Indian Geotechnical conference 2013

1. Proceedings of Indian Geotechnical Conference,
December 22-24, 2013, Roorkee.
EFFECT OF COIR MAT AND COIR FIBRE REINFORCEMENT OF
SHEAR STRENGTH OF DRY SAND.
R.Sridhar, Research scholor, Ghousia college of Engg. sridharrajagopalg@gmail.com.
M.T.Prathap Kumar, Prof, Reva Institute of Engg & Technology,drmtprathap@gmail.com.
INTRODUCTION
The term “Reinforced Soil” refers to a soil
that is strengthened by a material able to
resist tensile stresses within the soil
through friction and or adhesion. The
beneficial effect of inclusion of
reinforcement is largely dependent on the
form in which it is used. For example, the
a reinforcement when used in planar layers
such as a mat or in the form of discrete
fibres, comprising exactly the same
quantity of material, will give different
strength improvements.This difference in
strengths achieved is mainly due to the
different mechanisms of failure in soil in
different forms.The behaviour of mat
reinforced soil differs from fibre
reinforced soil with regard to interaction of
soil particles between the reinforcement
and fibres. If a soil grain is in contact with
reinforcement and if the contact force
makes an angle with the normal line
smaller than the sliding angle between the
grain and the reinforcement, it occurs as if
the grain was tied by the reinforcement
and in that case, it can be considered that
all the grains along reinforcement are tied
together. For the case of fibre reinforced
soil, the isotropic compression causes
relative movement among particles and
consequently produces tensile stresses in
the fibres located among them. There is
also the possibility of an additional
mechanism of fibres breaking during
testing by squeezing and crushing of the
sand particles, cutting the fibres trapped
between them[8].
Short discreet fibres
provide isotropic increase in the strength
of the soil composite without introducing
continuous planes of weakness. The
improvement of the engineering properties
due to the random inclusion of discrete
fibres is determined to be a function of a
variety of parameters such as fibre content,
aspect ratio, fibre length confining
pressure and sand type.
The effect of the reinforcement properties
on the overall behaviour of reinforced sand
needs to be established properly for the
selection of the reinforcement for any earth
structures through systematic element test.
Few studies are available in literature
regarding the effect of amount and form of
reinforcement on the response of
reinforced sand, viz Broms (1977), Mc
Gown et.al, (1978), Chandrashekarn et.al,
(1989), Gray and Ohashi (1983), Futaki
et.al, (1990), Lee and Manjunath (2000),
Haeri et.al (2000) and Venkatappa roa
(2005)[3]
. Study using direct shear and
triaxial test on small samples and model
footing test on large samples of randomly
distributed fibre reinforced sand (Praveen
kumar Guptha,Swami saran and Ravikant
Mittal) shows the percentage increase in
the strength of randomly distributed fibre
reinforced sand is more at 50% relative
density compared to 70% relative
density[5]
. Studies on comparative strength
behaviour of mat and fibre reinforced sand
are rare in literature.
Countries where the availability and cost
of reinforcing materials of major
constraining factors, reinforced soil
systems using natural materials have
tremendous application possibilities. These
natural materials include coir, which is the
husk of coconut, a common natural
material, where coconuts are grown and
subsequently processed. Coir is reputed to

2. R. Sridhar, M. T. Prathap Kumar
be the among the strongest and most
durable of these natural materials (Girish
and Kamanatha Ayyar,2000, Kamanatha
Ayyar et.al, 2002). The objective of the
present experimental study is to provide an
insight in to shear strength characteristics
of coir mat and fibre reinforced sand, so as
to understand the difference in mechanism
of their interaction with sand grains.
EXPERIMENTAL PROGRAMME
Materials used.
Sand
Locally available sand was used for the
present experimental investigation. The
properties of sand used are as shown in
table 1 and thje grain size distribution of
the sand used is as shown in Fig.1.
Coir Fibres
Coir is a biodegradable organic Fibre
material which is rigid, strong, and tensile
in nature. However, the rate of
decomposition of coir is much less than
any other natural Fibre. This
characteristic is due to the high lignin
content in the Fibre. For the present study
coir fibre was procured from Karnataka
coir development Board, Gubbi near
Tumkur. Different Properties of coir fibre
are as shown in Table1. 2, 1.3 and 1.4.
Table 1 Properties of sand
Figure 1 Grain size distribution of sand
used
Coir Mat
Coir mat with different opening size of
10mm, 20mm and 30mm square size was
procured from Karnataka coir industries
Ltd, Bangalore. The properties of Coir mat
are shown in Table 1.5 and 1.6.
Methodology
Direct Shear Test with Randomly
Distributed Coir Fibres.
Direct shear test were conducted on dry
sand specimen of size 60x60x25mm depth
in a direct shear box. Fibre content was
calculated on the basis of dry weight of
sand, defined by, Fc = Wf /WS, Where Wf
is dry weight of fibres and Ws is dry
weight of sand.Intialy unreinforced dry
sand was compacted at 70% relative
density and tested at three different normal
stresses 50KPa, 100KPa and 150KPa.
Similarly tests were conducted on
randomly distributed coir reinforced sand
of different lengths viz., 10mm, 20mm,
30mm mixed and compacted to relative
density of 70% and fibre content was
varied from 0.05%, 0.1%, 0.2% and 0.3%
each test was done at three different
normal stresses 50KPa, 100KPa and
150KPa. The fibre added to sand was
considered as a part of the solid fraction.
The fibres were mixed by hand and
transferred to the direct shear box.
Coefficient of uniformity, Cu 4.48
Coefficient of curvature, Cc 0.960
Specific gravity, G 2.66
Maximum density of sand, ϒd(max.),
KN/m3
16.7
Minimum density of sand, ϒd(min.),
KN/m3
14.0
Classification of Sand SP

