standardized with three standard buffer solutions of pH 7.00, 4.00 and 10.00 at 25 0C.
The suspension was stirred well and allowed to come to room temperature (25 1°C) before
taking the pH measurement.
3.2.2.2.10 Compaction test
Higher compaction is needed to meet the bearing capacity for heavy vehicle
transportation and typically the machineries operating in surface coal mines weigh about 80
tonne. Modified Proctor compaction test is typically used to give a higher standard of
compaction. It was performed to determine the maximum dry density and optimum moisture
content of the material as per IS: 2720-Part 8 (1983). The sample was compacted in the
mould in five layers using a rammer of 4.9kg mass with a fall of 450mm by giving 25 blows
per layer. The compacted energy value given was 2674 KJ/m3.
3.2.2.2.11 Triaxial compression test
The undrained, compression triaxial test was carried out as per IS: 2720-Part 11
(1993) to determine the shear strength parameters of the mine overburden and fly ash. Three
identical samples of 38mm diameter and 76mm length were prepared at optimum moisture
74
Chapter 3: Methodology
content and maximum dry density of the materials obtained from the modified Proctor
compaction test. The samples were tested by giving confining pressures 1 kg/cm2, 2 kg/cm2
and 3 kg/cm2 respectively. Average values of three tests for each type were considered for
analysis. Mohr-Coulomb relation between two normal stresses (τ=C+σntanφ) has been used
to determine Cohesion and angle of internal friction of the materials.
3.2.2.2.12 Permeability test
The permeability was measured by falling head permeability test as per IS: 2720-Part
17 (1986). Sample was compacted in the permeability mould by taking maximum dry density
and optimum moisture content values of the mine overburden, fly ash and mixes obtained
from the modified Proctor compaction tests.
3.2.2.2.13 California bearing ratio test
California bearing ratio (CBR) tests were performed in accordance with IS: 2720-Part
16 (1987). The samples were statically compacted to 95% of maximum dry density in the
mould for CBR test. The samples were soaked for four days in water and were allowed to
drain for 15 min before test to obtain soaked condition results. The curing periods adopted
were immediate, 7 days (3 days moist curing + 4 days soaking) and 28 days (24 days moist
curing + 4 days soaking) (Krishna, 2001; Leelavathamma et al., 2005; Cetin et al., 2010).
CBR tests were carried out at the end of respective curing period. Two surcharge disks, each
weighing 2.5 kg, were placed over the sample and a plunger, 50 mm in diameter, was used to
penetrate the sample at a rate of 1.25 mm/min during CBR test. Figure 3.8, 3.9 and 3.10 show
the prepared CBR samples in the moulds, soaking of CBR samples in the water tank and an
experimental set up for CBR test.
75
Chapter 3: Methodology
The general relationship between CBR and quality of sub-grade soil suggested for
pavement construction has been referred to in the analysis (Table 3.3).
Table 3.3: Relationship between CBR and quality of subgrade soil (Bowels, 1992)
CBR
Quality of sub-grade
0-3%
Very poor sub-grade
3-7%
Poor to fair sub-grade
7-20%
Fair sub-grade
20-50%
Good sub-grade
75%
Excellent sub-grade
Figure 3.8: Prepared CBR samples inside the moulds
Figure 3.9: Soaking of CBR samples
76
Chapter 3: Methodology
Figure 3.10: An experimental setup for CBR test
3.2.2.2.14 Unconfined Compressive strength test
Unconfined compressive strength is a common criterion to determine its resistance to
any external loading. The specimens prepared for compressive strength test were of 38 mm
diameter and 76 mm long. The availability of free lime and reactive silica and aluminum etc.
are vital to strength gain. Moisture content of the composite has performed effect on the
reactivity. Hence it is preserved by curing the specimen in a controlled chamber with >95%
humidity at 30 ± 20C for the required days (Figure 3.11). The unconfined compressive
strength tests were conducted as per IS: 2720-Part 10 (1991) at a strain rate of 1.2 mm/min.
Load and deformation data was recorded till failure of the specimen. The experimental set up
for unconfined compression test of the specimens is shown in Figure 3.12.
The general relationship between UCS and quality of sub-grade soil suggested for
pavement construction is also referred (Table 3.4).
77
Chapter 3: Methodology
Table 3.4: Relationship between UCS and quality of subgrade soil (Das, 1994)
UCS (kPa)
Quality of Sub-grade
25-50
Soft sub-grade
50-100
Medium sub-grade
100-200
Stiff sub-grade
200-380
Very stiff sub-grade
> 380
Hard sub-grade
Figure 3.11: Sample of UCS specimens prepared (undergoing curing)
78
Chapter 3: Methodology
Figure 3.12: An experimental setup for UCS test
3.2.2.2.15 Brazilian tensile strength test
The determination of direct tensile strength of soil or rock material is difficult. So,
indirect way (Brazilian tensile strength) of its determination is practiced. The Brazilian
tensile test make the sample fail under tension though the loading pattern is compressive in
nature. The tensile strength is determined as per ASTM D3967. In this test, the length
(thickness) to diameter ratio is 0.5. The specimens prepared for tensile strength test were of
38 mm diameter and 19 mm thick. The specimens were placed diametrically during test. The
sample fails diametrically in tension by application of load. The indirect tensile strength is
calculated as follows:
where, σt = Brazilian Tensile Strength; P = Failure Load, D = Diameter of the specimen; L =
length of the specimen.
79
Chapter 3: Methodology
3.2.2.2.16 Ultrasonic Pulse velocity test
Ultrasonic pulse velocity test is a nondestructive testing technique which is used to
determine the dynamic properties of materials.
This method is valid for wave velocity
measurement in both anisotropic and isotropic materials though velocities obtained in
anisotropic materials may be influenced by factors as direction, material composition,
dampness, weakness present, travel distance and diameter of transducer.
The ultrasonic pulse velocity test is a measurement of the transit time of a longitudinal
vibration pulse through a sample which has a known path length. It is carried out by applying
two transducers (transmitting and receiving) to the opposite end surfaces of the samples. The
schematic representation of Ultrasonic velocity measurement system is shown in Figure 3.13.
The electrical impulses of a specified frequency are generated by pulse generator. These
impulses are converted into elastic waves which propagate through a sample by transmitter.
The mechanical energy of the propagating waves that propagate through the sample are
received by the second transducer called receiver placed on the opposite side of the specimen
and then turns into electrical energy of the same frequency.
The signal travel time is
measured electronically through the specimen and is registered in the oscilloscope.
80
Chapter 3: Methodology
Figure 3.13: Schematic representation of ultrasonic velocity measurement
All pulse velocity measurements were determined using an Ultrasonic Velocity
Measurement System (make: GCTS, USA) (Figure 3.14). This system includes 10 MHz
bandwidth receiver pulse raise time less than 5 nano-seconds, 20MHz acquisition rate with
12bit resolution digitizing board, transducer platens with 200 KHz compression mode and
200KHz shears mode.
The GCTS pulse velocity device operates in a through transmission
mode of testing, that is, a signal is produced at one end of the specimen and received at the
other end.
The two, 54-mm diameter test platens are wired to a data acquisition and
processing unit. The equipment provides pulse and shear wave velocity to determine elastic
constants. The measurement was carried out according to ASTM D 2845-05.
The relationship between various parameters on pulse velocity, density, elastic
constants, modulus values are given by the following equations.
81
Chapter 3: Methodology
………………………………………… (1)
………………………………………. (2)
………………………………………… (3)
…………………………………………………. (4)
where,
= compression P wave velocity, m/s
= shear S wave velocity, m/s
= density, kg/m3
= Poisson’s ratio
E= Young’s Modulus, Pa
K= Bulk Modulus, Pa
G= Shear (Rigidity) Modulus, Pa
Figure 3.14: An experimental setup for Ultrasonic velocity measurement
82
Chapter 3: Methodology
3.2.2.2.17 Leaching study
Leaching analysis was carried out by using an Atomic Absorption Spectrophotometer (AA
200, Perkin Elmer). Atomic absorption spectroscopy (AAS) is a spectroanalytical procedure
for the qualitative and quantitative determination of chemical elements employing the
absorption of optical radiation (light) by free atoms in the gaseous state. The technique is
used to determine the concentration of a particular element (the analyte) in a sample. It
analyzes the concentration of elements in a liquid sample based on energy absorbed from
certain wavelengths of light (usually 190 to 900 nm). Atomic absorption spectrophotometer
typically includes a flame burner to atomize the sample (most commonly a hollow cathode
lamp), a monochromator and a photon detector.
3.3
Experimental Size
The investigation included many characterization studies including major laboratory tests as
compaction, California bearing ratio (CBR), unconfined compressive strength (UCS),
Brazilian tensile strength (BTS), Ultrasonic tests in addition to other index tests. The reported
results represent average values of three to four samples for each test type except that for
compaction to determine MDD and OMC. The best results that were not within 5% to 10%
of each other were discarded and fresh samples were prepared for retesting. A total of about
2000 samples were prepared and tested for various parameters. The details of the test size are
given below (Table 3.5).
83
Chapter 3: Methodology
Table 3.5: Experimental Design Chart
Sl
Compositions(%):
Major laboratory tests of composites (curing period)
No.
(FA+O/B)+L
Compaction
(L= Lime)
for MDD &
CBR
Soaking
OMC
UCS
UTS
Ultrasonic
7
28
7
14
28
56
28
56
7
14
28
56
days
days
days
days
days
days
days
days
days
days
days
days
1
15FA + 85O/B
√
*
*
*
+
+
+
+
×
×
+
+
+
+
2
20FA + 80O/B
√
*
*
*
+
+
+
+
×
×
+
+
+
+
3
25FA + 75O/B
√
*
*
*
+
+
+
+
×
×
+
+
+
+
4
30FA + 70O/B
√
*
*
*
+
+
+
+
×
×
+
+
+
+
5
35FA + 65O/B
√
*
*
*
+
+
+
+
×
×
+
+
+
+
6
40FA + 60O/B
√
*
*
*
+
+
+
+
×
×
+
+
+
+
7
45FA + 55O/B
√
*
*
*
+
+
+
+
×
×
+
+
+
+
8
50FA + 50O/B
√
*
*
*
+
+
+
+
×
×
+
+
+
+
9
(15FA+85O/B)+2L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
10
(15FA+85O/B)+3L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
11
(15FA+85O/B)+6L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
12
(15FA+85O/B)+9L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
13
(20FA+80O/B)+2L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
14
(20FA+80O/B)+3L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
15
(20FA+80O/B)+6L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
16
(20FA+80O/B)+9L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
17
(25FA+75O/B)+2L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
18
(25FA+75O/B)+3L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
19
(25FA+75O/B)+6L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
20
(25FA+75O/B)+9L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
84
Chapter 3: Methodology
21
(30FA+70O/B)+2L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
22
(30FA+70O/B)+3L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
23
(30FA+70O/B)+6L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
24
(30FA+70O/B)+9L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
25
(35FA+65O/B)+2L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
26
(35FA+65O/B)+3L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
27
(35FA+65O/B)+6L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
28
(35FA+65O/B)+9L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
29
(40FA+60O/B)+2L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
30
(40FA+60O/B)+3L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
31
(40FA+60O/B)+6L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
32
(40FA+60O/B)+9L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
33
(45FA+55O/B)+2L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
34
(45FA+55O/B)+3L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
35
(45FA+55O/B)+6L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
36
(45FA+55O/B)+9L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
37
(50FA+50O/B)+2L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
38
(50FA+50O/B)+3L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
39
(50FA+50O/B)+6L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
40
(50FA+50O/B)+9L
√
*
*
*
+
+
+
+
×
×
+
+
+
+
85
Chapter 3: Methodology
No. of tests with curing period:
Compaction
CBR
UCS
UTS
Ultrasonic
40
3×40= 120
4×40 = 160
2×40 = 80
3×40 = 120
Total = 40+120+160+80+120 = 520
Total No. of samples (@ 1 to 4 for each type of test)
Compaction
CBR
UCS
UTS
Ultrasonic
40
3×120= 360
4×160 = 640
3×80 = 240
3×160 = 480
Total = 40+360+640+240+480 = 1760
86
Chapter 4: Results and Discussion
CHAPTER 4
RESULTS AND DISCUSSION
4.1
Introduction
A typical surface coal mine in India has about 5km of permanent haul road with a life
span of 25 to 30 years. The carrying capacity of transportation trucks as well as other heavy
machineries undergoes many upward revisions in the life of an operating coal mine as the
demand for fuel continues to increase. However the design of haul road and its construction
do not undergo corresponding improvement. It is often observed that mine operators simply
cut or fill haul roads with the materials existing at the location to save capital cost. In case of
acute adverse condition, only the surface course of the haul road is repaired without, any
change to other courses, specifically to subbase course. So, it is most important to design and
build the haul road sub-base with material of sufficient bearing capacity. The engineering
properties of a material are dependent on the composition of material to a great extent. There
exists wide variation in the composition of fly ash depending on coal type, type of furnace,
temperature, collection mechanism, etc (Mishra, 2003). The overburden material over coal
strata in the Gondwana basin also exhibit similar heterogeneous attributes. The pertinent
geotechnical properties of the develop materials were determined as per established
mechanism. All the results of the above investigation and their corresponding analyses have
been presented in different section as mentioned below:
I.
