Geotech Geol Eng (2012) 30:15–25
DOI 10.1007/s10706-011-9445-6
ORIGINAL PAPER
Effect of Random Inclusion of Polypropylene Fibers
on Strength Characteristics of Cohesive Soil
Pradip Kumar Pradhan • Rabindra Kumar Kar
Ashutosh Naik
•
Received: 18 November 2010 / Accepted: 5 September 2011 / Published online: 14 September 2011
Ó Springer Science+Business Media B.V. 2011
Abstract This paper presents the effect of random
inclusion of polypropylene fibers on strength characteristics of soil. Locally available cohesive soil (CL) is
used as medium and polypropylene fibers with three
aspect ratios (l/d = 75, 100 and 125) are used as
reinforcement. Soil is compacted with standard Proctor’s maximum density with low percentage of
reinforcement (0–1% by weight of oven-dried soil).
Direct shear tests, unconfined compression tests and
CBR tests were conducted on un-reinforced as well as
reinforced soil to investigate the strength characteristics of fiber-reinforced soil. The test results reveal that
the inclusion of randomly distributed polypropylene
fibers in soil increases peak and residual shear
strength, unconfined compressive strength and CBR
value of soil. It is noticed that the optimum fiber
content for achieving maximum strength is 0.4–0.8%
of the weight of oven-dried soil for fiber aspect ratio of
100.
P. K. Pradhan (&)
Department of Civil Engineering, Veer Surendra Sai
University of Technology, Burla 768018, India
e-mail: pkpradhan1@yahoo.co.in
R. K. Kar
Department of Civil Engineering, Indira Gandhi Institute
of Technology, Sarang 759146, India
A. Naik
Department of Earth Science, Sambalpur University,
Jyotivihar, Burla 768019, India
Keywords Aspect ratio CBR Reinforced soil Polypropylene fibers Shear strength Unconfined
compressive strength
1 Introduction
Applications of soil strengthening or stabilization
range from the mitigation of complex slope hazards to
increasing the subgrade stability. Over the years,
number of methods has been developed for soil
stabilization in particular and ground improvement
in general. These methods can be broadly divided into
three types, such as mechanical methods, chemical
methods, and physical methods. Reinforced soil
technique is one of the physical methods of ground
improvement, the concept of which was first given by
Vidal of France in 1966. Since then significant
advances have been made in the design and construction of geotechnical structures such as retaining walls,
foundations, embankments, pavements, etc. The function of the reinforcements in the soil matrix is to
increase the strength (shearing resistance) and reduce
the deformation. Reinforcements may vary either in
form (strips, sheets, grids, bars or fibers), textures
(rough or smooth) or relative stiffness (high such as
steel or low such as fabrics and fibers). McGown et al.
(1978) pointed out the distinction between high
modulus and low modulus reinforcements and
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16
classified the reinforcements in two major categories:
(a) ideally inextensible inclusions (metal strips and
bars) and (b) ideally extensible inclusions (natural and
synthetic fibers, plant roots and polymeric fabrics).
The fundamental concepts of reinforced soil are
summarized in Shukla et al. (2009). Heimdahl and
Drescher (1999) reported that the orientation of
reinforcement in a particular direction might result in
anisotropy of the soil mass that could result in a
decrease of directional strength. On the contrary, the
primary advantages of randomly distributed fibers are
the absence of potential planes of weakness that can
develop parallel to oriented reinforcement (Maher and
Gray 1990).
Kumar et al. (1999) studied the engineering
behaviour of randomly distributed fiber-reinforced
pond ash and silty sand based on laboratory investigation and arrived at optimum fiber content of
0.3–0.4% of dry weight. Maher and Gray (1990)
studied the static response of sand reinforced with
randomly distributed fibers and observed that the
advantages of randomly distributed fibers were the
absence of potential planes of weakness that could
develop parallel to oriented reinforcement. Consoli
et al. (2003) studied the load–settlement response by
conducting plate load tests on a thick homogeneous
stratum of compacted sandy soil, reinforced with
randomly distributed polypropylene fibers. The
strength was found to increase continuously at a
constant rate, regardless of the confining pressure
applied, not reaching an asymptotic upper limit, even
at axial strains as large as 25%. The results of series of
triaxial compression tests on randomly distributed
discrete reinforced soils have been reported by Ranjan
et al. (1994a, b) and Charan et al. (1995). They studied
the strength of reinforced soils by varying the influencing parameters of both soil and fibers. Both
synthetic (polypropylene) and natural (coir and bhabar) fibers have been used for reinforcing soils ranging
from medium sand to non-plastic silt. Theoretical
models have also been developed to study the
mechanics of fiber-reinforced soil, which show adequate accuracy when compared with experimental
results (Gregory 1999; Zornberg 2002). Prabhakar and
Sridhar (2002) used randomly distributed sisal fiber as
reinforcement in a c- u soil at four different percentages of fiber content, i.e. 0.25, 0.5, 0.75 and 1% by
weight of raw soil and four different lengths of fiber,
i.e. 10, 15, 20 and 25 mm and found significant
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Geotech Geol Eng (2012) 30:15–25
improvement in the shear strength parameters (c and
u) of the soil. Kumar et al. (2005) found that the CBR
value of fiber reinforced fly ash increased by 41%
when compared with that of fly ash and the optimum
dose of polypropylene fiber was 0.5%. Nagrale et al.
