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International Journal of Geosynthetics and Ground Engineering (2022) 8:33
https://doi.org/10.1007/s40891-022-00377-z
ORIGINAL PAPER
Experimental Study onShear Strength Performance ofCoal Bottom
Ash Reinforced withGeosynthetics
AmitKumarRam1· YogeshKumarSharma1· SupriyaMohanty1
Received: 5 July 2021 / Accepted: 15 April 2022
© The Author(s), under exclusive licence to Springer Nature Switzerland AG 2022
Abstract
A productive utilization of ash materials may offer a sustainable solution by reducing the waste disposal problems and
overcoming the scarcity of natural, high-quality granular material. Additionally, it provides an economical alternative to
conventional geomaterials, which will encourage the economic benefits to both electricity producers and construction agen-
cies. This study focuses on the assessment of the shear strength performance of bottom ash reinforced with geotextile and
geonet. It also examines the influence of number of reinforcement layers, confining pressure, type of reinforcement and
imposed strain levels on the strength performance of reinforced bottom ash. The test results revealed that geonet has many
dominant factor over geotextile when used as a reinforcement in bottom ash. The cohesion (c) value of the reinforced bot-
tom ash was found to be increased from 4.26 to 115.21kPa and 4.26 to 107.42kPa with geonet and geotextile as reinforcing
material respectively. Similarly, the friction angle (ϕ) elevates from 46.05° to 48.80° and 46.05° to 47.48° with geonet and
geotextile as reinforcing material respectively. The application of reinforced bottom ash in the road pavement has also been
studied. The improved performance proved that the reinforced bottom ash is a promising material to be used as foundation
base material and sub-base material in the pavement.
Keywords Bottom ash· Strength performance· Geonet· Geotextile· Reinforcement
Introduction
The expansion in power consuming appliances exerts sig-
nificant pressure on the power production sectors that are
primarily fulfilled by the thermal power stations. This pri-
mary source is a significant contributor of coal ash residue.
Although these are not categorized as hazardous material,
their application in the environment results in the genera-
tion of harmful toxic compounds. Coal bottom ash particles
generally contain unburnt carbon with flaky shaped parti-
cles that can easily break into small parts when subjected to
external loads. The Central Electricity Authority [1] reported
fly ash production of the order of 217.04 Million tonnes with
an average ash content of 32.52%. The ratio of the produc-
tion of fly ash and bottom ash was reported as 80:20 [2].
Coal bottom ash is a purely non-plastic material with the
same chemical ingredients as that of fly ash. It is highly inert
in nature because of this it does not possess any pozzolanic
behavior, unlike fly ash. The absence of the binding property
supports the bottom ash application in the pavement as a
base or sub-base course and as a foundation material. The
cementation action in fly ash attracts multiple applications
in different sectors, whereas bottom ash can only be used in
a place where the binding property is insignificant. Some of
the applications of fly ash in construction industry are listed
as follows: cement replacement [3], light weight aggregates
[4], brick/cellular light weight concrete blocks [5], base/
sub-base of road [6], mine/embankment fill material [7] and
geopolymer composite [8]. There are various researches are
available on the utilization of coal bottom ash and fly ash.
However, studies related to geosynthetics reinforced coal
bottom ash are limited. The bottom ash usually have rough
texture, irregular/angular shape, coarser physical property
which gives better interlocking in the application of base
material or as substitute of fine aggregates [9]. However,
its high permeability provides better alternative source of
filter material in ash dikes [10]. Bottom ash also contains
high percentage of SiO2 and Al2O3 that is responsible for
the pozzolanic and geopolymerization property same as fly
* Supriya Mohanty
supriya.civ@iitbhu.ac.in
1 Department ofCivil Engineering, Indian Institute
ofTechnology (BHU), Varanasi221005, India
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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ash [11]. Balasubramaniam and Thirugnanam [12] found
better compressive strength of bottom ash concrete at the
later stage of curing and shows high workability due to the
presence of large particles [13]. Because of the porous and
glassy nature of the bottom ash, it shows light weight and
lower density as compared with the soil [9, 10]. It shows
more or less similar particle size distribution and more brit-
tle in nature as compared with natural sand [14]. Bottom
ash has significant water absorption capacity as compared
to natural fine aggregates [15]. The potential applications
of bottom ash in construction industry are in pavement con-
struction, as substitute of cement or aggregate in concrete,
aggregate in asphalt concrete, embankment fill, acoustic
absorbing material, pozzolanic material, and in mortar or
geopolymer paste [16–19]. The geopolymerization of bottom
ash (BA) shows lower reactivity as compared to fly ash [20]
and when it combined with calcined paper sludge (CPS) then
it shows best geopolymer synthesis only for 2:1 (BA:CPS)
proportion [21]. Also, it has been evaluated in terms of sand
replacement and found significant strength in comparison
with conventional material without compromising the com-
pressive strength of the concrete [9, 15].
Generally, soils are stable against compressive and shear
load as compared to that of tensile load. Hence, there is a
need of reinforcement in soil in order to mitigate the fail-
ure of soil against tensile load. Henry Vidal [22] first intro-
duced the concept of reinforcement in soil in the form of
steel strips. A higher modulus of reinforcement is preferred
in the structure where the strain is allowed to a marginal
extent. However, few structures are permitted to large strain
without affecting its load transfer behavior and are gener-
ally improved by using low modulus reinforcement [23].
The inclusion of any reinforcement such as natural fibers,
geosynthetics polymers in the soil provides an aid in reduc-
ing the lateral earth pressure and enhances its resistance by
virtue of which the thickness of all the structures reduces
significantly. Reinforcement was initially practiced in the
form of randomly oriented fibers followed by planar and
cellular. Among all the reinforcement forms, cellular form
proved to be the best for strength enhancement [24]. The
reason for this strength enhancement is that the cellular rein-
forcement provides lateral confinement to the specimen that
provides additional load resistance due to the involvement
of friction between the cellular material and the specimen.
A number of studies have been conducted to establish the
response between foundation and reinforced soil (e.g., clayey
soil, pond ash, fly ash) in the laboratory [25–29] and in large
scale field test [30, 31]. Xie etal. [32] analyzed the different
failure mechanism of footing resting on reinforced soil and
established dimensionless charts for the determination of the
ultimate bearing capacity of strip footing. A number of Cali-
fornia bearing ratio (CBR) experiments have been conducted
by Singh etal. [33] in order to obtain the optimum location
of a geosynthetics for ultimate load carrying capacity and
recommended that it should be kept either in the middle
or in between the upper 1/3rd layer with a middle layer for
the improvement in subgrade soil. Similarly, based on CBR
results, Ullagaddi etal. [34] observed a decrement in thick-
ness by more than 50%, considering granular soil and black
cotton soil as subgrade materials. Indraratna etal. [35, 36]
examined the breakage of recycled ballast used under the
railway tracks. They suggested that the geogrid and geo-
composite are better in reducing the breakage by ensuring
good railway track performance. Higher number of geotex-
tile reinforcement results in an increment in stiffness of geo-
polymerized fly ash mixed with residual soil [37].
Tauahmia [38] and Pando etal. [39] performed pullout
performance test of geogrid compacted between soil and
bottom ash. Since bottom ash exhibits coarser grain particle,
the interlocking resistance between geosynthetic& bottom
ash will be high enough, and resembles the resistance of
sand with geosynthetics, which was supported by Pant etal.
[40] as backfill in mechanically stabilized earth wall (MSE).