II.
Results of geotechnical properties of ingredients.
Results of geotechnical properties of developed composite materials.
87
Chapter 4: Results and Discussion
III.
Results of microstructural analyses.
IV.
Results of leaching studies.
V.
4.2
Development of model relationship between geotechnical parameters.
Results of geotechnical properties of ingredients
The aim was to develop suitable engineering material for the subbase course with fly
ash and overburden. So a detail analysis of the constituent materials as well as the developed
composites was carried out.
4.2.1 Physical Properties
The fly ash was collected in dry state and was in loose stage.
content was less than 1%.
Its average water
The fly ash used had a powdery structure with medium to dark
grey colour indicating low lime (Meyers et al., 1976).
The physical properties of fly ash and mine overburden are reported in Table 4.1. The
specific gravity of fly ash and mine overburden obtained are 2.16 and 2.6. The specific
gravity of fly ash is found to be less than that of mine overburden, due to the presence of
cenospheres and less iron content. The materials with higher iron content have relatively high
specific gravity (Sridharan and Prakash, 2007).
The grain size distribution of the construction material has a strong influence on the
density that can be achieved.
The grain size distribution curves for the fly ash and
overburden are shown in Figure 4.1.
The overburden contains sand size fraction with
appreciable amount of non-plastic fines or fines with low plasticity. It is typically described
as poorly graded sand-silt mixtures and belongs to SM group. It contains 32.91% of sand,
43.73% of silt and 13.65% of clay particles.
88
Chapter 4: Results and Discussion
The United States classification systems (USCS) of soils do not classify the coal ashes
satisfactorily because of its non-plastic nature. Hence geotechnical classification system
developed for the purpose has been followed in this investigation (Sridharan and Prakash,
2007). The fly ash belongs to non-plastic inorganic coarse silt sized fractions (MLN) group
as it contains more than 50% of fines is in the range 20 μm to 75 μm.
The particle sizes
corresponding to the 10% finer are 6.4 µm and that to 60% finer is 68 µm. The coefficient of
uniformity (Cu = D60/D10) as calculated is 10.62. The fly ash used is of medium graded soil.
Specific surface area is a measure of the fineness of the material which influences the
reactivity with other ingredients. The specific surface areas of fly ash particles and over
burden materials are measured by Blaine’s Air permeability method. The respective surface
area of fly ash is 458 m2/kg and that of over burden material is 943 m2/kg. The low value of
fly ash is due to more percent of silt particle as compared to that in overburden material
(Sridharan and Prakash, 2007).
Consistency limits as liquid limit (LL), plastic limit (PL), plasticity index (PI) and
shrinkage limit are important factors in material identification and classification. These
parameters reflect a few geotechnical problems as swelling potential and workability. The
respective values of LL for fly ash and overburden are 30.7% and 25.7% respectively.
Though both values are close overburden material is less workable than that for fly ash. Lime
in general is suitable for soils with plasticity index (PI) ≥ 20 % (Anon, 1985).
The
overburden material has PI of about 15% and hence is not suitable to be modified by lime
addition for stabilization. However stabilization can be achieved by addition of fly ash that
are pozzolanic.
The tests also confirm that fly ash is non-cohesive and has negligible
shrinkage index thus better suited to geotechnical application. Free swell index of the fly ash
89
Chapter 4: Results and Discussion
is found to be negligible due to flocculation which confirms to that reported elsewhere
(Pandian, 2004).
Table 4.1: Physical properties of fly ash and mine overburden
Property
Fly ash
Overburden
Specific gravity
2.16
2.6
Gravel (>4.75 mm)
----
9.71
Sand (4.75 mm – 0.075 mm)
22.17
32.91
Silt (0.075 mm – 0.002 mm)
75.04
43.73
Clay (<0.002 mm)
2.79
13.65
458
943
Liquid limit (%)
30.75
25.7
Plastic limit (%)
Non-plastic
15.04
Shrinkage limit (%)
----
13.44
Plasticity index (%)
----
10.66
Free swell index (%)
Negligible
20
Particle size analysis (%)
Specific Surface Area (m2/kg)
Consistency limits
Figure 4.1: Grain size distribution curves of fly ash and mine overburden
90
Chapter 4: Results and Discussion
4.2.2 Chemical Properties
The chemical composition of fly ash, overburden and lime are important indicators of
suitability of a material for geotechnical applications. The chemical compositions of mine
overburden and fly ash are presented in Table 4.2. The chemical composition of fly ash
indicates that it has not only less calcium content but also the quantity of SiO2+Al2O3+Fe2O3
exceed 70%. Thus, it is classified as “Class F” fly ash as per ASTM 618 specifications. The
fly ash and mine overburden consist mostly of silica (SiO2), alumina (Al2O3) and iron oxide
(Fe2O3). Oxides of calcium, magnesium, potassium, titanium, sodium are also present in
small quantities. The major constituent of lime is calcium oxide (75.82%).
The morphology of fly ash indicates the presence of glassy solid spheres, hollow
spheres (cenospheres), rounded and smooth porous grains as well as irregular agglomerates
((Figure 4.2 (a)). These particles affect the compaction behaviour (Leonards and Bailey,
1982; De Santayana and Mazo, 1994).
The micrograph is without any formation of
cementitious compound. It confirms that the fly ash used in the investigation has low calcium
content. It compares favourably to those observed elsewhere (Baker and Lagurous, 1985;
Krishna, 2001).
The morphology of overburden indicates the presence of irregular shape solid particles
(Figure 4.2 (b)). The X-ray diffraction profiles of the overburden and fly ash indicate the
presence of crystalline phases (Figure 4.3 (a and b)). It is noticed from the XRD analysis that
Kaolinite and Quartz are the major mineral constituents of the overburden. The major mineral
constituents of fly ash are Quartz, Silliminate and Mullite. Quartz is the most prominent
mineral present in both overburden and fly ash materials. It peaks at 950 counts/s near 2θ =
26.660 for fly ash samples and at 450 counts/s near 2θ = 26.66 0 for overburden samples.
91
Chapter 4: Results and Discussion
The pH of overburden and fly ash are found to be 4.85 and 7.2. The pH values
indicate that fly ash is alkaline and overburden is acidic depending on alkaline oxide content
and free lime content. The silica lime reaction is pH dependent. The higher the pH, the better
is the solubility of silica and lime-silica reaction in producing pozzolanic products. The pH of
the solution increases if presence of lime is in excess of the amount required for the silica to
react. But addition of lime in excess than that required for the reactions makes pH constant as
the solution becomes saturated (Sivapullaiah et al., 1995).
(a)
(b)
Figure 4.2: Scanning electron micrograph of (a) mine overburden material and (b) fly ash
92
Chapter 4: Results and Discussion
(a)
(b)
Figure 4.3: X-ray diffractogram of (a) mine overburden and (b) fly ash
93
Chapter 4: Results and Discussion
Table 4.2: Chemical compositions of fly ash, mine overburden and lime (wt. %)
Constituents
Fly ash
Mine overburden
Lime
SiO2
50.88
49.8
0.92
Al2O3
34.78
28.49
0.29
Fe2O3
6.31
8.32
0.45
CaO
0.52
1.09
75.82
K2O
1.42
0.39
--
MgO
0.51
1.23
3.94
TiO2
2.95
0.69
--
Na2O
0.2
--
--
P2O5
--
--
0.04
SO3
--
--
0.55
LOI
2.4
10
17.99
4.2.3 Engineering Properties
The engineering properties of a material such as compressive strength, tensile strength,
CBR, permeability etc. is dependent on the moisture content and density at which the
composite is prepared. Typically the higher the compaction the better is its geotechnical
characteristics. Hence it is necessary to achieve the desired degree of compaction which is
necessary to meet the expected properties (Nicholson et al., 1994).
Compaction is the process of increasing the density of material by the application of
mechanical energy such as tamping, rolling and vibration. It is achieved by forcing the
particles closer with a reduction in air voids. Optimum moisture content (OMC) is the
moisture content at which compacted material reaches the maximum dry density of solid
particles. The compaction characteristics of ingredients as fly ash and mine overburden were
carried out to determine the optimum moisture content and maximum dry density (Figure
94
Chapter 4: Results and Discussion
4.4). The maximum dry density of fly ash was lower than that obtained for mine overburden
due to low specific gravity and non-cohesive in nature. Fly ash has higher optimum moisture
content due to the fact that particles themselves are hollow or cenospheres and holds a
considerable quantity of water internally. The compaction curve of fly ash is almost flat and
indicates it’s insensitive to moisture variation. It confirms to similar observations reported
elsewhere (Sridharan et al., 2000; Das and Yudhbir, 2006; Sridharan and Prakash, 2007). The
maximum dry density of fly ash is less than that of overburden due to less iron content and
specific gravity of the fly ash. The MDD of overburden and fly ash were 2040 kg/m3 and
1396 kg/m3 respectively (Table 4.3).
Corresponding values for OMC were 8.15% and
20.06% respectively. The low density value of fly ash was due to the high moisture content,
unlike the overburden which has less water content.
The permeability of overburden and fly ash are 3.06×10-6 cm/s and 1.01×10-5 cm/s
respectively (Table 4.3). Both represent low permeability though fly ash exhibit more value
than that of overburden. This phenomenon is because of uniform particle sizes in fly ash
where as overburden grains are heterogeneous.
The shear strength parameters of compacted overburden and fly ash are represented in
Table 4.3. As the sand and clay contents of overburden are more than that of fly ash, the
cohesion and angle of internal friction of overburden are relatively higher than that of fly ash.
Angle of repose is the maximum inclination of the sloping surface of a material exhibiting the
limiting stability. The angle of repose of over burden material is close to its angle of internal
friction value. It is very low for fly ash as fly ash exhibits pseudo-cohesion, even in its dry
state by virtue of its fineness.
95
Chapter 4: Results and Discussion
Unconfined compressive strength (UCS) of a material is its resistance to any
externally applied load. It reflects inter granular cohesion as well as strength of cementing
material holding those grains. The UCS values of fly ash and overburden material are 142
KPa and 313 KPa respectively (Table 4.3). The value of overburden is more than twice to
that of fly ash due to presence of high quantity binding material (CaO) as compared to fly ash.
The California Bearing Ratio values for overburden and fly ash materials are almost
same in unsoaked condition at around 22%. But the values reduce drastically at soaked
condition. Over burden exhibited value at 2.95% and fly ash at 0.72%. Both these values are
less than 3 % that make the materials unsuitable for sub-base application (Bowels, 1992).
Table 4.3: Engineering properties of overburden and fly ash
Property
Overburden
Fly ash
(a) Maximum dry density (kg/m3)
2040
1396
(b) Optimum moisture content (%)
8.15
20.06
3.06E-06
1.01E-05
(a) Cohesion (kPa)
58.09
39.35
(b) Angle of internal friction
36.230
28.450
4. Angle of repose
33.770
120
5. Unconfined compressive strength (kPa)
313.91
142.76
(a) Unsoaked condition
23.65
22.42
(b) Soaked condition
2.95
0.72
1. Compaction characteristics
(from Heavy compaction or Modified Proctor test)
2. Permeability (cm/sec)
3. Shear strength parameters
6. CBR value (%)
96
Chapter 4: Results and Discussion
Figure 4.4: Compaction curves of fly ash and overburden
4.3
Geotechnical properties of developed composite materials
The developed composite materials were subjected to various engineering tests as
compaction behaviour, UCS, CBR, Brazilian tensile strength, ultrasonic pulse velocity and
micro- structural analyses.
