The document summarizes a study on the settlement characteristics of sand reinforced with coir mats and coir fibers. Load-settlement tests were conducted on model footings placed on reinforced sand beds. It was found that the settlement reduction factor (SRF), a measure of settlement, increases with increasing stress for both mat and fiber reinforcement. For mat reinforcement, SRF increases with decreasing mat opening size and decreasing reinforcement depth. For fiber reinforcement, an optimum depth of 0.6 times the footing width was found to provide the highest SRF. Reinforcement form significantly influences strength, with mat reinforcement performing better at shallow depths and fiber reinforcement at greater depths.

1. Proceedings of Indian Geotechnical Conference IGC-2014
December 18-20, 2014, Kakinada, India
SETTLEMENT CHARACTERISTICS OF COIR MAT AND COIR
FIBER REINFORCED SAND
Dr.M.T.Prathap Kumar. Prof.,Dept. of Civil Engg., Reva Inst. of T & M, Blore.,drmtprathap@gmail.com
R Sridhar, Research Scholar, Ghousia college of Engg. Ramanagara, sridharrajagopalg@gmail.com
ABSTRACT: The use of natural fibers for soil improvement is highly attractive in countries where such materials
are locally and economically obtainable, in view of the preservation of natural environment and cost effectiveness.
Very few literatures are available to assess comparative performance of two forms of reinforcement, viz., mat type
and fiber type, that are extensively used as reinforcing the soils. In the present study, load-settlement tests using
model footing of 50mm diameter resting on coir mat of different opening size and coir fibers of average length of
20mm were conducted. The performance of both the forms of reinforcement were analyzed by calculating
Settlement Reduction Factor (SRF).. Variations between SRF versus normalized stress were plotted to acess the
performance of model footings for settlement. It was found that, SRF increases with increase in normalized stress.
With decrease in mat opening size, there is significant increase in SRF for a given u/B ratio. Settlement of footings
on fiber reinforced sand is dependent on depth of fiber reinforced zone. SRF obtained corresponding to u/B=0.6 is
significant for all values of normalized stress, indicating u/B=0.6 becomes optimum depth of fiber reinforced zone
from consideration of both bearing capacity and settlement.
Key words: Settlement Reduction Factor, model footings, Coir mat
INTRODUCTION
The use of natural fibers for soil improvement is
highly attractive in countries where such materials
are locally and economically obtainable, in view of
the preservation of natural environment and cost
effectiveness. Several investigations have been
carried out [1-5] with regard to the bearing
capacity of mat type and fiber type of
reinforcements and have indicated that both
synthetic and natural materials when used as
reinforcement in soils are beneficial in increasing
the bearing capacity of soil and in reducing the
settlement of reinforced soil. Guido et al. (1986)
[6]performed a series of laboratory model tests on
rectangular and square footing. They indicated that
bearing capacity ratio (BCR) at a settlement of
0.1Bincreases rapidly with increasing strip length
up to a length of about 0.7B after which it remains
relatively constant. Omar et al. (1993)[7] have
conducted laboratory model test results for the
ultimate bearing capacity of strip and square
foundations on sand reinforced with geogrid layers.
Based onthe model test results, the critical depth of
reinforcement and the dimensions of the geogrid
layers for mobilizing the maximum bearing
capacity ratio have been determined and compared
Dash et al. (2004)[8] have done model studies on
circular footing supported on geocell reinforced
sand underlain by soft clay. The test beds are
subjected to monotonic loading by a rigid circular
footing. The influence of width and height of
geocell mattress as well as that of a planar geogrid
layer at the base of the geocell mattress on the
overall performance of the system has been
systematically studied through a series of tests. The
test results indicate that the provision of geocell
reinforcement in the overlaying sand layer
improves the load carrying capacity and reduces
the surface heaving of the foundation bed
substantially. The performance improvement
increases with increase in the width of the geocell
layer up to b/D = 5, beyond which it is negligible.
(Here, b =width of geocell layer, D = diameter of
footing). The overall performance improvement is
significant up to a geocell height of about two
times the diameter of the footing and beyond this,
the improvement is marginal. Das et al. (1998) [9]
have conducted laboratory tests to find out 1the

2. M.T.Prathap kumar,R.Sridhar
effect of transient loading over a foundation
supported by geogrid reinforced sand. In the test, a
square foundation is used and through out the test
one relative density is maintained. In al the tests,
the peak value of the transient load per unit area of
the foundation exceeded the ultimate static bearing
capacity of foundation supported by unreinforced
sand. The conclusion drawn this test is that the
geogrid reinforcement reduces the settlement due
to transient loading. Madhavi Latha and
AmitSomwanshi(2007)[10] Presented the result
from laboratory model test on square footing
resting on sand. Laboratory model test and
numerical simulation on square footing supported
by sand bed with or without geosynthetic
reinforcement are discussed. MadhaviLatha and
Vidya (2006)[11].Presented the effects
of reinforcement form on strength improvement of
geosynthetic-reinforced sand through triaxial
compression tests. Samples of sand reinforced with
geosynthetics in three different forms, viz.