The junction between soil and geosynthetic is the most crit-
ical point responsible for the load transfer mechanism in
reinforced soil. Lack of knowledge regarding the interface
behavior results in successive failure, especially in slopes of
embankments, MSE walls, top portions of reinforced back-
fill retaining wall, etc. Juran etal. [41] investigated the soil
retaining wall reinforced with different geosynthetics and
found failure due to excessive facing displacement and slid-
ing of the reinforcement in the case of woven geotextile. In
addition, breakage of the reinforcement was observed in the
case of nonwoven geotextile and plastic grid. In the same
way four types of mechanism, log-spiral mechanism, two
part wedge mechanism, one/two side general shear mecha-
nism was noticed for the strip footing resting on reinforced
soil [32]. A number of researchers [42–44] using a direct
shear test has been explored the strength of interface and
these test results were used by Anubhav and Basudhar [45]
for the development of constitutive models. Most of the stud-
ies have been done considering soft soil, sand, or any other
soil for the determination of strength of reinforced soil. In
recent years there have been interesting studies on the rein-
forced structures with geosynthetics and different types of
waste [46–48]. Several researchers have reported research
on recycled waste materials considering construction and
demolition waste, recycled glass and bricks as aggregate,
and various materials like red mud, foundry sand, alumina
in the application of road base/subbase materials [49–53].
Rai etal. [54] found remarkable improvement in the use of
marble powder and magnesium phosphate cement in soft
soil. Similarly, Reddy and Krishna [55] reported significant
reduction in deformation and earth pressure of retaining
structures with the use of recycled tire chips mixed with
sand as backfill. Considering bulk utilization of industrial
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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waste material, the fly ash and bottom ash can be success-
fully applied in the embankment and road base material.
Most of the studies recommended that the mixture of fly
ash and bottom ash would be a better alternative and found
comparable engineering properties with conventional fill
when used in equal proportion or slightly high percentage
of fly ash [2, 56]. In addition, to ensure better results, it is
suggested to compact the mixture at greater than 95% of
MDD and should be access from a single source of produc-
tion [57, 58]. The utilization of coal ash led to the several
environmental benefits such as saving of natural material,
no emissions of pollutant in the air, less requirement of area
for waste disposal, elimination of pollution of the surface
sources, and reduction in health related issues due to coal
ash. Geosynthetic reinforced soil helps in the enhancement
of strength, compressibility, stiffness, etc., because of this,
material requirement decreases remarkably compared with
unreinforced soil. These improved results in an increment in
load carrying capacity, reduction in deformation and thick-
ness [59] and responsible for the low maintenance cost &
high durable roads, which are very useful from the economic
point of view.
Therefore, here an attempt has been made to study the
strength behavior of coal bottom ash reinforced with geo-
textile and geonet. Most of the earlier investigations on the
behavior of reinforced and unreinforced soils were empha-
sized failure strength from the peak point or corresponding
to higher axial strain. However, as we know in field applica-
tion the value of strain (or settlement) should be limited to
an acceptable limit, thus the comparison between reinforced
and unreinforced soil should be considered at different val-
ues of imposed strain level to estimate the correct mobilized
strength. The influence of the number of reinforcement lay-
ers and confining pressure at various imposed strain levels
on the strength of the reinforced bottom ash was studied and
discussed. A series of strain controlled conventional triaxial
compression tests under undrained conditions on samples
of dry bottom ash reinforced with geotextile and geonet
were performed. All the tests were conducted on samples
of 38mm diameter with aspect ratio of two by maintaining
relative density of 70%. The strength behavior of the com-
posite material has been investigated through varying test
parameters like number of reinforcement layers, confining
pressure, type of reinforcement and imposed strain levels.
The effect of reinforcement on stress–strain behavior, shear
strength parameters, deformation behavior, strength ratio
and failure envelope has been studied. The test results were
analyzed and significant conclusions were drawn. A com-
parison of the present study with the previous literature sup-
ports the better utilization of reinforced bottom ash as base/
sub-base material. The outcomes of this study can help with
the implementation of industrial waste resource materials in
various fields with improved engineering properties, which
will not only provide an alternative material over conven-
tional granular material but also minimize the problem of
ash disposal in the country.
Materials andMethodology
The present study deals with the coal bottom ash collected
from Grasim Industries Ltd. Renukoot (Uttar Pradesh, India)
whereas TUFLEX India (Division of Parry Enterprises
India Ltd.) supplied the reinforcing materials (nonwoven
geotextile and geonet). The burning of pulverized coal into
the furnace of the thermal power station lead to the for-
mation of two types of ashes, i.e. lighter and heavier. The
lighter particle travels through flue gases and gets collected
at the electrostatic precipitator known as fly ash. Whereas
the heavier that contains unburnt coal, which settled down
at the bottom of the furnace is termed as bottom ash. The
principal properties of these materials are discussed in the
subsequent sections.
Coal Bottom Ash
Few laboratory tests have been performed to obtain the
required geotechnical as well as chemical characteris-
tics of the collected bottom ash and reinforcing materials.
The details of laboratory testing program are presented in
Table1. From the SEM analysis (Fig.1) it can be noticed
that the bottom ash particles are composed of irregular and
angular shape with complex pore structures. The XRD curve
given in Fig.2a shows that, quartz is predominant in the
present bottom ash. The test result of EDX analysis is shown
in Fig.2b and it can be inferred that, elements like Si, O, Al,
Ti, Ca and Fe are present in the bottom ash sample.
Geosynthetics
Geotextile, geogrid, and geocell are the most common geosyn-
thetics used for soil reinforcement among the various avail-
able geosynthetics. Geogrids are especially good at reinforc-
ing road cross sections, and are hence commonly utilized in
their design [60]. The strength and bearing capacity enhance-
ment of reinforced soil system has been examined by several
researchers considering uniaxial/biaxial geogrid, geotextile,
geonet, geomembrane as reinforcing materials [61–65]. The
layout and configuration of reinforcement play an important
role in enhancing the bearing capacity rather than the tensile
strength [66]. And also the improvement in the performance
was a result of the reinforcing material's tensile stiffness rather
than its tensile strength [62]. One of the most crucial features
of flexible pavement design is drainage. Geonet, which is gen-
erally utilized for drainage, also functions as reinforcement,
giving it a double benefit [60]. Considering the past utilization
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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of reinforcing materials, the geotextile and geonet has been
selected as a reinforcement for the present coal bottom ash. Its
utilization will be limited to low-volume traffic roads during
the initial phase application of the coal bottom ash as sub-
grade material. Geosynthetics with low tensile strength will
also suffice as reinforcement. The basic properties and picto-
rial illustration of geonet and geotextile used in the present
study are shown in Table2 (provided by the manufacturer),
and Fig.3 respectively. The change in orientation of the rein-
forcing elements can clearly visible from the figure. The use
of geosynthetics in reinforced soil gives appreciable benefits
such as high tensile strength, flexibility, high durability, reduc-
tion in actual thickness, etc., because of which it can with-
stand large strain [67]. In the reinforced sample, due to the
Table 1 Details of the laboratory testing program
Material Characteristics Details of experiments
Bottom ash from Grasim Industries Ltd., Renukoot, Uttar Pradesh, India Geotechnical Grain size distribution (IS:2720-Part IV, 1985)
Relative density test (IS:2720-Part XIV, 1983)
Specific gravity test (IS:2720-Part-III,1980)
Compaction test (IS:2720-Part VII, 1980)
Permeability test (IS:2720-Part XXXVI, 1987)
Direct shear test (IS:2720-Part XIII, 1986)
Triaxial test (UU) (IS:2720-Part XI,1993)
Chemical Scanning electron microscopy (SEM)
X-ray diffraction (XRD)
Energy dispersive X-ray (EDX)
Fig. 1 Scanning electron micrographs of bottom ash at (i) ×200, (ii) ×100, (iii) ×50 and (iv) ×5.00K magnifications, respectively
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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Page 5 of 26 33
existence of friction and adhesion bonding at the interface,
the deformation reduces remarkably. In other words, if there
exists a perfect bond between geosynthetics and soil, then
due to the application of vertical load, the geosynthetics has a
Fig. 2 a X-ray Diffraction pattern and b energy dispersive X- ray pattern of bottom ash respectively
Table 2 Physical/mechanical
properties of the geosynthetic
reinforcement
Properties of reinforcement Nonwoven geotextile Geonet
Constituent polymer Polypropylene High density polyethylene
Mass per unit area (g/m2) 120 730
Thickness (mm) 1.20 3.0
Cell size (mm) – Diamond (8 × 6)
Tensile strength (kN/m) 10 7.68
Maximum elongation at break (%) 50 30
Secant modulus (kN/m) 7 30
CBR puncture strength (N) 1600 –
Permeability (m/s) 115 × 10–3 –
Fig. 3 Pictorial representation of geosynthetics with scale a geotextile, and b geonet, respectively
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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tendency to mobilize the tensile force by extending its length.
