GEO-ENGINEERING PROPERTIES OF SEDIMENTED FLYASH DEPOSIT STABILIZED BY LIME PILE

GEO-ENGINEERING PROPERTIES OF SEDIMENTED FLYASH DEPOSIT STABILIZED BY LIME PILE
GEO-ENGINEERING PROPERTIES OF
SEDIMENTED FLYASH DEPOSIT STABILIZED
BY LIME PILE
A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of
Master of Technology
In
Civil Engineering
(Geotechnical Engineering)
Ganesh R
DEPARTMENT OF CIVIL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
JUNE 2014
GEO-ENGINEERING PROPERTIES OF SEDIMENTED FLYASH
DEPOSIT STABILIZED BY LIME PILE
A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of
Master of Technology
In
Civil Engineering
(Geotechnical Engineering)
Under the guidance and supervision of
Prof S. P. Singh
Submitted by
Ganesh R
(ROLL NO. 212CE1018)
DEPARTMENT OF CIVIL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
JUNE 2014
Department of Civil Engineering
National Institute of Technology Rourkela
Rourkela – 769008, India www.nitrkl.ac.in
CERTIFICATE
This is to certify that the project entitled “Geo-Engineering Properties of Sedimented
Flyash Deposit Stabilized by Lime Pile” submitted by Mr. Ganesh R (Roll No.
212CE1018) in partial fulfilment of the requirements for the award of Master of
Technology Degree in Civil Engineering at NIT Rourkela is an authentic work carried
out by him under my supervision and guidance.
To the best of my knowledge, the matter embodied in this report has not been
submitted to any other university/institute for the award of any degree or diploma.
Place: Rourkela
Date:
Prof. Suresh Prasad Singh
Department of Civil Engineering
National Institute of Technology Rourkela
Acknowledgement
I would like to express my deepest thanks, great indebtedness and gratitude to my
thesis supervisor Prof. Dr. S. P. Singh, Department of Civil Engineering, National
Institute of Technology Rourkela, Odisha, India, for his kind supervision, valuable
comments during courses of my research work. I express my sincere thanks to all my
colleagues at the Department of Civil Engineering, especially Mr. Mohanty, Mr.
Chamru suniani, Mr. Dillip and Mr. Loboon for their both field and laboratory help
and others who helped directly/ indirectly for my work. I am also grateful to all my
friends at the National Institute of Technology Rourkela, India, especially Mr.
Srikanth, Ms. Pani for the kind help and to Ms. Pavani for her motivation and
continuous support. Thanks to all staff members and HOD of Civil Engineering
Department, National Institute of Technology Rourkela, Odisha, India, for providing
me all the necessary facilities needed for the experimental work. I extend my heartily
thanks to RSP personals for providing me the flyash to carry out this study. I would
like to acknowledge my seniors at the Geotechnical Engineering Department for their
continuous moral support.
I extend my special and heartily thanks and gratitude to my Institute, National
Institute of Technology Rourkela, Odisha, India, for giving me the opportunity to
carry out research. Finally, I would like to express my deepest gratitude to my
beloved parents, my sister and also her husband who made all of this possible, for
their endless encouragement, support, love and patience throughout the research
period.
(Ganesh R)
Table of Contents
Abstract ..................................................................................................................................... i
List of Tables........................................................................................................................... vi
Organization of Thesis .......................................................................................................... vii
CHAPTER 1 ............................................................................................................................ 1
Introduction ............................................................................................................................. 1
CHAPTER 2 ............................................................................................................................ 4
Review of Literature and Scope of the Present Study ........................................................... 4
2.1 General ............................................................................................................................ 4
2.1.1 Characteristics of Sedimented Flyash Deposits........................................................ 4
2.1.2 Characteristics of Compacted Flyash ....................................................................... 6
2.2 Deep Mixing Method ...................................................................................................... 8
2.2.1 Lime Column Method .............................................................................................. 8
2.2.2 Lime Column Method- Field Trials............................................................................ 10
2.3 Shallow Mixing Methods .............................................................................................. 11
2.3.1 Properties of Flyash Modified With Chemical Additives ...................................... 11
2.3.2 Stabilization/Densification of Deposited Flyash- Field/Laboratory Trials............. 13
2.4 Scope of the Present Study ............................................................................................ 15
CHAPTER 3 .......................................................................................................................... 17
Materials and Methods ......................................................................................................... 17
3.1 General .......................................................................................................................... 17
3.2 Materials ........................................................................................................................ 17
3.2.1 Fly ash .................................................................................................................... 17
3.2.2 Lime........................................................................................................................ 19
3.3 Methods ......................................................................................................................... 20
3.3.1 General ................................................................................................................... 20
3.3.2 Preparation of Flyash Sample in Test Tank................................................................ 20
3.3.3 Simulation of Sedimentation of Ash Slurry ........................................................... 21
3.3.4 Compaction of Flyash in Test Tank ....................................................................... 22
3.3.5 Installation of Lime Column................................................................................... 24
3.3.6 Sampling Program .................................................................................................. 25
3.4 Test procedures................................................................................................................ 28
3.4.1 Specific Gravity ...................................................................................................... 28
3.4.2 Particle Size Distribution ........................................................................................ 28
3.4.3 Compaction Test ..................................................................................................... 28
3.4.4 Unconfined Compressive Strength Test ................................................................. 28
3.4.5 Direct Shear Test .................................................................................................... 29
3.4.6 Permeability Test .................................................................................................... 29
CHAPTER 4 .......................................................................................................................... 30
Results and discussion ........................................................................................................... 30
4.1 Introduction ................................................................................................................... 30
4.2 Characteristics of Materials Used .................................................................................. 31
4.2.1 Physical Properties ................................................................................................. 31
4.2.2 Specific Gravity ...................................................................................................... 31
4.2.3 Particle Size Distribution ........................................................................................ 31
4.2.4 Engineering Properties ........................................................................................... 32
4.3 Geotechnical Properties of Sedimented Flyash slurry Surround Lime Column ............ 29
4.3.1 Water content.......................................................................................................... 29
4.3.2 Dry density ............................................................................................................. 31
4.3.3 Unconfined compressive strength .......................................................................... 32
4.3.4 Shear strength parameters....................................................................................... 34
4.3.5 Permeability............................................................................................................ 36
4.3.6 Temperature............................................................................................................ 37
4.4 Geotechnical Properties of Compacted Flyash Surround Lime Column....................... 39
4.4.1 Water content.......................................................................................................... 39
4.4.2 Dry density ............................................................................................................. 41
4.4.3 Unconfined compressive strength .......................................................................... 42
4.4.4 Shear strength parameters....................................................................................... 43
4.4.5 Permeability............................................................................................................ 45
4.4.6 Temperature............................................................................................................ 46
CHAPTER 5 .......................................................................................................................... 49
Summary and Conclusions ................................................................................................... 49
5.1 Scope for Further Research ........................................................................................... 51
References ............................................................................................................................... 52
Abstract
Coal based thermal power plant has created over 50,000 acres of ash ponds in
India, with approximately 2500 acres of additional ponds created for a 500-MW power
plant and is filled with ash up to 10 m in height within a period of 5 years. Presently,
160 MT of fly ash is generated every year and it is likely to increase up to 300 MT by
2016-17. In the process of sluicing and sedimentation of ash in the storage ponds
considerable segregation of particles occurred resulting in formation of complex,
heterogeneous, sedimentary profiles. The in situ water content of deposits typically
varied from 10% to 110% and ultimate bearing capacity of not more than 95kN/m2.
Various ground improvement Techniques have been applied to improve the
geotechnical characteristics of these lands and or to enhance storage capacity and or to
make it suitable for construction purposes. Since construction of buildings or utilities on
these lands by conventional methods is not possible because of low strength flyash
forms a very soft ground and highly compressible due to high water content. Also
ponding of the ash generally found to reduce its self-hardening or pozzolanic properties.
However, in most geotechnical projects, ground modification is needed to obtain a
construction site that will meet the design requirements. For this reason, a need exists to
develop an economical and practical methods may bring about improvement in the
geotechnical properties of the ash deposit as a whole, converting it to a usable land can
be utilized for a broad range of purposes, such as suburban housing, light commercial
building, and utilities etc.
Several attempts have been made in the past with number of methods like
electro osmosis, vacuum dewatering, vibroflotation, densification by blasting,
densification by vibration and heavy compaction method (HCM) to stabilize soft flyash
deposits. Among all lime stabilization of coal ash by mechanical mixing is the
commonly adopted method. In the present work, emphasis has been given on
application of the in-place lime column method for stabilization of sedimented pond ash
deposits. Since various disadvantages such as excavation, mixing, and transportation of
huge quantity of ash from the ash ponds or disposal sites in the case of conventional
mixing method can be avoided and at the same time improvement in the engineering
properties of the whole deposit can be achieved thereby these abandoned sites may be
used for construction purposes. Generally, lime column method has been found to
i
modify the properties by increasing unconfined compressive strength and reducing
hydraulic conductivity of pond ash deposits in addition to modifying other geotechnical
properties such as water content, density, and particle size etc.
In the present work, investigations were made to study the strength distribution
of sedimented flyash deposit surrounded by lime column over a stabilization period of
90 days. Flyash slurry was prepared and allowed to fall from a constant height of 1 m in
a test tank having 1m diameter and 1.2 m height. Prior to saturation, a single lime
column of 0.1 m diameter over full length of deposited slurry was installed in the test
tank after the initial sedimentation period of 30 days. It is reasonable to assume that the
lime will flow easily downward into flyash deposit in vertical direction and the strength
may also increase with the availability of lime. To obtain variation of strength in
vertical direction, lime column of 0.2 m height was made in other tank in a similar
manner. A series of uniaxial strength and direct shear tests were performed on the
samples extracted at various depths and radial distances. It was observed that the lime
column inclusion enhance the strength of sedimented flyash deposit with stabilization
time. Also significant improvement in strength was observed up to a horizontal distance
of 3 D (where D is the diameter of lime column) from the center of column and vertical
distance of 4 D from bottom of lime column. A comparative study showed that the
strength of stabilized mass is much higher than the un-stabilized one. The method has
also proved to be useful in reducing the contamination potential of the ash leachates,
thus mitigating the adverse environmental effects of ash deposits.
Similar investigations were made to study the potential of lime column method
for stabilization of pond ash compacted at its standard proctor density. After LC has
been installed, it is expected that the lime or calcium ions migrate into the surrounding
flyash by the pathways developed in the hydration process. Since quick lime is used to
form LC which produces enormous amount of heat in the hydration process. For lime
column stabilization to be efficient, calcium and hydroxyl ions should migrate into the
surrounding flyash. Therefore flyash becomes highly alkaline due to the migration of
hydroxyl ions give rise to the slow solution of alumino-silicates in the pore water which
are then precipitated as hydrated cementitious reaction products.
As a result
flocculation occurs by bonding of adjacent flyash particles that leads to the
ii
improvement in strength and other geotechnical parameters. The above pozzolanic
reactions are time dependent and hence the development of soil strength.
The experimental results showed that the improvement in geotechnical
characteristics of sedimented flyash deposit is considerably higher than the compacted
flyash. Upon saturation the strength of both compacted and sedimented flyash deposit
found to decreases initially. However, inclusion of lime column was found to enhance
the strength and other geotechnical properties of saturated flyash (compacted and
slurry) with time of curing.