4.3.1 Compaction characteristics
The compaction characteristics of untreated composites varied between 1965 kg/m 3 to
1686 kg/m3 (Figure 4.5). It reduced as the quality of fly ash percentage increased in the
composites (Figure 4.6). But the trend is reverse for the optimum level of moisture holding
capacity of the composites. As the fly ash percentage was increased the values for OMC also
increased (Figure 4.7).
97
Chapter 4: Results and Discussion
Figure 4.5: Compaction curves of untreated composites
Figure 4.6: Variation of maximum dry density with fly ash content
Figure 4.7: Variation of optimum moisture content with fly ash content
98
Chapter 4: Results and Discussion
The aim of the investigation was to develop an engineering material with mine
overburden and fly ash stabilized with lime. Accordingly samples were prepared (Table 3.2).
The specimen behaviour changed when lime added in various proportions. The maximum dry
density values of all composite materials decreased and optimum moisture content increased
with increase in lime content (Figures 4.8, 4.9 and 4.10). It confirms to similar observations
for fly ash-soil-lime mixtures (Consoli et al., 2001; Arora and Aydilek, 2005; Kaniraj and
Havanagi, 1999; Krishna, 2001; Sahu, 2005; Das and Yudhbir, 2006; Jadhao and Nagarnaik,
2008). The variation of MDD and OMC values with lime content are given in Appendix.
Figure 4.8: Compaction curves of the composites containing 15, 20 and 25% fly ash
99
Chapter 4: Results and Discussion
Figure 4.9: Compaction curves of the composites containing 30, 35 and 40% fly ash
Figure 4.10: Compaction curves of the composites containing 45 and 50% fly ash
100
Chapter 4: Results and Discussion
4.3.2 California Bearing Ratio behavior
CBR value is used as an index of material strength and its bearing capacity. This
method is well established and popular for design of the base and subbase material for
pavement. In this investigation CBR tests were carried out to characterize the strength and
the bearing capacity of the untreated fly ash-overburden composite as well as lime treated fly
ash-overburden composite materials.
4.3.2.1Effect of curing on the CBR of untreated composites
The variation of CBR with the addition of fly ash to mine overburden for both
unsoaked and soaked conditions were reported (Figure 4.11). The results of penetration test
(CBR test) carried out for different composites exhibit the weakening in soaked conditions as
compared to that of unsoaked conditions (Appendix). The CBR values of untreated fly ashoverburden composite materials ranged from 26% to 45% in unsoaked conditions (Figure
4.12). Similar values in soaked condition are less than 3%. The higher CBR value in
unsoaked condition is due to the capillary forces created at optimum moisture content and
maximum dry density condition in addition to the friction resisting the penetration of the
plunger. However when the samples were tested after 96 hour soaking in water, the CBR
values exhibited very low due to the destruction of the capillary forces (Figure 4.11). Soaked
condition though a conservative estimate, yet considered for worst case scenario.
The
obtained CBR value less than 3% is unsuitable for subgrade material (Bowels, 1992) and
hence need to be stabilized with additives for pavement applications.
The CBR values of composites when tested at different curing periods exhibited little
change over uncured results. The CBR values of 28 days cured samples almost doubled to
101
Chapter 4: Results and Discussion
that at soaked conditions. However in all cases, the CBR values were less than 5% and hence
unsuitable for subbase material (Figure 4.12). Hence it was decided to improve the CBR of
fly ash and mine overburden mixes by using additive.
Figure 4.11: Variation of CBR with the addition of fly ash to mine overburden
Figure 4.12: Effect of curing on the CBR of fly ash and mine overburden composites
102
Chapter 4: Results and Discussion
4.3.2.2 CBR behaviour of lime stabilized fly ash composites
Generally additives like cement or lime improve the strength of the soil or fly ash
(Consoli et al., 2001; Ghosh and Subbarao, 2006; Mackos et al., 2009). CBR values were
determined for fly ash-overburden material stabilized with lime. Lime was added between
2% to 9% to observe the effect of lime. As the lime was added, each composition showed
significant improvement in CBR values. The results represented are for the soaked and cured
conditions. In soaked condition the composite with 15% fly ash exhibited maximum CBR
value 77.08 at 9% lime content (Figure 4.13). However as the lime content increased the
composites with higher fly ash content showed more CBR values (Figure 4.14) thus confining
that availability of silica, alumina added to strength gain over time. The maximum CBR
value obtained was for composite with 40%FA+60%O/B with 3% lime at 28 days curing at
140 MPa (Figure 4.15). The corresponding values for composites with 45% and 50% fly ash
content were almost same. It shows that there exists a maximum limit for fly ash content to
add to the increased CBR value. But the results show curing and fly ash content has strong
effect on the bearing capacity of the composite.
elsewhere for soil with class C fly ash (Ismaiel, 2006).
103
It confirms to the observation found
Chapter 4: Results and Discussion
Figure 4.13: Effect of lime on CBR behavior of composites in soaked condition
Figure 4.14: Effect of lime on CBR behavior of composites at 7 days curing
104
Chapter 4: Results and Discussion
Figure 4.15: Effect of lime on CBR behavior of composites at 28 days curing
It is observed from the results that the CBR values increased from 42% to 149% and
85% to 195% at 7 and 28 days of curing respectively. Thus, increasing in curing period
revealed that increase in percentage of lime increases the CBR value.
This is because
hydration of lime forms calcium silicate hydrate gel and with addition of more and more lime,
more amount of this gets formed. Hence there is a continuous increase in the CBR values
with different curing period. The mine overburden mixed with 30% fly ash and 9% lime
resulted in maximum bearing ratio as compared to that of other mixes at 7 and 28 days curing
respectively. There is a strong binding between the fly ash and mine overburden particles by
the cementitious products and hence higher CBR in the composite. The value compares
favourably with the CBR result of the soil stabilized with fly ash (10% and 20%) and lime
kiln dust (2.5% and 5%) to be 69 to 142 at 7 days curing and greater than 164 at 28 days
curing (Cetin et al., 2010).
105
Chapter 4: Results and Discussion
CBR gain factor is the strength enhancement due to addition of lime in the treated
composites with respect to that of untreated composites. The maximum gain factor was
obtained for composite (40%FA+60%O/B)+9%L at 61.1 and the minimum factor was for the
composite (15%FA+85%O/B)+2%L at soaked condition (Figure 4.16).
The composite
(30%FA+70%O/B)+9%L gained by 19.76 times. The maximum and minimum gain factors at
7 days curing were for (25%FA+75%O/B)+6%L and (50%FA+50%O/B)+2%L composites
with values 56 and 20 respectively (Figure 4.17). The corresponding gain factor for the
composite (30%FA+70%O/B)+9%L was 51.4. At 28 days curing minimum gain was with
the composite (35%FA+65%O/B)+2%L at 22 and maximum at 64.5 was for
(45%FA+55%O/B)+9%L
(Figure
4.18).
The
gain
factor
for
the
composite
(30%FA+70%O/B)+9%L at 28 days was 56. The maximum gain factor values were 42.5, 55,
56.7 and 61 at 2, 3, 6 and 9% lime respectively in soaked condition (Figure 4.16). Similarly,
the maximum gain factors were 45.5, 56, 56.3 and 55 at 7 days curing and 32.5, 48, 53.1 and
64.5 at 28 days curing with varying percentage of lime respectively (Figures 4.17 and 4.18).
Figure 4.16: Influence of Lime in CBR Gain for all composites at soaked condition
106
Chapter 4: Results and Discussion
Figure 4.17: Influence of Lime in CBR Gain for all composites at 7 days curing
Figure 4.18: Influence of Lime in CBR Gain for all composites at 28 days curing
4.3.3 Unconfined compressive strength characteristics
The unconfined compression test is one of the widely used laboratory tests in
pavement and soil stabilization applications. It is often used as an index to quantify the
improvement of materials due to treatment. The results of UCS tests for both untreated and
treated composites are reported and discussed here.
107
Chapter 4: Results and Discussion
4.3.3.1Unconfined compressive strength of untreated composites
The UCS values of all untreated fly ash and overburden compositions immediately
after preparation could not be obtained due to weak development of bonds. The strength
values were less than 0.04 MPa. Marginal increase in corresponding values was observed at
different curing periods and hence those were also not reported here.
4.3.3.2Unconfined compressive strength of treated composites
The compressive strength values changed dramatically with addition of lime and
curing period. At 7 days curing all composites showed more than 0.7MPa strength values
with maximum values at 3% lime exhibited by 15% fly ash and 85% overburden (Figure
4.19). The composite containing 30% fly ash and 70% overburden with 3, 6 and 9% lime
resulted in higher strength (1.58 to 2.04 MPa) as compared to others in 14 days curing period.
The sample with 30% fly ash and 70% overburden exhibited 3.14MPa with 9% lime at 56
days curing (Figure 4.22). It showed availability of additional lime has produced additional
bonding between reactive elements. Each composition exhibited higher strength values with
an increase in lime content and curing periods.
These values are above the minimum
suggested for subgrade quality (Das, 1994).
The composite containing 30% fly ash and 70% mine overburden with 9% lime
exhibited maximum compressive strength as compared to that for other composites at 7, 14,
28 and 56 days of curing (Figures 4.19 – 4.22). Addition of lime improved the strength of fly
ash – overburden composites. Typically the stress values at the base/subbase layers of mine
haul road for 35-170T dumpers are 300 to 650 kPa respectively (Tannant and Kumar, 2000).
The strength achieved in all the mixes in this study is above these values after a period of
curing and hence useful for mine haul road construction.
108
Chapter 4: Results and Discussion
Figure 4.19: Effect of lime on compressive strength of composites at 7 days curing
Figure 4.20: Effect of lime on compressive strength of composites at 14 days curing
109
Chapter 4: Results and Discussion
Figure 4.21: Effect of lime on compressive strength of composites at 28 days
Figure 4.22: Effect of lime on compressive strength of composites at 56 days
All the samples in unconfined compressive loading conditions exhibited shear type failure
(Figure 4.23). Except a few samples all samples failed by shear which reflect the influence of
110
Chapter 4: Results and Discussion
sample and machine characteristics (Singh and Ghosh, 2006). Load bearing capacity and
longitudinal displacement recording were made till failure i.e. peak strength of all the
specimen. The axial strain values could not be recorded for post failure investigation as the
weakened specimen got disintegrated soon after its peak strength. Typical axial stress- strain
behaviour of a sample is shown in Figure 4.24. The Young’s modulus values were computed
for each specimen (Table 4.4).