This extension leads to the development of tensile stress in
the reinforcement members, which can be counterbalanced by
the surrounding soil in the form of compressive stress. This
compressive stress acts as a confining pressure that helps in
reducing the lateral strain [68]. The shearing action depends
much on the type of reinforcing material used. Like in geo-
textile reinforcement, its entire length contributes to resist the
shear load. Whereas in geogrid reinforcement, the resistance is
attributed due to two components, i.e., (1) resistance between
the soil and rib of the geogrid, and (2) resistance between soil
and soil-filled in the aperture of geogrid. Jewell etal. [69]
first theoretically explained the interaction between geogrid
through different mechanism and also given the equation to
determine interface shear strength.
Sample Preparation
To investigate the soil-geosynthetics interaction one
approach could be by treating the soil and reinforcement as
a composite material, and then investigating the stress–strain
and strength characteristics. The reinforced bottom ash as a
composite material can be used as an alternative construc-
tion material in various civil engineering fields. To study
the strength properties of bottom ash reinforced with geonet
and geotextile, a series of unconsolidated undrained (UU)
triaxial compression tests have been conducted on reinforced
and unreinforced specimens of bottom ash. The response of
reinforced bottom ash under the influence of confining pres-
sure, reinforcement layers, and reinforcement type has been
studied. A sample size of 38mm diameter and 76mm height
was considered for all the tests. The relative density of the
bottom ash sample was maintained constant around 70% for
all the test specimens. For the triaxial shear test, the deviator
stress was applied at a rate of 1.2mm/min with a load cell
of capacity 5 kN. Samples were compacted in several layers
with a tamping rod. The diameter of the reinforcement disc
was considered slightly less than that of the sample (37mm
for geotextile and 36mm for geonet). It was considered to
account for the damage of membrane and proper placement
of reinforcement during sample preparation. The reinforce-
ment was placed horizontally within the specimen after
completion of compaction and leveling of each layer. Here,
most of the tests were continued up to a strain level of 14%.
Results andDiscussion
Fundamental Characterization ofCoal Bottom Ash
The percentage of fines in the present bottom ash sample
was found to be less than 5% and was classified as poorly
graded sand (SP) type material. The particle shape of bottom
ash substantially remains flaky or in the form of chips that
tend to break into small pieces when subjected to an exter-
nal load. This breaking phenomenon has been experimen-
tally proved by taking a sample of post standard Proctor test
shown in Fig.4. Hence, sieve analysis was performed before
and after compaction to investigate the breaking phenom-
enon of bottom ash in particle size distribution. All the basic
properties of bottom ash before/after compaction have been
tabulated in Table3. The gradation of post compacted sam-
ple was found to be in the range of well-graded sand (SW).
The breaking tendency decreases the sand fraction from
95.50 to 89%; on the other hand, silt fraction was increased
from 3.50 to 11%.
The degradability of bottom ash under the standard Proc-
tor compaction test was evaluated using crushing coefficient
and increase in percentage of fines. For quantifying the deg-
radation, the weighted mean size of the sample before and
after the compaction was calculated. The crushing coeffi-
cient was obtained by expressing the percentage reduction
between two mean sizes compared to their initial mean size
and was found to be 12.23%. The crushing index of bot-
tom ash was found to be higher than that of natural granu-
lar aggregates. This may be due to the breakdown of weak,
porous particles and the crushing of cenospheres (hollow
burnt coal particles) present in the sample. Fly ash shows
high specific gravity as compared to other coal ashes for the
same collection site [70]. The bottom ash has an average
specific gravity of 2.33 that is 10% lower than the average
specific gravity of soil, and it falls in the range of Indian coal
ash [71]. Sridharan etal. [72] studied various properties of
bottom ash collected from India and reported specific gravity
in the range of 1.82–2.15 whereas Kim etal. [2] had found
high specific gravity of bottom ash i.e., 2.32–2.62.
According to Lovell etal. [73], bottom ash exhibit maxi-
mum dry density (MDD) at either an air-dried condition or
a wet or flushed condition on a compaction curve. Compared
Fig. 4 Particle size distribution curve of bottom ash before and after
subjected to compaction
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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to the conventional granular materials, bottom ash was
found to have lower maximum dry density. The MDD and
OMC of the granular material normally lies in the range of
1.55–1.89g/cm3 and 11.6–22.4% [74–76]. The maximum
dry density and optimum moisture content (OMC) of pre-
sent bottom ash was found to be 1.44g/cm3 and 26.50%
respectively. Similarly, the MDD and OMC of various bot-
tom ash reported by Sridharan etal. [72] was found to be
0.77–1.07g/cm3 and 36.5–63.4% respectively. The MDD
and OMC of fluidized bed combustion and pulverized coal
combustion type bottom ash was 1.64 and 1.02g/cm3 and
28.78 and 38.31% respectively [10]. From the past study
results, it can be deduced that the present bottom ash has
better dry density which was achieved at low OMC. In
most of the cases, the addition of water in coal ash follows
a decreasing trend of dry density initially, then reaches to a
minimum value and finally increases to reach the maximum
value on the MDD-OMC profile. The fundamental cause
behind this decreasing phenomenon of dry density is the
existence of capillary action during the initial water content,
which counters the external compactive effort responsible
for the particle movement in a closed state of contact [77].
After maximum value, the decrease in dry density has been
explained by the electrical double layer theory by Lambe
[78].
As per the grain size distribution, bottom ash lies in the
range of granular soil; hence, here the permeability of bot-
tom ash was determined by constant head permeability test.
The coefficient of permeability of bottom ash sample pre-
pared at OMC and MDD was found to be 9.08 × 10–3cm/s.
The materials not suitable for subgrade are Peat, log, stump,
perishable material and also soil that are classified as OL,
OI, OH (organic soil of low, intermediate, and high plastic-
ity) [79] whereas the present bottom ash falls in the category
of favorable material. The coefficient of permeability of pre-
sent bottom ash is within the range of Indian bottom ash
(9.9 × 10–5 to 7 × 10–4cm/s) [71]. Therefore, it can be used
in place where high water drainage properties are required,
such as base material, backfill material, foundation mate-
rial, etc.
The shear strength parameters of the bottom ash sample
have been determined by conducting both direct shear test
and triaxial shear test. The test specimen was prepared by
measuring the volume of the shear box and finding the mass
of dry bottom ash required to maintain a relative density of
70%. For the present study, a direct shear test equipment
having box dimension, 6cm × 6cm × 4.5cm and load cell of
capacity 0.25 kN was used. The specimen was subjected to
shear load at the rate of 1.25mm/min in the direct shear test.
Similarly, for triaxial test a specimen size of 38mm diameter
and 76mm height was maintained. It is usually preferred
to perform direct shear test over triaxial shear test for the
assessment of shear strength of cohesionless soils [80, 81].