(2005) studied the improvement of CBR value of
subgrade soil with inclusion of polypropylene fibers
and concluded that 1.5% fiber with aspect ratio 100
and 84 would be the optimum quantity in clayey soil
and fine sand respectively. Marandi et al. (2008)
studied the strength and ductility of fiber reinforced
silty sand with palm fibers and concluded that palm
fibers could be used as a reinforcing material in
improving the strength and ductility characteristics of
soil. Jadhao and Nagarnaik (2008) studied the influence of polypropylene fibers on the engineering
behavior of soil fly ash mixtures by using different
fiber lengths (6, 12 and 24 mm) in the range of 0–1.5%
by dry wet of soil and observed that maximum
improvement in strength was achieved at a fiber length
of 12 mm with fiber content of 1%. Ramesh et al.
(2010) studied the compaction and strength behavior
of lime-coir fiber treated black cotton soil with aspect
ratio of 20, 40, 60, 80, 100 and 120 and observed that
addition of 4% lime to the soil reinforced with 0.5%
coir fiber increased the strength of the composite
matrix, the effect of which was more at fiber aspect
ratio 80. Jiang et al. (2010) conducted a series of direct
shear tests on clayey soil reinforced with
0.02–0.05 mm diameter and 15, 20 and 25 mm long
polypropylene fibers on 61.8 mm diameter samples at
fiber content of 0–0.4% by weight of soil and reported
that the cohesion and internal friction angle of fiber
reinforced soil was greater than those of the parent
soil. Further, they reported that the cohesion and
internal friction angle of fiber-reinforced soil exhibited an initial increase followed by a rapid decrease
with increasing fiber content and fiber length.
Construction of buildings, roads and other civil
engineering structures on weak or soft soil is highly
risky because such soil is susceptible to differential
settlements due to its poor shear strength and high
compressibility. Hence, there is a need to improve
certain desired properties like bearing capacity, shear
strength (c and u) and CBR of subgrade soil. In
tropical countries like India, the locally available soil
(cohesive material) is too plentiful to be ignored.
Furthermore, in terms of cost, the use of locally
available materials will result in reducing the cost of
Geotech Geol Eng (2012) 30:15–25
2 Materials and Test Methods
2.1 Materials
The soil sample was locally collected from near
Sambalpur town of India. The soil lumps were broken
into small pieces and screened through 4.75 mm size
sieve to make it free from roots, pebbles, gravel etc.
The soil was screened to have a homogeneous mass
containing sand to clay. Polypropylene fibers were
obtained from the local market and used as
reinforcement.
2.1.1 Properties of Soil
The soil used in the investigation was classified as CL
according to Unified Soil Classification System. The
liquid and plastic limits of the soil were found to be 50
and 21, respectively. The grain size distribution curve
shown in Fig. 1 indicated that the soil was composed
of 33% fine sand, 28% silt and 39% clay. The soil had
a maximum dry density of 1.8 Mg/m3 with optimum
moisture content (OMC) of 11%.
2.1.2 Properties of Reinforcement
Polypropylene fibers were used as reinforcement. The
diameter (d) of the fibers used was 0.2 mm. The fibers
were cut into small pieces of 15, 20 and 25 mm
(average) lengths. Thus, three aspect ratios (l/d = 75,
100 and 125) were used in the study. The properties of
polypropylene fibers are presented in Table 1.
