IGC 2009, Guntur, INDIA
Coir Geotextiles as Separation and Filtration Layer for Low Intensity Road Bases
COIR GEOTEXTILES AS SEPARATION AND FILTRATION
LAYER FOR LOW INTENSITY ROAD BASES
K. Rajagopal
Professor, Department of Civil Engineering, IIT Madras, Chennai–600 036, India. E-mail: [email protected]
S. Ramakrishna
Formerly M.S. Research Scholar, Department of Civil Engineering, IIT Madras, Chennai–600 036, India.
ABSTRACT: Coir is a naturally occurring fibre available at relatively low cost in India. A number of coir products are
manufactured by coir board in Kerala for various geotechnical applications. The strength and other properties of these products
compare favourably with those of non-woven geosynthetic materials. The hydraulic properties of coir geotextiles are found to be
superior to the synthetic properties because of higher thickness and larger number of openings in the coir textiles. Depending
on the soil environment, these products can retain the strength in the soil environment for four to six years. Hence, these can
be used very effectively in many geotechnical applications for separation and filtration purposes in low intensity highway
pavements. This paper describes some studies performed at IIT Madras on the strength, stiffness and durability of coir geotextiles.
Through laboratory plate load tests the strength of coir reinforced road bases was studied. All the tests were performed in a
test tank of plan dimensions 1.5 m × 1.5 m and depth of 1 m. The results have shown that the coir geotextiles can increase the
strength and stiffness of soft soil bases. The durability aspects of coir geotextiles were studied by exposing them to acidic and
alkaline environments to accelerate their degradation. The strength of the coir geotextiles after different periods of exposure
was used to develop some simple equations to develop simple equations to relate the strength of coir geotextiles with time.
1. INTRODUCTION
The coir is a naturally occurring fibre derived from the husk
of coconut fruit. It is abundantly available at very low cost in
India. A large number of coir products are manufactured by
coir board in Kerala for various geotechnical applications in
the form of grids, textiles and mats. These applications include
filtration and drainage applications, reinforcement, erosion
control, etc. These products were found to last for as long as
four to six years within the soil environment depending on
the physical and chemical properties of the soil, Ramakrishna
(1996) and Rao & Balan (1994). Details of this work could
not be presented here due to lack of space. When it is used as
a reinforcement, the coir layers can share the load with soil
until its degradation thus increasing the load bearing capacity
of the subgrades. When coir geotextiles are used, they also
serve as good separators and drainage filters. In many
instances, the strength of subgrade soil increases in course of
time as the soil undergoes consolidation induced by the
traffic loads. At this stage, the subgrade may be strong enough
to support the loads on its own without the necessity for
reinforcement. For such applications, where the strength of
subgrade increases with elapsed time, the natural reinforcement
products are extremely suitable. After the degradation of the
coir geotextiles, the organic skeleton remains in place in
compressed form which will act as a filter cake keeping the
moisture content of the subgrade soil constant.
In the first part of this paper, results from many index tests
carried out on coir geotextile samples are reported. In the second
part, results from model plate load tests are reported along
with a discussion on the use of this data for design purposes.
2. MATERIALS AND PROPERTIES
2.1 Clay Soil
The clay soil used in this investigation had plastic and liquid
limits of 15% and 42% respectively. This soil can be classified
as CI according to the Indian Standard Classification System,
IS 1498–1978.
2.2 Gravel Soil (murum)
The gravel soil (murum) contains 48% gravel size particles,
48% sand size particles and 4% silt and clay size particles.
The specific gravity of this soil was found to be 2.75. This
soil is classified as A-2-4 (0) as per AASHTO classification
system and is rated as a good sub-base material. The
maximum dry density and optimum moisture content of this
soil are 21.6 kN/m3 and 10.2%. The cohesive strength and
friction angle of this soil are 10 kPa and 45° respectively.