Triaxial test is a universal test developed in order to elimi-
nate the limitations of the direct shear test [82]. Therefore,
several researchers focused on the direct shear test [83–85]
whereas various researchers started giving significance to
both the methods [80, 81, 86, 87]. So that the deviation in
the strength parameters can be predicted from both triaxial
and direct shear test. The triaxial test gives 1–8° higher angle
of internal friction (ϕ) than that obtained from direct shear
test in the case of fine grained and alluvial soil, however in
the case of sand, direct shear test gives 2–8° or 2–10% higher
ϕ as compared to triaxial test [80, 81, 88]. Since, the present
bottom ash contains nearly 90% of sand size particles, thus
found similar observations as noticed in the case of sand but
does not show remarkable difference in both the test. Hence,
for strength evaluation of the composite material triaxial test
has been considered. The shear strength parameters c and ϕ
values were found to be 4.57kPa and 45.57° (direct shear
test) and 4.26kPa and 46.05° (triaxial test) respectively. On
the other hand, the bottom ash generated from municipal
solid waste (incinerator bottom ash) shows zero cohesion
and high friction angle, which was approximately 54° [89].
Mandal etal. [10] also investigated two types of bottom ash
Table 3 Fundamental properties of the bottom ash before and after
compaction
Geotechnical properties Conditions Values
D10 (mm) Before compaction 0.17
After compaction 0.04
D30 (mm) Before compaction 0.29
After compaction 0.20
D50 (mm) Before compaction 0.38
After compaction 0.34
D60 (mm) Before compaction 0.45
After compaction 0.40
D90 (mm) Before compaction 1.50
After compaction 1
Coefficient of uniformity (Cu) Before compaction 2.73
After compaction 10
Coefficient of curvature (Cc) Before compaction 1.12
After compaction 2.5
SPAN Before compaction 3.48
After compaction 2.85
Sand fraction (%) Before compaction 95.5
After compaction 89
Silt fraction (%) Before compaction 3.5
After compaction 11
Minimum dry density (g/cm3) 1.26
Maximum dry density (g/cm3) 1.44
Optimum moisture content (%) 26.50
Specific gravity 2.33
Coefficient of permeability (cm/s) 9.08 × 10–3
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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and observed that the cohesion of 12.35 and 15.29kPa and
friction angle of 35.75° and 39.69°. The present bottom ash
shows high friction angle and low cohesion whereas past
studies results indicate high cohesion and comparable fric-
tion angle. This is due to the variation in the particle size
distribution of the bottom ash. The variation of shear stress
and shear displacement with failure envelop determined by
direct shear test has been depicted in Fig.5.
Behavior ofReinforced andUnreinforced Coal
Bottom Ash
A total of 30 laboratory triaxial compression tests under
unconsolidated undrained condition were carried out for dif-
ferent reinforcement configuration. The confining pressure
range from 41 to 124kPa represents the confining pressure
at a depth in between 2 to 7m [90] whereas the embank-
ment of height 3–5m can introduce confining pressure of
10–50kPa [91]. Moreover, these confining pressures are
frequently used by many of the researchers, which can help
in the comparison of the study results [92–95]. Therefore,
considering the application and comparison aspects in mind,
the confining pressure of 50, 100, and 150kPa has been
considered for the present study. The stress–strain behavior
of the unreinforced and reinforced bottom ash at confining
pressures of 50kPa, 100kPa and 150kPa, are shown in
Fig.6. From the figure, it is clear that with the increase
in confining pressure the deviator stress and corresponding
strain at failure increases. The shear strength parameters,
i.e. c and ϕ of the unreinforced bottom ash at 70% relative
density are found to be 4.26kPa and 46.05° respectively. The
experimental consequences of systematically reinforced soil
are more prominent as compared with randomly distributed
fiber-reinforced soil. It shows efficient contribution in bottom
ash when subjected to a higher strain under triaxial shearing.
During the initial loading condition, the compressive load
is appreciable, which is taken by the bottom ash itself, and
at the time of high strain, there is development of tension
which is counterbalanced by the geosynthetic reinforcement
and ultimately helps in the reduction in deformation and
tendency of early failure. The effect of reinforcement on
stress–strain behavior, shear strength parameters, strength
ratio, strength difference and failure pattern of the reinforced
bottom ash are discussed in the following sections. In addi-
tion, the effect of confining pressure on strength response of
the reinforced bottom ash has also been explained.
Effect ofReinforcement onStress–Strain Behavior
oftheReinforced Bottom Ash
The stress–strain curves of reinforced bottom ash samples
under the confining pressure of 50, 100, and 150kPa with
different number of reinforcement layers and types are
shown in Fig.6a–f. These figures indicate that the reinforced
bottom ash samples show improved stress–strain behavior
in terms of increased peak deviator stress and axial strain
at failure irrespective of the type of geosynthetic material
used. This is due to the internal pseudo confinement pro-
vided by the reinforcement to the specimen. The increase
in peak strength was observed to be more pronounced for
a greater number of reinforcement layers. From the figures
it can be concluded that the beneficial effect of geotextile
to enhance the strength of reinforced samples appear at
high strain, while at low strain, the geotextile layers does
not show significant effects. The stiffness of the geotextile
reinforced specimen was observed to be less as compared
to the stiffness of the unreinforced specimen at small strain,
which could be attributed due to the compressibility and low
stiffness of the geotextile material. The stress–strain behav-
ior was consistent with the reported previous studies [24,
Fig. 5 Representation of direct shear test results in the form of a shear stress versus shear displacement, and b shear stress versus normal stress
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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Page 9 of 26 33
96, 97]. The stiffer initial response of the geonet reinforced
bottom ash could be attributed to the faster development of
interfacial friction due to the better interlocking of geonet as
compared to that of geotextile used. From Fig.6a–f a very
important conclusion can be drawn that inclusion of rein-
forcement (either geotextile or geonet) reduces the post-peak
loss of strength. In some cases of reinforced bottom ash, the
post-peak loss of strength was not observed significantly for
Fig. 6 Representation of deviator stress with axial strain for different confining pressure under multiple reinforcement layers of geonet and geo-
textile
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33 Page 10 of 26
more number of reinforcement layers, which were three in
the present study. The increase in the number of reinforce-
ment layers results more ductile behavior of the reinforced
bottom ash samples.
The results indicate that the axial strain at failure
increases with the increase in the number of reinforcement
layers (Fig.7). These findings demonstrate that the rein-
forced bottom ash samples possess a greater axial strain at
failure as compared to that of unreinforced samples. This
property ensures sufficient flexibility of the reinforced earth
structures, which is important for dynamic loading condi-
tions. From figure (Fig.8), it is clear that with the increase
in the number of layers of reinforcement, the peak devia-
tor stress increases for a particular confining pressure. This
may be due to the fact that the inclusion of any type of
reinforcement into a sample induces extra confining pres-
sure, which is analogous to an addition of externally applied
confining pressure and in turn there is an increment in the
peak deviator stress.
Another important finding to be noticed is that the rate of
increment of peak deviator stress was maximum for three-
layer configuration. One of the reasons may be that for the
reinforcement layers more than two, the reinforcement lay-
ers at the ends of the sample come closer to the end platens.
The portion of the triaxial sample nearer to the end plat-
ens develops compressive radial stress due to the frictional
effect of the end platens, which restricts the lateral expansion
of the samples. As the number of layers of reinforcement
increases, the failure surface for unreinforced bottom ash is
intercepted completely throughout the height of the sample
Fig. 7 Axial strain at failure versus number of reinforcement layers under different confining pressures a geotextile reinforcement and b geonet
reinforcement, respectively
Fig. 8 Peak deviator stress versus number of reinforcement layers curves for reinforced bottom ash a geotextile reinforcement and b geonet rein-
forcement, respectively
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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Page 11 of 26 33
by the reinforcement layers. With increase in the number of
layers of reinforcement in the triaxial samples, the vertical
spacing between two consecutive layers of reinforcement
decreases.