Figure 4.23: Post failure profiles of a few UCS specimens
111
Chapter 4: Results and Discussion
Figure 4.24: Stress- strain behaviour of a sample
Table 4.4: Young’s modulus values of the fly ash composites for 7, 14, 28 and 56 days curing
Curing Period
7 days
14 days
28 days
56 days
curing
curing
curing
curing
Young's modulus (MPa)
Composites
(15FA+85O/B)+2L
62
98
102.3
123
(15FA+85O/B)+3L
68
67.1
115.1
136
(15FA+85O/B)+6L
55
65.8
130.8
157
(15FA+85O/B)+9L
54.2
137.5
139
166
(20FA+80O/B)+2L
57
87.32
115.6
114
(20FA+80O/B)+3L
59
101.43
108.8
113.4
(20FA+80O/B)+6L
55.24
99.1
136
148
(20FA+80O/B)+9L
62
105
138.8
201
(25FA+75O/B)+2L
60
87.4
98
87.5
(25FA+75O/B)+3L
57.52
84
87.6
141
(25FA+75O/B)+6L
62
97
115.2
131
(25FA+75O/B)+9L
55
87.23
138.3
192
(30FA+70O/B)+2L
48.24
100
108.5
127
(30FA+70O/B)+3L
60
112
122
186
(30FA+70O/B)+6L
57.1
122.3
140.5
164
112
Chapter 4: Results and Discussion
4.3.4
(30FA+70O/B)+9L
69.3
140
142
183
(35FA+65O/B)+2L
49
50.3
51.5
112
(35FA+65O/B)+3L
48.7
76.8
95.8
114
(35FA+65O/B)+6L
62
105
133.4
127
(35FA+65O/B)+9L
57.4
123
141
166
(40FA+60O/B)+2L
39.5
59.16
83.5
100
(40FA+60O/B)+3L
43.8
87.3
90.8
116
(40FA+60O/B)+6L
43.42
107
120
143
(40FA+60O/B)+9L
51.22
111
121.2
174
(45FA+55O/B)+2L
41.33
45.2
49.2
75
(45FA+55O/B)+3L
48.22
52
69.8
105
(45FA+55O/B)+6L
57
84
89
136
(45FA+55O/B)+9L
53
105
145
156
(50FA+50O/B)+2L
43.52
45.4
52.5
52.2
(50FA+50O/B)+3L
46.2
103
106.3
122.2
(50FA+50O/B)+6L
57.45
104
106
165
(50FA+50O/B)+9L
62
119
157
192
Brazilian Tensile strength characteristics
The tensile strength is a vital parameter to evaluate the suitability of the stabilized soil
or fly ash as road base material. Tensile strength is an important property to predict the
cracking behaviour of pavement, structures using stabilized soils (Baghdadi et al., 1995). In
the present study split tensile test was conducted on specimen to characterize the tensile
strength and the cracking behaviour of lime treated fly ash–overburden material as described
in section 2.3. The tensile strength of untreated fly ash, untreated overburden as well as
untreated fly ash–overburden composite materials was very low and hence not reported here.
However the behavior of composites changed dramatically and values could be recorded as
113
Chapter 4: Results and Discussion
lime was added. The treated fly ash–overburden composite materials at 7 and 14 days
exhibited marginal values due to low strength and hence not reported here. All the specimens
failed more or less at the middle through an induced force which is tensile in nature (Figure
4.25). The failure occurred within 60 to 100 seconds.
At 28 days curing all composites showed more than 50kPa strength values with
maximum values at 9% lime with 30% fly ash and 70% overburden. The sample exhibited
291kPa at 28 days curing (Figure 4.26). Brazilian tensile strength of all the composites was
between 73 to 357 kPa at 56 days of curing (Figure 4.27). The mine overburden mixed with
30% fly ash and 9% lime exhibited maximum tensile strength as compared to that of other
composites at 28 and 56 days of curing respectively.
The strength achieved in all the
composites in this investigation is above these values after a period of curing and hence useful
for mine haul road construction.
Figure 4.25: Post failure profiles of a few Brazilian tensile test specimens
114
Chapter 4: Results and Discussion
Figure 4.26: Effect of lime on tensile strength of composites at 28 days curing
Figure 4.27: Effect of lime on tensile strength of composites at 56 days curing
The Brazilian tensile strength as a percentage of unconfined compressive strength
against curing period is plotted (Figures 4.28 and 4.29). The average value of Brazilian
tensile strength as a percentage of unconfined compressive strength was 8.4% at 28 days and
10% at 56 days. The percentage increased from 5% to 11% at 28 days curing and 6.3% to
115
Chapter 4: Results and Discussion
13% in 56 days curing. The tensile strength was a function of the amount of cementitious
compounds formed, which increased with increase in curing period. It compares favourably
with that observed for the fly ash stabilized with 10% lime only (Ghosh and Subbarao, 2006).
Consoli et al. (2001) observed that the average ratio of Brazilian tensile strength and
unconfined compressive strength increased from 4% at 7 days to about 15% at 180 days for
the mixes containing soil and 25% fly ash stabilized with 4%, 7% and 10% of lime. The
current investigation confirms these observations.
Figure 4.28: Effect of curing period on tensile strength as percentage of unconfined
compressive strength of the composites containing 15, 20, 25 and 30% fly ash
116
Chapter 4: Results and Discussion
Figure 4.29: Effect of curing period on tensile strength as percentage of unconfined
compressive strength of the composites containing 35, 40, 45 and 50% fly ash
4.3.5
Ultrasonic pulse velocity
The velocity of Ultrasonic P (pulse) wave is measured over a distance and it depends
on the quality of transmission, cohesiveness of constituent materials, dampness, presence of
weaknesses as crack, voids, etc. Its accuracy also depends on the homogeneity of the sample.
The P wave tests conducted in all the samples confirm it. The P wave velocities of untreated
fly ash, overburden and composites were not conducted as those did not exhibit any
significant strength values.
The P wave values of treated samples are reported here and analyzed. The ultrasonic
pulse velocities varied in the range of 797 m/s to 1699 m/s for varying curing periods.
Maximum velocity values were obtained at 56 days curing, thus confirming the increased
conductivity in the samples. The conductivity is a result of enhanced pozzolanic activities
117
Chapter 4: Results and Discussion
due to enhanced reactivity of calcium, aluminium, silicon which is a time dependent
behaviour. The increase in P wave velocity was steep between 2 to 3% lime content at 7
days, 14 days and 28 days curing period (Figures 4.30, 4.31 and 4.32). The rise is marginal at
56 days curing, though the values are much higher (Figure 4.33). The maximum P wave
velocity values were obtained for composite with 30% fly ash+70% overburden and 9% lime
which is similar to the results obtained for UCS, UTS and CBR tests. P wave velocities
obtained at 7, 14, 28 and 56 days curing ranged from 797 to 1170 m/s, 1012 to 1350 m/s,
1057 to 1553 m/s and 1187 to 1699 m/s respectively. There was a continuous increase in the
UPV with time for all the composites. Addition of lime improved the pulse velocity of fly ash
composites. The P wave velocity signal plot of fly ash composite is shown in Figure 4.34.
Figure 4.30: Effect of lime on pulse wave velocity of fly ash composites at 7 days curing
118
Chapter 4: Results and Discussion
Figure 4.31: Effect of lime on pulse wave velocity of fly ash composites at 14 days curing
Figure 4.32: Effect of lime on pulse wave velocity of fly ash composites at 28 days curing
119
Chapter 4: Results and Discussion
Figure 4.33: Effect of lime on pulse wave velocity of fly ash composites at 56 days curing
Figure 4.34: A typical P wave velocity signal plot of fly ash composite
The Poisson’s ratio is a measure of the behaviour of material under loading. The
ultrasonic tests produced the Poisson’s ratio between 0.30 and 0.41 (Table 4.5) except a few
composites with 0.24 to 0.29. The Poisson’s ratio values of each composite did not change
significantly either with higher lime content or longer curing periods which are the typical
characteristics of any material.
120
Chapter 4: Results and Discussion
Young’s modulus (E) values were also obtained from nondestructive testing. Its values
ranged between 776 to 3107 MPa (Table 4.6). These values are very high compared to that
obtained from static tests. Nondestructive tests do not cause either generation or extension of
flaws unlike static tests. The Young’s modulus (E) values increased with lime content as well
as curing periods confirming to enhance pozzolanic activities resulting in higher stiffness of
the composites. The velocity of propagation increases with increased stiffness of the material
(Yesiller et al., 2000).
Tables 4.5: Poisson’s ratios of the composites for 7, 14, 28 and 56 days
Sample
Curing
Fly ash
Overburden
period
(% weight)
(% weight)
(days)
15
20
25
30
35
85
80
75
70
65
Lime content (% weight)
2
3
6
9
Poisson’s ratios
7
0.35
0.33
0.34
0.33
14
0.35
0.32
0.35
0.35
28
0.34
0.34
0.36
0.35
56
0.31
0.39
0.35
0.29
7
0.33
0.33
0.29
0.27
14
0.37
0.37
0.36
0.36
28
0.37
0.35
0.37
0.39
56
0.37
0.38
0.38
0.33
7
0.35
0.34
0.24
0.3
14
0.36
0.29
0.41
0.38
28
0.37
0.36
0.36
0.35
56
0.39
0.35
0.38
0.35
7
0.33
0.36
0.37
0.28
14
0.31
0.36
0.36
0.26
28
0.34
0.37
0.37
0.35
56
0.39
0.3
0.35
0.35
7
0.35
0.34
0.32
0.34
121
Chapter 4: Results and Discussion
40
60
45
55
50
50
14
0.37
0.3
0.37
0.33
28
0.34
0.29
0.36
0.32
56
0.31
0.4
0.34
0.36
7
0.3
0.33
0.32
0.32
14
0.3
0.32
0.35
0.35
28
0.29
0.32
0.34
0.36
56
0.4
0.36
0.38
0.34
7
0.31
0.37
0.37
0.37
14
0.35
0.38
0.36
0.34
28
0.31
0.35
0.34
0.33
56
0.32
0.31
0.3
0.3
7
0.36
0.36
0.35
0.37
14
0.31
0.32
0.39
0.36
28
0.33
0.33
0.38
0.33
56
0.31
0.27
0.31
0.34
Tables 4.6: Young’s (dynamic) modulus values of the composites at 7, 14, 28 and 56 days
Sample
Curing
Fly ash
Overburden
period
(% weight)
(% weight)
(days)
15
20
25
85
80
75
Lime content (% weight)
2
3
6
9
Young’s moduli (MPa)
7
1501
1644
1679
1740
14
1528
1855
1794
1893
28
1755
1866
2291
2536
56
1904
2358
2992
3190
7
1353
1512
1786
1893
14
1240
1435
1908
1931
28
1605
1909
2111
2298
56
1617
1810
2354
2727
7
1212
1328
1691
1676
14
1098
1793
1501
1574
122
Chapter 4: Results and Discussion
30
35
40
45
50
4.3.6
70
65
60
55
50
28
1740
2081
2306
2395
56
1784
2354
2455
2927
7
1066
1230
1215
1732
14
1539
1857
2085
2633
28
1803
1879
2102
2822
56
1921
2645
2974
3107
7
1084
1287
1532
1440
14
1108
1755
1427
1686
28
1759
2075
1895
2200
56
2713
2146
2454
2573
7
1092
1173
1413
1512
14
1426
1438
1514
1766
28
1528
1634
1876
1999
56
2036
2331
2733
2829
7
776.6
915.1
1111
1061
14
1187
1300
1436
1693
28
1296
1439
1532
1731
56
1628
1987
2116
2781
7
954.8
974.3
1037
1114
14
1257
1302
1320
1354
28
1532
1237
1507
1985
56
1550
1801
1910
2269
Microscopy analysis
It is observed that the glassy portions of the micro-particles of the composite are
potentially attacked by lime. The micrographs show the development of hardened paste at
different stages of pozzolanic activity. Particles exhibit evidence of corrosion and etching on
surface. There was evidence of formation of ettringite rods (Appendix). It confirms to the
observation that during early stages, the reactive particles in the fly ash-overburden-lime
123
Chapter 4: Results and Discussion
composite served as nucleation sites for hydration and pozzolanic reaction products (C-S-H,
CH and Ettringite) (Lav and Lav, 2000).
The cementitious compounds formed around the fly ash and overburden particles are
shown in Figure 4.35(a), (b) and (c). The composite ((30%FA + 70%O/B)+9%L) at 28 days
exhibited dense gel - like mass covering all reactive particles completely and filling up the
inter-particle spaces with blurred grain boundaries (Figure 4.35(c)). It appears like a massive
unit compared to the other composites. The dense gel acted as a binding substance and
appears to be evenly distributed to form compact structure, thus creating more contact and
higher cohesion that in turn reflects in greater strength values. It was observed from static
laboratory tests that all samples exhibited maximum strength values at 28 days. So its SEM
analysis was carried out to understand the microstructural aspects.
Cetin et al. (2010)
reported that the CBR of lime kiln dust amended soil-fly ash mixtures increase with
increasing curing time due to the formation of calcium silicate hydrate (CSH) and calcium
aluminate silicate hydrate gels (CASH) around soil and fly ash particles.
Figure 4.35 (a): SEM photograph of (15FA+85O/B) +2L
124
Chapter 4: Results and Discussion
Figure 4.35 (b): SEM photograph of (30FA+70O/B) +6L
Figure 4.35 (c): SEM photograph of (30FA+70O/B) +9L
4.3.7
Energy dispersive X-ray analysis
EDX is an analytical technique for elemental analysis or chemical characterization.