2.3 Coir Geotextile
The coir geotextiles used in this investigation are commercially
available floor mats which are woven from coir fibres. The
coir fibres are twisted into a rope form and these ropes are
woven in weft and warp directions to form the mat. These
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Coir Geotextiles as Separation and Filtration Layer for Low Intensity Road Bases
mats have approximately the same strength in both the
principal directions. The thickness of this mat is dependent
on the diameter of the twisted ropes used in forming the mat.
The particular geotextile (coir mat) used in the investigation
had a thickness of 7.2 mm at a standard normal pressure of
2 kPa. The mass per unit area of this mat was 1,396 g/m2 at a
room temperature of 30°C.
The compressibility coefficient and compressibility modulus
(inverse of compressibility coefficient) were found to be
0.46 mm/kPa and 2.17 kPa/mm up to normal pressures of
50 kPa. At higher pressures, these values were found to be
0.025 mm/kPa and 40 kPa/mm respectively.
The ultimate tensile capacity of this mat was found to be
37 kN/m at a strain of 43% from wide-width tensile tests
performed according to ASTM D4595 standards. The secant
modulus of this material at 10% strain is 110 kN/m. The seam
strength, determined according to the suggested ASTM
procedures, was found to be 24 kN/m. The seams were stitched
using strong nylon thread with 25 mm overlap on both sides
of the seam.
Some fatigue tests were also performed to study the strength
under load repetitions. The tensile strength of coir geotextile
was found to decrease with number of load repetitions. The
lowest fatigue strength was found to be approximately 55%
of the ultimate capacity, i.e. if the applied load is less than this
level, the material will not fail under load repetitions. The
modulus was also found to decrease with the number of cycles.
The trapezoidal tear strength was found to be 12 kN from
tear tests performed according to ASTM D4533 standards.
The impact test performed by dropping a metal cone having
an apex angle of 45° from 500 mm height on coir geotextile
clamped in modified CBR mould resulted in a hole of
diameter 6.8 mm.
The puncture resistance from CBR push-through test using a
50 mm diameter plunger rod was obtained as 30 kN/m. The
larger plunger diameter of 50 mm was used to account for
the irregular aperture sizes and imperfections in the weave
pattern. The 8 mm diameter rod as suggested in ASTM D3787
was found to be too small for the relatively large aperture
opening sizes in the sample.
The cohesive strength and friction angle of clay-coir geotextile
interface were found to be 23 kPa and 8° respectively. The
same properties for gravel-coir interface are 40 kPa and 32°
respectively. A comparison of these strength properties with
those of individual materials shows that coir geotextile has
excellent interface strength properties. This is because of the
rough nature of the coir and its natural affinity towards the
water and clay because of different surface charges.
The Apparent Opening Size (AOS) of the coir geotextile was
determined using wet sieving (hydrodynamic) technique
using single sized sand particles. The uniform size sand
particles were used instead of glass beads recommended in
ASTM D4751 test procedures. As the glass beads get
repelled away from the coir (because of surface charges),
they could not be used in this test. The test was conducted by
fixing the coir geotextile in a open ended CBR mould and
repeatedly dipping the mould in a water tank up to 100 times
at a rate of 10 times per minute for each sand particle size.
The AOS [O(95)] was obtained as the size of sand particles
when only 5% of sand by weight passes through the geotextile.
The O(95) of coir geotextile used in this investigation was
found to be 1.18 mm.
The permittivity (k/t, k = coefficient of permeability and t =
thickness) of coir geotextile was determined from conventional
constant head tests as 0.07 mm/sec. The geotextile was sandwiched in these tests between graded sand (particle size 1–2
mm). The coefficient of permeability of the sand medium was
found to increase with the introduction of coir geotextile by
20%.
3. LABORATORY PLATE LOAD TESTS
3.1 Test Facility
The plate load tests were performed within a test tank of plan
dimensions 1.5 m × 1.5 m and 1.2 m deep. The loading was
applied through a 100 kN capacity proving ring using a
hydraulic jack. The test tank was centrally located below a
reaction frame for applying plate loads through a hydraulic
jack. The plate was of 300 mm diameter (D) to simulate the
Equivalent Single Wheel Load (ESWL). The test tank was
connected to a vacuum pump to suck water from soft clay
bed and accelerate its pre-consolidation.