In the field, designing a structure using peak strength at
failure can cause uncertainty and damage of the reinforced
earth structures. The value of the settlement or strain should
be limited to the allowable strain for the safety of the struc-
ture. Thus, it is important to investigate the strength of rein-
forced soil compared with unreinforced soil at the imposed
strain level. Figure9 shows the plot of deviatoric stress ver-
sus the number of reinforcement layers at different level of
strains, i.e. 2.63%, 5%, 7.63%, 10%, and 13.15% for different
values of confining pressures i.e. 50, 100 and 150kPa for
geotextile and geonet reinforced bottom ash respectively. It
is very clear from the Fig.9 that at low strain value, the
effect of geotextile reinforcement is very little in improving
the shear strength of reinforced soil even for three layers of
reinforcement. For example, in two layer configuration under
confining pressure 50kPa the deviator stress is maximum for
a strain level of 5.00%, but, for higher confining pressure,
i.e. 150kPa, the maximum deviator stress developed at a
strain level of 7.63%. Thus imposed strain level and applied
confining pressure on the sample plays an important role in
the strength improvement of the reinforced samples as com-
pared to that of the unreinforced samples. From Fig.9, it can
be concluded that geotextile as reinforcement having lower
stiffness requires some deformation to mobilize its tensile
capacity in the form of soil-geotextile interface friction to
improve the shear strength of reinforced soil. Unlike geotex-
tile reinforcement, geonet reinforced bottom ash specimens
show better strength even at lower confining pressure as the
number of reinforcement layer increases, which may be due
to its high stiffness and low compressibility as compared to
geotextile. Thus, permitting improved interaction with bot-
tom ash particles mechanically through surface friction and
by interlocking. For a certain specific value of axial strain,
the deviator stress increases under different confining pres-
sure as the number of reinforcement layers increases after
that specific value of strain either the strength improvement
is marginal or decreases. Thus, it is very important to con-
sider the strength of the reinforced soil as compared to the
unreinforced soil at the same imposed strain level.
Effect ofReinforcement onShear Strength Parameters
oftheReinforced Bottom Ash
From p–q plot (Fig.10), it can be concluded that the inclu-
sion of reinforcement imparts cohesive strength to cohe-
sionless bottom ash. The values of c and ϕ for the bottom
ash reinforced with different geosynthetics in different layer
configurations are given in Table4. The increase in shear
strength of the bottom ash due to inclusion of reinforcement
was not directly related to the tensile strength of the reinforc-
ing material as observed from the figures. Geotextile found
to give significant benefit in terms of strength improvement
at any strain level as compared to geonet, although the ulti-
mate tensile strength and secant modulus of the geotextile is
being lower as compared to that of geonet. To verify this, the
repetition of some tests has been done and the results from
these repeated tests gave almost identical results.
The above discussion concludes that the interaction
between reinforcement materials play a very important
role in enhancing the strength of reinforced soil. From the
physical observations of the reinforcement discs after the
completion of triaxial tests reveals that the granular bottom
ash particles tried to penetrate through the flexible geotex-
tile sheet due to applied pressure during shearing which in
turn developed a very high interlocking between both the
materials. This will ultimately cause high interface friction
mobilization and improved shear strength.
Effect ofReinforcement onStrength Ratio andStrength
Difference oftheReinforced Bottom Ash
This section discussed the effect of reinforcement layers
and the threshold value of percentage strain for the devel-
opment of strength in the case of reinforced bottom ash.
Figure11 shows the plots between strength ratio and axial
strain under different confining pressures and numbers of
reinforcement layers. In order to evaluate the effects of the
imposed strain level on the strength of the reinforced soil, a
parameter strength ratio at any specific strain value is intro-
duced which is nothing but the ratio of deviator stress of
the reinforced specimens to that of unreinforced specimens
under the same axial strain.
From the figures, it is clear that the strength ratio
increases as axial strain and the number of reinforcement
layers increases, and confining pressure decreases. It is very
important to notice that for a range of axial strain of approxi-
mately 1–3%, the mobilized shear strength of the reinforced
bottom ash exceeded that of the unreinforced sample. This
can be explained that during initial shearing the geotextile
requires a significant deformation for the mobilization of its
tensile capacity to improve the shear strength of the rein-
forced bottom ash. To appear the effect of geotextile, the
reinforced bottom ash requires sufficient deformations (i.e.
larger axial strain) to reach or exceed strength ratio of one,
when the number of reinforcement layers and confining pres-
sure value was increased. The geonet reinforced specimens
shows higher shear strength response in the initial stages
of shearing due to its high stiffness as compared to geo-
textile, lower compressibility, and better interlocking in an
open aperture space with granular bottom ash particles. Even
for very low strain values geonet reinforced specimens have
strength ratio more than one. For higher confining pressure
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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Fig. 9 Deviator stress versus number of reinforcement layers under various confining pressures at different strain levels (%) for geonet reinforced
bottom ash and geotextile reinforced bottom ash
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Page 13 of 26 33
as shown in Fig.11c–e the initial downward trend in strength
ratio values may be attributed due to the crushing of bottom
ash and hollow cenospheres.
Further evaluation of the shear strength of reinforced
specimens is done using peak strength ratio and strength
difference to check the efficiency of reinforcement. The peak
strength ratio is defined as the peak deviator stress of the
reinforced specimen to that of the unreinforced specimen
(Peak strength ratio =
(
𝜎d,max
)
reinfor ced
(
𝜎d
,max)unr einforced
). The strength difference
(Δσd) is defined as the difference between the maximum
shear strength of the reinforced specimen and that of the
unreinforced specimen under the same confining pressure,
for a particular reinforcement layer, which also indicates the
net strength improvement by reinforcement inclusion
(Strength difference =
(
𝜎d
,max)r einfor ced
−
(
𝜎d
,max)unr einfor ced
).
Tables5 and 6 summarizes the results and shows the varia-
tion of the peak strength ratio with numbers of reinforcement
layers for different confining pressures.
Both the strength difference and the peak strength ratio
increase as the number of reinforcement layer increases.
From Tables5 and 6 it can also be observed that, as the
confining pressure increases, the strength difference
increases but the peak strength ratio decreases. The
increasing strength difference can be explained as the
confining pressure increases the net strength improvement
by reinforcement increases due to the mobilization of
larger tensile force in it at higher confining pressure. The
percentage contribution of the reinforcement at higher
confining pressure to the overall improvement of shear
strength of the reinforced specimen relatively decreases
compared with the percentage of the soil’s contribution
(i.e., unreinforced soil) which could be the reason for
the decrease in the peak strength ratio at high confining
pressure.
For a geotextile, at higher confining pressure (150kPa),
the strength difference is not consistent, i.e. its value
decreases this may be due to the full utilization of the ten-
sile strength of geotextile in the form of rupture of the rein-
forcement disc after shearing at higher confining pressure.
Figure12 shows the variation of strength difference with
reinforcement spacing for geonet reinforced bottom ash. It
is clear from the figure that as the reinforcement spacing
increases strength difference decreases significantly. The
strength difference observed to be insignificant for spacing
called influence spacing of reinforcement, which can be
found out using extrapolation of curves. The reinforce-
ment may not contribute to the increase in shear strength
of the reinforced soil if the reinforcement spacing exceeds
the influence spacing. By extrapolation, it was found that
influence spacing of reinforcement was in the range of
73–77mm for geonet and 54–58mm for geotextile, which
is equivalent to spacing/diameter ratios of 1.92–2.02 and
1.42–1.53 for geonet and geotextile, respectively. This
observation is in good agreement with the earlier work on
Fig. 10 p–q plot of distinct geosynthetic inclusion condition for a Geonet reinforcement and b Geotextile reinforcement, respectively
Table 4 Shear strength parameters of the reinforced bottom ash
Number of
layers
Geotextile reinforced bottom
ash
Geonet reinforced
bottom ash
c (kPa) ϕ (°) c (kPa) ϕ (°)
0 4.26 46.05 4.26 46.05
1 27.03 47.22 38.42 48.07
2 61.40 47.31 57.51 48.76
3 107.42 47.48 115.21 48.80
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Fig. 11 Variation of strength ratio with axial strain for different number of geonet and geotextile reinforcement layers at confining pressure of
50kPa, 100kPa, and 150kPa, respectively
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Page 15 of 26 33
geotextile reinforced sand [97, 98] where they concluded
that geotextile reinforcement placed at spacing/diameter
ratio of 1 had very little effect on increasing the shear
strength of the reinforced soil.