The EDX analyses of all the mixes were carried out to observe the effect of hydration at 28
days of curing. There were variations in elemental composition in various mixes (Table 4.7).
In all mixes calcium content increased due to increase in lime content. Alumina content also
increased with increase in fly ash content in the mixes. But, increase in lime content reduces
the alumina percentage in all mixes. The silica content decreased with increase in fly ash
125
Chapter 4: Results and Discussion
content at 3 and 6% of lime in the mixes. But, the silica content was more in 20% of fly ash
with 2 and 9% of lime. These results confirm to similar observation reported elsewhere
(Cetin et al., 2010). All the mixes contain very small or negligible percentages of Na, Mg, P,
S, Mn, Cu, Ba and Mo elements. The chemical composition of all the mixes indicates that
they contain high percentages of silica (SiO2), alumina, iron and calcium oxides as well as
small percentages of other elements. Among all these oxides, calcium oxide (CaO) is very
reactive. In the presence of aqueous solution, calcium oxide undergoes hydration. The
formation of calcium silicate hydrate (CSH) gel and calcium aluminate hydrate (CAH) gel
leads to increase in calcium content. The effectiveness of the lime treatment depends on the
quality and quantity of lime as well as the chemical composition of the soil/fly ash. The
strength developed is obviously influenced by the quantity of cementitious gel produced and
consequently by the amount of lime consumed (Abdelmadjid and Muzahim, 2008).
CaO/SiO2 is a good indicator of pozzolanic reactions which yields higher strength
values (Janz and Johansson, 2002). The CaO/SiO2 and CaO/(SiO2 + Al2O3) ratios ranged
from 0.03 to 1.06 and 0.02 to 0.59 respectively in the composites. The composite containing
30% fly ash and 70% overburden treated with 9% lime gave CaO/SiO 2 and CaO/(SiO2 +
Al2O3) ratios of 1.06 and 0.59 which exhibited maximum strength value. The CaO content is
more (i.e 27%) in the composite containing 30% fly ash and 70% overburden treated with 9%
lime as compared to other composites. The strength values increased with increase in CaO
content, CaO/SiO2 and CaO/(SiO2 + Al2O3) ratios reported elsewhere (Cetin et al., 2010).
126
Chapter 4: Results and Discussion
Table 4.7: Chemical compositions of the composites, cured for 28 days
Elements in (15FA+ (15FA+(15FA+ (15FA+ (20FA+ (20FA+ (20FA+ (20FA+ (25FA+ (25FA+(25FA+(25FA+ (30FA+ (30FA+ (30FA+ (30FA+
oxide form 85O/B) 85O/B) 85O/B) 85O/B) 80O/B) 80O/B) 80O/B) 80O/B) 75O/B) 75O/B)75O/B) 75O/B) 70O/B) 70O/B) 70O/B) 70O/B)
+2L
+3L
+6L
+9L
+2L
+3L
+6L
+9L
+2L
+3L
+6L
+9L
+2L
+3L
+6L
+9L
Na
1.71
0
0.28
0.97
0.11
0
0.7
0.01
0.05
0
0.04
0
0
0.4
0.5
0.15
Mg
1.66
0.09
0.43
0.79
0.96
1.23
0.93
0.46
0.92
0
1.41
0
1.01
1.3
1.68
0.59
Al
26.3
21.5
20
13.2
26.7
24
22.5
18.3
30
27.7
19
23.4
30.2
25
22.8
20.4
Si
38.8
50.3
37.8
36.5
43
42.7
36.7
38.2
41.3
34
32.7
22.8
42.6
41
34.2
25.5
P
0.35
0
0.03
0
0.5
0.33
0
0
0.18
0.33
0
0
0
0.8
0.17
0.34
S
0.08
0.55
0.33
0
0.48
0
1.08
0.95
0
0.71
0.06
0
0.41
1.2
0.26
0.5
K
0.07
2.97
3.82
1.26
2.24
1.34
2.28
0.65
2.24
2.34
1.25
1.82
2.05
2.3
1.99
1.21
Ca
3.29
4.36
6.59
16.2
3.44
6.28
14.4
19.7
3.46
13
19.5
22.6
4.31
8.5
15.6
27
Ti
1.98
0
1.27
1.74
3.05
1.08
0.89
0.2
3.2
7.4
2.7
3.74
2.22
0.5
0.51
0.97
Mn
0
0
0.5
0.29
0
0
0
0.12
0
0.35
0
1.76
0
0
0.91
0.48
Fe
10.1
5.03
10.7
1.85
4.5
6.25
4.2
3.9
4.57
2.03
7.68
8.1
8.39
6.9
2.15
4.93
Cu
2.06
0
2.79
6.49
1.51
0
1.58
1.08
0.67
0.47
1.37
0
0
0
3.65
1.61
Zn
1.96
2.23
1.05
1.11
1.32
2.07
1.75
0.93
0.63
1.87
0.58
2.75
0.88
2.6
3.76
3.86
Ba
0.75
1.67
0
1.9
2.02
2.64
0.35
1.8
3.23
0
2.19
0
0
1.4
2.14
2.28
Mo
0
0
0.73
2.98
0
1.63
0
0
0
0
0.04
0
0
0
0
0
LOI
10.9
11.3
13.6
14.8
9.9
10.4
12.7
13.9
9.6
9.69
11.3
13
7.89
7.8
9.6
10.2
CaO/SiO2
0.08
0.09
0.17
0.44
0.08
0.15
0.39
0.52
0.08
0.38
0.6
0.99
0.1
0.21
0.46
1.06
0.05
0.06
0.11
0.33
0.05
0.09
0.24
0.35
0.05
0.21
0.49
0.38
0.06
0.13
0.27
0.59
CaO/(SiO2
+ Al2O3)
127
Chapter 4: Results and Discussion
4.3.8
X-ray diffraction analysis
The mineralogical analyses of the composites are very important to determine the
changes in the mineralogical phases due to pozzolanic reactions. The formation of reaction
products such as calcium silicate hydrate (CSH), calcium aluminate hydrate (CAH) and
calcium aluminate silicate hydrates (CASH) confirmed from X-ray diffraction analysis
(Figures 4.36). These new cementitious compounds induce aggregation effect in fly ash and
overburden particles, bind the particles together to form crumbs of fly ash-overburden clusters
and result in the overall improved behaviour of the composites.
The X-ray diffraction
analysis of the selected composite ((30%FA+70%O/B) +9%L) show the influence of lime
content on the hydration products (CAH, CSH, CASH). The intensity increased with increase
in lime content, maximum being at 9% lime. The peaks for the composite vary with the lime
contents, minimum at 2% and maximum observed at 9% (Figure 4.36). Quartz, the primary
mineral present in fly ash is indicated by sharp peaks.
Cementing compounds such as CSH, CAH and CASH were identified in 3% cement
stabilized fly ash only and fly ash – black cotton soil mixes after 28 days curing by XRD
analysis (Krishna, 2001). The strength development is also dependent on the amount of
hydration products as well as their interlocking mechanisms (Lav and Lav, 2000). X-ray
diffraction patterns of all composites are given in Appendix.
128
Chapter 4: Results and Discussion
Figure 4.36: XRD patterns of (30FA+70O/B) stabilised with 2, 3, 6 and 9% lime at 28 days
4.3.9
Leachate characteristics
The concentration of metals in leachate of all the treated samples were analyzed for
Ni, Cr, Pb, Cu, Zn, As, Hg, Cd and Se on 7th day of flow. The presence of metals As and Hg
were below the detection level (Table 4.8). Threshold value for maximum contaminant level
is considered as 100 times the allowable limit reported elsewhere (IS 10500; Gidley and Sack,
1984; Ghosh and Subbarao, 1998). It is observed that the leachate effluents contain Ni, Cr,
Pb, Cu and Zn between 0.002 to 1.1 ppm. Concentrations of Ni and Pb were above the
allowable limits of drinking water quality for some of the composites. Concentration of Cr
was above the allowable limits for the composites. The concentrations of Cu and Zn in the
leachate were below the allowable limit for drinking water quality. It is observed from the
leachate effluents that concentration of Cadmium is in between 0.001 to 0.009 ppm. The
concentration of Selenium was below the detection level. However, the concentrations of all
129
Chapter 4: Results and Discussion
the metals were below the threshold limits. These results compare favourably with those
obtained elsewhere (Ghosh and Subbarao, 1998) for high (4 % to 10 %) lime and gypsum (0.5
% to 1 %) to content with only fly ash.
Table 4.8: Leachate concentration (ppm) at 7 days curing period
Leachate concentration (ppm)
Mix
Ni
Cr
Pb
Cu
Zn
Allowable limits
0.02
0.05
0.05
1
5
Threshold limits
2
5
5
100
500
(15FA+85O/B)+2L
0.025
0.312
0.116
0.05
0.108
(15FA+85O/B)+3L
0.028
0.415
0.208
0.078
0.245
(15FA+85O/B)+6L
0.204
0.64
0.681
0.245
0.160
(15FA+85O/B)+9L
0.21
0.642
0.684
0.248
0.191
(20FA+80O/B)+2L
0.004
0.081
0.002
0.033
0.08
(20FA+80O/B)+3L
0.006
0.1
ND
0.068
0.162
(20FA+80O/B)+6L
0.011
0.72
0.704
0.156
0.355
(20FA+80O/B)+9L
0.015
0.728
0.706
0.159
0.36
(25FA+75O/B)+2L
0.01
0.61
0.006
0.023
0.155
(25FA+75O/B)+3L
0.231
0.656
ND
0.058
0.237
(25FA+75O/B)+6L
0.116
0.735
0.385
0.214
0.535
(25FA+75O/B)+9L
0.121
0.731
0.388
0.218
0.539
(30FA+70O/B)+2L
0.102
0.501
0.064
0.129
0.26
(30FA+70O/B)+3L
0.107
0.542
0.075
0.278
0.490
(30FA+70O/B)+6L
0.137
0.529
0.194
0.085
0.053
(30FA+70O/B)+9L
0.14
0.545
0.211
0.112
0.1
(35FA+65O/B)+2L
0.05
0.009
0.071
0.056
0.351
(35FA+65O/B)+3L
0.071
0.125
0.095
0.134
0.538
(35FA+65O/B)+6L
0.014
0.416
0.3
0.046
0.109
(35FA+65O/B)+9L
0.035
0.424
0.315
0.1
0.46
(40FA+60O/B)+2L
0.024
0.445
0.072
0.083
0.45
130
Chapter 4: Results and Discussion
(40FA+60O/B)+3L
0.078
0.491
0.077
0.189
0.504
(40FA+60O/B)+6L
0.164
0.619
0.147
0.097
1.031
(40FA+60O/B)+9L
0.142
0.625
0.154
0.182
1.1
(45FA+55O/B)+2L
0.13
0.634
0.24
0.053
0.4
(45FA+55O/B)+3L
0.152
0.669
0.705
0.065
0.435
(45FA+55O/B)+6L
0.180
0.823
0.693
0.122
0.578
(45FA+55O/B)+9L
0.184
0.825
0.711
0.13
0.583
(50FA+50O/B)+2L
0.147
0.803
0.15
0.048
0.65
(50FA+50O/B)+3L
0.258
0.845
0.345
0.066
0.842
(50FA+50O/B)+6L
0.346
0.855
0.562
0.071
0.640
(50FA+50O/B)+9L
0.348
0.858
0.567
0.081
0.8
ND: Not detected
4.4 Development of empirical models
A part of the objectives is to develop model equations for the investigation with the
parameters like unconfined compressive strength, tensile strength, CBR, CaO/SiO2,
CaO/(SiO2 + Al2O3) and P-wave velocity. Those are reported for the best fit correlations as
below.
4.4.1
Relation between Tensile Strength, CBR and UCS
The investigation involved 1760 specimens for various parametric determinations.
Each parameter has been discussed separately earlier. A few empirical models have been
developed to establish mutual relationships between UCS and Brazilian tensile strength and
CBR and UCS.