Keywords: lime column, sedimented flyash, standard proctor density, pozzolanic
reaction, unconfined compressive strength, shear strength parameters.
iii
List of Figures
Figure 2.1 Strength distribution and wet density of flyash slurry (Source: Horiuchi et al. 2000)
.................................................................................................................................................... 5
Figure 2.2 Schematic diagram of installation of lime columns.................................................. 9
Figure 2.3 Shear strength of phosphatic clay ponds stabilized by lime column (Source:
Hardianto and Ericson, 1994) .................................................................................................. 12
Figure 2.4 Variation of cone resistance before and after blasting (source: Gandhi et al. 1999)14
Figure 2.5 Variation of SPT ‘N’ along depth of fly ash bed before and after blast (source:
Gandhi et al. 1999) ................................................................................................................... 15
Figure 3.1 Photograph of slurry flyash in test tank and other setups ....................................... 21
Figure 3.2 Schematic diagram of lime column installed along full length of deposit in a test tank
with other setups ...................................................................................................................... 22
Figure 3.3 Schematic diagram of test tank for 0.2 m lime column and other setups ............... 23
Figure 3.4 Photograph shows the preparation of flyash bed at standard proctor density......... 24
Figure 3.5 Elevation of test tank with detailed Sampling location ......................................... 27
Figure 3.6 Plan view of test tank with detailed Sampling location .......................................... 27
Figure 4.1 Particle size distribution curves of fly ash .............................................................. 31
Figure 4.2 Relation between moisture content and dry density ............................................... 28
Figure 4.3 Variation of moisture content with depth (lime column installed at full depth) ..... 30
Figure 4.4 Variation of moisture content with depth (lime column installed at 0.2m depth) .. 30
Figure 4.5 Variation of dry density with depth (lime column installed at full depth) .............. 31
Figure 4.6 Variation of moisture content with depth (lime column installed at 0.2m depth) .. 31
Figure 4.7 Variation of unconfined compressive strength with depth (lime column installed at
full depth) ................................................................................................................................. 32
Figure 4.8 Variation of unconfined compressive strength with depth (lime column installed at
0.2m depth) .............................................................................................................................. 33
Figure 4.9 Variation of peak friction angle with depth (lime column installed at full depth).. 34
Figure 4.10 Variation of cohesion with depth (lime column installed at full depth) ............... 34
Figure 4.11 Variation of peak friction angle with depth (lime column installed at 0.2m depth)35
Figure 4.12 Variation of peak friction angle with depth (lime column installed at 0.2m depth)35
Figure 4.13 Variation of temperature with RD (lime column installed at full depth) .............. 38
Figure 4.14 Variation of temperature with RD (lime column installed at 0.2m depth) ........... 38
Figure 4.15 Variation of temperature with vertical distance (lime column installed at 0.2m
depth) ....................................................................................................................................... 38
Figure 4.16 Variation of moisture content with depth (lime column installed at full depth) ... 40
Figure 4.17 Variation of peak friction angle with depth (lime column installed at 0.2m depth)40
Figure 4.18 Variation of peak friction angle with depth (lime column installed at full depth) 41
Figure 4.19 Variation of moisture content with depth (lime column installed at 0.2m depth) 41
Figure 4.20 Variation of UCS with depth (lime column installed at full depth) ...................... 42
Figure 4.21 Variation of UCS with depth (lime column installed at 0.2m depth) ................... 42
Figure 4.22 Variation of shear parameters with depth (lime column installed at full depth) .. 43
Figure 4.23 Variation of shear parameters with depth (lime column installed at 0.2m depth) 44
Figure 4.24 Variation of temperature with RD (lime column installed at full depth) .............. 46
Figure 4.25 Variation of temperature with RD (lime column installed at 0.2m depth) ........... 46
iv
Figure 4.26 Variation of temperature with vertical distance (lime column installed at 0.2m
depth) ....................................................................................................................................... 47
v
List of Tables
Table 4.1 Physical Properties of Flyash ................................................................................... 32
Table 4.2 Geotechnical properties of fly ash ........................................................................... 32
Table 4.3 Permeability of flyash with depth (lime column installed at full depth) .................. 36
Table 4.4 Permeability of flyash with depth (lime column installed at 0.2m depth) ............... 36
Table 4.5 Permeability of flyash with depth (lime column installed at full depth) .................. 45
vi
Organization of Thesis
Chapter 1 describes the introductory about the efficacy of lime column
technique for the stabilization of flyash deposits. It also states about the usefulness and
advantages of Lime column methods over other stabilization techniques.
Chapter 2 In this, a detailed review of literature performed towards highlighting
the need of stabilization of flyash deposit using lime column technique. A detailed
literature about stabilization of flyash deposit carried out in the field using different
methods was also presented.
Chapter 3 presents a detailed procedure of various experimental programs
conducted in this study. Four tons of flyash with approximate residual moisture content
of 15% was used in the present experimental works and is collected from RSP. Flyash
slurry was prepared in a test tank having 1m diameter and 1.2 m height. Quick lime was
used to form lime column. A single lime column of 0.1 m diameter over full length of
deposited slurry was installed in the test tank after the initial sedimentation period of 30
days. After successful installation of lime column, the whole tank was saturated. The
improvement in geotechnical properties like unconfined compressive strength, water
content, dry density, shear strength parameters, hydraulic conductivity, and grain size
distribution were evaluated by sampling flyash specimens at different radial distances
from the central lime column at different depths. The improvement was monitored over
a period of 90 days.
Similar studies have been attempted in case of compacted flyash. It is
reasonable to assume that the lime will flow easily downward into flyash deposit in
vertical direction and the strength may also increase with the availability of lime.
Hence, a lime column of 0.2 m height was made in other tank in a similar manner for
both sedimented and compacted flyash deposit. Radial and vertical migration of lime
from the central lime column was examined and results are compared with those of the
untreated specimen.
Chapter 4 examines the potential of lime column in stabilizing sedimented
flyash deposits. Results showed that the lime column method possibly to be the most
efficient and economical method to improve soft soils like deposits of flyash. Generally,
vii
lime columns caused migration of dissociated calcium and hydroxyl ions into the
surrounding flyash mass thus enhance the strength. However the gain in strength of top
portion found to be lower than middle. Also the alteration of geotechnical properties
has taken place due to migration of lime up to three times the lime pile diameter. The
flyash becomes more and more alkaline with time due to increase in pore salinity and
pH in the migration of calcium and hydroxyl ions. The laboratory results hence bring
out that lime column treatment in the field can substantially increase the unconfined
compressive strength in addition to other geotechnical parameters at least to a radial
extent of 2 to 3 times the diameter of lime column.
The lime column installed in one fifth to the height of deposit showed the improvement
in strength possibly occurs up to a distance of 3D to 4D from the bottom of column. For
a given time and depth, the downward migration of lime much faster into flyash deposit
than in radial direction and the strength may also increase with the availability of lime.
As a result, cementation compounds formed by the pozzolanic reactions are responsible
for the improvement in strengths of lime stabilized mass.
Chapter 5 examines the potential of lime column in stabilizing compacted
flyash. The initial strength of compacted flyash mass reduces due to saturation effect.
However inclusion of lime column under same conditions found to increase the strength
of compacted flyash with time. The quick lime was used to form lime column which
allows generating enormous amount of heat in the hydration process. Thus the heat of
hydration results in the formation of number of micro cracks around the lime column
depending on the reactivity of lime. This crack helps in the migration of dissociated
calcium and hydroxyl ions into the surrounding flyash mass thus enhance the strength.
However the gain in strength of top portion found to be lower than middle. Finally it is
sown that the increase in unconfined compressive strength in addition to other
geotechnical parameters occurs at least to a radial extent of 2 to 3 times the diameter of
lime column.
Similar studies has been extended to the lime column installed in one fifth to the
height of deposit showed the improvement in strength possibly occurs up to a vertical
distance of 3D to 4D from the bottom of column.
Chapter 6 summarizes the findings of the study
viii
CHAPTER 1
Introduction
Coal ash, is a waste residue from thermal plant produced large amount thought
the world every year. Coal ash is a general name given to both bottom ash and flyash.
Current production of coal ash is estimated typically around 600 MT/year worldwide,
with fly ash constituting about 75-80% of the total ash produced. Thus, the amount of
fly ash generated from thermal power plants has been increasing throughout the
world, and the Safe disposal of such large quantities of flyash from thermal power
plants is a major concern. The percentage utilization of flyash is limited in India
compared to most of the advanced countries and it is a mere of 5%. In India, most of
the power plants adopt wet disposal system for disposing coal ash. In wet disposal
system, large quantity of flyash along with bottom ash is mixed with 70–80% of
water, transported in the form of slurry and deposited of in the ash pond, resulting in
very soft deposits. Typically around 50,000 acres of such ash ponds has been located
in various parts of India. The height of ash pond is raised every year due to scarcity of
land in and around thermal power plant in order to increase the storage capacity of an
ash pond. To increase storage capacity of ash pond various raising methods are in use
which includes upstream, downstream and central raising methods. However, in many
places the total height of the deposit exceeds 30 m and further increase in height may
result in stability problem. Generally, the ash deposit placed in slurry form has a very
low density and leads to problems such as liquefaction during earthquake, poor
bearing capacity, large settlement, etc.
Number of research has been conducted in field as well as in laboratory to
improve the density of ash by different techniques such as vacuum dewatering, electro
osmosis, vibro compaction, stone columns, blasting (Gandhi et al. 1997). Chand and
Subbarao 2007 reported the effectiveness of in-place treatment of an ash deposit by
hydrated lime column. Hydrated lime column was applied to laboratory model of
deposited flyash slurry. It was reported that the lime column method found to increase
the unconfined compressive strength and reduce hydraulic conductivity of pond ash
deposits in addition to modifying other geotechnical parameters. Also showed the
contamination potential of the ash leachates from deposited ash slurry is greatly
1
reduced by lime column method that helps in mitigating the adverse environmental
effects of ash deposits. Raju 2011 applied ground improvement using vibro
techniques in stabilizing the flyash deposits. Initial field trials were carried out to
assess the bearing capacity and also the lateral capacity of bored cast-in-situ pile
foundations as a result of stone column installation. They showed that the successful
application of vibro techniques to enhance the bearing capacity and lateral capacity of
deep pile of the fly ash deposits and also mitigation of liquefaction potential of the
site. More recently Kokusho et al. 2012 applied heavy compaction method (HCM)
normally used for sandy soils for compacting flyash deposit. The improvement in
flyash deposit was found from cone penetration tests were carried before and after the
compaction which indicating obvious effects on soil properties and strength increase.
With the knowledge of above successful application of various methods, an
attempt has been made in this research to study the potential of lime column method
in stabilizing sedimented flash deposit and compacted flyash under saturation.
Normally the strength of flyash was found to decrease under water table. This may be
due to reduction in the development of suction in the pore fluid. Also most of the
thermal power plant produces class F flyash which has less or no self-hardening
property. Successful application of lime column for stabilization soft soils by various
researchers like Barnes et al. 1993 presented both laboratory and field test results of
in-place stabilization of waste phosphatic clays using lime column. Results of their
study showed that the clay shear strength increased by 2 to 3 orders of magnitude and
the time of primary consolidation was reduced by 1 to 2 orders of magnitude.