Effect ofConfining Pressure onStrength Response
oftheReinforced Bottom Ash
The deviator stress at failure of samples versus various con-
fining pressures for one, two, and three layers of reinforce-
ment are shown in Fig.13a, b. For the analysis of reinforced
earth structures, confining pressure plays an important role.
From the figures, it is clear that the strength of reinforced
sample increases with the increase in confining pressure
irrespective of the reinforced layers. This enhancement in
strength is due to the direct proportionality of sliding resist-
ance at each contact point to the normal force acting at that
contact point [99]. The failure envelope for unreinforced bot-
tom ash is observed to be linear. However, for geotextile and
geonet reinforced bottom ash the failure envelope is found
to be nonlinear and nearly more or less linear, respectively.
For different number of reinforcement layers, the strength
ratios of reinforced specimens are compared under different
confining pressure as shown in Fig.14. The figures illustrate
that for any number of reinforcement layers, the strength
ratio decreases with an increase in confining pressure. The
reason could be the decrease in the interaction between the
geosynthetic and bottom ash by the reduction in the dilation
tendency at higher confining pressure, which restricts the
interlocking, and interface friction development.
Failure Pattern oftheReinforced Bottom Ash
Figure15a–e shows the typical images of the failure pat-
tern of unreinforced and reinforced bottom ash samples. If
we do a close examination of the photographs of the failed
specimens then it can be seen that unreinforced bottom
ash samples failed along a planar shear plane at an angle
of (45 + ϕ/2) as predicted by the classical soil mechanics.
However, the reinforced bottom ash samples failed by bulg-
ing between two adjacent reinforcement layers, moreover
for one or two layers of reinforcements slip planes were also
Table 5 Strength difference
and Peak strength ratio for
geonet reinforced bottom ash
at different confining pressures
and numbers of reinforcement
layers
Confining pressure,
σ3 (kPa)
Strength difference, Δσd (kPa) Peak strength ratio
1 layer 2 layers 3 layers 1 layer 2 layers 3 layers
50 211.98 328.00 512.38 1.75 2.16 2.82
100 239.07 369.29 537.65 1.45 1.69 2.00
150 273.42 413.65 507.98 1.34 1.52 1.63
Table 6 Strength difference
and peak strength ratio for
geotextile reinforced bottom ash
at different confining pressures
and numbers of reinforcement
layers
Confining pressure,
σ3 (kPa)
Strength difference, Δσd (kPa) Peak strength ratio
1 layer 2 layers 3 layers 1 layer 2 layers 3 layers
50 149.50 298.93 568.00 1.53 2.06 3.01
100 180.21 364.56 559.50 1.33 1.68 2.04
150 151.34 333.69 605.00 1.19 1.42 1.76
Fig. 12 Extrapolation of influence of spacing for a geonet reinforce-
ment, and b geotextile reinforcement, respectively
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Fig. 13 Deviator stress at failure versus confining pressure curve for reinforced bottom ash a for geonet reinforcement, b for geotextile reinforce-
ment, respectively
Fig. 14 Strength ratio versus confining pressure for various number of reinforcement layers a geonet and b geotextile, respectively
Fig. 15 Failure pattern of
a unreinforced, b, c single-
layer reinforced, d two layer
reinforced and e three-layer
reinforced bottom ash samples
respectively
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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observed within the bottom ash between two adjacent layers
of reinforcement. Figure15b shows the one layer composite
failure bulging with a slip plane, which was developed in the
upper portion between top end platens. It is clearly visible
that inclusion of reinforcement intercepted the shear plane
and results in improved shear strength of reinforced speci-
men. From the close examination of deformed geotextile
disc, it is clear that there is tensile deformation of reinforce-
ment as spacing between the marked lines had changed.
Mainly upper and middle reinforcement layer contributes
in strength improvement for the reinforced specimen. As
lower layers do not show any significant deformations and
this finding is true for all the confining pressures and for
both types of reinforcement used in the study. From the
close observations of retrieved reinforcement discs after
the test, it is concluded that the residual tensile strain in
the reinforcement reaches its peak value at the center and
decreases along the radial direction towards the periphery.
For higher confining pressure, i.e. 150kPa, the rupture of the
middle layer of the geotextile for three-layer configuration
was observed near the center of the reinforcement disc. The
quantification of residual tensile strain and mobilized tensile
force in the reinforcement could be done using digital image
processing which is one of the limitations of the present
study. Figure15d, e shows the failure pattern of the bottom
ash samples at confining pressure of 100kPa for two and
three layers of reinforment respectively. The failure shape
of reinforced and unreinforced bottom ash follows the simi-
lar fashion compared to the sand reinforced with different
geosynthetic materials [24, 100, 101].
Comparison
The comparison of shear strength parameters (c and ϕ) of
the present results with past literature considering various
reinforcements such as systematically reinforced and fiber
reinforced has been presented in Figs.16 and 17. For the
unreinforced sample, the shear failure is very common,
which follows an approximate failure angle of 45 + ϕ/2 with
the horizontal. However, in the presence of geosynthetic
inclusion, this failure pattern of unreinforced soil converted
from shear failure to bulging failure in the case of reinforced
soil [24]. This conversion of failure mechanism has been
explained by Jayawardane etal. [37], who stated that with
the increment in reinforcement layers, there would be a
high tendency of the intersection of the failure plane with a
reinforcement layer. Because of this, the resistance against
shear increases in the reinforced soil specimen and results in
uniform distribution of stress exerted from the external load,
which helps in the enhancement of overall strength. The
inclusion of more reinforcement will not always strengthen
the reinforced soil. So, it should be applied to the critical
portions of the unreinforced soil to avoid shear failure of the
specimen. A similar kind of increment can be observed in
shear strength parameters with an increase in reinforcement
for all results in Fig.16. A better agreement of the results
can be seen for cohesion, but friction angle shows more or
less same magnitude because, in the present study, the triax-
ial test was performed under undrained condition. The angle
of internal friction (ϕ) of reinforced soil was found to be
increased under drained condition; similarly, cohesion was
Fig. 16 Comparison of present shear strength parameters with past
literatures in terms of different reinforcement layers: a cohesion,
b angle of internal friction. aGeogrid, bGeotextile, b*Woven geotex-
tile, b**Nonwoven geotextile, b50geotextile with 50% relative density,
b50geotextile with 85% relative density, cGeonet
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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33 Page 18 of 26
increased in the case of undrained loading condition [102].
The reinforcement does not affect the friction angle very
much as compared with cohesion, and similar behavior has
been noticed in the present study. The friction angle is higher
compared with other shown results, due to the variability in
the materials and the presence of a high percentage of coarse
grain particles. The outcome of reinforced bottom ash has
also compared with various fiber-reinforced soils and does
not find any clear agreement. Figure17 shows a higher rate
of change of the cohesion curve than that of friction angle,
same as observed in Fig.16. There is no direct relation-
ship between systematic and fiber-reinforced soil; this may
be because of distinct load distribution mechanisms. Fiber-
reinforced soil can absorb a high amount of energy with a
higher content of fiber and much dependent on its aspect
ratio [103, 104]. In this type of reinforcement, it is difficult
to identify peak strength even at a large strain (15%) that
results in ductile behavior [105]. The advantages of geosyn-
thetic reinforcement are dominating over fiber reinforcement
because (1) during sample preparation, it is challenging to
maintain a vertical orientation, which becomes horizontal
at the time of tamping [106]; (2) major fibers naturally have
biodegradable properties. The present bottom ash exhibits
high percentage increase in the cohesion than that of fric-
tion angle with the increase in the number of reinforcement
layers. It shows increment of about 25 times (geotextile)
and 27 times (geonet) of cohesion of unreinforced bottom
ash. Noorzad and Mirmoradi [107] observed similar trend of
increase in the cohesion for geotextile reinforced clay, which
shows increment of c from 258 to 323kPa and ϕ from 8.4°
to 8.7° with the increase in reinforcement. Benessalah etal.