100
90
80
Percentage finer
construction. However, the interaction mechanism of
the reinforced residual soil and the mobilization of the
tensile strain in the reinforced composites are not yet
well understood due to limited study. As a result of the
growing interest in utilizing clayey soils in reinforced
soil structures, research on the subject of the reinforcement-cohesive soil interface behaviour has been
intensified (Athanasopoulos 1996). For the above
reasons, there is a need for thorough investigation of
the properties of cohesive soil, reinforced with locally
available synthetic (polypropylene) fibers. Thus, the
authors are motivated to study the strength characteristics of randomly distributed fiber-reinforced c- u soil
using polypropylene fibers as reinforcement, at different aspect ratio.
17
70
60
50
40
30
20
10
0
1E-3
0.01
0.1
1
10
Particle size (mm)
Fig. 1 Grain size distribution curve
Table 1 Properties of polypropylene fibers
Properties
Values
Specific gravity
0.91
Linear density (denier)
260
Young’s modulus (GPa)
Tensile strength (MPa)
3
120
Provided by supplier-Rebuild Technologies, Mumbai, India
2.2 Test Methods
In the present investigation an attempt was made to
study the effects of inclusion of polypropylene fibers
(with aspect ratio, l/d = 75, 100 and 125) on the
strength of locally available c- u soil compacted to
standard Proctor’s maximum density. In order to
quantify the increase in the both peak and residual
strength due to inclusion of fibers, a series of direct
shear tests were conducted with un-reinforced as well
as reinforced soil at three different normal stresses (rn)
i.e. 100, 200 and 300 kPa. For reinforced soil, the fiber
content was varied from 0.1 to 0.5% with an increment
of 0.1%. Thus, a total of 48 direct shear tests were
conducted. Improvement in the strength was also
studied through 19 numbers of unconfined compression tests and 31 CBR tests for the above three aspect
ratios. The fiber content was varied from 0.1 to 0.6%
and 0.1–1.0% with an increment of 0.1% for unconfined compression tests and CBR tests respectively.
2.2.1 Sample Preparation
The fibers were cut into average lengths of 15, 20 and
25 mm and thus, three different aspect ratios for the
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fibers were considered in the investigation. Ovendried soil was ground and sieved through 2 mm sieve.
The fibers were added to this soil at different
percentages varying from 0 to 1. The fibers to be
added to the soil were considered as a part of the solids
fraction in the void-solid matrix of the soil. The
content of fiber reinforcement (q) is defined herein as
q = Wf/W, where Wf is the weight of the fibers, and
W is the weight of the oven-dried soil. The mixing of
soil was felt very difficult beyond q = 1%, as the same
sticked together to form lumps. This also caused
pockets of low density. So, it was decided to stop with
1% fiber content. Soil samples were prepared by initial
dry mixing of oven-dried soil and corresponding
quantity of fiber (according to percentage by weight of
oven-dried soil). Then optimum water obtained from
standard Proctor compaction test was added and
mixed again until the water spreads all over the soil.
The dry and wet mixing of soil–fiber–water was
carried out in a non-porous metal tray in order to avoid
loss of water. The soil, fiber and water were mixed
manually spending sufficient time with proper care to
get homogeneous mix. The soil mixed with fibers and
water was kept in closed polyethylene bags for 24 h in
the laboratory at room temperature (27 ± 2°C) for
uniform mixing of soil with water. The mix thus
obtained was used for preparation of direct shear,
unconfined compression and CBR test specimens. The
above tests were conducted on both unreinforced and
reinforced soil specimens to make comparison
between the strength of unreinforced soil with that of
reinforced soil by varying the fiber content and fiber
aspect ratio.
2.2.2 Direct Shear Test
The experimental study involved performing a series
of direct shear tests. Soil-fiber mix was compacted in
the shear box of 60 9 60 mm in plan and 25 mm in
depth by tamping to standard Proctor’s maximum
density to obtain the specimens for direct shear tests.
The specimens were prepared at q = 0.1%; 0.2, 0.3,
0.4 and 0.5% for all the three aspect ratios. Three
specimens were prepared for each test. The specimens
were tested at normal stresses of 100, 200 and 300 kPa
in unconsolidated undrained conditions as per Indian
Standards Specifications IS 2720 (Part-13) (1986).