The variation of tensile strength and California bearing ratio with unconfined
compressive strength are shown in Figures 4.37 and 4.38. The data are analyzed using linear
regression model by the method of least squares. There exists relation between compressive
131
Chapter 4: Results and Discussion
strength of fly ash/lime and fly ash/lime and gypsum mixes with chemical composition, loss
on ignition, CBR and tensile strength using power model (Das and Yudhbir, 2006; Ghosh and
Subbarao, 2006).
It is observed that linear regression model suits for the fly ash and
overburden mixes stabilized with lime. It is observed from the relationship between tensile
strength and compressive strength that R2 value is more at 56 days than that at 28 days curing
(Figure 4.37(b)). It confirms that the relationship between tensile strength and compressive
strength become stronger with increasing curing period. Similar results were also observed
between CBR and unconfined compressive strength (Figure 4.38(b)). The results of linear
regression model between California bearing ratio values, unconfined compressive strength
and tensile strength at different curing period are reported (Table 4.9).
Figure 4.37 (a): Relationship between Brazilian tensile strength and unconfined compressive
strength for all samples at 28 days of curing
132
Chapter 4: Results and Discussion
Figure 4.37 (b): Relationship between Brazilian tensile strength and unconfined compressive
strength for all samples at 56 days of curing
Figure 4.38 (a): Relationship between bearing ratio and unconfined compressive strength for
all samples at 7 days of curing
133
Chapter 4: Results and Discussion
Figure 4.38 (b): Relationship between bearing ratio and unconfined compressive strength for
all samples at 28 days of curing
Table 4.9: Best fit regression models between California bearing ratio values, unconfined
compressive strength and tensile strength at different curing period
Dependent
Best fit Equation
R2
value
R value No. of
Curing
Observation period
(days)
Brazilian tensile strength, σt
σt = 121.5σc – 61.211
0.8122 0.9012 32
28
Brazilian tensile strength, σt
σt = 127.96σc – 55.752
0.9022 0.9498 32
56
California bearing ratio (CBR)
CBR = 108.38σc – 13.818 0.7202 0.8486 32
7
California bearing ratio (CBR)
CBR = 56.369σc + 39.447 0.7603 0.8719 32
28
4.4.2
Effect of chemical composition on CBR, UCS, tensile strength and Ultrasonic
velocity results
In order to evaluate the effect of chemical composition of the mixtures on observed
CBR, compressive strength, tensile strength and ultrasonic velocity values, the best suit
regression model at 28 days curing were plotted against CaO content, CaO/SiO2 and
CaO/(SiO2 + Al2O3) ratios of the mixes (Figure 4.39) and results were reported in Table 4.10.
134
Chapter 4: Results and Discussion
The best fit curves to the data produced high and modest correlation coefficients (R) for CBR,
UCS, tensile strength and ultrasonic velocity values. It confirms from the linear regression
model that the CaO content, CaO/SiO2 and CaO/(SiO2 + Al2O3) ratios exhibited good
correlation with CBR, UCS, tensile strength and ultrasonic velocity values. Cetin et al.
(2010) observed that the CBR value increased with increasing CaO content as well as with
CaO/SiO2 and CaO/(SiO2 + Al2O3) ratios.
Table 4.10: Best fit regression models at 28 days curing period
Dependent
Best fit Equation
R2 value
R value
No. of
Obs.
CBR
CBR = 3.6743x + 94.234
0.6941
0.8331
16
CBR
CBR = 84.359y + 106.64
0.5828
0.7634
16
CBR
CBR = 162.43z + 102.1
0.6516
0.8072
16
UCS
UCS = 0.049x + 1.2989
0.7091
0.842
16
UCS
UCS = 1.1443y + 1.457
0.6166
0.7852
16
UCS
UCS = 2.1146z + 1.4146
0.635
0.7968
16
Tensile strength (UTS)
UTS = 8.4767x + 50.41
0.7489
0.8653
16
Tensile strength (UTS)
UTS = 198.75y + 77.539
0.6558
0.8098
16
Tensile strength (UTS)
UTS = 368.57z + 69.885
0.6801
0.8246
16
Ultrasonic velocity (UV)
UV = 13.927x + 1215.8
0.7797
0.8830
16
Ultrasonic velocity (UV)
UV = 309.17y + 1266.7
0.6121
0.7823
16
Ultrasonic velocity (UV)
UV = 604.77z + 1248
0.7063
0.8404
16
Note: x: CaO; y: CaO/SiO2; z: CaO/ (SiO2 + Al2O3)
135
Chapter 4: Results and Discussion
Figure 4.39 (a): Effect CaO content, CaO/SiO2 and CaO/(SiO2 + Al2O3) ratios on CBR and
compressive strength values
136
Chapter 4
Figure 4.39 (b): Effect CaO content, CaO/SiO2 and CaO/(SiO2 + Al2O3) ratios on tensile
strength and ultrasonic velocity value
137
Chapter 5: Numerical Investigation
CHAPTER 5
NUMERICAL INVESTIGATION
5.1
General
Availability of high speed computer has facilitated and enhanced the simulation of
different conditions and situations. There exist many approaches as Finite element method
(FEM), Finite boundary method (FBM), Finite difference method (FDM) for designs of
structural elements in all field of engineering. FEM is very popular because of its flexibility
to adapt variables as irregular geometric shapes and form, unusual loading conditions, varying
material characteristics, different field conditions, etc. It is now gaining wide acceptance in
other disciplines like thermal analysis, fluid mechanics, electromagnetic etc. It is a numerical
analysis technique to obtain the stress, strain and deflection of haul road pavement layers. It
is a mathematical method of converting governing differential equations into a system of
linear algebraic equations which is solved by using matrix techniques. Finite element method
handles irregular and complex geometries, linear and nonlinear problems, non homogenous
and anisotropic, structural interactions, various boundary conditions and multiple loading
conditions.
Analytical method usually uses elastic modulus and Poisson’s ratio of the
pavement materials as design parameters.
This method involves identification of the problem, its geometric and material
characteristics and development of model with the Finite Element Software. The problem
geometry is created using a CAD system or Geometric Modeler and then the material
138
Chapter 5: Numerical Investigation
properties, boundary conditions either on FE Model or Geometric Model and loads on the
model are assigned. Meshing is done to discretize the model elements. Then the solution is
carried out and evaluated in terms of stress, strain and temperature etc.
The first FEM analysis of a pavement structure was reported with an axisymmetric
formulation and specified the stiffness of each element in the granular layer as a function of
the stresses in the element (Duncan et al., 1968). The use of a FEM model allows the model
to accommodate the load dependant stiffness of the base/subbase and subgrade materials,
although most models still use linear elastic theory as the constitutive relationship. The FEM
analysis involves two approaches as force based and displacement based. In mining and
allied areas the displacement (or strain) approach is popular (Mishra, 2003).
Numerical simulation of cement of varying percentage stabilized fly ash uses in road
pavement has been observed to exhibit lower strain values (Lav et al., 2006). These exists
numerous reports on both linear and non linear numerical analysis of pavement design using
different materials (Witczak et al., 2004; Kim and Tutumluer, 2007; Kettil et al., 2007; Park,
2008; Kaliske et al., 2008; Nahi et al., 2011).
5.2
Modeling and Boundary condition
FEM divides the structures into a large of pieces of similar behaviour.
It finds
solutions for each piece and then combines the individual to find the result of whole. The
assumption behind the analysis is that structure represents a continuous mass of physical
body. All the analyses were carried out using the code ANSYS 10. It is an event simulation
programme. It is capable of carrying out static and dynamic analysis in both linear and nonlinear criteria. The program has been validated and used extensively for solutions (Stankus
and Gou, 2001; Mishra, 2003).
139
Chapter 5: Numerical Investigation
A haul road represents structural mass experiencing wheel load which is modeled as a
semi-infinite circular loading (Desai and Abel, 1987).
The three dimensional loading
conditions has been simplified as two dimensional modeling and the stress-strain behaviour is
axisymmetric about the axial loading. In this analysis an axisymmetric pavement model
under wheel load and a road profile width of 10m and depth of 5m had been considered to
represent the actual filed condition. A typical of pavement and load on it has been shown in
Figure 5.1. Haul road pavement is a multilayer structure consists of four layers surface, base,
subbase and subgrade. One of the main objectives of the investigation is to evaluate the
performance of haul road by replacing its subbase with the developed fly ash based
composites.
Figure 5.1: A typical haul road pavement under wheel load
The boundary condition of the model of the haul road has been adopted to simulate
field behaviour (Figure 5.2). It is assumed that the flour of the pavement is strong enough not
to undergo any settlement under loading. There is no movement of the bottom surface and
hence all degrees of freedom are constrained in both horizontal and vertical direction. The
tire load is applied on the top of the haul road and hence it is allowed to move freely along the
140
Chapter 5: Numerical Investigation
vertical directions. It is also expected that sides of the haul road is laterally constrained as the
objective was to evaluate the behaviour of subbase in vertical direction as well as the sides of
the pavement would not be excavated.
Figure 5.2: Schematic layout of FEM modelling of haul road
Many engineering problems involve axisymmetric solids subjected to axially
symmetric loading. A circular footing on a soil mass is a semi-infinite half space loaded by a
circular area. Vehicle loading on the pavement represents an axisymmetric problem as shown
in Figure 5.3.
Though the pavement layers are horizontally layered, the stiffness
characteristics of these layers vary in lateral as well as in the vertical direction because of the
stress-dependent nature of the materials. The finite element method is best suited for such
circumstances (Siddharthan et al., 1991). According to the symmetry, both geometrical and
mechanical, around a vertical axis which passes through the loading centre, it is advantageous
to use cylindrical coordinates, z, r and h for analysis. The radial symmetry makes the problem
independent of the coordinate h. Thus, the strains and stresses at any point are only functions
of its vertical and radial coordinates, z and r, respectively. The displacements is described in
terms of the components in the vertical and radial directions v and u. The corresponding
vectors have the following form:
141
Chapter 5: Numerical Investigation
(1)
where, σzz, σrr, σөө and τzr are the stress components.
εzz, εrr, εөө and γzr are the strain components.
According to the hypothesis of small displacements, strains are expressed in terms of
the first derivations of the displacements:
(2)
The constitutive relation between stresses and strains is:
(3)
where, [D] = matrix of elastic constants.
[D] =
where, E = Young’s modulus, μ = Poisson’s ratio.
142
Chapter 5: Numerical Investigation
Figure 5.3: Haul road cross-section under axisymmetry loading
Numerical modeling to determine the stress-strain behaviour of pavement was carried
out using both conventional materials and the developed composite materials. Typically the
construction materials used in haul road are dense sand, gravel, silty sand, sandy clay, clay
shale, medium clay, soft clay, silty clay, etc. The elastic constants of these vary over a wide
range. In this study the higher values (elastic modulus and Poisson’s ratio) of these materials
have been considered for analysis. The Young’s modulus values for conventional material in
respective layers of haul road pavement considered are 200, 100, 50 and 50 MPa (McCarthy,
2007).
The conventional material for subbase was replaced with the composite material
((30%FA+70%O/B) +9%L) exhibiting high strength values among all composites and
analyzed for corresponding strain in the pavement. The respective value for the composite
(Case I) is 142 MPa as determined from laboratory test (Table 5.1). The Poisson’s ratio
considered for analysis is 0.4 (Tannant and Regensburg, 2001; McCarthy, 2007; Lav et al.,
2006). The thickness of various layers of the haul road pavement was considered as per
CMPDI (2000) guidelines.
143
Chapter 5: Numerical Investigation
Table 5.1: Young’s modulus, E (MPa) and Thickness, t (m) of the pavement layers for
different cases
Layer
Construction
Materials
Conventional
Surface
Base
Subbase
Subgrade
E
t
E
t
E
t
E
t
200
0.22
100
0.3
50
0.83
50
Semi-
Material (CM)
infinite
Case I
200
0.22
100
0.3
142
0.83
50
Do
Case II
500
0.5
350
1
142
0.83
50
Do
Case III
500
0.5
350
1
142
1
50
Do
Case IV
500
0.5
350
1
142
1.5
50
Do
The maximum strain at the surface course just below the wheel loading point was
4775 microstrain with the conventional materials (CM) (Figure 5.4). The maximum strain
was 4498 microstrain after replacement of subbase material with the developed fly ash
composite (Case I) thus exhibiting a 6% reduction. The strain at various positions of the
pavement had been determined using both conventional and fly ash composite materials. The
locations were the interface between each layer (below the wheel loading). These are A, B, C
and D representing the position between surface course and base course, base course and
subbase course, subbase course and subgrade and bottom of the subgrade respectively (Figure
5.5). Those positions are 0.5m, 1.5m, 3m and 5m from the top of the surface course. All the
discussion has been made based on the results of those locations.
experienced stress-strain of lesser magnitude.