[100] performed drained triaxial test on geotextile reinforced
sand and noticed that with the increase in the reinforcement
there exist exponential increment of c (20–85kPa) and lin-
ear increment of ϕ (22–38). Shao etal. [108] examined the
strength of the randomly distributed fibre reinforced sand,
which depicts maximum strength at 0.9% fiber. Fibre rein-
forced soil also follows similar trend and increases c from
3.5kPa to 30.9kPa and ϕ from 29.8° to 37.1°. Kwon etal.
[109] investigated the shear property of the bottom ash
mixed with crumb rubber and reinforced with waste fish-
ing net. The reinforcement enhanced the strength parameter
but not as observed in the past studies. Similar study on
fly ash reinforced with waste plastic geocell has been done
by Lal and Mandal [110]. They found that the maximum
strength was obtained when the reinforcement was placed
at 1/3rd from both the side. The c and ϕ shows increment
of 58.64% and 40.05%, respectively. The reinforced bottom
ash shows comparable shear strength parameters same as
3D reinforced sand with galvanized iron sheet reported by
Zhang etal. [111].
After analysis of the entire results, it can be stated that
the performance of geonet is relatively higher than that of
geotextile used. The reason for its greater contribution is due
to the resistance from the mesh structure as well as soil filled
in that structure, which helps it to behave as a single unit,
whereas in geotextile, there exists a weak bond between two
layers because of the separation phenomenon. Moreover,
the dissipation of pore water pressure possibly shows high
for geonet reinforcement compared with others. Kamalzare
and Moayed [112] experimentally proved that geogrid gives
more shear strength in comparison with woven/nonwoven
Fig. 17 Comparison of present shear strength parameters with past
literatures in terms of different fiber reinforcement: a cohesion, b
angle of internal friction. aCoconut fiber, b(50&70polypropylene fiber
reinforced in soil having relative density of 50&70%, cGlass fiber,
dCoir fiber, eGeonet, fGeotextile
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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Page 19 of 26 33
geotextile, and it provides good tensile resistance and lateral
support to the unpaved road [113]. The efficiency of geogrid
is directly proportional to stiffness, aperture size, and appro-
priate position [114, 115]. Chen etal. [25] and Infante etal.
[44] have noticed similar types of consequences for geogrid
inclusion. Chen etal. [116] found that the strength advance-
ment of geogrid reinforcement is predominantly by cohesion
and less by friction angle, which resembles the present study.
Therefore, geonet can be effectively used for improving the
bearing capacity of soft soil and similar type of situations.
Benet andApplication Aspects
Despite tremendous growth in the production of coal ash,
the acceptance of ash products is unsatisfactory in India
because of the following reasons: (a) lacking of awareness,
(b) need of consumer attention regarding its benefits com-
pared with clay bricks and other products, (c) transportation
cost involves for far places from power stations, (d) scarcity
of structures for the application of ash products. Consider-
ing its harmful impact on the environment, the Ministry of
Environment, Forest and Climate Change (MoEFCC) had
imposed regulation of complete utilization of coal ash in
every construction activity within the radius of 50km from
the thermal power stations, which was further extended to
100km and then 300km [117]. To diminish the destruc-
tive effect towards the environment, the Government of
India made a mission named as Fly Ash Mission (FAM) for
successful applications [118]. Moreover, several dedicated
research institute such as the Coal ash institute of India and
Centre for fly ash research and management, come-up with
concrete outcomes for this environmental problem. In order
to minimize the hurdle in its implications, Indian Road Con-
gress (IRC) &Ministry of Road Transport and Highways
(MoRTH) have given recommendations in their standards
(IRC:SP:58:2001, MoRTH (Section305)) regarding its
uses in the road embankment. Ministry of Power has devel-
oped an ASHTRACK application, to make the procurement
from power stations and its utilization process as simple as
possible.
Fly ash is a pozzolanic material that shows the cement-
ing behavior in the presence of water, and also has several
application areas (Cement, Bricks, geopolymer, etc.); however,
bottom ash is non-reactive coal ash having limited application
areas (pavement structure, fill material). Pavement subsurface
layers are the appropriate area of application of bottom ash/
pond ash for bulk consumption. For the design of flexible/
rigid pavement layers, the California bearing ratio (CBR) is
the fundamental property [119]. The CBR of subgrade plays a
crucial role in the determination of the thickness of subsequent
layers in the flexible pavement design (IRC: 37:2018) [120].
The subgrades are generally composed of low bearing capacity
soil such as high- to low-plasticity clay, red earth, black cotton
soil, and expansive soil having CBR in the range of 2.80–8%,
4.25–12.60%, 1.35–2.50 and 4.52%, respectively [121–123].
Most of the studies reported CBR value of minimum 1.10%
and maximum 34% for different kind of soils (8.90–30.4%
[124], 2.80–8.94% [125], 2.90–6.50% [126], 12–34% [127]).
The subsurface layers of pavement are applied based on CBR
ratings like 0–3 (very poor), 3–7 (poor to fair), 7–20 (fair),
20–50 (good) and > 50 (excellent) [128]. Very poor and poor
to fair are used for subgrade and rests are used as base/sub-
base layer. Low strength subgrade requires high strength sub-
base/base and superior quality of dense bituminous macadam
(DBM) which will, in turn, increase the cost of the construc-
tion. As per IRC: 37:2018, the thickness of subgrade should
be greater than 500mm; therefore, in the present work, bottom
ash is considered as a subgrade layer over low-strength soil.
This will help in the bulk utilization of bottom ash and reduce
the requirement of the material of superior quality and ground
improvement needed to make it fit for the construction, which
will ultimately cut the construction cost as well. The design
of pavement thickness composed of bottom ash and soil as
subgrade has been briefly discussed in the subsequent sections.
Influence ofTraffic Variations inPavement Thickness
The CBR value of locally available subgrade soil is very low,
but on an average 15% has been taken for the comparison
with bottom ash. The design of pavement thickness has been
accomplished by the use of IRC: 37:2018 [120] and IITPAVE
software. IRC 37 is based on the fatigue strain developed at
the bottom of the bituminous layer and rutting strain, which
is developed at the top of the subgrade layer. Both of these
strains should be less than the limiting strain (Eqs.9 and 10)
for the safe design of pavement. The dense bituminous mac-
adam (DBM) + bearing course (BC) of viscosity grade 40
having a resilient modulus of 3000MPa has been considered
here. The practical relevance of presenting fatigue strain cor-
responding to 80% and 90% reliability is that there will be
20% or 10% chance of getting higher strain than that of strain
computed from Eqs.7–10. Generally, pavement fails if the
developed strain exceeds the permissible strain estimated from
the mentioned equations. Also, as per IRC 37 (2018) the 80%
reliability model can be implemented upto the traffic load of 20
million standard axle (MSA), whereas 90% reliability model
has been made especially for highways of prime importance
or (traffic load > 20 MSA). The resilient modulus of different
layers can be estimated using the following Eqs.4–6:
Subgrade resilient modulus (MR).
Sub-base and Base resilient modulus
(1)
M
R
=10 ×CBR for CBR
≤
5%
MR=17.6 ×CBR
0.64
for CBR
>
5%.
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
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33 Page 20 of 26
Fatigue strain (limiting) at the bottom of the bituminous
layer
Rutting strain (limiting) at the top of the subgrade layer
where MR is the subgrade resilient modulus, MR_sg is the
resilient modulus of subgrade, MR_gsb is the granular sub-
base resilient modulus, MR_gb is the granular base resilient
modulus, εt: maximum tensile strain, Nf is the fatigue life,
MRB is the modulus of bituminous layer, εv is the vertical
strain and N is the number of cumulative standard axles.