Fransiscus et al. 1993 successfully used lime column in stabilizing phosphatic clays.
Lime column was formed by continues mixing of clay and quick lime (CaO). The insitu test results shows that there is a reduction in plasticity, increased permeability and
strength, and lower the water content through hydration and pozzolanic reaction.
Gupta et al. 1998 presented the results of field trials for improvement of soft soils.
They embankment made with black cotton soil was modified with lime columns and
pressure injection of lime slurry and found that both techniques resulted in significant
improvement in strength and settlement characteristics. Deep mixing method
commonly used for soft soil stabilization in which column of whole was made
through hollow, rotated mixing shafts tipped with some type of cutting tool and
cementitious materials or any suitable binders were injected (Terashi, 1997). The
2
lime-column method was formed by injecting the dry or wet lime under preferable
pressure into soil in-situ thereby the soil surrounded by column get stabilized through
physico-chemical reactions (Rogers and Glendinning, 1997). Lime column method
would increase soil bearing capacity and reduces soil settlement helps in improving of
soil strength and stiffness. Therefore, this technique was preferable for soft soil
stabilization (Broms and Boman, 1975). Based on full-scale model, Baker (2000)
showed that the stiffness of the improved soil using lime-column increased more
significantly than that of lime-cement column. Other researchers like Shen et al.,
2003; Tonoz et al., 2003; Budi, 2003 studied the strength of the soil surrounding the
lime-column. Most of their results showed that the soil strength increased near the
column to a distance up to 2 to 3 times of the column diameter in radial direction.
However, the effect of strength change beneath the bottom of lime-column was not
studied. Muntohar (2010) presented laboratory scale model test results of soft clay
stabilized with lime column technique. The lime-column of 50 mm in diameter (D),
and the depth was 200 mm is used. The CPT results showed that the installation of LC
affected the soil strength to a depth of 4xD beneath the bottom of LC and the water
content of soil decreased near the LC, but beyond the distance of 4D in radial
direction the water content remained its original value.
A laboratory program was undertaken to systematically investigate the
potential of the Lime Column Method (LCM) normally used for stabilizing soft soils
for improving sedimented flyash deposit. A series of uniaxial strength and direct shear
tests were performed on the samples collected at various depths and radial distances.
It was observed that the lime column inclusion enhance the strength of sedimented
flyash deposit with stabilization time. Also significant improvement in strength was
observed up to a horizontal distance of 3 D (where D is the diameter of lime column)
from the center of column and vertical distance of 4 D from bottom of lime column. A
comparative study showed that the strength of stabilized mass is much higher than the
un-stabilized one. The method has also proved to be useful in reducing the
contamination potential of the ash leachates, thus mitigating the adverse
environmental effects of ash deposits.
3
CHAPTER 2
Review of Literature and Scope of the Present Study
2.1 General
This chapter describes the detailed review of literature performed towards
highlighting the need of stabilization of flyash deposit using lime column technique.
A detailed literature about stabilization of flyash deposit carried out in the field using
different methods was presented and discussed. The various stabilization methods
both shallow and deep are outlined.
2.1.1 Characteristics of Sedimented Flyash Deposits
Due to rapid growth in industrialization and economy, the large number of
coal based thermal power plant has set up in various parts of country to meet the
electricity demand. At the same time these power plants producing large amount of
coal ash and its safe disposal of such ash is major concern. Usually flyash along with
bottom ash mixed to form slurry and is disposed of in the storage ponds. In the
process of sluicing and sedimentation of ash in the storage ponds considerable
segregation of particles occurred resulting in formation of complex, heterogeneous,
sedimentary profiles. The engineering behavior of flyash slurry after sedimentation
and consolidation processes under its own self-weight found to vary considerably than
the compacted after dewatering. A metastable fabric formed in the sedimentation
process which shows collapse potential of the materials ranged between 0.5 and 1%
and also the flyash slurry exhibits a pseudo over consolidation effect, moderate
collapsible behavior, and high compressibility at applied stresses Madhyannapu et al.
(2008). The compressibility of sedimented fly ash is considerably greater than that of
compacted fly ash specimens. The compression indices values of fly ash beds are
dependent on the source material, sedimentation, and compaction procedures
followed and the stress range over which it was subjected to consolidation test.
Fig 2.1 shows the distribution of strength and wet density of flyash slurry used for
back filling on the wall of cofferdam. From Fig. 2.1, it is observed that the strength of
fill at 6–10 m is very large and the strength primarily depends on the type of coal ash
used. Horiuchi et al. 2000 used coal ash slurry as a back fill material on the wall of
4
cofferdam, there was significant improvement in strength development with time was
observed.
Also they presented the effective use of flyash slurry in variety of
applications listed below
(1) Underwater fills
(2) Light weight backfills
(3) Light weight structural fills etc.
Figure 2.1 Strength distribution and wet density of flyash slurry (Source: Horiuchi et
al. 2000)
Among various possible applications of flyash slurry, placement of fly ash
slurry underwater as a fill found to be feasible to form a stable artificial ground for
construction of various structures such as harbor or airport construction. Development
of strength of flyash slurry with time is affected by parameters such as temperature
and additives and considered to be major concern for making appropriate slurries.
Also high calcium content of flyash helps to gain in strength of slurry with time due to
pozzolanic reaction. From the one-dimensional consolidation of sedimented stowed
pond of the mines, Mishra and Das (2012) studied experimentally the variation of
coefficient of consolidation of the sedimented stowed pond ash and were found to be
in range of 0.0195–0.1882 cm2/min. The value of consolidation coefficient decreases
with increment in applied load and time indicating that the stowed pond ash mass will
undergo gradual settling and not suffer large deformation.
5
2.1.2 Characteristics of Compacted Flyash
As discussed in the previous session, the compressibility of compacted fly ash
specimens is considerably lesser than sedimented fly ash. Soaking of compacted fly
ash generally found to decreases the strength of compacted fly ash and also exhibits
high compressibility. Indraratna et al. (1991) reported that the high value of
compressive strength in case of unsoaked specimen possibility due to suction
development in the pore fluid. There are three possible mechanisms which responsible
for gain/ loss of strength of flyash while soaking. 1) Soaking of the specimens may fill
the specimen voids to certain extent and thereby it reduces development of suction in
the pore fluid. 2) Soaking may cause softening of the specimens and thus reducing the
shear strength. 3) During soaking, the specimens may get sufficient moisture required
for pozzolanic reaction which may help to increase the shear strength by the
formation of reaction products. Also the density found to be an important parameter
responsible for the strength, compressibility and permeability of fly ash.
Densification of ash by any suitable techniques improves the engineering
properties. The unit weight of the material mainly depends on the amount and method
of energy application, grain size distribution, plasticity characteristics and moisture
content at compaction Pandian, 2004. Flyash normally have air void content ranging
between 5 to 15% at maximum dry density. Toth et al. (1988) reported that the higher
void content tend to limit the buildup of pore pressures during compaction allowing
the fly ash to be compacted over a larger range of water content. One of interesting
result provided by Gatti and Tripiciano (1981) that the compaction tests on coal ashes
were collected from Vado Ligure Power Plant, Italy indicating maximum dry density
varied between 11.4kN/m3 and 45kN/m3 and corresponding optimum moisture
contents ranging between 28% and 36%. Also standard Proctor compaction curves
provided by DiGioia et al. (1986) for Western Pennsylvania Class F fly ash shows
that the maximum dry density ranged from 11.9 to 18.7 kN/m3 and optimum water
content ranged from 13 to 32%.
The permeability value of flyash mainly depends on its grain size distribution,
degree of compaction and pozzolanic activity (Sridharan and Prakash, 2007). The
values of coefficient of permeability of flyash were in the same range as those of nonplastic silts and also the permeability value of fly ashes produced from bituminous
6
coals is in the range of 1×10-5 to 3×10-6 cm/s. Shenbaga and Gayathri (2004).
Therefore the compacted fly ash deposits have moderate permeability value. 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). Leonards and Bailey (1982) reported that the value of unconfined
compressive strengths for fine ash is higher than those for the coarser ash specimens.
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 Gray and Lin (1972). This is due the smaller
fraction of lime, present as free lime in the form of calcium oxide or calcium
hydroxide in the flyash which also controls self-hardening characteristics of fly ashes
Sherwood and Ryley (1966). Many others like Yudhbir and Honjo (1991) reported
that the UCS of fly ash increased exponentially with the free lime content and also
presence of carbon in fly ashes found to give reduced strength. 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 sample. When density increases its friction
also increases. 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).
Ramasamy and Pusadkar 2007 presented the method to estimate settlement of
7
footings on compacted ash considering the effect of capillary and preconsolidation
stresses. Experimental results showed that the compressibility of compacted coal ash
fills is greatly influenced by capillary and preconsolidation stresses, overestimation of
settlement greater than 100% occurs when capillary and preconsolidation stresses are
not taken into account.
2.2 Deep Mixing Method
Deep mixing method commonly used for soft soil stabilization at large depth in which
column of whole was made by suitable means and cementitious materials or any
suitable binders were injected and blended with in-situ soils.
2.2.1 Lime Column Method
Lime column method, where quicklime are mixed in situ with soft soil as
shown in Fig 2.2, are common in Sweden and Finland, to stabilize soft clay and silt as
well as organic soils. This method has been used to increase the stability and to reduce
the settlements of road and railroad embankments and to increase the stability of
trenches for sewer lines, water mains and heating ducts etc. Also Lime in addition
with cement columns have also been used to stabilize organic soils, where unslaked
lime alone has not been effective. Lime columns found to have the high permeability
and the ductility. Normally the ground temperature is increased by the heat generated
during the slaking. Therefore, correspondingly the shear strength increased, caused by
the reduction of the water content. There are number of factors found to affect the
behavior of lime columns hence it is necessary to determine for each site the effect of
different stabilizers (e.g. lime, cement, gypsum, industrial waste and different ashes)
on compressibility, shear strength and permeability of the stabilized soil. Extensive
field and laboratory tests are usually required.
It is expected that inclusion of LC in soil mass increases the strength by the
migration of lime or calcium ions into the surrounding soil mass. The modification/
alteration of soil properties around LC possibly occur due to consolidation,
densification followed by hardening by the chemical reaction between lime and soil.
Migration of calcium and hydroxyl ions into the surrounding mass found to increases
the alkalinic conditions in soil.
8
Figure 2.2 Schematic diagram of installation of lime columns
Hydrated cementitious reaction products are formed as a precipitate from high
concentration of alumino- silicates. The strength of soil mass is increased with time
since the reaction products contributing to flocculation by bonding adjacent soil
particles together and when curing is allowed. The LC stabilized soil strength will
vary with the distance from the center of the LC both in radial and vertical directions.
The strength characteristics of stabilized mass can be used to predict the migration
zone of the calcium ion of lime. The initial reaction between lime and soil takes place
nearly 24 – 72 hours after installation of LC and the soil properties of soil mass
surround LC are altered /modified. At the end of initial reaction, Secondary reaction
i.e pozzolanic reactions between lime and soils take place for a prolonged period.
After that, the soil undergoes a permanent change in mechanical properties. However
the strength develops gradually over a long period of time (Bell 1996; Sivapullaiah et
al., 2000; Muntohar, 2003).