144
All other location
Chapter 5: Numerical Investigation
Figure 5.4: Maximum strain of haul road pavement with conventional materials
Figure 5.5: Haul road pavement model with various positions in layers
145
Chapter 5: Numerical Investigation
The strain values at different depths from top of haul road pavement model are shown
in Figure 5.6. There is not much reduction in strain values after replacement of developed fly
ash composite. The maximum strain limit at surface course immediately under the wheel load
is 1500-2000 microstrains (Thompson and Visser, 1997; Tannant and Regensburg, 2001). So,
a haul road cannot adequately support haul trucks if strain values exceed the maximum strain
limit.
Figure 5.6: Strain values at different depth of the pavement
Further analysis by varying the thickness of layers (Table 5.1) has been conducted to
evaluate the performance of developed fly ash composite material. The thickness of subbase
layer was varied between 0.83 and 1.5 m for the use of more quantity of fly ash (Table 5.1).
The stress and strain values obtained for 0.83m, 1m and 1.5m thickness of subbase layer were
452.5, 453.5 and 456 kPa and 1008, 1010 and 1016 microstrains respectively. It confirmed
from the simulation that with varying thickness of the subbase layer, there was not much
variation in stress and strain values. Figure 5.7 shows strain values at different depth from top
of haul road pavement model and also stress values at different depth.
146
Chapter 5: Numerical Investigation
Figure 5.7 (a): Strain values at different depth of the pavement with varying subbase thickness
Figure 5.7 (b): Stress values at different depth of the pavement with varying subbase thickness
However the layer with 1.5m thickness offers maximum fly ash consumption
possibilities. Numerical modeling for various fly ash composites were carried out to observe
the stress-strain behaviour of haul road pavement with 1.5m thickness.
147
Chapter 5: Numerical Investigation
It was determined from the laboratory tests that the composite with 30% fly ash and
9% lime produced maximum strength values.
So this composition was selected for its
performance evaluation in replacing the conventional subbase material.
Accordingly
numerical analysis was carried out with the elastic parameters obtained for this composite at
7, 14 and 28 days curing. All the three types of materials exhibited almost equal maximum
strain values (Figure 5.8(a)) at immediately below the tire load (at 0.5m). However at a depth
of 1.5m the strain values using 7 days cured sample was higher than that of using 14 and 28
days cured samples. Respective values are 709 and 502 microstrain. The strain values at 3m
and 5m were almost same for the three specimens. The values were 351 and 317 microstrain.
Similar trend was also observed for the stress vs. depth analysis using the three different
composites (Figure 5.8(b)).
The stress and strain values for the composite at 7days cured condition were less than
the maximum values due to wheel loading (Kumar, 2000).
Further numerical analysis were carried out using the elastic parameters to evaluate
the performance of the composite materials cured at 28 days only as those are comparable to 7
days and 14 days cured behaviour of the best developed composite.
148
Chapter 5: Numerical Investigation
Figure 5.8 (a): Strain values at different depth of the pavement with 1.5subbase thickness with
(30PA+70OB)+9L composite
Figure 5.8 (a): Stress values at different depth of the pavement with 1.5subbase thickness with
(30FA+70OB)+9L composite
The dynamic Young’s modulus values and Poisson’s ratios of the developed
composite material containing 30% fly ash and 70% mine overburden stabilized with 9% lime
determined from Ultrasonic pulse velocity test were used in subbase layer for numerical
modeling (Table 5.2).
The dynamic properties of other layers of the haul road pavement
149
Chapter 5: Numerical Investigation
were adopted from published literature (Lav et al., 2006) (Table 5.2). The static tests involve
destructive approaches where load is applied at an incremental step. During testing flaws,
cracks develop and close till hydrostatic conditions exist beyond which cracks/ flaws keep on
extending till failure occurs.
But in dynamic loading condition, often carried by non-
destructive testing there is little scope for flaws/cracks to change their position and hence the
modulus values obtained are high.
The stress and strain values obtained were very low (Figure 5.9). The maximum strain
observed at the top of the surface course was 228 microstrains at 0.5m below the tire loading.
The minimum strain values 28 microstrains was determined at 3m and beyond.
Corresponding stress values at 0.5m and 3m depth were 512kPa and 12kPa respectively.
These values are very low as compared to that obtained by considering static loading
parameters. Hence the stress/strains related to static loading are only reported here. The
elastic modulus values for respective composites considered are given Table 5.3.
The
Poisson’s value has been kept constant at 0.4.
The stress and strain at different depth vary between 460 to 10 kPa and 1020 to 220
microstrains respectively for all the composites (Figures 5.10 and 5.11). These values are less
than the corresponding values for conventional materials as well as less than the maximum
strain limits experienced (Thompson and Visser, 1997; Tannant and Regensburg, 2001).
Relevant analysis for each material type is given in Annexure.
150
Chapter 5: Numerical Investigation
Table 5.2: Dynamic Elastic parameters and Thickness of the pavement layers
Layer
Thickness, t (m)
Dynamic Elastic parameters of materials
Young’s modulus, E Poisson’s ratio
(MPa)
Surface
0.5
2500
0.4
Base
1
1500
0.4
Subbase
1.5
2822
0.35
Subgrade
Semi-infinite
500
0.45
Figure 5.9 (a): Strain at different depth of the pavement using dynamic elastic parameters
Figure 5.9 (b): Stress at different depth of the pavement using dynamic elastic parameters
151
Chapter 5: Numerical Investigation
Table 5.3: Young’s modulus, E (MPa) of fly ash composites
Young’s modulus,
Composite (s)
E (MPa)
(15%FA+85%O/B) +2%L
102.3
(15%FA+85%O/B) +3%L
115.1
(15%FA+85%O/B) +6%L
130.8
(15%FA+85%O/B) +9%L
139
(20%FA+80%O/B) +2%L
115.6
(20%FA+80%O/B) +3%L
108.8
(20%FA+80%O/B) +6%L
136
(20%FA+80%O/B) +9%L
138.8
(25%FA+75%O/B) +2%L
98
(25%FA+75%O/B) +3%L
87.6
(25%FA+75%O/B) +6%L
115.2
(25%FA+75%O/B) +9%L
138.3
(30%FA+70%O/B) +2%L
108.5
(30%FA+70%O/B) +3%L
122
(30%FA+70%O/B) +6%L
140.5
(30%FA+70%O/B) +9%L
142
(35%FA+65%O/B) +2%L
51.5
(35%FA+65%O/B) +3%L
95.8
(35%FA+65%O/B) +6%L
133.4
(35%FA+65%O/B) +9%L
141
(40%FA+60%O/B) +2%L
83.5
(40%FA+60%O/B) +3%L
90.8
(40%FA+60%O/B) +6%L
120
(40%FA+60%O/B) +9%L
121.2
152
Chapter 5: Numerical Investigation
Figure 5.10 (a): Strain values at different depth of the pavement with composites containing
15 and 20% fly ash as subbase material
Figure 5.10 (b): Strain values at different depth of the pavement with composites containing
25 and 30% fly ash as subbase material
153
Chapter 5: Numerical Investigation
Figure 5.10 (c): Strain values at different depth of the pavement with composites containing
35 and 40% fly ash as subbase material
Figure 5.11 (a): Stress values at different depth of the pavement with composites containing
15 and 20% fly ash as subbase material
154
Chapter 5: Numerical Investigation
Figure 5.11 (b): Stress values at different depth of the pavement with composites containing
25 and 30% fly ash as subbase material
Figure 5.11 (c): Stress values at different depth of the pavement with composites containing
35 and 40% fly ash as subbase material
155
Chapter 6: Summary and Conclusions
CHAPTER 6
SUMMARY AND CONCLUSIONS
In this investigation, potential of lime stabilized fly ash and mine overburden was
evaluated for haul road construction and improved the road condition by reducing strain in the
road pavement through experimental and numerical study. Lime stabilization is widely used
to improve the strength of road building materials. The characterisation of fly ash and its
interaction behaviour with mine overburden and lime is likely to provide viable solutions for
its large scale disposal and utilisation in geotechnical applications as mine haul road
construction. Strength characteristics of any construction material are vital parameters to
judge its suitability. Strength characteristics of the fly ash composites are studied through
different conventional test methods such as CBR, unconfined compressive strength, Brazilian
tensile strength and ultrasonic velocity which cover a broad area of design parameters, useful
for mine haul road application and to understand the engineering behaviour of composite
materials. Microstructural analyses carried out to gain better understanding of the mechanism
of lime, fly ash and overburden interaction. The change in surface morphology and variation
in chemical composition due to formation of hydration products were analyzed through
scanning electron micrographs and energy dispersive X-ray results.
X-ray diffraction
analyses were carried out to identify the hydration production phases. Leaching studies were
carried out to analyze heavy metal concentration in the composites. The numerical studies (2dimenstional finite element modeling) were carried out to study the effectiveness of the
developed composite materials on the stress-strain behaviour of haul road pavement.
156
Chapter 6: Summary and Conclusions
The experimental and numerical investigation results and discussions presented in
previous chapters, following conclusions have been drawn by dividing into two categories:
6.1
1.
Untreated materials
Mine overburden has major chemical constituents of silica, alumina and iron oxide
and mineral constituents of Kaolinite and Quartz and acidic in nature.
2.
The overburden material is poorly graded sand-silt mixtures.
3.
Mine overburden has high maximum dry density and low optimum moisture content
as compared to fly ash. It exhibits minimum CBR value at soaked condition.
4.
It is not amenable to be used as road material with additive alone. It posses low
compressive strength.
5.
The fly ash used for the investigated was class F type.
The major chemical
constituents of fly ash are silica, alumina and iron oxide with major mineral
constituents as Quartz, Silliminate and Mullite.
6.
Fly ash has high optimum moisture content and low maximum dry density due to the
fact that particles themselves are hollow or cenospheres and holds a considerable
quantity of water internally. It exhibits minimum CBR value at soaked condition.
7.
The maximum dry density decreased and optimum moisture content increased with
increase in fly ash content in the untreated composites.
8.
The UCS values were not significant.
9.
The CBR values of untreated fly ash-overburden composite materials were low and
hence those are unsuitable for road construction.
10.
There was not much strength gain in the untreated composites with varying curing
periods.
157
Chapter 6: Summary and Conclusions
6.2
1.
Treated materials
The maximum dry density values of all the treated composite materials decreased and
optimum moisture content increased with increase in lime content.
2.
The CBR values increased with increase in lime content. It increased from 22 to
77.08% in soaked condition. Similar trend exist for increasing curing period.
3.
Unconfined compressive strengths increased from 0.71 to 3.14 MPa with curing
period. The composition with 30%FA, 70% O/B and 9% lime exhibited maximum
compressive strength 3.14 MPa at 56 days curing as compared to that of other mixes.
4.
The unconfined compressive strength increased with increase in lime content and
curing period.
5.
Brazilian tensile strengths increased from 55.7 to 357 kPa with curing period. Similar
trend like CBR and unconfined compressive strength was also observed for the
Brazilian tensile strength.
6.
The mine overburden mixed with 30% fly ash and 9% lime produced highest
compressive strength, tensile strength and bearing ratio value as compared to that of
other composites with curing period.
7.
The ultrasonic pulse velocities varied in the range of 797 m/s to 1699 m/s for varying
curing periods, maximum at 56 days.
8.
The composite containing 30% fly ash and 70% overburden treated with 9% lime
produced highest ultrasonic velocities as compared to that of other composites at 7, 14,
28 and 56 days of curing respectively.
9.
The pulse wave velocity increased with an increase in lime content though the rate of
increase varies.
158
Chapter 6: Summary and Conclusions
10.