horizontal layers, geocells, and randomly
distributed discrete fibers are tested in triaxial
compression and results are analyzed to understand
the strength improvement in sand due to
reinforcement in different forms.In all these
investigations, it has been observed that the layout
and configuration of reinforcement play a vital role
in bearing capacity improvement rather than the
tensile strength of the material. In contrast to grid
or mat form of reinforcement, randomly distributed
fibre-reinforced soils exhibit some advantages.
Preparation of randomly distributed fibre-
reinforced soils mimics soil stabilization by
admixtures.[12]. Discrete fibres are simply added
and mixed with soil, much like cement, lime, or
other additives and offer strength isotropy and limit
potential planes of weakness that can develop
parallel to the oriented reinforcement as included
in reinforced soil[13].Interaction between soil and
grid/mat basically depends on mechanical
properties of soil (density, grain size distribution,
particle size, shape and orientation) and
geometrical and mechanical properties of
reinforcement.When grids/mats are used, aperture
size of the grid, thickness and shape of rib cross
section, extensibility of longitudinal ribs, flexibility
and shear stiffness of transversal ribs, strength of
knots matters Degree of interaction is influenced
by interrelation of soil particles and structure of the
grid: ratio between particle. The interaction of fiber
reinforcement is much simpler, where in, the
isotropic compression causes relative movement
among particles and produces tensile stresses in the
fibres located among them. There is also the
possibility of an additional mechanism of fibre
breaking during testing by squeezing and crushing
of the sand particles, cutting the fibres trapped
between them. Very few investigations are
available on the comparative assesment of
behaviour of mat/grid form of reinforcement versus
fiber reinforcement, as essentially there exists a
completely different mechanism between two
forms of reinforcement. The objective present
study thus is to understand the performance of two
forms of reinforcement, viz., mat type and fiber
type of reinforcement. For this study, the locally
available coir based materials have been used as
reinforcement.
MATERIALS AND METHODS
Sand Used
Sand, a naturally occurring granular material
composed of finely divided rock and mineral
particles was used which was obtained locally at
Bangalore. Properties of sand used in the present
experimental study are as shown in Table 1
Table1: Properties of Sand Used
Coir and Coir Mat Used
Coir mat and coir fibres obtained from coir
industry, Gubbi, Tumkur district, Karnataka, India,
were used in the present study.
Coefficient ofuniformity, (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. Settlement characteristics of coir fibre and coir mat reinforced sand
Model footings and test tank
The load tests on model footings resting on
unreinforced sand and reinforced sand were
conducted in a load frame that can apply load at a
continuous rate of 1.25mm/minute. Sand beds were
prepared in a cylindrical steel tank of diameter
300mm and height 350 mm. The model footing
used for the tests was circular in shape and is of 50
mm diameter and is of sufficient thickness to
withstand bending stress.Fig.1 shows the typical
layout of the reinforced sand bed adopted in the
model tests.
Methodology for load testing
Sand bed was prepared up to the height of 30cm by
compaction in three layers and a relative density
of 80% was maintained for all the tests. Coir mats
of opening 10x10 mm,20x20 mm and 30x 30 mm
and of diameter slightly less than the inner
diameter of tank, to avoid side friction, were used
and placed at specific depths while preparing the
sand bed for each model test. Coir fibers of average
length 20mm were randomly reinforced for the
required depth. The depth of layer of reinforcement
in case of coir mat and the depth of randomly
treated fibre reinforced sand from the bottom of the
footing is measured as u, and model footing tests
for various depth of reinforcement to width of
footing ratio(u/B) 0.6, 0.3, 1.0 and 2.0 were
conducted.
Fig. 1. Schematic diagram of the test set-up
Tests with reinforced sand beds were carried out by
placing the coir mat at the predetermined depths
while preparing the sand beds. After preparing the
bed, surface was leveled and the footing was
placed exactly at the center to avoid eccentric
loading. The footing was loaded and the load was
applied at the rate of 1.25mm/min, measuring the
corresponding footing settlements through the dial
gauges D1, D2 and D3.Average of the three
readings were considered as final settlement for a
given load intensity. Model footings resting on
unreinforced sand bed were conducted to compare
the results in terms of ‘Bearing Capacity Ratio
(BCR).Experiments was repeated with
unreinforced sand for comparison purpose.