Laboratory reagent grade quick lime (CaO) was used in this study. Quick lime
was used in the construction of the lime piles so that cracks could be generated in the
compacted soil mass by the heat generated during hydration of the quick lime (Bell,
9
1988). The generated cracks were expected to provide pathways for migration of
calcium and hydroxyl ions from the lime pile into the soil mass.
2.2.2 Lime Column Method- Field Trials
Successful application of lime column for stabilization soft soils by various
researchers like Barnes et al. 1993 presented both laboratory and field test results of
in-place stabilization of waste phosphatic clays using lime column. Results of their
study showed that the clay shear strength increased by 2 to 3 orders of magnitude and
the time of primary consolidation was reduced by 1 to 2 orders of magnitude.
Hardianto and Ericson (1994) successfully used lime column in stabilizing phosphatic
clays ponds in IMC Haynsworth Mine, southwest of Bradley, Florida. Lime column
was formed by continues mixing of clay and quick lime (CaO). The in-situ test results
shows that there is a reduction in plasticity, increased permeability and strength, and
lower the water content through hydration and pozzolanic reaction.
Fig 2.3 shows the average shear strength before and after lime column
installation in phosphatic clays. Gupta et al. 1998 presented the results of field trials
for improvement of soft soils. Lime to water ratio of 30% was applied to study the
efficacy of lime column in improving soft soil. The embankment made with black
cotton soil was modified with lime columns and pressure injection of lime slurry and
found that both techniques resulted in significant improvement in strength and
settlement characteristics. Deep mixing method commonly used for soft soil
stabilization in which column of whole was made through hollow, rotated mixing
shafts tipped with some type of cutting tool and cementitious materials or any suitable
binders were injected (Terashi, 1997).
The lime-column method was formed by injecting the dry or wet lime under
preferable pressure into soil in-situ thereby the soil surrounded by column get
stabilized through physico-chemical reactions (Rogers and Glendinning, 1997). Lime
column method would increase soil bearing capacity and reduces soil settlement helps
in improving of soil strength and stiffness. Therefore, this technique was preferable
for soft soil stabilization (Broms and Boman, 1975). Other researchers like Shen et
al., 2003; Tonoz et al., 2003 studied the strength of the soil surrounding the limecolumn. Most of their results showed that the soil strength increased near the column
10
to a distance up to 2 to 3 times of the column diameter in radial direction. However,
the effect of strength change beneath the bottom of lime-column was not studied.
Muntohar (2010) presented laboratory scale model test results of soft clay stabilized
with lime column technique. The lime-column of 50 mm in diameter (D), and the
depth was 200 mm is used. The CPT results showed that the installation of LC
affected the soil strength to a depth of 4xD beneath the bottom of LC and the water
content of soil decreased near the LC, but beyond the distance of 4D in radial
direction the water content remained its original value.
Wilkinson et al. (2010) showed the applicability of lime slurry pressure
injection (LSPI), stabilization technique for improving the geotechnical properties of
problematic soils. This method involves the use of a hirail rig and is usually attached
with three vertical probes inserted to a target depths in problematic soils, usually
within the seasonal moisture fluctuation zone at approximately 2–4 m. Cementitious
agents like slurry of lime and fly ash is injected under a typical hydraulic pressure of
800–1,000 kPa and ceases when slurry is observed to break out at the surface, or
when a maximum pressure of 1,450 kPa is reached (Kayes et al. 2000). Also
presented the detailed field and laboratory studies of a lime/fly ash stabilized site at
Breeza, NSW, Australia.
2.3 Shallow Mixing Methods
Used for stabilizing weak or contaminated ground at relatively shallow depth.
In this method, mixing shaft (kelly bar) is attached to a single flight auger which
sluice the soil loose and lifts it slightly to six beater bars on the mixing shaft. As the
auger penetrates the soil, a slurried reagent is pumped through the mixing shaft and
exits through jets located on the auger flighting.
2.3.1 Properties of Flyash Modified With Chemical Additives
There are number of chemical additives such as lime, gypsum, cement etc. are
used to modify/ improve the geotechnical characteristics of flyash materials and hence
the strength of flyash materials used for fills can be improved as to meet the design
requirements. Moghal and Sivapullaiah (2011) presented the effects of addition of
lime and lime along with gypsum on the compressibility behavior of two class F fly
ashes.
11
Figure 2.3 Shear strength of phosphatic clay ponds stabilized by lime column (Source:
Hardianto and Ericson, 1994)
It was found that the compressibility of lime modified specimens slightly
higher than the specimen stabilized with both lime and gypsum. However, in general
it is observed that addition of lime to flyash found to reduce the compressibility value.
The optimum gypsum value was found out to be 2.5%. It was also found that, the
effect of improvement in compressibility characteristics is significant up to certain
lime content beyond which it becomes less.
Similarly Ghosh and Subbarao 2007 studied the shear strength characteristics
of a low lime class F fly ash modified with lime alone or in combination with
gypsum. They observed the gain in unconfined compressive strength of the fly ash
was 2,853 and 3,567% at 28 and 90 days curing under unsoaked condition
respectively, for addition of 10% lime along with 1% gypsum to the fly ash. Also the
value of cohesion has increased up to 3,150% after 28 days cured sample. However
reduction in strength varying from 30 to 2% was observed after 24 h soaking of
modified specimens depending on mix proportions and curing period. The modified
fly ash shows the values of Skempton’s pore-pressure parameter, Af similar to that of
over consolidated soils. Ghosh and Subbarao (1998) modified class F flyash with lime
12
and combination of lime and gypsum to make it suitable for structural fill in road
bases and embankments and for use in impermeable barriers, such as covers and liners
and cutoff trench walls, minimizing the potential for ground water contamination.
Their study showed all the mixes of fly ash and lime or fly ash lime and gypsum,
reduction in hydraulic conductivity with an increase in the curing period. Also
stabilized compacted low lime fly ash mixed with 10% lime and 1% gypsum after 28
days of curing could produce an impermeable layer useful for base layers or waste
containment liners with permeability on the order of 8 X 10-8 cm/s from fly ash with
permeability 4.5 X 10-' cm/s.
The leaching of calcium in lime modified fly ash is more for a lower
percentage of lime addition and decreases with addition of higher lime content up to
10% for class F fly ash. Addition of only 1% gypsum is very effective in reducing the
concentration of calcium in the leachate from compacted fly ash–lime–gypsum
specimens that reduces the concentration of calcium in the leachate from 540 to 80
ppm for the 28-day cured specimen Ghosh and Subbarao (2006).
2.3.2 Stabilization/Densification of Deposited Flyash- Field/Laboratory Trials
Number of research has been conducted in field as well as in laboratory to
improve the density of ash by different techniques such as vacuum dewatering, electro
osmosis, vibro compaction, stone columns, blasting (Gandhi et al. 1997). Gandhi et al.
(1999) presented the results of 90 deep blasts carried out to densify a 12-m-thick fly
ash deposit in an ash pond at Mettur Thermal Power Station, Tamil Nadu, India. Fig
2.4-2.5 shows the variation of SPT ‘N’ and cone penetration resistance along depth of
fly ash bed before and after blast.
The results of CPTs show that there is no significant improvement near the
surface. The surface may require secondary compaction. Chand and Subbarao 2007
reported the effectiveness of in-place treatment of an ash deposit by hydrated lime
column. Hydrated lime column was applied to laboratory model of deposited flyash
slurry. It was reported that the lime column method found to increase the unconfined
compressive strength and reduce hydraulic conductivity of pond ash deposits in
addition to modifying other geotechnical parameters.
13
Figure 2.4 Variation of cone resistance before and after blasting (source: Gandhi et al. 1999)
Also showed the contamination potential of the ash leachates from deposited
ash slurry is greatly reduced by lime column method that helps in mitigating the
adverse environmental effects of ash deposits. After one year, an increase of 160% in
compressive strength was achieved at a radial distance of 10 cm and the reduction in
hydraulic conductivity of 54% and 62% at top and bottom levels respectively. This is
may be due to the pozzolanic nature of the ash, and thus its capability to react with
lime and develop substantial strength. The reduction in hydraulic conductivity with
lime addition may possibly due to the formation of cementitious compounds which
causes reduction in the void spaces and in the interconnectivity of pore channels. Raju
2011 applied ground improvement using vibro techniques in stabilizing the flyash
deposits. Initial field trials were carried out to assess the bearing capacity and also the
lateral capacity of bored cast-in-situ pile foundations as a result of stone column
installation. They showed that the successful application of vibro techniques to
enhance the bearing capacity and lateral capacity of deep pile of the fly ash deposits
and also mitigation of liquefaction potential of the site. More recently Kokusho et al.
2012 applied heavy compaction method (HCM) normally used for sandy soils for
compacting flyash deposit. The improvement in flyash deposit was found from cone
penetration tests were carried before and after the compaction which indicating
14
obvious effects on soil properties and strength increase. For effective rehabilitation of
ash ponds, densification of the slurry deposit is essential to increase the bearing
capacity and to improve its resistance to liquefaction
Figure 2.5 Variation of SPT ‘N’ along depth of fly ash bed before and after blast (source:
Gandhi et al. 1999)
2.4 Scope of the Present Study
Pond ash located extensively in many places of India occupying typically
20,000 acres of land and it is likely to be increase in future. Due to scarcity of land for
constructions purposes or for storage of coal ash, there is a need to stabilize the
abandoned ash pond. There are number of methods for the stabilization of ash ponds
have been attempted in the past. This study is an attempt to utilize lime column
method for stabilization of ash ponds. The present study also includes the
development of new experimental setup to carry out the investigations systematically.
The objective of this study is mentioned below
To determine the potential of lime column in improving the strength and other
geotechnical properties of sedimented and compacted flyash deposit.
15
To examine the efficacy of lime column in mitigating the contamination potential of
sedimented and compacted flyash mass.
To study strength distribution of lime column improved sedimented and compacted
flyash mass with curing period.
To study the strength distribution of lime column improved sedimented and
compacted flyash mass in vertical direction with curing period.
16
CHAPTER 3
Materials and Methods
3.1 General
The aim of the investigation is to improve geotechnical characteristics of
sedimented and compacted flyash deposit as well as the potential of lime column
method to achieve this and to study the strength distribution surround lime column.
This chapter describes the methodology and materials used to achieve the objectives.
The flyash collected from local power plant and commercially available quick lime
are two major materials used in the present investigation. Procedure for Sample
preparation, sampling and testing techniques used for characterization of materials as
well as development of experimental setup for investigation are reported in the
following session.
3.2 Materials
The details of materials used in this study are given as follows.
3.2.1 Fly ash
3.2.1.1 Background
Generally coal based thermal power plants produces two kinds of ashes, viz.
fly ash and bottom ash in the combustion process. Flyash, finer fractions of ashes
carried by the flue gas and collected from the electrostatic precipitators of thermal
power plants. Mostly flyash particles are spherical in shape whose size ranges from
0.5 μm to 100 μm. According to ASTM C618, 75% to 80% is constituted by low lime
flyash in the total production of coal ash which generally comes under class F flyash.