3. Effect Of Coir Mat And Coir Fibre Reinforcement Of Shear Strength Of Dry Sand.
Adequate compaction was achieved by
light tamping, to achieve the desired
relative density.
Direct Shear Test with Coir Mat
Direct shear test were conducted on dry
sand specimen of size 60x60x25mm. Coir
mat was pasted on wooden plate of
thickness 12.0mm and was placed in the
bottom half of the direct shear box, such
that the coir mat was placed exactly in the
plane of horizontal shear. The sand in the
upper half was compacted with dry sand at
70% relative density at three different
normal stresses of 50KPa, 100KPa and
150KPa. A total of three direct shear tests
were conducted to study the influence of
coir mat with different size openings, viz
10mm, 20mm, 30mm. Desired compaction
was achieved by light tamping, using
circular iron rod.
Results and Discussions.
Effect of Fibre Content on Angle of
Internal Friction.
Figure 2, .3 and .4 show the variation of
shear stress versus normal stress obtained
from direct shear test. Table 2 shows the
summarized values of angle of internal
friction (ø) obtained corresponding to
different lengths of fibre and fibre
content(FC). It can be seen that the ø
values of reinforced sand is more than that
of the unreinforced sand at 70% relative
density, The ø value of the unreinforced
sand is 33o
where as with 10mm length of
the fibre and 0.05 % of FC, the ø value is
39o[2]
. The increase in the length of the
fibre also increases ø, but the increase is
marginal, particularly at high FC. It can be
seen that the optimum FC corresponds to
0.2% FC at which maximum value of ø
has been obtained.
Figure 2, Direct shear test for 10mm length
coir fiber with different percentage
Figure 3 Direct Shear Test for 20mm
Length Coir Fiber
0
50
100
150
200
0 50 100 150 200
ShearstressKPa
Normal stres KPa
unreinforced case (ɸ=33°)
0.05% coir fiber (ɸ=39°)
0.1% coir fiber (ɸ=42°)
0.2% coir fiber (ɸ=44°)
0.3% coir fiber (ɸ=43.5°)
0
50
100
150
200
0 50 100 150 200
ShearstressKPa
Normal stress KPa
unreinforced case (ɸ=33°)
0.05% coir fiber(ɸ=41°)
0.1% coir fiber(ɸ=44°)
0.2% coir fiber(ɸ=46°)
0.3% coir fiber(ɸ=45.5°)