RESULTS AND DISCUSSIONS
Effect of Coir Mat and Coir Fiber
Reinforcement on Peak Stress at Failure
Results of the load settlement measurement were
plotted in terms of Load-intensity versus Percent
Strain for model footings resting on unreinforced
and reinforced sand beds. Typical curves at U/B
ratio of 0.3,0.6 and 1.0 for coir mat reinforced sand
are as shown in Figures 2,3 and 4. The peak stress
was obtained from these plots and the strain
corresponding to the peak stress was considered
peak strain.
Fig2: Typical Load intensity versus Settlement for
U/B=0.3 for coir mat reinforced sand
Load–settlement behaviour of sand showed general
shear failure,indicating that the sand was stiffened
with the inclusion of reinforcement. Lifting of coir
mat placed at shallow depth ratio of u/B=0.3 was
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 10 20 30 40
LOADINTENSITYKN/m2
% STRAIN

4. M.T.Prathap kumar,R.Sridhar
observed beyond peak stress value measured
during loading process.Considerable bulging was
the edges of model footiongs were observed for
sand reinforced with coir fibers upto a shallow
depth ratio of u/B=0.3 anmd 0.6..
It can also be seen that the peak stress obtained is
significantly larger for mat form of reinforcement,
when placed at lower u/B ratio. However, the peak
stress at failure at higher u/B ratio for fiber type of
reinforcement is significantly larger than that
obtained for coir mat form of reinforcement. The
trend in the result thus has a significant bearing on
design aspects of foundations on reinforced soil
bed
Fig3: Typical Load intensity versus Settlement for
coir Fiber reinforced sand
Fig 4 shows similar comparative variation of
Strain at peak stress, called as peak strain, for both
coir mat and coir fiber reinforced sand. The trend
obtained was similar to variation of peak stress for
both form of reinforcement. The increase in peak
strain with u/B ratio for coir fiber reinforced sand
indicates that the introduction of randomly
distributed fibers in soil increases the ductility of
soil, as there is no plane of weakness unlike coir
mat reinforced sand. Thus a fiber form of
reinforced soil will have a significant improvement
in resisting cyclic or dynamic loads.
Bearing Capacity ratio (BCR) was calculated as the
ratio between Peak stresses at failure of reinforced
sand to peak stress at failure for unreinforced sand.
Fig.5 shows such a variation with increase in u/B
ratio for both coir mat form of reinforcement and
coir fiber form of reinforcement.. As can be seen,
that the BCR increases with decrease in size of the
opening and is maximum for mat opening of size
10x10mm.For the case of coir fiber reinforced
sand, the maximum BCR corresponds to u/B=1.0,
indicating that there always exists an optimum u/B
ratio at which strength gain of fiber reinforced will
be maximum.
Fig 4: Comparative Variation of Peak Strain with
u/B ratio for coir mat and Coir Fiber Reinforced
Sand
Fig 5: Comparative Variation of BCR with u/B
ratio for coir mat and Coir Fiber Reinforced Sand
Effect of Coir Mat and Coir Fiber
Reinforcement on Settlement Reduction Factor
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 10 20 30 40
LOADINTENSITYKN/m2
% STRAIN
10mmX10m
m COIR MAT
20mmX20m
m COIR MAT
30mmX30m
m COIR MAT
UNREINFOR
CED
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 0.5 1 1.5 2 2.5
PEAKSTRESS(KN/m2)
u/B RATIO
10X10
mm
20X20
mm
30X30
mm
FRS
0
1
2
3
4
5
6
0 0.5 1 1.5 2 2.5
BCR
u/B ratio
FOR UNTREATED MAT
10X10mm
20X20mm
30X30mm
FRS

5. Settlement characteristics of coir fibre and coir mat reinforced sand
Variations between SRF versus normalized stress
were plotted to assess the performance of model
footings for settlement. Figure 6 shows such a
variation for model footings resting on reinforced
sand with different u/B ratios and different size of
mat openings. In majority of the cases, SRF
increases with increase in normalized stress.
Further, with decrease in mat opening size, there is
significant increase in SRF for a given u/B ratio.
With increase in u/B ratio, there is decrese in SRF,
indicating that when mat reinforcement is placed
at shallow depth, the settlement of the model
footings also decreases significantly. Increase in
the mat opening size, also increases settlement.
Fig 6 : Variation of model footings resting on coir
mat reinforced sand
Figure 7 shows similar variation for model footings
resting on coir fiber reinforced sand It can be seen
that there is significant increse in SRF with
increase in depth of fiber reinforced zone i.e u/B
ratio. Hence settlement of footings on fiber
reinforced sand is dependent on depth of fiber
reinforced zone. SRF obtained corresponding to
u/B=0.6 is significant for all values of normalized
stress, indicating u/B=0.6 becomes optimum depth
of fiber reinforced zone from consideration of both
bearing capacity and settlement for the case of
fiber reinforced sand.