3.2 Preparation of Test Bed
The soft clay soil bed was prepared by sedimentation technique
under vacuum pressure to simulate the soft natural subgrade.
The soil was initially mixed manually using crowbars at 160%
water content. This slurry was kept under a low vacuum
pressure of 19.6 kPa for three days to drainout the water and
remove the entrapped air. Then it was subjected to a
consolidation pressure of 98 kPa until the rate of deformation
has decreased to less than 0.01 mm per minute. The method
of test bed preparation was adapted from the work reported
by Kuntiwattanakul et al. (1995). The consolidation pressure
was applied uniformly over the entire area through 20 mm
thick mild steel plate stiffened with I-sections. The entire
consolidation process took approximately 8 to 10 days for
each preparation. This procedure had created soft subgrades
with uniform properties with a CBR value of approximately
0.4. The thickness of soft clay layer was maintained at
around 900 mm for all the tests.
The gravel sub-base course was prepared directly on top of
this clay layer. The gravel layer was compacted at optimum
moisture content using a 10 kg drop hammer falling through
a height of 500 mm. The number of blows and the height of
fall were decided by equating to the standard compaction
energy, Equation 1.
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Coir Geotextiles as Separation and Filtration Layer for Low Intensity Road Bases
 number
 number

of 
 of blows ×
 per layer 
 layers 
E=
volume

×
weight
height


 of
of
 × drop
 of
 hammer 
 hammer 
of soil layer
(1)
was continued until a total plate settlement of 150 mm has
occurred which took approximately 6 days.
in which E is the standard compaction energy (≅ 592.5 kJ/m3)
The gravel layer was compacted to 80% maximum density in
all the tests. The compacted sub-base course was allowed to
mature for one day by covering it with a polythene sheet. The
coir geotextile reinforcement was introduced during the
compaction stage itself.
The above procedure of preparing the clay and gravel layers
was repeated for all the tests performed in this present
investigation so that the test conditions remain uniform for
the entire range of tests.
3.3 Test Programme
The following series of plate load tests were carried out in
this investigation with the following four configurations.
• Type I: on soft clay subgrade alone
• Type II: on gravel sub-base course over soft clay subgrade.
• Type III: on gravel sub-base course over soft clay subgrade
and one layer of coir geotextile at clay-gravel interface.
• Type IV: on gravel sub-base course over soft clay subgrade
and two layers of coir geotextile, one at clay-gravel
interface and the other at mid-depth of gravel layer.
After each test, the gravel layer was carefully removed. After
that, the top 400 mm thick clay soil in the tank bed was
replaced with puddled clay having 40% water content. This
test bed was once again pre-consolidated under 98 kPa
surcharge pressure which took 2 to 3 days to stabilise. This
re-formed clay bed had the same properties as the originally
prepared clay bed as confirmed by results from in situ vane
shear tests. On this, fresh gravel layer of required thickness
was laid in the same manner as discussed earlier.
4. RESULTS FROM PLATE LOAD TESTS
The plate load tests carried out on clay bed showed that it is
an extremely soft subgrade having an ultimate pressure of
20 kPa. Hence, it can be expected that the provision of gravel
sub-base with or without coir reinforcement will significantly
improve its load bearing capacity. Typical improvement in
the performance obtained with the provision of gravel layer
with and without reinforcement layers is illustrated in Figure 1.
In general, the performance has improved with the increase
in the thickness of gravel layer. The provision of a geotextile
layer at the clay-gravel interface has further increased the
load bearing capacity of subgrade. When an additional layer
of geotextile was placed at the mid-height of the gravel layer,
the ultimate capacity and stiffness has tremendously increased.