The loading rate was 0.002 mm/s in the tests. Shear
stresses were recorded as a function of horizontal
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Geotech Geol Eng (2012) 30:15–25
displacement up to a total displacement of 15 mm to
observe the post-failure behavior as well. The proving
ring dial gauge readings were noted at fixed interval of
horizontal dial gauge readings to study the stressdisplacement behaviour of both unreinforced and
fiber-reinforced soil. The shear strength parameters
were also studied.
It is to be noted that the choice of a small direct
shear apparatus as the testing platform brings
some inherent problems into the experimental study
(Yetimoglu and Salbas 2003). This limits the amount
of fiber inclusion. Other problems such as the imposed
plane of shear failure, ambiguous stress state, and end
effect in such a small sample size make it more
difficult to model fiber-reinforced soil behavior realistically. Despite these limitations, direct shear device
has been widely used for different theoretical and
practical research projects in most laboratories all over
the world due to its simplicity and other advantages
(Athanasopoulos 1996; Izgin and Wasti 1998; Kumar
et al. 1999; Wasti and Ozduzgun 2001; Yetimoglu and
Salbas 2003; Naeini and Sadjadi 2008). The device
was also employed in some research similar to this
study to highlight the complexity of fiber-reinforced
soil behavior (Gray and Ohashi 1983).
2.2.3 Unconfined Compression Test
The soil-fiber mix was compacted in a cylindrical
mould of 50 mm diameter and 100 mm high to
standard Proctor’s maximum density. Then the specimen was extracted for unconfined compression test.
Specimens were prepared at q = 0.1%; 0.2, 0.3, 0.4,
0.5 and 0.6% for all the three aspect ratios. The
unconfined compression tests were conducted on both
unreinforced and reinforced specimens at a constant
strain rate of 0.125 mm/min as per Indian Standards
Specifications IS 2720 (Part-10) (1991). Three specimens were tested for each combination of variables.
The stress–strain response was plotted from which
unconfined compressive strength was studied with
variation in fiber content for all the three aspect ratios.
2.2.4 CBR Test
California Bearing Ratio (CBR) tests were carried out
to examine the effects of polypropylene fiber on the
ultimate strength of fiber-reinforced soil. CBR test
specimens were prepared in a cylindrical mould of
Geotech Geol Eng (2012) 30:15–25
19
150 mm diameter and 175 mm height by compacting
the soil-fiber mix to standard Proctor’s maximum
density. Specimens were prepared at q = 0.1%; 0.2,
0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0% for all the three
aspect ratios. Three specimens were tested for each
combination of variables. The specimens were submerged in drinking water and soaked for 96 h before
testing. All the tests were conducted at a penetration
rate of 1.25 mm/min until a penetration of 12.5 mm
was achieved. The load-penetration curves were
plotted and the CBR values were computed. Soaked
CBR tests were conducted with unreinforced and
reinforced soil specimens in accordance with Indian
Standards Specifications IS 2720 (Part-16) (1987).
3 Results and Discussions
As per the testing programme, various tests were
conducted on soil without reinforcement and with
randomly distributed discrete fiber-reinforcement
(0.1–1.0% by weight of dry soil) at three different
aspect ratio (l/d = 75, 100 and 125). The effect of fiber
inclusion on stress-displacement behaviour, shear
parameters, unconfined compressive strength and
CBR values were studied. The results are presented
in Tables 2, 3, 4 and Figs. 2, 3, 4, 5, 6, 7, 8, 9.
3.1 Direct Shear Test
The stress-displacement behaviour of soil reinforced
with varying fiber content and length, obtained from
direct shear tests are presented in Figs. 2, 3, 4. From
these Figures, it is observed that the peak and residual
strength of fiber-reinforced soil occur at higher
horizontal displacement in majority of cases
investigated compared to the unreinforced soil. Observation of these Figures indicates that with increase in
normal stress both peak and residual strength of
reinforced soil increase. Also, these strengths increase
with increase in fiber content up to 0.4%, beyond
which they decrease, irrespective of fiber lengths used
in the investigation. Thus, the optimum fiber content is
found to be 0.4%. With inclusion of polypropylene
fibers in the soil, the maximum increase in the peak
strength is observed at low normal stress (100 kPa),
where as maximum increase in the residual strength is
observed at high normal stress (300 kPa), for all the
fiber lengths considered in the investigation. With
inclusion of fibers, the peak and residual shear stresses
are increased by factors 2.0, 2.7, 2.5 and 1.9, 2.3, 2.1
respectively for fiber lengths of 15, 20 and 25 mm.
Thus, the maximum increase in both peak and residual
strengths occurs at fiber length of 20 mm (l/d = 100).