However, heavier and coarser coal ash collected from the bottom of furnace is
generally referred as bottom ash which constitutes around 20–25% of the total ash
production. Although these three kinds of coal ashes possess different engineering
properties, they are synonymously called ‘fly ash’ unless otherwise specifically
referred. Coal ashes mainly consist of silicon dioxide (SiO2), aluminum oxide (Al2O3)
and iron oxide (Fe2O3). However, silicon dioxide (SiO2) may present in two forms: 1)
amorphous, which is rounded and smooth. 2) Crystalline, which is sharp, pointed and
hazardous. Fly ashes are generally highly heterogeneous, consisting of a mixture of
17
glassy particles with various identifiable crystalline phases such as quartz, mullite,
and various iron oxides.
According to ASTM C618 fly ash can be classified into Class F fly ash and
Class C fly ash depending on the amount of calcium, silica, alumina, and iron content
present in the ash. There are various types of coal used such as anthracite, bituminous,
and lignite and hence the chemical properties of the fly ash produced in combustion
process are largely influenced by the chemical content of the coal burned.
Class F fly ash:
Class F fly ash produces in the burning process of harder, older anthracite and
bituminous coal. Class F fly ash is pozzolanic in nature, and contains less than 10%
lime (CaO). Usually requires a cementing agent, such as Portland cement, quicklime,
or hydrated lime, with the presence of water in order to react and produce
cementitious compounds. It contains more percentage of glassy silica and alumina.
Alternatively, the addition of a chemical activator such as sodium silicate (water
glass) to a Class F ash can leads to the formation of a geopolymer.
Class C fly ash:
This is produced from the burning of younger lignite or sub bituminous coal,
in addition to having pozzolanic properties, also has some self-cementing properties.
In the presence of water, Class C fly ash will harden and gain strength over time.
Class C fly ash generally contains more than 20% lime (CaO). Unlike Class F, selfcementing Class C fly ash does not require an activator. Alkali and sulfate (SO4)
contents are generally higher in Class C fly ashes.
Fly ashes may contain toxic and trace elements such as arsenic, boron,
chromium, copper, zinc, vanadium, and nickel. Disposing large amounts of fly ashes
into landfills can cause leaching of these heavy metals which contaminate the
groundwater and may threaten aquatic life, the environment, as well as human health.
There have been efforts to reuse fly ash materials in construction in order to decrease
the disposal rate.
18
Adequate amount (approximately 40 kN) of representative sample of Fly ash,
a by-product of thermal power units was collected from Rourkela Steel Plant (RSP),
SAIL. The above was selected due to: (1) it produces huge quantities of fly ash and
dumping is a problem. There are many adverse environmental issues associated with
the ash ponds; (2) the proposed method if found to be suitable would help in
mitigating leachate and in improving the geotechnical characteristics of ash pond.
3.2.2 Lime
The commercially available superior grade quick lime was used to prepare
lime column. Quicklime is manufactured by chemically transforming calcium
carbonate (limestone – CaCO3) into calcium oxide (CaO).
3.2.2.1 Background
Lime can be used to improve or modify some of the engineering properties of
fine grained soils. Thereby the strength and durability in the stabilized matrix can be
improved. The amount of lime additive will depend upon number of parameters such
as fines, liquid limit, plastic limit etc. The lime required for treatment of fine grained
soils is more than the coarse grained soils.
The improvement of the geotechnical properties of the soil mainly achieved by
two basic chemical reactions (1) Short-term reactions including cation exchange and
flocculation, where lime is a strong alkaline base which reacts chemically with clays
causing a base exchange. Calcium ions (divalent) displace sodium, potassium, and
hydrogen (monovalent) cations and change the electrical charge density around the
clay particles. This results in an increase in the inter particle attraction causing
flocculation and aggregation with a subsequent decrease in the plasticity of the soils.
(2) Long-term reaction including pozzolanic reaction, where calcium from the lime
reacts with the soluble alumina and silica from the clay in the presence of water to
produce stable calcium silicate hydrates (CSH), and calcium aluminate hydrates
(CAH), and calcium alumino silicate hydrates (CASH) which generate long-term
strength gain and improve the geotechnical properties of the soil.
The use of lime for soil stabilization is either in the form of quicklime (CaO)
or hydrated lime Ca(OH)2. The chemical reaction between quicklime (CaO) and water
19
resulting in formation of hydrated lime Ca(OH)2 . The addition of water to quicklime
(CaO) is referred to as slaking.
High calcium quicklime + water = Hydrated lime + Heat
CaO + H2O = Ca(OH)2 + Heat
Various forms of lime sometimes used in lime stabilization applications are
dehydrated dolomitic lime, monohydrated dolomitic lime, and dolomitic quicklime.
In the present study, quicklime (CaO) in a dry form was used to form lime column. In
general, the properties of lime treated sols are dependent on many factors such as soil
type, lime type, lime percentage, and curing conditions (time, temperature, and
moisture) and hence the strength and durability
3.3 Methods
3.3.1 General
The present experimental program investigates the efficacy of lime column
method to stabilize sedimented flyash deposits. A large scale laboratory model test
tank was made in which flyash slurry allowed to fall from constant height. Both
sedimentation and consolidation under its own weight of flyash slurry were allowed to
occur for a period of 30 days in a laboratory environment simulated as close to the
same expected in ash ponds. Lime column was installed in center of sedimented
flyash deposit after initial sedimentation period and the sample was saturated.
Unconfined compressive strength tests, direct shear tests and hydraulic conductivity
tests were conducted on flyash specimens extracted from the sedimented fly ash
deposits at different radial and vertical distances. The improvement in the
geotechnical parameters was observed over a stabilization period of 90 days. Similar
studies were extended to compacted flyash samples. A detailed experimental program
adopted in the present study is given in the following sections.
3.3.2 Preparation of Flyash Sample in Test Tank
In the present study, four numbers of test tanks of size 1.1 m diameter and 1.2
m height was used. The schematic diagrams of test tank with sample and other
arrangements are shown in Fig 3.1. Two types of flyash samples were prepared i.e.
20
sedimented flyash slurry deposit and compacted flyash. Detailed procedures adopted
for preparation of samples have been mentioned in the following sections.
3.3.3 Simulation of Sedimentation of Ash Slurry
The amounts of water required for the flowable flyash slurry were determined
from step-by-step water addition, and mixing of flyash. Significant variation in
viscosity was observed with mixing time of flyash slurries. A conventional mixer
machine was used to prepare the slurries in the laboratory. Mixing time of 10 minutes
was adopted to obtain good workable flyash slurry. The average initial moisture
content was determined by random sampling method. Finally, to obtain good flowable
flyash slurry water to flyash ratio was fixed at 75%.
The prepared flyash slurries were allowed to fall from a constant height of 1 m
into the test tank. Fig. 3.1 shows the photograph of flyash slurry preparation in test
tank with arrangements to measure variation of temperature. Fig. 3.2 and Fig. 3.3
shows the test setup for sedimentation consisted of a circular tank open at the top and
fitted with a drainage bed and a perforated base plate at the bottom. Before placing
slurry in the test tank, a steel casing of size equal to size of lime column covered with
fiber mesh of small aperture was placed exactly at the center of test tank and a number
of temperature sensors were also inserted at different predetermined places of tank to
record the variation of temperature in the installation and hydration of lime column.
Figure 3.1 Photograph of slurry flyash in test tank and other setups
21
0.1 m
W.T
0.1 m
3
1
0.5 m
2
1.0 m
5
0.1 m
4
1.0 m
Figure 3.2 Schematic diagram of lime column installed along full length of deposit in a test
tank with other setups
Fig 3.1 shows the casing and sensors holder. After filling the test tank with
flyash slurry, consolidation due to the self-weight of fly ash slurry deposited
hydraulically in an ash pond/lagoon has been simulated in the laboratory environment.
After all the ash particles settled into the bottom test tank, the excess water was
removed through the bottom drainage arrangement. The test tank was covered with
polythene sheets and the ash was allowed to remain in place for an initial
sedimentation period of 30days to facilitate sedimentation and consolidation under
self-weight. Mitchell and Soga 2005 reported that the average moisture content of the
slurry typically varied from 74 to 81% and is higher than the liquid limit of 60%.
Hence sedimented deposits are likely to induce metastable open fabric in such higher
liquid limits.
3.3.4 Compaction of Flyash in Test Tank
Similar test tanks and setups as shown in Figs 3.2 and Fig 3.3 were used to
carryout lime column experiments in case of compacted flyash. Flyash was
22
compacted at Standard Proctor MDD 11.4 kN/m3 and 92.7% of OMC (41.04 %)
values.
0.1 m
W.T
0.1 m
3
0.2 m
1
2
1.0 m
5
0.1 m
4
1.0 m
Figure 3.3 Schematic diagram of test tank for 0.2 m lime column and other setups
(1) Stand pipe
(2) Temperature sensors
(3) Lime column with casing
(4) Sand bed (drainage layer)
(5) Drainage pipe (outlet)
The values of dry density and water content of the flyash used in the study were
guided by the values typically expected in field situations. For each test tank, the
weight of flyash required to compact at proctor density is 8.59 kN. The volume of test
tank was divided into ten equal parts by marking horizontal lines. The required bulk
mass of flyash for each layer was then divided into 8 equal fractions. Each fraction of
flyash was thoroughly mixed using conventional laboratory mixer machine with
desired amount of water to get wet flyash mass with average moisture content of 38
23
%. At the end of thorough mixing, the fractions were compacted in the test mould
using rammer.
Figure 3.4 Photograph shows the preparation of flyash bed at standard proctor density
The thickness of each compacted layer was approximately 10 cm. To ensure
homogeneity and bonding between the separate layers, each layer was scarified
mechanically before compacting the next soil layer. Similarly each layer was
compacted in a test tank. Fig 3.4 shows Photograph of flyash bed preparation at
standard proctor density. The sample in the test tank was then saturated by external
water supply units. A small thickness of about 1cm free standing water was
maintained throughout the stabilization period.
3.3.5 Installation of Lime Column
After the initial sedimentation period of 30 days, a small amount of water was
standing in the casing and was removed using vacuum pump, and a quick lime
powder was poured at the central casing in the test tank. The mass of lime required for
lime column at full length of flyash bed was found to be 5.5 kg. The required bulk
mass was divided into ten equal portion. After placing each portion of total mass, a
slight compaction using specially fabricated hammer was adopted. Thus, a column of
lime was neatly formed at the center of the sedimented flyash bed. (Note: This study
aims at to study strength distribution of flyash mass due to lime column inclusion and
24
hence a volume change due to expansion of lime column was prevented with the help
of casing). However in practical cases the improvement was normally achieved by
both expansions and migration of ions due to inclusion of lime column.
Similarly lime of required size was formed in all the test cases. After
successive installation of lime column, a weight of around 10 kg was placed over
casing, and sample was then saturated. The initial cracks at the surface of bed were
formed followed by reduction in the water content.
This is due generation of
enormous amount of heat in the hydration of lime. Since no expansion of lime column
was allowed in the hydration process, the improvement of any geotechnical properties
was solely due to migration of calcium and hydroxyl ions into the surrounding soil
mass. Water was continued to be supplied at the top portion to the quick lime until
the exothermic process had subsided and the lime was in a slurry state. A small
thickness of about 1cm free standing water was maintained throughout the
stabilization period. Further, a quick lime was used to form the lime column so that
cracks could be generated in the compacted mass by the heat generated during
hydration of the quick lime (Bell, 1988). These generated cracks were expected to
provide pathways for migration of calcium and hydroxyl ions from the lime pile into
the soil mass.