The pulse velocity changed marginally with varying fly ash content.
Young’s
modulus values and Poisson’s ratios also followed the same trend as the pulse wave
velocity.
11.
The morphology of all the mixes showed the formation of hydrated gel at 28 days
curing. The voids between the particles were filled by growing hydrates with curing
time.
12.
Microanalysis and compositional analysis confirmed the formation of new
cementitious compounds such as calcium silicate hydrate (CSH), calcium aluminate
hydrate (CAH) and calcium aluminate silicate hydrate (CASH) which leads to increase
in strength of the material over time.
13.
The major chemical constituents are alumina, silica iron oxide, calcium oxide in the
composites. Very small or negligible percentages of Na, Mg, P, S, Mn, Cu, Ba and
Mo elements are found in the mixes.
14.
The CaO content, CaO/SiO2 and CaO/(SiO2 + Al2O3) ratios are highest (27%, 1.06
and 0.59) in composite the containing 30% fly ash and 70% overburden treated with
9% lime as compared to other mixes which exhibited maximum strength value.
15.
XRD patterns indicate CAH is the most dominant formation followed by CSH and
CASH.
16.
Lime content showed a significant effect on the strength development and pozzolanic
reaction rate of natural pozzolans.
17.
The concentration of heavy metals (Ni, Cr, Pb, Cu and Zn) in the leachate effluent was
below the threshold limits acceptable for contaminants flowing into ground water.
159
Chapter 6: Summary and Conclusions
18.
The model equations governing the relationship between Brazilian tensile strength and
unconfined compressive strength and CBR and unconfined compressive strength and
CBR, compressive strength, tensile strength and ultrasonic velocity values against
CaO content, CaO/SiO2 and CaO/(SiO2 + Al2O3) ratios.
19.
All the model equations developed have correlation coefficient (R) values are about
80% to 90%.
20.
The strain at the surface course with conventional material is above the critical limit
with fixed subbase thickness (0.83m). Replacement with fly ash composite material
did not show any significant improvement.
21.
The maximum strain reduced drastically when subbase thickness changed from 0.83 to
1.5m. The strain value was less than the critical strain limit with dynamic constants of
the best developed composite ((30%FA+70%O/B)+9%L).
22.
The maximum stress and strain values were very low with dynamic elastic parameters.
23.
All the composites exhibited less than the critical strain and stress values for 1.5m
thick subbase.
24.
About 16MT of fly ash can be used in 5 km long, 20m wide and 5m thick haul road
for 200 opencast mines.
25.
The developed fly ash composite material ((30%FA+70%O/B)+9%L) has strong
potential to be used as subbase material.
6.3
Scope for Further Research
The investigation has certain limitation and hence all the factors that contribute to the haul
road performance could not be addressed in time. So the future research should incorporate
the following aspects in detail.
160
Chapter 6: Summary and Conclusions
i. Performance of developed composites was evaluated numerically. Same should be
carried out in field conditions.
ii. The CBR value is considered for the road design. The resilient modulus values should
be considered and evaluated.
161
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184
Appendix
APPENDIX
185
Appendix
Figure 4.40: Variation of maximum dry density with lime content
Figure 4.41: Variation of optimum moisture content with lime content
186
Appendix
Figure 4.42: Load vs penetration curves of untreated composites in unsoaked condition
Figure 4.43: Load vs penetration curves of untreated composites in soaked condition
187
Appendix
Figure 4.44 (a): SEM photograph of
(15FA+85O/B) +3L
Figure 4.44 (d): SEM photograph of
(20FA+80O/B) +9L
Figure 4.44 (b): SEM photograph of
(15FA+85O/B) +6L
Figure 4.44 (e): SEM photograph of
(25FA+75O/B) +9L
Figure 4.44 (c): SEM photograph of
(20FA+80O/B) +6L
Figure 4.44 (f): SEM photograph of
(30FA+70O/B) +2L
188
Appendix
Figure 4.44 (g): SEM photograph of
(30FA+70O/B) +3L
Figure 4.44 (j): SEM photograph of
(40FA+60O/B) +9L
Figure 4.44 (h): SEM photograph of
(35FA+65O/B) +2L
Figure 4.44 (k): SEM photograph of
(45FA+55O/B) +9L
Figure 4.44 (i): SEM photograph of
(35FA+65O/B) +6L
Figure 4.44 (l): SEM photograph of
(50FA+50O/B) +9L
Figure 4.44: SEM photographs of fly ash composites at 28 days curing
189
Chapter 6
Figure 4.45 (a): XRD patterns of (15FA+85O/B) stabilised with 2, 3, 6 and 9% lime
Figure 4.45 (b): XRD patterns of (20FA+80O/B) stabilised with 2, 3, 6 and 9% limes
190
Appendix
Figure 4.45 (c): XRD patterns of (25FA+75O/B) stabilised with 2, 3, 6 and 9% lime
Figure 4.45 (d): XRD patterns of (35FA+65O/B) stabilised with 2, 3, 6 and 9% lime
191
Appendix
Figure 4.45 (e): XRD patterns of (40FA+60O/B) stabilised with 2, 3, 6 and 9% lime
Figure 4.45 (f): XRD patterns of (45FA+55O/B) stabilised with 2, 3, 6 and 9% lime
192
Appendix
Figure 4.45 (g): XRD patterns of (50FA+50O/B) stabilised with 2, 3, 6 and 9% lime
Figure 4.45: XRD patterns of fly ash composites at 28 days curing
Figure 5.12 (a): Total strain at various layers of haul road pavement with
((15FA+85O/B)+9L) composite
193
Appendix
Figure 5.12 (b): Total stress at various layers of haul road pavement with
((15FA+85O/B)+9L) composite
Figure 5.12 (c): Total strain at various layers of haul road pavement with
((20FA+80O/B)+3L) composite
194
Appendix
Figure 5.12 (d): Total stress at various layers of haul road pavement with
((20FA+80O/B)+3L) composite
Figure 5.12 (e): Total strain at various layers of haul road pavement with
((20FA+80O/B)+9L) composite
195
Appendix
Figure 5.12 (f): Total stress at various layers of haul road pavement with
((20FA+80O/B)+9L) composite
Figure 5.12 (g): Total strain at various layers of haul road pavement with
((25FA+75O/B)+9L) composite
196
Appendix
Figure 5.12 (h): Total stress at various layers of haul road pavement with
((25FA+75O/B)+9L) composite
Figure 5.12 (i): Total strain at various layers of haul road pavement with
((30FA+70O/B)+6L) composite
197
Appendix
Figure 5.12 (j): Total stress at various layers of haul road pavement with
((30FA+70O/B)+6L) composite
Figure 5.12 (k): Total strain at various layers of haul road pavement with
((30FA+70O/B)+9L) composite
198
Appendix
Figure 5.12 (l): Total stress at various layers of haul road pavement with
((30FA+70O/B)+9L) composite
Figure 5.12 (m): Total strain at various layers of haul road pavement with
((35FA+65O/B)+9L) composite
199
Appendix
Figure 5.12 (n): Total stress at various layers of haul road pavement with
((35FA+65O/B)+9L) composite
Figure 5.12: Total strain and stress at various layers of haul road pavement with fly ash
composites as subbase material
200
Appendix
Calculation of fly ash for haul road
Any permanent haul road length = 5km
Haul road width = 20m
Subbase thickness = 1.5m
No. of surface coal mine = 200
Volume of the road pavement = 5km×20m×1.5m
= 5000×20×1.5 m3
= 150000 m3
Density of the fly ash composite material = 1740 kg/ m3
Quantity of material used = 1740 kg/ m3×150000 m3
= 261000000 kg
= 0.261 MT
Material used for 200 opencast mines = 0.261×200 = 52.2 MT
Fly ash usage @ 30% of the material = 30%×52.2 MT = 16 MT
201
LIST OF PUBLICATIONS
Journals
1.
Behera, B., Mishra, M. K. and Mallick, S. R. (2010), “California Bearing Ratio behavior
of mine overburden stabilized fly ash”, Journal of Indian Mineral Industry, May 2010, pp.
133-135.
2.
Mishra, M. K. and Behera, B. (2010), “Laboratory investigation on behaviour of surface
coal mine overburden stabilized with fly ash and lime”, Journal of Mine Metal and Fuel,
May 2010, ISSN 0022-2755, pp. 129-133.
3.
Behera, B. and Mishra, M. K. (2010), “Strength behaviour of Surface coal mine
overburden-fly ash mixes stabilized with quick lime”, International Journal of Mining,
Reclamation and Environment, Vol. 26, Issue 1, pp.38-54.
4.
Behera, B. and Mishra, M. K. (2011), “Effect of lime on the California Bearing Ratio
behaviour of fly ash - mine overburden mixes”, Division of Civil and Environmental
Special Journal Issue, WASET, ISSN 2010-3778, vol. 75, pp. 161-166.
5.
Behera, B. and Mishra, M. K. (2012), “California Bearing Ratio and Brazilian Tensile
Strength of Mine Overburden-Fly Ash-Lime Mixtures for Mine Haul Road Construction”
International Journal of Geotechnical and Geological Engineering, Vol. 30, Issue 2, pp.
449-459.
6.
Behera, B. and Mishra, M. K. (2012), “Strength assessment and compositional analysis of
lime stabilized fly ash and mine overburden mixes upon curing” International Journal of
Solid Waste Technology and Management, Vol. 38, Issue 3, pp. 211-221.
Conferences
1.
Behera, B., Mishra, M. K. and Naik, H. K. (2008), “Critical Review of Fly Ash
Utilization in Mines”, Conference on Emerging Trends in Mining and Allied Industries
(ETMAI-2008), NIT Rourkela, Feb. 2-3, pp 277 – 283.
2.
Behera, B. and Mishra, M. K. (2009), “Utilisation of fly ash composite material for
surface coal mine haul road stabilization”, International Symposium on Rock Mechanics
and Geo-Environment in Mining and Allied Industries (RGMA-09), IT BHU, Varanasi,
Feb. 12-14, pp 327 – 333.
3.
Behera, B. and Mishra, M. K. (2009), “Stabilization of Surface Coal Mine Overburden by
Fly ash and lime”, National Conference on Advances in Environmental Engineering
(AEE-09), NIT, Rourkela, Nov. 14-15, pp. 98-103.
202
4.
Behera, B. and Mishra, M. K. (2010), “Mechanical properties of lime treated fly ashoverburden mixes in relation to their use in mine haul road construction”, International
Conference on Developments in Road Transportation (DRT-10), NIT Rourkela, Oct. 0810, pp. 572-578.
5.
Behera, B. and Mishra, M. K. (2011), “Effect of lime on the California Bearing Ratio
behaviour of fly ash - mine overburden mixes”, International Conference on
Environmental and Civil Engineering, World Academy of Science, Engineering and
Technology (WASET), Bangkok, Thailand, Mar. 29-31, vol. 75, pp. 213-218.
6.
Behera, B. and Mishra, M. K. (2011), “The use of lime-stabilized fly ash-overburden
mixtures as haul road construction materials” International Conference on Technological
Challenges and Management Issues for Sustainability of Mining Industries (TMSMI
2011), NIT Rourkela, Aug. 04-06, pp. 117-123.
7.
Behera, B. and Mishra, M. K. (2011), “Some aspects of lime treated fly ash and mine
overburden composite samples” 34th International Conference of Safety in Mines
Research Institutes, New Delhi, Dec. 7-10, pp. 257-264.
8.
Behera, B. and Mishra, M. K. (2012), “Microstructure and leaching characteristics of fly
ash-mine overburden-lime mixtures” International Conference on Chemical, Civil and
Environment Engineering, Dubai, Mar. 24-25, pp. 256-260.
203
Curriculum Vitae
Name
Banita Behera
Date of Birth
01-06-1983
Educational Qualification
2006
M.Sc. (Physics), NIT, Rourkela, India
2003
B.Sc. (Physics Hons.), Sambalpur University, Odisha, India
Experience
5 years, Research experience
Permanent Address
C/O: Lokanath Behera
Q. No. – F/7, NIT Campus,
Rourkela – 769 008
Sundargarh, Odisha
Email: [email protected],
[email protected]
Phone: 07204732598 (M)
204
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