In Types 2–4, six sub-base layer thickness (h) values of 100,
150, 200, 250, 300 and 350 mm were considered.
3.4 Test Procedure
The general test procedure for plate load tests as described in
Indian standard IS 1888:1982 (Reaffirmed 1988) was
adopted for all the tests. The loading was applied through a
300 mm diameter plate. The applied load was measured
using a pre-calibrated 100 kN capacity proving ring. The
settlement of plate and the soil surface were measured using
totally six dial gauges. Three of these were fixed on the plate
and the other three on the soil surface at distances of 100,
300 and 400 mm away from the plate. The surface settlement
of soil was measured through small settlement plates.
In the case of Type-I series of tests, the clay soil was covered
with 5 mm thick fine sand layer. In the case of reinforced
tests, the coir geotextile was placed at the required levels
after wetting.
Each load increment was applied as either 10% of the estimated
ultimate load or the load required to produce 1 mm settlement,
whichever is lesser. Each load increment was kept constant
until the rate of settlement reduces to less than 0.025 mm per
minute. The load and the corresponding deformations were
recorded after the settlements have stabilised under each load
increment, which was typically 6 to 12 hours. The loading
Fig. 1: Performance with 150 mm Thick Gravel Layer
The ultimate pressures developed from various tests are
compared in Table 1 which clearly shows the improvement
in the performance with the provision of coir geotextile
reinforcement.
From the results shown in Figure 1 and Table 1, it can be
seen that the provision of a single layer reinforcement at the
clay-gravel interface does not improve the load bearing
capacity very much. The effect of single layer of geotextile is
significant when gravel layer is thicker than 200 mm. When
a thin layer of gravel is provided, there may not have been an
943
Coir Geotextiles as Separation and Filtration Layer for Low Intensity Road Bases
adequate bond with the coir geotextile for the load transfer to
take place. In the case of two layers of coir geotextile, the
reinforcement layer at the mid-depth of gravel prevents its
lateral spread and hence higher loads are mobilised in coir
reinforcement which contributes to the increase in ultimate
pressures. This can also be explained by the good bond
between the coir and the gravel as shown from the interface
shear strength properties (section 2.3).
standard wheel loads. It is evident from this chart that the
thickness of subbase can be substantially reduced with the
use of coir geotextile reinforcement. At rut depths less than
approximately 10 mm, the gravel layer can not mobilise
enough shear strength which results in the requirement of
very thick subbases, as is evident from the initial steep slope
of curves.
In the case of unreinforced and single layer reinforcement
cases, the ultimate pressures have developed within a
settlement of 15 to 40 mm whereas the two layer system had
developed ultimate pressures at much higher settlements in
the range of 100 mm (Figure 1). This result once again confirms
the advantage of placing the additional reinforcement layer
within the gravel layer.
Table 1: Ultimate Pressures (kPa) from Plate Load Tests
Thickness
of gravel
layer (h)
100 mm
150
200
250
300
350
Unreinforced
subbase layer
100
200
240
300
370
400
One layer of
reinforcement
120
220
250
360
440
500
Two layers
of reinforcement
400
550
750
900
1000
1150
Fig. 2: BCR for a Rut Depth of 75 mm
In addition to the reinforcement action, the geotextile layer at
clay-gravel interface functions as a separator and filtration
and drainage medium as it has good compressibility
characteristics. On the other hand, the layer within the subbase
contributes mainly to the strength and stiffness of subgrade.
It is evident from this experimental results that stiffness of
the coherent mass is also an important parameter as also
reported by Douglas & Valsangkar (1992).
From the plate load test data, the pressures developed for
different thicknesses of gravel layers at various settlement
levels were developed. This data is plotted in a nondimensional form in Figure 2 for a rut depth of 75 mm.