During the stabilization period, the test tank was uncovered at the end of every
7 days and a small amount of water each time was added to maintain the hydrated
lime in a slurry condition. At the end of 45 and 90 days curing period, samples were
extracted from pre-determined locations for evaluation of improvements in
geotechnical properties of lime column treated flyash specimens.
3.3.6 Sampling Program
Fig. 3.5 and Fig 3.6 shows the locations and depths of Samples extracted to
study improvements in geotechnical characteristics of lime column treated specimens.
Sampling tubes having 10 cm external diameter and 15 cm length were used to extract
samples in order to determine the geotechnical parameters such as water content,
density, shear strength parameters, unconfined compressive strength and hydraulic
conductivity. Shear strength parameters were determined for samples collected from
radial distance of 20 cm, 30cm and 45cm at various depths of 10cm, 30cm, 50cm,
70cm and 90cm respectively. In order to obtain samples after stabilization periods of
25
30 days and 90 days, a partition was made by inserting thin GI sheet into the flyash
bed to avoid caving and or heaving at the time of sampling. The partition was done by
inserting individual GI sheets as shown in Fig 3.5. Hydraulic jack reaction frame
assembly was used for inserting GI sheets into the flyash mass so that the disturbance
of the surrounding mass will be avoided. The excess water standing over the flyash
bed was removed by using vacuum pump. A sampling tube was pushed into the top
layer flyash bed at various locations and depths as shown in Fig 3.5. The various tests
as mentioned above were carried out from the extracted samples. The diameter of
sampler used is exactly equal to the diameter of permeability mould i.e 10 cm
diameter and also the height of mould was 12.5 cm which is quite less than the height
of sampler. The samples obtained in the sampler were safely transferred into the
permeability mould and excess portion was then trimmed off. The mold assembly
with the ash sample was connected to a constant head permeameter system, which
accommodate three numbers of molds that could be operated independently. Average
permeability was determined for each extracted samples by allowing water to flow
through the samples under a constant pressure head of 1.5 m.
To conduct unconfined compression test, a thin sampling tube of 36mm
diameter and 78mm height was inserted in the permeability mould and samples were
collected. The samples for unconfined compressive strength were obtained in the thin
sampling tube are trimmed to make final size of 36mm diameter and 72mm height.
For direct shear test, a square sampling device of size 6cm x 6cm x 2.5 cm was
pushed in the samples obtained directly from sampler. Prior to testing, a
representative samples were trimmed and made surface flat to bring required testing
size of samples.
The density and moisture content of flyash for different predetermined
locations and positions were determined from the samples directly obtained from the
bed. Enough care was taken in the process of insertion of sampler into the ash bed to
obtain least disturbed samples for the representative testing. Similar procedure was
adopted for sampling corresponding to different stabilization periods.
26
0.2
0.2
0.1 0.1
0.05 0.1 0.1 0.1 0.1 0.05
0.1
0.2
0.2
1.1
0.1
1.0
All dimensions are in m
Figure 3.5 Elevation of test tank with detailed Sampling location
30 day
Figure 3.6 Plan view of test tank with detailed Sampling location
27
3.4 Test procedures
3.4.1 Specific Gravity
The specific gravity of fly ash were determined using pycnometer method as per IS:
2720-Part 3 (1980).
3.4.2 Particle Size Distribution
Particle size distribution of flyash was determined using hydrometer method in
accordance with IS: 2720- part 4 (1975). The flyash sieved through 75μm sieve size
and samples was collected carefully. The flyash passing 75 μm was used for particle
size analysis and the analysis was performed using Hydrometer method.
3.4.3 Compaction Test
Compaction curves of flyash were obtained for both standard and modified
compaction energies. The water – density relation of flyash using light compaction
was determined in accordance with IS: 2720-Part 7 (1983). The Modified Proctor
compaction test helps to provide a higher standard of compaction. The same was
performed to determine the relationship between dry density and moisture content of
the flyash as per the procedure given in IS: 2720-Part 8 (1983). In case of modified
proctor test, the required amount of sample was compacted in the standard mould in
five layers using 4.9kg rammer with 450mm height of fall and by giving 25 blows in
each layer. The compaction energy value of modified proctor is approximately 4.55
times the compaction energy of standard proctor.
3.4.4 Unconfined Compressive Strength Test
Unconfined compression strength (UCS) tests used mostly in order to verify the
effectiveness of the stabilization with lime and or to study the influencing factors for
the strength of lime-treated mass. Since this test has several advantages such as
simple, fast, reliable and cheap. The UCS tests were conducted according to IS: 2720Part 10 (1991). The UCS test was performed on stabilized and unstabilized fly
samples by using conventional compression testing machine. The 2kN capacity
proving ring was used since the soaked flyash samples soft in nature while having
lesser compressive strength. The size of the tested specimens is 72 mm height and 36
mm diameter. The test was continued till failure or maximal vertical strain according
28
to IS: 2720-Part 10 (1991) is equal to 20% of the height of the specimen which
corresponds to a deformation of 14.4 mm (whichever is earlier). More specifically,
unconfined compressive strength of specimen can be defined by the strength
corresponding either at the failure stage or at the maximal vertical strain (ε) equal to
20% of the original height whichever occurring first. In the present study, the
specimens were sheared at a strain rate of 1.2mm/min for both stabilized and
unstabilized samples.
3.4.5 Direct Shear Test
The shear parameters of flyash specimens were determined as per IS: 2720 (Part 13)
1986. The specimens were collected by inserting sampling device of size
60mm×60mm×25mm into the samples collected in the sampler. The specimens were
trimmed and levelled prior to testing. All the specimens were sheared at a rate of 0.2
mm/ min in a motorized direct shear machine. The shear strength parameters (i.e. cp
and ϕp values) were determined by varying normal stress of 0.5 kg/cm2, 1kg/cm2, 1.5
kg/cm2 and 2kg/ cm2.
3.4.6 Permeability Test
The coefficient of permeability of flyash specimens (both stabilized and unstabilized)
were determined as per IS: 2720 (Part 36) 1975. The samples were collected by
inserting sampler of size 10 cm diameter and 15 cm height into the flyash bed at
predetermined locations. Then these samples were transferred to the permeability
mould of size 10cm diameter and 12.5 cm height. The excess portion was trimmed off
and levelled. The permeability mould consists of detachable collar, drainage base and
cap. Average permeability was determined for each sample by allowing water to flow
through the samples under a constant pressure head of 1.5 m.
29
CHAPTER 4
Results and discussion
4.1 Introduction
Huge quantities of coal fly ash are produced every year as a residue from coal
based thermal power plants in all over the world. Safe disposal and utilization of such
large quantities of flyash is a major concern. The percentage utilization of flyash is
rather limited in India than most of the advanced countries. Coal ash is a general term
given to both flyash and bottom ash. Normally both fly ash and bottom ash from
thermal power plant is sluiced with sufficient amount of water to form flyash slurry,
transported and deposited in pond in the vicinity of plants. The clear decanted water
after settling of flyash particle is discharged into a natural stream. Such ash ponds
normally forms a soft ground of high water content and high fines content with small
strength and high deformability particularly under the water table. Typically 20,000
ha of land are occupied by ash ponds. Various problems being encountered with the
ash ponds includes dusting problems, increasing the level of solid suspended
particulate materials in the air, low bearing capacity and large settlement. Hence it is
considered to be unsuitable for supporting any structural load. Also, the leachates
emanating from the ash ponds may lead to contamination of surface water and
groundwater bodies, as well as soils depending on the amount of toxic elements it
contains.
A more economical and suitable soil improvement method such as the lime
column method may possibly be used to convert suitable for construction purposes.
Present work used lime column method to improve geotechnical characteristics of
sedimented flyash deposits. The variation of strength (both vertical and horizontal
direction) surround lime column in compacted and slurry fly ash materials are studied
in the large scale laboratory model. All the results of the above investigation and their
corresponding analyses have been presented in different section as mentioned below:
I. Characteristics of materials used.
II. Results of geotechnical properties of sedimented flyash slurry surround lime
column.
30
II. Results of geotechnical properties of compacted flyash surround lime column.
4.2 Characteristics of Materials Used
4.2.1 Physical Properties
Approximately 4 tons of Loose fly ash samples was collected in dry form
which having average water content less than 1%. Most of the fly ash had a powdery
structure with medium to dark grey color indicating low lime.
4.2.2 Specific Gravity
The specific gravity of fly ash was obtained as per IS: 2720-Part 3 (1980) and
it is found to be 2.44. The specific gravity of fly ash is considerably lesser than other
conventional filling materials possibly due to the presence of small hollow spherical
particle called cenospheres and lesser iron content. In general, materials having higher
iron content will have high specific gravity.
4.2.3 Particle Size Distribution
The particle size distribution curve of fly ash is shown in Fig 4.1. Form the
graph, the value of D10 (Diameter of particle corresponding to the 10% finer), D30
(Diameter of particle corresponding to the 30% finer) and D60 (Diameter of particle
corresponding to the 60% finer) are obtained.
D10 = 0.0046; D30 = 0.0192; D60 = 0.0384
100
Percentage Finer, (%)
90
80
70
60
50
40
30
20
10
0
0.001
0.01
0.1
Particle Size, (mm)
Figure 4.1 Particle size distribution curves of fly ash
31
1
The coefficient of uniformity i.e Cu = D60/D10 and coefficient of uniformity i.e Cc =
D302/ (D10* D60) are calculated and representative values are 8.34 and 2.08
respectively.
Table 4.1 Physical Properties of Flyash
Physical parameters
Values
Colour
Medium grey
Silt size
8.13%
Shape
Rounded/sub-rounded
Uniformity coefficient, Cu
8.34
Coefficient of curvature, Cc
2.08
Specific Gravity, G
2.44
Plasticity Index
Non- plastic
4.2.4 Engineering Properties
The engineering properties of untreated flyash are shown in Table 4.2 which
includes compaction and strength characteristics at different states. The Strength
parameters (Peak values) from direct shear tests of flyash slurry are comparably lower
than the compacted flyash mass at its standard proctor density.
Table 4.2 Geotechnical properties of fly ash
Property
Fly ash
1. Compaction characteristics
From Light compaction or Standard Proctor test
a) Maximum dry density (kN/m3)
11.40
b) Optimum moisture content (%)
41.04
From Heavy compaction or Modified Proctor test
a) Maximum dry density (kN/m3)
12.24
32
b) Optimum moisture content (%)
34.17
2. Permeability (cm/sec) of
a) sample compacted at standard Proctor density
1.86x10-5
b) sample poured in slurry form
2.84X10-5
3. Shear strength parameters from direct shear test of
sample compacted at Standard Proctor density
a) cohesion, cp (kPa)
25.23
b) friction angle, ϕp
42.8
3. Shear strength parameters from direct shear test in Slurry
state
a) cohesion, cp (kPa)
16.94
b) friction angle, ϕp
37.2
14
Dry unit weight, γd (kN/m3)
13.5
Standard Proctor
13
Zero air void line
12.5
Modified Proctor
12
11.5
11
10.5
10
9.5
9
15
20
25
30
35
40
Moiture content, w(%)
Figure 4.2 Relation between moisture content and dry density
28
45
50
The optimum moisture content of flyash sample corresponding to maximum
dry density in case of sample compacted at standard proctor energy and modified
proctor energy was found to be 41.04 % and 34.17% respectively. The maximum dry
density (MDD) achieved through standard proctor effort of fly ash is less than that of
sample compacted corresponding to modified proctor effort. The MDD of flyash at
standard and modified proctor effort were 11.4 kN/m3 and 12.24 kN/m3 respectively.