The failure envelopes corresponding to both peak
and residual shear stresses obtained from direct shear
tests are presented in Figs. 5, 6, 7. The observed shear
parameters (c and u) presented in Tables 2, 3, 4
indicate that the reinforced soil exhibits an increase in
the angle of internal friction (u) and the cohesion
(c) with increase in fiber content corresponding to both
peak and residual strength, up to the optimum dose and
decreases or nearly remain the same thereafter for all
fiber lengths investigated. This effect may be due to
interaction between the soil and fiber. In case of
15 mm fiber lengths (l/d = 75), both peak and residual angle of internal friction are increased to maximum by a factor 1.6 and observed at 0.4% fiber
content. The peak and residual cohesion are increased
to maximum by factors 2.2 and 1.6 respectively,
observed at 0.4% fiber content. For 20 mm fiber
lengths (l/d = 100), peak and residual angle of
Table 2 Shear parameters of fiber reinforced soil obtained from direct shear test (l/d = 75)
Sl. no.
Fiber
content (%)
Cohesion (c), kPa
Peak
Angle of internal friction (u), degrees
Residual
Peak
Residual
1
0.0
45
40
26
26
2
0.1
50
48
33
34
3
0.2
58
53
39
35
4
0.3
68
54
40
38
5
0.4
100
65
42
42
6
0.5
94
52
39
39
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Geotech Geol Eng (2012) 30:15–25
Table 3 Shear parameters of fiber reinforced soil obtained from direct shear test (l/d = 100)
Sl. no.
Fiber content (%)
Cohesion (c), kPa
Angle of internal friction (u), degrees
Peak
Residual
Peak
Residual
1
0.0
45
40
26
26
2
0.1
90
90
30.3
30.1
3
0.2
130
90
38
40.9
4
0.3
130
95
41
44.1
5
0.4
145
96
45.85
44.4
6
0.5
165
50
42
52
Table 4 Shear parameters of fiber reinforced soil obtained from direct shear test (l/d = 125)
Sl. no.
Fiber content (%)
Cohesion (c), kPa
Angle of internal friction (u), degrees
Peak
Peak
Residual
Residual
1
0.0
45
40
26
26
2
0.1
100
85
27.47
27.47
3
0.2
118
92
36.12
37.2
4
0.3
118
92
40
42.1
5
0.4
135
100
42.9
43.97
6
0.5
120
72
43.7
46.85
internal friction are increased to maximum by factors
1.76 and 1.71 respectively, observed at 0.4% fiber
content. Also, the peak and residual cohesion are
increased to maximum by factors 3.2 and 2.4 respectively, observed at 0.4% fiber content. For 25 mm fiber
lengths (l/d = 125), peak and residual angle of
internal friction are increased maximum by factors
1.6 and 1.8 respectively, observed at 0.5% fiber
content. Further, the peak and residual cohesion are
increased maximum by factors 3.0 and 2.5 respectively, observed at 0.4% fiber content. Thus, the
maximum increase in shear parameters is observed
at 0.4% fiber content with 20 mm fiber length
(l/d = 100). Similar results were obtained by Kumar
et al. (1999).
3.2 Unconfined Compression Test
The test results of unconfined compression tests are
presented in the form of stress–strain relationships in
Fig. 8. The results show that the inclusion of
reinforcement increases the unconfined compressive
strength (qu) and the strain at failure. It is observed
that the failure strain is increased by a factor 3.1
when compared with that of unreinforced soil and
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occurs at a fiber content of 0.5% for 15 mm fiber
length. The corresponding qu of the reinforced soil is
increased by a factor 2.2. Similarly, for fiber lengths
of 20 and 25 mm, the failure strains are increased by
factors 3.4 and 3.3 and the corresponding qu by
factors 2.9 and 2.7 respectively when compared with
that of unreinforced soil and occur at a fiber content
of 0.5%. Thus, the unconfined compressive strength
and the corresponding strain at failure increase up to
20 mm fiber length (l/d = 100) and decrease thereafter. Increase in the length of fiber beyond 20 mm
reduces the soil–fiber interlocking, which may be the
reason for the reduction in failure strain and the
corresponding qu. Hence, the optimum fiber content
is observed to be 0.5% in the range of fiber lengths
considered in the study. Further, it is observed that
with the inclusion of fibers, ductility of the reinforced
soil improves when compared with the unreinforced
soil up to optimum content and length of fiber.