Similar charts were developed for other rut depths also. In
these figures the thickness of gravel layer (h) is normalised
with respect to the diameter of plate (D) and the plate
pressure at any settlement is normalised with respect to the
ultimate capacity of soft clay layer (20 kPa) denoted as the
Bearing Capacity Ratio (BCR). This term BCR indicates the
relative improvement in the bearing capacity of subgrade
with the provision of gravel layers with or without
reinforcement. These charts can be used for designing the
thickness of subbase layer over soft subgrades for a given
BCR and the diameter of wheel base.
Fig. 3: Design Chart for Wheel Load of 22.25 kN
5. ILLUSTRATIVE EXAMPLE
Design a subbase course to increase the load bearing capacity
to 500 kPa of soft clay subgrade whose ultimate bearing
pressure is 25 kPa. Consider the Equivalent Single Wheel
Loads (ESWL) of 28 kN and 48 kN having a tire pressure of
580 kPa. Design the subbase course for allowable rut depths
of 25, 50 and 75 mm.
Design: The following step-by-step procedure illustrates the
design process using the design charts developed in this
investigation.
Step 1: Calculate the Bearing Capacity Ratio (BCR) at the
ultimate bearing pressures.
The above data is presented in a different form in Figure 3
for Indian Road Congress (IRC) standard wheel load of
22.25 kN. Similar curves have also been developed for other
944
BCR =
required ultimate bearing pressure on subbase course
ultimate bearing capacity of subgrade
Coir Geotextiles as Separation and Filtration Layer for Low Intensity Road Bases
In the present example, BCR =
500
= 20
25
Step 2: Calculate the contact area of the wheel as the ratio of
ESWL and the tire pressure. From these, the equivalent
diameters of contact area are calculated for the two wheel
loads as 250 mm and 325 mm respectively.
Step 3: Obtain the required h/D ratios for rut formations of
25, 50 and 75 mm from the design charts. The results are
given in Table 2.
In the following, r is the rut depth and P is the wheel load. As
can be seen, the single layer reinforcement does not result in
much savings in subbase thickness. On the other hand, when
an additional layer is provided within the subbase, significant
savings are achieved. This response directly follows the
response of plate load test from Cases 3 and 4.
6. DEGRADATION OF COIR GEOTEXTILES
Any long term applications of natural geotextiles should
consider the possibility of degradation of the properties over
the period of time. The ideal situation is when the subgrade
soil gains the required strength due to consolidation before
the degradation of the coir geotextile, Datye (1983).
The degradation due to chemical, hydrolysis and biological
factors are most predominant in the soil environment. This
paper examines some of these aspects through laboratory
tests. The effect of various parameters on the durability of
coir was characterised using wide-width tensile strength of
coir geotextile samples. These tests were carried out at a rate
of 10 mm/minute on samples 200 mm wide and 100 mm long.
6.1 Degradation Due to Hydrolysis and Biological Actions
The moisture content of the coir fibre greatly affects the
strength and hence it is important to perform hydrolysis test.
The tests were performed by performing wide-width tensile
tests on samples immersed in tap water for varying periods of
time. Some of these samples were dried and then tested in
dry condition. In general, it was found that the strength of
coir geotextiles in wet conditions is very much less. The
strength in the wet conditions drops by more than 70%
whereas the samples dried after wetting had almost the same
strength as the virgin samples. The water content of coir had
increased from 9% to 120% due to the immersion in water,
Kulkarni et al. (1983).
6.2 Degradation due to Chemical Action
The degradation studies of coir fibre (which is an organic
material) acid reagents have been used in the past to wash
away the lignin, e.g. Uma et al. (1994). In the absence of
standard method of testing the coir fibre for chemical
degradation, a general procedure was adopted to test the fibre
for both acid and alkaline reagents. The tests were performed
under the laboratory conditions at different concentrations of
acid and alkaline reagents for different duration of exposures.
Both the concentration and the number of days of exposure
also influence the loss of strength.