Fig 7 : Variation of model footings resting on coir
fiber reinforced sand
CONCLUSIONS
On the basis of present experimental study, the
following conclusions have been drawn:
The forms of reinforcement, viz.mat/grid and fibers
have significant influence on strength of reinforced
sand. The peak stress and hence BCR is decreases
with increases with increases in u/B ratio, for the
case of mat form of reinforcement.
The size of the mat opening has a significant
influence on BCR of coir mat reinforced sand.
Smaller the opening size of mat, greater
interlocking effect, which increase the BCR of mat
form of reinforcement
.For fiber form of reinforcement, there always exist
an optimum u/B ratio at which the peak stress and
hence BCR reaches a maximum value.
The increase in peak strain with u/B ratio for coir
fiber reinforced sand indicates that the introduction
of randomly distributed fibers in soil increases the
ductility of soil. Thus a fiber form of reinforced
soil will have a significant improvement in
resisting cyclic or dynamic loads.
-200
-150
-100
-50
0
50
100
150
0 0.5 1 1.5
SRF
Normalized Stress
0.3 10X10
0.6 10X10
1.0 10X10
0.3 20X20
0.6 20X20
1.0 20X20
0.3 30X30
0.6 30X30
1.0 30X30
-10
0
10
20
30
40
50
60
70
0 0.2 0.4 0.6 0.8 1 1.2
SRF
Normalised stress
0.3
0.6
1
2

6. M.T.Prathap kumar,R.Sridhar
REFERENCES
1. Vidal.H.(1969),The principle of reinforced
earth, Highway research record, 282, 1: 1-
163
2. Sreekantaiah, H.R. (1988), “Stability of
Loaded footings on Reinforced sand”,
Proc. Indian Geotech. Conf. on Reinforced
soil and Geotextiles, Bombay, Vol.1, C.3-
C.7.
3. Yetimoglu, T., Wu, J.T.H., & Saglamer, A.
(1994). Bearing capacity of rectangular
footings on geogrid-reinforced sand.
Journal of Geotechnical Engineering, 120
(12), 2083-2099.
4. Temel Yetimoglu*, Omer Salbas(2002), A
study on shear strength of sands reinforced
with randomly distributed discrete fibers
Jou. of Geotextiles and Geomembranes 21
(2003) 103–110, 5 December 2002
5. Consoli, N.C., Casagrande, M.D.T., preitto,
P.D.M., and Thome, A. (2003), Plate load
test on fiber reinforced soil, Jou. of
Geotech. Geoenviron. Eng., 129(10), 0951-
0955
6. Guido, V. A., Chang, D. K., Sweeney, M.
A. (1986) ,Comparison of geogrid and
geotextile reinforced earth slabs. Canadian
Geotechnical Journal 23: pp. 435-440
7. Omar, M.T., Das, B.M., Puri, V.K. and
Yen, S.C. (1993) Ultimate bearing capacity
of shallow foundations on sand with
geogrid reinforcement,Canadian
Geotechnical Journal, 30, 545–549.
8. Dash.S.K,Krishnaswamy.N.R.and Rajgopal
(2001),Bearing capacity of strip footing
supported on Geocell reinforced
sand,Geotextiles and Geomembranes.19(4),
235-236
9. Das, B.M. and Shin, E. C. (1998) Strip
foundation on geogrid-reinforced clay:
behavior under cyclic loading, Geotextiles
and Geomembranes,13(10), 657–666
.
10. Madhavi Latha .G &Vidya S. Murthy (
2007), Effects of reinforcement form on the
behavior of geosynthetic reinforced sand,
Geotextiles and Geomembranes, Vol. 25,pp
23–32.
11. 11 Madhavi Latha.G,Amit Somwanshi, ,
2009. Effect of reinforcement form on the
bearing capacity of square footings on sand.
Geotextiles and Geomembranes 27 (6), 409
- 422
12. 12.VenkatappaRao,G.,Dutta.R.K.,andUjwal
a,D.,(2005), Strength characteristics of
Sand Reinforced with coir fibre and coir
Geotextiles, Jou. of Geotechnical
Engineering, USA, Vol.10/G,
http://www.ejge.com .
13. 13.Praveen kumar gupta, Swami Saran and
Ravikant Mittal (2006), Behavior of Fibre
reinforced sand in different test conditions,
Indian Geotechnical Journal, vol. 36, No.
3, 2006, 272-282.