Similar observations were made by Kumar et al.
(1999) on fiber reinforced silty sand and Maher and
Gray (1990) on fiber reinforced sand. At fiber content
higher than the optimum, qu decreases compared to
its maximum value. This may be due to the fact that
with higher fiber content, the quantity of soil matrix
Geotech Geol Eng (2012) 30:15–25
21
300
200
σ n=100kPa
σ n = 100 kPa
250
100
Fiber content (%)
0.0
0.1
0.2
0.3
0.4
0.5
50
0
0
5
10
15
Shear Stress (KPa)
Shear Stress (kPa)
150
200
150
Fiber Content (%)
0.0
0.1
0.2
0.3
0.4
0.5
100
50
0
20
0
Horizontal Displacement (mm)
5
10
15
20
Horizontal Displacement (mm)
400
300
σ n=200kPa
σ n = 200 kPa
350
250
200
150
Fiber content (%)
0.0
0.1
0.2
0.3
0.4
0.5
100
50
0
Shear Stress (KPa)
Shear Stress (kPa)
300
250
200
Fiber Content (%)
0.0
0.1
0.2
0.3
0.4
0.5
150
100
50
0
0
5
10
15
0
20
5
10
15
20
Horizontal Displacement (mm)
Horizontal Displacement (mm)
500
400
σn=300 kPa
350
σ n = 300 kPa
450
400
Shear Stress (KPa)
Shear Stress (kPa)
300
250
200
Fiber Content (%)
0.0
0.1
0.2
0.3
0.4
0.5
150
10
0
50
0
350
300
250
150
100
50
0
0
5
10
15
Fiber Content (%)
0.0
0.1
0.2
0.3
0.4
0.5
200
20
Horizontal Displacement (mm)
Fig. 2 Stress-displacement curves for fiber reinforced soil
obtained from direct shear test (l/d = 75)
0
5
10
15
20
Horizontal Displacement (mm)
Fig. 3 Stress-displacement curves for fiber reinforced soil
obtained from direct shear test (l/d = 100)
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22
Geotech Geol Eng (2012) 30:15–25
500
250
σ n = 100 kPa
150
Fiber Content (%)
0.0
0.1
0.2
0.3
0.4
0.5
50
Peak Shear Stress (kPa)
Shear Stress (KPa)
200
100
5
10
15
400
350
300
250
200
150
100
50
0
0
0
Fiber content (%)
0.0
0.1
0.2
0.3
0.4
0.5
450
0
20
100
200
300
400
300
400
Normal Stress (kPa)
Horizontal Displacement (mm)
400
350
σ n = 200 kPa
250
200
Fiber Content (%)
0.0
0.1
0.2
0.3
0.4
0.5
100
50
0
0
5
10
15
300
250
200
150
100
50
0
0
100
200
Normal stress (kPa)
20
Horizontal Displacement (mm)
Fig. 5 Failure envelopes for fiber reinforced soil obtained from
direct shear test (l/d = 75)
450
σ n = 300 kPa
400
Residual shear stress (kPa)
Shear Stress (KPa)
300
150
Fiber content (%)
0.0%
0.1%
0.2%
0.3%
0.4%
0.5%
350
Shear Stress (KPa)
350
3.3 CBR Test
300
250
200
Fiber Content (%)
0.0
0.1
0.2
0.3
0.4
0.5
150
100
50
0
0
5
10
15
20
Horizontal Displacement (mm)
Fig. 4 Stress-displacement curves for fiber reinforced soil
obtained from direct shear test (l/d = 125)
available for holding the fiber is insufficient to
develop an effective bond between fibers and soil,
causing balling of fibers and poor mixing.
123
The results of soaked CBR tests are presented in
Fig. 9. The results indicate that with inclusion of
fibers, the soaked CBR values increase up to fiber
content 0.8–0.9% for all the three aspect ratios
investigated. The CBR values are increased by factors
2.6, 3.0 and 2.7 for fiber lengths 15, 20 and 25 mm
respectively, when compared with that of unreinforced
soil. It is also observed that the CBR values increase
with increase in fiber length up to 20 mm and then
decrease. Thus, the optimum dose of fiber is 0.8% for
the fiber length of 20 mm (l/d = 100). Similar observations were made by Kumar et al. (2005) on
polypropylene fiber-reinforced fly ash with optimum
fiber content of 0.5%.