6.3 Degradation of Coir Fibre in Organic Clayey Soil
Biological and chemical agencies in the presence of water
are the most likely causes of the degradation of coir. The
study of coir fibre degradation by single agencies like alkali,
acidic reagent, or a particular type of micro-organism strain
may not simulate the real conditions and also the degradation
by a particular strain alone was not significant as discussed in
the previous sections. Therefore, degradation of coir fibre in
organic clay media by synergistic activity of chemical
reagents and micro-organisms was employed in this
investigation. The coir samples were exposed to this mixture
for varying periods of time.
Table 2: Design of Subbase Thickness over Soft Subgrades
Design
case
1.
2.
3.
Design base course thickness (mm)
Savings in thickness (mm)
gravel alone
(A)
gravel + one
layer reinf. (B)
gravel + two
layer reinf. (C)
Savings for
B over A
Savings for C
over A
r = 25 mm
P = 28 kN
P = 48 kN
300
360
268
321
153
183
32
39
147
177
r = 50 mm
P = 28 kN
P = 48 kN
288
345
255
310
108
129
33
35
180
216
r = 75 mm
P = 28 kN
P = 48 kN
275
330
250
300
83
100
25
30
192
230
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Coir Geotextiles as Separation and Filtration Layer for Low Intensity Road Bases
Four naturally occurring chemical compounds, Calcium
Chloride (CaCl2), Magnesium Sulphate (MgSO4), Sodium
Hydroxide (NaOH), and Potassium Carbonate (K2CO3), were
mixed in different proportions to a specially prepared organic
clay in these studies. The chemical compositions had been
decided based on a second order rotatable theory proposed
by Box & Hunter (1957). This theory is most suitable to
study the effect of various parameters. More details of this
application can be found in Ramakrishna (1996).
6.4 Tests on Polymer Coated Samples
comparable to those of non-woven geotextiles. The large
thickness of coir geotextiles helps in good transmissivity
properties. Hence, the use of these coir geotextiles can be
considered in unpaved roads where the traffic intensity is
low, e.g. in rural roads.
The accelerated degradation tests on coir fibres show that the
coir can have a life span of about four years. The full tabular
data and the analysis of degradation tests will be presented
during the conference.
ACKNOWLEDGEMENTS
Some tests were performed by giving polymer coating to the
coir geotextiles to understand their degradation behaviour. It
was found that both strength and stiffness of coir geotextiles
increase due to polymer coating. The reinforcement products
made of natural coir fibre develop their ultimate strength at
very high strains because of their low modulus. Hence, their
tensile capacity is not fully utilised if there is a restriction on
the maximum deformations that a structure can undergo. To
overcome this problem, the natural coir fibres can be given a
coat of thermosetting polyester to increase their strength and
stiffness, Prasad et al. (1983). The results from their studies
showed that the polyester coating improves the durability,
strength, stiffness and decreases the water absorption of coir
fibres. The effectiveness of polymer coating was found to
depend on the pre-treatment methods used before applying
the polymer coating. The variation of ultimate strength and
stiffness of polymer coated coir samples (with alkali, acidic
and copper pre-treatment) was determined before and after
exposing them to organic clay for six months.
Different pre-treatments were given for the efficient polymer
coating of geotextiles.
6.5 Results from Degradation Tests
Large amount of data was generated from these accelerated
degradation tests. Based on these results, some equations
were developed to quantify the rate of degradation of the
strength of the coir geotextiles in natural clay soils.
7. CONCLUSIONS
The use of coir geotextiles for construction of subbase layer
over soft subgrades is studied in this paper. Various
engineering properties of coir geotextiles have been reported
in this paper. These properties are comparable to those of
intermediate to high density polypropylene based geotextiles.
The plate load tests have clearly indicated the capability of
coir geotextiles in improving the stiffness and load bearing
capacity of soft subgrades. Hence, the coir geotextiles are
suitable for cost-effective field applications. The physical
and hydraulic properties of these coir geotextiles are quite
The help received from Coir Board, Kerala is gratefully
acknowledged for much data received from them. The
samples of free coir geotextiles received from them are also
gratefully acknowledged.
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