4. R. Sridhar, M. T. Prathap Kumar
Figure 4 Direct Shear Test for 30mm
Length Coir Fiber
Table 2 Values of ø obtained
corresponding to different lengths of
Fiber and Fiber contents.
Angle of internal friction(Φ) in degrees
Length of fibre
Fibre
content
(%)
10mm 20mm 30mm
0 33° 33° 33°
0.05 39° 41° 42.5°
0.1 42° 44° 43.5°
0.2 44° 46° 47°
0.3 43.5° 45.5° 46°
Effect of Fibre Content on Peak Shear
Stress
Table 3. Shows the percentage increase in
peak shear stress randomly distributed coir
fiber(RDCF) at different fibre content in
comparison to the unreinforced sand. As
the fibre content increases the percentage
increase in the peak shear stress increases
up to 0.2% FC. At 0.3% FC the increase in
the peak shear stress decreases in most of
the cases. The maximum increase in peak
shear stress occurs at maximum normal
stress for a given fiber content.
Table3 Percentage increase in shear
stress of RDFS with different coir Fiber
length at failure.
Percentage increase in shear stress of
RDFS with different coir Fiber length at
failure
Coir
Fiber
length
in mm
Fiber
content
(%)
Normal stress (KPa)
50 100 150
10mm
0.05 25.31 19.7 40.51
0.1 35 34.30 63.93
0.2 53.12 43.06 74.47
0.3 50 34.30 65.33
20mm
0.05 31.25 20.43 50.46
0.1 31.56 33.43 63.23
0.2 51.56 44.37 72.83
0.3 56.56 44.31 84.54
30mm
0.05 35.62 33.43 59.25
0.1 45.34 34.89 65.57
0.2 57.50 88.32 80.91
0.3 62.5 53.28 85.40
Effect of Size of Coir Mat Opening on
Angle of Internal Friction (ø)
Results of the direct shear test on
unreinforced sand as well as sand
reinforcement coir mat with different sized
openings have been plotted in terms of
shear stress v/s normal stress. Fig .5 shows
the the variation of shear stress versus
normal stress from which angle of internal
friction (ø) was obtained.
0
50
100
150
200
0 50 100 150 200
ShearstressKPa
Normal stress KPa
unreinforced case (ɸ=33°)
0.05% coir fiber (ɸ=42.5°)
0.1% coir fiber (ɸ=43.5°)
0.2% coir fiber (ɸ=47°)
0.3% coir fiber (ɸ=46°)

5. Effect Of Coir Mat And Coir Fibre Reinforcement Of Shear Strength Of Dry Sand.
Figure 5 Direct Shear Test For coir mat
with different mat openings
Table 4. show the summary of angle of
internal friction obtained for coir mat with
different sized openings. It can be seen that
increase in the size of the opening of the
coir mat decreases the value of angle of
internal friction.The decrease in ø is
significant when the size of the opening
increases from 20mm to 30mm.
Table 4 Effect of size of coir mat
opening on angle of internal friction (Φ)
Effect of Coir Mat Opening on Peak
Shear Stress
Table 5 shows the percentage increase in
peak shear stress obtained corresponding
to different values of normal stress. It can
be seen that maximum value of peak shear
stress obtained corresponds to coir mat
with 10mm size opening. Thus, the effect
of increase in size of opening decreases the
interlocking effect between sand particles
and coir mat. Hence maximum ø value has
been obtained corresponding to smaller
size of the opening of coir mat[2]
. Thus the
size of the coir mat opening has
significant influence on the shear strength
characteristics of coir mat reinforced sand.
Smaller the size of opening, greater will be
the strength mobilized.
Table 5. Percentage increase in peak
stress of coir mat with different
openings
Percentage increase in shear stress of
coir mat with different opening sizes at
failure
Coir mat
opening
Normal stress (KPa)
50 100 150
10mm 184 107.30 151
20mm 78 60.60 95.55
30mm 68.90 54.74 87.35
Conclusions
On the basis of the present investigations,
the following conclusions have been
drawn:
1. The maximum value of ɸ has been
obtained corresponding to 30mm
length coir Fibre for the Fibre
content of 0.2% as compared to
unreinforced sand and also with
other lengths of coir Fibre.
2. In direct shear test, peak shear
stress of different lengths coir Fibre
0
50
100
150
200
250
0 50 100 150 200
ShearstressKPa
Normal stress KPa
unreinforced sand
10mm coir mat opening {ø=55°}
20mm coir mat opening {ø=48°}
30mm coir mat opening {ø=47°}
Relative
density
of sand
used
=70%
Coir mat opening in mm
unrei
nforc
ed
10x
10
20x
20
30x
30
Angle
of
internal
friction
in
degrees
33° 55° 51° 48

6. R. Sridhar, M. T. Prathap Kumar
is more than that of the
unreinforced sand. The effect of
Fibre content at smaller values of
normal stress is more at relative
density 70%
3. As the Fibre content increases the
percentage increase in peak shear
stress increases up to 0.2% Fibre
content. Beyond 0.2% of FC, rate
of percentage increase in the peak
shear stress decreases. As the
normal stress increases, the stress
in the peak shear stress of RDFS
(Randomly Distributed Fibre
Reinforced Soil) decreases.
4 The maximum value of ɸ has been
obtained corresponding to 10mm
coir mat opening as compared to
unreinforced sand.
5 In direct shear test, peak shear
stress of different coir mat
openings is more than that of the
unreinforced sand. The effect of
coir mat at smaller values of
normal stresses is more in
increasing peak shear stress.
6 As the normal stress increases, the
percentage increase in the peak
shear stress decreases and as the
size openings of coir mat
decreases, the peak shear stress
increases.
References
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11 December 2009

7. Effect Of Coir Mat And Coir Fibre Reinforcement Of Shear Strength Of Dry Sand.