Geotech Geol Eng (2012) 30:15–25
23
450
Fiber content (%)
450 Fiber content (%)
0.0
400
0.1
0.2
350
0.3
0.4
300
0.5
250
400
Peak shear stress (kPa)
Peak Shear Stress (kPa)
500
200
150
100
350
300
250
200
150
100
50
50
0
0.0
0.1
0.2
0.3
0.4
0.5
0
100
200
300
0
400
0
100
400
0.0
0.1
0.2
0.3
0.4
0.5
400
350
300
Residual shear stres (kPa)
Residual shear stress (kPa)
450
250
200
150
100
350
300
250
200
150
100
0
0
100
200
300
400
Fig. 6 Failure envelopes for fiber reinforced soil obtained from
direct shear test (l/d = 100)
100
200
300
400
Fig. 7 Failure envelopes for fiber reinforced soil obtained from
direct shear test (l/d = 125)
4 Conclusions
On the basis of the above experimental investigations
the following conclusions are drawn.
The shear strength of soil increases with inclusion
of polypropylene fiber up to 0.4%, for all the three
fiber lengths considered in the investigation,
beyond which it decreases. For fiber length of
20 mm (l/d = 100) the increase is maximum,
when compared with the unreinforced soil.
Both angle of internal friction (u) and cohesion
(c) increase with increase in fiber content up to the
optimum dose for all the three fiber lengths and
then decrease or remain nearly the same.
For most of the cases, the maximum increase in
both peak and residual angle of internal friction is
observed at 0.4% fiber content and the factors of
0
Normal stress (kPa)
Normal stress (kPa)
3.
0.0
0.1
0.2
0.3
0.4
0.5
50
50
2.
400
Fiber content (%)
Fiber content (%)
1.
300
450
500
0
200
Normal stress (kPa)
Normal Stress (kPa)
4.
5.
increase being 1.76 and 1.71 respectively for fiber
length of 20 mm (l/d = 100). Similarly, the
maximum increase in both peak and residual
cohesion is observed at 0.4% fiber content and the
factors of increase being 3.2 and 2.4 respectively
for fiber length of 20 mm (l/d = 100). Thus, the
maximum increase in shear parameters occurs for
fiber length of 20 mm (l/d = 100) at 0.4% fiber
content.
The peak and residual shear stresses of fiber
reinforced soil are increased by factors 2.0, 2.7,
2.5 and 1.9, 2.3, 2.1 respectively for the fiber
aspect ratios 75, 100 and 125. Thus, the maximum
increase in both peak and residual strengths
occurs for fiber length of 20 mm (l/d = 100) at
0.4% fiber content.
With inclusion of polypropylene fibers, the unconfined compressive strength and the corresponding
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24
Geotech Geol Eng (2012) 30:15–25
300
Fiber content (%)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
260
240
220
Stress (kPa)
200
180
160
8
l/d = 75
7
6
CBR (%)
280
140
120
5
100
4
80
60
40
l/d =75
l/d =100
l/d =125
3
20
0
0
1
2
3
4
5
6
7
8
9
2
0.0
10
Strain (%)
300
260
240
220
Stress (kPa)
200
180
160
l/d = 100
120
100
80
60
6.
40
20
0
1
2
3
4
5
6
7
8
9
10
Stress (kPa)
Strain (%)
300
Fiber content (%)
280
0.0
260
0.1
240
0.2
220
0.3
200
0.4
180
0.5
160
0.6
140
120
100
80
60
40
20
0
-20
0
1
2
3
0.8
1.0
lengths investigated and qu and failure strain are
increased to maximum by factors 2.9 and 3.4
respectively compared to unreinforced soil, occurring at fiber length of 20 mm (l/d = 100). Thus,
fiber inclusion makes the soil more ductile.
Compared to unreinforced soil, the soaked CBR
value of fiber reinforced soil is increased to
maximum by a factor 3.0 for 20 mm fiber length
(l/d = 100) at optimum fiber content of 0.8%.
l/d = 125
References
4
5
6
7
8
9
10
Strain (%)
Fig. 8 Stress-strain response for fiber reinforced soil obtained
from unconfined compression test
strain at failure increase up to an optimum fiber
length and decrease thereafter. The optimum fiber
content is observed to be 0.5% for all the three fiber
123
0.6
Fig. 9 CBR values for fiber reinforced soil at different fiber
contents
140
0
0.4
Fiber content (%)
Fiber content (%)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
280
0.2
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