Fig 1 show the relationship between water content and dry density of flyash
specimens compacted at standard and modified proctor effort.
Shear strength
parameters (cp, ϕp) from direct shear test of flyash samples of different states i.e slurry
and compacted was found to vary significantly at soaked condition. However it was
observed that the strength parameters of compacted specimens decrease under
soaking. This is possibly due to reduction in the suction pressure in the increase of
degree of saturation.
4.3 Geotechnical Properties of Sedimented Flyash slurry Surround Lime
Column
The changes in varies geotechnical properties such as water content, dry
density, shear strength parameters and unconfined compressive of flyash samples
extracted at different locations and positions of lime column included flyash bed at
different stabilization periods of 30 day and 90 day was reported. The variation of
temperature in the flyash bed after inclusion of lime column was measured using
temperature sensors and the values of which are also reported.
4.3.1 Water content
The variation of water content along depth of sedimented flyash slurry after
inclusion of lime column (both 0.2m and 1m) was shown in Fig 4.3 and 4.4. Form the
figures it is observed that the inclusion of lime column reduces the water holding
capacity of flyash mass. In both the cases, the water content at top of the flyash bed
shows higher value. However it is found to decrease with stabilization time. The
reduction in water content occurs higher at middle portion of stabilized mass and it
increases with stabilization time.
29
Moisture content, W (%)
43
48
53
58
0
Depth below surface, D (cm)
10
unstabilized flyash
20
30
Radial distance in cm
40
10
50
20
60
30
70
45
80
90
100
Figure 4.3 variation of moisture content with depth (lime column installed at full depth)
Moisture content, W (%)
40
42
44
46
48
50
52
54
56
58
0
Depth below surface, D (cm)
10
20
30
Radial distance in cm
40
50
10
20
30
45
60
70
80
90
100
- Indicate the Results obtained after 30 days of stabilization period
- Indicate the Results obtained after 90 days of stabilization period
Figure 4.4 Variation of moisture content with depth (lime column installed at 0.2m depth)
30
60
4.3.2 Dry density
The variation of dry density along depth of sedimented flyash slurry after inclusion of
lime column (both 0.2m and 1m) was shown in Fig 4.5 and 4.6.
Dry unit weight, γd (kN/m3)
9
9.5
10
10.5
11
11.5
12
0
Depth below surface, D (cm)
10
20
Radial distance in cm
30
10
unstabilized mass
40
20
50
30
60
45
70
80
90
100
Figure 4.5 Variation of dry density with depth (lime column installed at full depth)
Dry unit weight, γd (kN/m3)
9.6
9.8
10
10.2
10.4
10.6
10.8
0
Depth below surface, D (cm)
10
20
30
40
50
60
70
Radial distance in cm
10
20
30
45
80
90
100
- Indicate the Results obtained after 30 days of stabilization period
- Indicate the Results obtained after 90 days of stabilization period
Figure 4.6 Variation of moisture content with depth (lime column installed at 0.2m depth)
31
11
The dry density of flyash slurry is lower at top portion due to reduction in the
confinement pressure occurs with less surcharge and higher water content. The rate
of increase of dry density of flyash mass is higher in the middle portion compared to
top and bottom. Wide randomness in the parameters obtained possibly due to methods
adopted for testing, sampling and other factors. However, in general it is found that
the inclusion of lime column in flyash mass decreases the dry density with
stabilization time.
The dry density of sedimented flyash slurry stabilized by 0.2m lime column,
shows increasing linear trend with depth of flyash bed. Wide randomness in the
parameters obtained possibly due to methods adopted for testing, sampling and other
factors. However, in general it is found that the inclusion of lime column in flyash
mass decreases the dry density with stabilization time.
4.3.3 Unconfined compressive strength
The variation of unconfined compressive strength along depth of sedimented flyash
slurry after inclusion of lime column (both 0.2m and 1m) was shown in Fig 4.7 and
4.8.
Unconfined compressive strength, qu (kN/m2)
0
50
100
150
200
250
300
0
Range of qu after
30 days of curing
20
30
unstabilized
40
Influence
zone
Depth below surface, D (cm)
10
50
60
Radial distance in
70
80
10
20
30
45
Range of qu after
90 days of curing
90
100
Figure 4.7 Variation of unconfined compressive strength with depth (lime column installed at
full depth)
32
The Unconfined compression strength (UCS) is used as an indicator to find the
effectiveness of lime column. In the present study, the specimens were sheared at a
strain rate of 1.2mm/min for both stabilized and unstabilized samples. The middle
portion of flyash mass gets stabilized rapidly as compared to top and bottom portions.
However it is found that the strength of flyash mass around lime found to increase
with stabilization time. The enhancement in strength occurs to a distance of 2 D to 3
D (where D is the diameter of lime column). This also confirms from the results of
dry density and water content as discussed in the previous section.
Unconfined compressive strength, qu (kN/m2)
40
60
80
100
120
140
160
180
200
0
Depth below surface, D (cm)
10
20
30
40
Radial distance in cm
50
10
20
30
45
60
70
80
90
100
- Indicate the Results obtained after 30 days of stabilization period
- Indicate the Results obtained after 90 days of stabilization period
Figure 4.8 Variation of unconfined compressive strength with depth (lime column installed at
0.2m depth)
As disused in the last paragraphs, the UCS value of flyash mass shows
increasing linear trend with depth in case of 0.2m lime column. It is found that the
strength of flyash mass around lime found to increase with stabilization time. The
enhancement in strength occurs to a vertical distance of 3 D to 4 D (where D is the
diameter of lime column). This also confirms from the results of dry density and
water content as discussed in the previous section.
33
4.3.4 Shear strength parameters
The shear parameters of flyash specimens were determined as per IS: 2720 (Part 13)
1986. The shear strength parameters (i.e. cp and ϕp values) were determined by
varying normal stress of 0.5 kg/cm2, 1kg/cm2, 1.5 kg/cm2 and 2kg/ cm2. The wide
range of randomness in the test results may be due to methods adopted for sampling
and testing. The shear parameters (i.e. cp and ϕp values) were found to increase with
stabilization time. (See figure 2.1 reported by Horiuchi et al. 2000)
Peak friction angle, ϕp (Degrees)
0
10
20
30
40
50
60
70
Depth below surface, D (cm)
0
10
20
30
40
Radial distance in cm
50
20
60
30
70
45
80
90
100
Figure 4.9 Variation of peak friction angle with depth (lime column installed at full depth)
0
20
Cohesion, Cp (kN/m2)
40
60
80
100
Depth below surface, D (cm)
0
10
20
30
40
Radial distance in cm
50
20
60
30
70
45
80
90
100
- Indicate the Results obtained after 30 days of stabilization period
- Indicate the Results obtained after 90 days of stabilization period
Figure 4.10 Variation of cohesion with depth (lime column installed at full depth)
34
Peak friction angle, ϕp (Degrees)
30
35
40
45
50
0
Depth below surface, D (cm)
10
20
30
40
50
Radial distance in cm
60
20
70
30
80
45
90
100
Figure 4.11 Variation of peak friction angle with depth (lime column installed at 0.2m depth)
Cohesion, Cp (kN/m2)
0
10
20
30
40
50
60
70
80
0
Radial distance
in cm
Depth below surface, D (cm)
10
20
30
20
40
30
50
45
60
70
80
90
100
- Indicate the Results obtained after 30 days of stabilization period
- Indicate the Results obtained after 90 days of stabilization period
Figure 4.12 Variation of peak friction angle with depth (lime column installed at 0.2m depth)
35
4.3.5 Permeability
The average value of coefficient of permeability of flyash specimens were determined
as per IS: 2720 (Part 36) 1975. The samples were collected by inserting sampler of
size 10 cm diameter and 15 cm height into the flyash bed at predetermined locations.
Then these samples were transferred to the permeability mould of size 10cm diameter
and 12.5 cm height. The excess portion was trimmed off and levelled. Then the
average permeability was determined for each sample by allowing water to flow
through the samples under a constant pressure head of 1.5 m.
Table 4.3 Permeability of flyash with depth (lime column installed at full depth)
Depth Hydraulic Conductivity in (cm/sec)
(cm)
Radial Direction in cm
10
20
30
45
10
1.85X10-5 1.93X10-5 1.1X10-5
2.04X10-5
30
1.83X10-5 1.57X10-5 1.38X10-5
1.18X10-5
50
1.83X10-5 1.51X10-5 1.336X10-5 1.21X10-5
70
1.75x10-5
1.56X10-5 1.43X10-5
1.86X10-5
90
1.29X10-6 1.45X10-5 1.68X10-5
2.14x10-5
Table 4.4 Permeability of flyash with depth (lime column installed at 0.2m depth)
Depth
Hydraulic Conductivity in (cm/sec)
(cm)
Radial Direction in cm
10
10
20
30
45
2.47X10-5
2.8X10-5
3.00X10-5
2.14X10-5
36
30
1.56X10-5 1.56X10-5
1.5X10-5
1.73X10-5
50
1.62X10-5 1.62X10-5
1.5X10-5
1.55X10-5
70
1.23x10-5
1.4X10-5
1.46X10-5
1.87X10-5
90
9.86x10-6
1.13X10-5 1.86x10-5 1.11x10-5
The wide range of randomness in the test results may be due to methods adopted for
sampling and testing.
4.3.6 Temperature
The variation of temperature after inclusion of lime column (both 0.2m and 1m) in
sedimented flyash slurry was shown below
25
Initial
24
1 Hour
3 Hour
Temperature (0C)
23
5 Hour
7 Hour
22
9 Hour
21
20
19
18
1
11
21
31
41
51
61
Radial Distance, RD (D)
Figure 4.13 variation of temperature with RD (lime column installed at full depth)
In all the test cases samples were kept saturated. There are three possible mechanisms
which responsible for gain/ loss of strength of flyash while soaking. 1) Soaking of the
specimens may fill the specimen voids to certain extent and thereby it reduces
development of suction in the pore fluid. 2) Soaking may cause softening of the
specimens and thus reducing the shear strength. 3) During soaking, the specimens
37
may get sufficient moisture required for pozzolanic reaction which may help to
increase the shear strength by the formation of reaction products.
21
Temperature (0C)
20
19
18
Initial
1 Hour
2 Hour
3 Hour
10 Hour
21 Hour
24 Hour
17
16
15
1
11
21
31
41
51
Radial Distance, RD (cm)
Figure 4.14 Variation of temperature with RD (lime column installed at 0.2m depth)
21
Temperature (0C)
20
19
18
Initial
1 Hour
17
9 Hour
10 Hour
16
21 Hour
24 Hour
15
1
11
21
31
41
51
Vertical Distance in cm
Figure 4.15 variation of temperature with vertical distance (lime column installed at
0.2m depth)
In this investigation, it is observed that the strength of sedimented fly ash in the top
portion has reduced compared to bottom and middle portion for 45 days and 90 days
38
cured samples which implies that the former two mechanisms i.e., probability of low
suction development in soaked specimens and softening of the specimens have
dominated over the third mechanism of gain in shear strength due to pozzolanic
reaction in presence of sufficient moisture.