The CBR of present bottom ash is 37%, which has been
compared with soil of CBR 15%, and the optimum thickness
of various layers has been shown in Fig.18. Since the top
layers are more expensive than those of the under layers that
is the reason why base/sub-base layers are kept constant for
(2)
MR_gsb
=0.2 ×h
0.45
×M
R_sg,
(3)
MR_gb
=0.2 ×h
0.45
×M
R_gsb.
(4)
𝜀
t=
[
2.21 ×10−4×
(
1∕Nf
)
×
(
1∕MRB
)
0.854
](1∕3.89)
(80%reliability)
,
(5)
𝜀
t=[0.711 ×10−4×(1∕Nf)×(1∕MRB)0.854 ]
(1∕3.89)
(90%reliability).
(6)
𝜀v
=
[
4.1656 ×10−8×(1∕N)
](1∕4.5337)
(80%reliability)
,
(7)
𝜀v
=
[
1.41 ×10−8×(1∕N)
](1∕4.5337)
(90%reliability)
,
both bottom ash and soil. From the figure, it can be seen that
there is an efficient reduction in the thickness of the expen-
sive layer due to the application of bottom ash as subgrade.
The thickness reduction of 25mm, 45mm and 45mm has
been observed for the traffic load of 50, 100 and 150 MSA
respectively.
Influence ofDifferent Subgrade Soil inPavement
Thickness
The subgrade soil of a wide range of CBR (5–35%) has been
assumed for the present study. In addition, traffic load of 150
MSA has been considered for the design. The thickness plot
of different CBR subgrade is presented in Fig.19. The CBR
of the soil shows large variations (generally < 20%) because
of which the thickness of the above layers increases. The
reduction of the DBM + BC layer of 80mm can be wit-
nessed from the figure in comparison of soil with bottom
ash. This thickness reduction helps in fulfilling the following
two objectives: (1) bulk utilization of bottom ash is the first
priority, which can be achieved in pavement layers; (2) the
overall cost can be reduced by the reduction in the thickness
of different pavement layers.
Inuence ofReinforcement inBottom Ash
inPavement Thickness
In order to satisfy the minimum thickness criteria of IRC,
the resilient modulus of the DBM + BC layer is reduced
to 1500MPa and traffic load is increased to 250 MSA.
According to Singh etal. [33], the maximum advantage
Fig. 18 Illustration of pavement
thickness when Bottom ash and
soil as subgrade under various
traffic conditions
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
1 3
Page 21 of 26 33
of geosynthetic reinforcement can be achieved when it is
applied to either the middle or upper one-third layer of
the CBR mould. Therefore, CBR of the reinforced bottom
ash has been determined by keeping the geonet layer at
the mid-height and found 40% CBR value that is shown in
Fig.20. After reinforcement, the thickness of the bitumi-
nous layer has been reduced by 10mm, which can be seen
in Fig.21. Goud etal. [129] explored the design of rein-
forced (geogrid) flexible pavement in terms of layer coef-
ficient ratio (LCR) and traffic benefit ratio (TBR) based on
IRC:37, IRC:SP:59 [130], AASHTO [131]. In this process,
the resilient modulus of the reinforced layer has been back-
calculated using the method mentioned in AASHTO [131]
which overestimated the modulus of the reinforced layer. In
order to achieve the actual modulus value of the reinforced
layer, it is better to determine the CBR of the reinforced
layer and calculate the modulus. The decrease in thickness
of the pavement layer is because of the presence of better
strength subgrade material. In addition, higher strength of
the subgrade layer results in an increase in the modulus
of above layers and because of this increment, the thick-
ness of layer reduces. In order to study the realistic appli-
cation, Kumar and Rajkumar [132] determined the CBR of
the combined layer of pavement and reinforcement (base
course + reinforcement + subgrade). The CBR of 50% has
been observed for the unreinforced combination, whereas it
was increased to 84% with the inclusion of woven geotextile
and 70% with the inclusion of nonwoven geotextile. How-
ever, Raja etal. [133] found the CBR of 21.98% and 29.3%
when geotextile layer was placed at the centre (one layer)
and 1/4th H (2 layer) in the case of sandy soil subgrade.
Thakur etal. [134] investigated the clayey subgrade with
different kind of geotextiles and found increment in CBR
from 19 to 29% with the inclusion of nonwoven geotextile.
Similar study considering sand and fly ash mixture rein-
forced with geogrid shows increase in CBR value from 6%
Fig. 19 Representation of
thickness of pavement layers for
different CBR of soil subgrade
Fig. 20 Enhancement of California bearing ratio of unreinforced bot-
tom ash with the use of geonet as reinforcement
Fig. 21 Comparison of thickness of pavement layers for unreinforced
and reinforced sections
International Journal of Geosynthetics and Ground Engineering (2022) 8:33
1 3
33 Page 22 of 26
(unreinforced) to 13% (reinforced) [135]. Hence, the present
bottom ash shows better penetration resistance in compari-
son with clayey and sandy subgrade and found comparable
results with CBR of unpaved road reported by Singh etal.
[136]. Since geonet is suitable for drainage purpose in vari-
ous application of Civil engineering, it can also be used as
reinforcement that will provide dual advantage. Singh and
Mittal [60] also used the geonet in pavement application as
reinforcement and found comparable results with geogrid.
This study provides the information needed in the applica-
tion of reinforced bottom ash material in the road pavement
so that the stability and service life of the weak subgrade
can be improved significantly. Present study proves the
remarkable reduction in pavement thickness using this low
tensile strength reinforcement material. That will widen
the application of geosynthetics in various kind of works.
Similar types of effort have been made by many researchers
to consider low tensile strength geosynthetics for reinforce-
ment purposes [60, 137, 138].
Conclusions
In this study, an experimental investigation was conducted
to improve the mechanical behavior of coal bottom ash by
reinforcing it with different types of reinforcement. The
physical and engineering characterization of unreinforced
bottom ash shows very favorable unique properties that
make this material suitable for use in the construction of
embankments, for highways in subgrades and sub-base,
retaining structures, structural fills, and many other geotech-
nical applications. The key findings of the present investiga-
tion are as follows:
1. Bottom ashes possess high frictional strength (45° to 46°
for unreinforced case) despite its low unit weight when
compared with natural soils.
2. Bottom ash shows significant degradation (crushing
coefficient value 12.23%) under the standard Proctor
compaction test.
3. Inclusion of geotextile (0–3 layers) increases apparent
cohesion and friction angle from 4.26kPa to 107.42kPa
and 46.05° to 47.48°, respectively.
4. Inclusion of geonet increases apparent cohesion and fric-
tion angle from 4.26kPa to 115.21kPa, and 46.05° to
48.80° respectively.
5. The optimum spacing between two adjacent layers
of reinforcement was observed to be in the range of
54–58mm and 73–77mm for geotextile and geonet
reinforcement, respectively. The strength difference
decreases as the spacing between reinforcement layers
increases.
6. The failure pattern of unreinforced sample was noticed
to be planner shear plane at an angle of (45 + ϕ/2),
whereas reinforced sample failed by bulging between
two adjacent layers of reinforcement.
7. The thickness of the flexible pavement reduced up to
80mm with the application of bottom ash as subgrade
material as compared to soil as subgrade material.
So as far as the present study concern, it is concluded
that geonet as reinforcement proved to be more effective
than geotextile in terms of the stiffer initial response, higher
shear strength parameters (c and ϕ), and quick mobilization
of strength. Reinforced bottom ash is a simple composite
material with improved mechanical characteristics, which
encourage its use in various geotechnical applications such
as foundation base material, sub-base material for pave-
ments, and many more. Their utilization should be empha-
sized near to the thermal power stations that will discourage
the use of conventional material.
Data Availability Statement All data generated or analysed during this
study are included in this submitted article.
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