Test results of saturated specimens may be used in practice to avoid the
assessment of development of suction in partially saturated specimens. The reduction
in strength due to soaking is also governed by the hydraulic conductivity of the
stabilized matrix. (See variation of moisture content).The reduction in strength at
bottom of sedimented flyash occurs possibly due to low solubility of lime. In lime pile
technique, the soil is stabilized through the physico-chemical reactions assisted by ion
migration. Usually in lime piles, greater portion of the lime stays at the bottom of the
borehole without getting dispersed into the surrounding soil due to the low solubility
of lime, and takes longer time for migrating into the surrounding soil.
4.4 Geotechnical Properties of Compacted Flyash Surround Lime Column
The changes in varies geotechnical properties like water content, dry density, shear
strength parameters and unconfined compressive of flyash samples extracted at
different locations and positions of lime column included flyash bed at different
stabilization periods of 30 day and 90 day was reported.
4.4.1 Water content
The variation of water content along depth of sedimented flyash slurry after inclusion
of lime column (both 0.2m and 1m) was shown in Fig 4.16 and 4.17. It is observed
that the inclusion of lime column reduces the water holding capacity of flyash mass.
In both the tested cases, the water content at top of the flyash bed shows higher value.
However it is found to decrease with stabilization time. The reduction in water
content occurs higher at middle portion of stabilized mass and it increases with
stabilization time. Upon saturation, compacted flyash mass also shows similar trend
as observed in case of sedimented flyash ash slurry.
39
Moisture content, W (%)
40
45
50
55
60
65
70
0
Depth below surface, D (cm)
10
20
30
40
50
Radial distance in
60
10
20
70
30
80
45
90
100
Figure 4.16 variation of moisture content with depth (lime column installed at full
depth)
Moisture content, W (%)
40
45
50
55
60
65
0
Depth below surface, D (cm)
10
20
30
40
50
Radial distance in
cm
10
20
30
45
60
70
80
90
100
Figure 4.17 variation of peak friction angle with depth (lime column installed at 0.2m
depth)
40
4.4.2 Dry density
The variation of dry density along depth of sedimented flyash slurry after inclusion of
lime column (both 0.2m and 1m) was shown in Fig 4.18 and 4.19
9
Dry unit weight, γd (kN/m3)
9.5
10
10.5
11
11.5
0
Depth below surface, D (cm)
10
20
30
40
Radial
distance in cm
50
60
10
70
20
80
30
90
45
100
Figure 4.18 variation of peak friction angle with depth (lime column installed at full
depth)
Dry unit weight, γd (kN/m3)
8.5
9
9.5
10
10.5
0
Depth below surface, D (cm)
10
20
30
40
Radial distance in cm
50
10
60
20
70
30
80
45
90
100
Figure 4.19 variation of moisture content with depth (lime column installed at 0.2m
depth)
41
11
4.4.3 Unconfined compressive strength
The variation of unconfined compressive strength along depth of sedimented flyash
slurry after inclusion of lime column (both 0.2m and 1m) was shown in Fig 4.20 and
4.21.
Unconfined compressive strength, qu (kN/m2)
0
50
100
150
200
250
300
0
Depth below surface, D (cm)
10
20
30
40
50
60
Radial distance in cm
70
10
20
80
30
45
90
100
Figure 4.20 Variation of UCS with depth (lime column installed at full depth)
Unconfined compressive strength, qu (kN/m2)
0
20
40
60
80
100
120
0
Depth below surface, D (cm)
10
20
Radial distance in cm
30
40
10
50
20
60
30
45
70
80
90
100
Figure 4.21 variation of UCS with depth (lime column installed at 0.2m depth)
42
140
4.4.4 Shear strength parameters
The shear parameters of flyash specimens were determined as per IS: 2720 (Part 13)
1986. The specimens were collected by inserting sampling device of size
60mm×60mm×25mm into the samples collected in the sampler.
Peak friction angle, ϕp (Degrees)
20
25
30
35
40
45
50
55
0
Depth below surface, D (cm)
10
20
30
Radial distance in cm
40
20
50
60
30
70
45
80
90
100
Cohesion, Cp (kN/m2)
0
10
20
30
40
50
60
70
Depth below surface, D (cm)
0
10
20
30
Radial distance in cm
40
20
50
30
60
70
45
80
90
100
- Indicate the Results obtained after 30 days of stabilization period
- Indicate the Results obtained after 90 days of stabilization period
Figure 4.22 variation of shear parameters with depth (lime column installed at full
depth)
43
Peak friction angle, ϕp (Degrees)
20
25
30
35
40
45
50
55
60
70
60
0
Depth below surface, D (cm)
10
20
30
40
Radial distance in cm
50
20
60
30
70
45
80
90
100
Cohesion, Cp (kN/m2)
0
10
20
30
40
50
80
0
Depth below surface, D (cm)
10
20
30
40
Radial distance in cm
20
50
60
30
70
80
45
90
100
- Indicate the Results obtained after 30 days of stabilization period
- Indicate the Results obtained after 90 days of stabilization period
Figure 4.23 Variation of shear parameters with depth (lime column installed at 0.2m
depth)
44
4.4.5 Permeability
Table 4.4 Permeability of flyash with depth (lime column installed at full depth)
Depth Hydraulic Conductivity in (cm/sec)
(cm)
Radial Direction in cm
10
20
30
45
10
1.196X10-5 1.5X10-5
2.44X10-5 2.0X10-5
30
1.85X10-5
2.43X10-5 2.92X10-5 1.77X10-5
50
1.38X10-5
1.49X10-5 2.44X10-5 3.722X10-5
70
1.09x10-5
1.31X10-5 1.74X10-5 1.87X10-5
90
1.75X10-5
1.75X10-5 1.46X10-5 2.21x10-5
Table 4.5 Permeability of flyash with depth (lime column installed at full depth)
Depth
Hydraulic Conductivity in (cm/sec)
(cm)
Radial Direction in cm
10
20
30
45
10
1.23X10-5 1.62X10-5 2.29X10-5
1.32X10-5
30
1.28X10-5 2.63X10-5 2.44X10-5
2.00X10-5
50
1.0X10-5
2.0X10-5
1.28X10-5
70
1.05x10-5
2.34X10-5 1.95X10-5 1.125X10-5
90
6.86X10-6 7.72X10-6 6.64X10-6
45
1.39X10-5
8.28x10-6
4.4.6 Temperature
The variation of temperature after inclusion of lime column (both 0.2m and 1m) in
sedimented flyash slurry was shown below.
40
Initial
1 Hour
Temperature (0C)
35
3 Hour
5 Hour
30
7 Hour
9 Hour
11 Hour
25
24 Hour
48 Hour
20
15
0
10
20
30
40
50
Radial Distance, RD (cm)
Figure 4.24 Variation of temperature with RD (lime column installed at full depth)
Temperature (0C)
20
Initial
1 Hour
2 Hour
3 Hour
10 Hour
21 Hour
24 Hour
19
18
1
11
21
31
41
Radial Distance, RD (cm)
Figure 4.25 Variation of temperature with RD (lime column installed at 0.2m depth)
46
51
20
Initial
1 Hour
Temperature (0C)
2 Hour
3 Hour
10 Hour
21 Hour
24 Hour
19
18
1
6
11
16
21
26
31
36
41
Vertical Distance, (cm)
Figure 4.26 Variation of temperature with vertical distance (lime column installed at 0.2m
depth)
In all the test cases samples were kept saturated. There are three possible
mechanisms which causes increase or decrease of strength. The development of
suction in the pore fluid may reduce due to saturation of the specimens that fill the
voids to certain extent. Saturation may soften the specimens and thus leads to
reducing the shear strength. During saturation the specimens may get sufficient
moisture for pozzolanic reaction and hence the shear strength may increase on
formation of reaction products. In this investigation, it is observed that the strength of
sedimented fly ash in the top portion has reduced compared to bottom and middle
portion for 45 days and 90 days cured samples which implies that the former two
mechanisms i.e., probability of low suction development in soaked specimens and
softening of the specimens have dominated over the third mechanism of gain in shear
strength due to pozzolanic reaction in presence of sufficient moisture. Test results of
saturated specimens may be used in practice to avoid the assessment of development
of suction in partially saturated specimens. The reduction in strength due to soaking is
also governed by the hydraulic conductivity of the stabilized matrix. (See variation of
moisture content).
47
The reduction in strength at bottom of sedimented flyash occurs possibly due
to low solubility of lime. In lime pile technique, the soil is stabilized through the
physico-chemical reactions assisted by ion migration. Usually in lime piles, greater
portion of the lime stays at the bottom of the borehole without getting dispersed into
the surrounding soil due to the low solubility of lime, and takes longer time for
migrating into the surrounding soil.
48
CHAPTER 5
Summary and Conclusions
In this investigation, potential of lime column for stabilization of ash pond was
evaluated for converting it to a usable land can be utilized for a broad range of
purposes, such as suburban housing, light commercial building, and utilities etc. Two
different states of flyash (slurry and compacted) were considered as expected in the
field. The improvements in strength of the flyash mass surround lime column are
studied through different conventional test methods such as unconfined compressive
strength and direct shear test. An experimental investigation to assess the potential of
in-place treatment of an ash deposit was carried out. In the present work, emphasis
has been given on application of the in-place lime column method for stabilization of
sedimented pond ash deposits. Since various disadvantages such as excavation,
mixing, and transportation of huge quantity of ash from the ash ponds or disposal sites
in the case of conventional mixing method can be avoided and at the same time
improvement in the engineering properties of the whole deposit can be achieved
thereby these abandoned sites may be used for construction purposes.
The lime column method was found to be effective in increasing the UCS and
reducing hydraulic conductivity of pond ash deposits along with modifying other
geotechnical parameters including water content, density. An increase of 263.26% of
UCS at a radial distance of 10 cm at top portion compared to the unstabilized ash was
observed. This may due to in-place lime stabilization confirms the pozzolanic nature
of the ash, and thus its capability to react with lime and develop substantial strength.
The formation of cementitious compounds reduces the void spaces and in the
interconnectivity of pore channels, thereby reducing hydraulic conductivity. Also this
method is also found to be useful in reducing the contamination potential of the ash
leachates.
It was observed that the lime column inclusion enhance the strength of
sedimented flyash deposit with stabilization time. Also significant improvement in
strength was observed up to a horizontal distance of 3 D (where D is the diameter of
lime column) from the center of column and vertical distance of 4 D from bottom of
lime column. A comparative study showed that the strength of stabilized mass is
49
much higher than the un-stabilized one. The method has also proved to be useful in
reducing the contamination potential of the ash leachates, thus mitigating the adverse
environmental effects of ash deposits.
50
5.1 Scope for Further Research
The investigation has certain limitation and hence all the factors that could not be
addressed in time. So the future research should incorporate the following aspects in
detail
i. The migration of lime in the ash bed was to be simulated numerically. Same should
be checked in field conditions.
ii. Stress strain behaviour of stabilized flyash mass has to be studied numerically.
iii. Design procedures of lime column should be made by conducting large number of
small scale model tests.
51
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