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Geotextiles and Geomembranes 27 (2009) 204–210

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Geotextiles and Geomembranes

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Laboratory performance of unpaved roads reinforced withwoven coir geotextiles

E.A. Subaida*, S. Chandrakaran, N. SankarDepartment of Civil Engineering, National Institute of Technology, Calicut, 673 601, India

a r t i c l e i n f o

Article history:Received 6 June 2008Received in revised form5 November 2008Accepted 7 November 2008Available online 22 January 2009

Keywords:Woven coir geotextileUnpaved roadBearing capacityPlastic deformation

* Corresponding author. Tel.: þ91 495 2286230 or þ2287250.

E-mail addresses: easubaida@yahoo.co.in (E.A.(S. Chandrakaran), sankar@nitc.ac.in (N. Sankar).

0266-1144/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.geotexmem.2008.11.009

a b s t r a c t

The results of an experimental study conducted to investigate the beneficial use of woven coir geotextilesas reinforcing material in a two-layer pavement section, are presented. Monotonic and repeated loadswere applied on reinforced and unreinforced laboratory pavement sections through a rigid circular plate.The effects of placement position and stiffness of geotextile on the performance of reinforced sectionswere investigated using two base course thicknesses and two types of woven coir geotextiles. The testresults indicate that the inclusion of coir geotextiles enhanced the bearing capacity of thin sections.Placement of geotextile at the interface of the subgrade and base course increased the load carryingcapacity significantly at large deformations. Considerable improvement in bearing capacity was observedwhen coir geotextile was placed within the base course at all levels of deformations. The plastic surfacedeformation under repeated loading was greatly reduced by the inclusion of coir geotextiles within thebase course irrespective of base course thickness. The optimum placement position of coir geotextile wasfound to be within the base course at a depth of one-third of the plate diameter below the surface.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Unpaved roads are usually used for low volume traffic and serveas access roads. Being basically an agricultural country low volumeroads play a very important role in the rural economy and resourceindustries in India. When unpaved roads are built on soft founda-tion soils, large deformations can occur, which increase mainte-nance cost and lead to interruption of traffic service. The use ofgeosynthetic products as an inclusion in flexible pavements forreinforcement has been demonstrated to be a viable technologythrough studies conducted over the last three decades (Cancelli andMontanelli, 1999; Chan et al., 1989; Collin et al., 1996; Fannin andSigurdsson, 1996; Gopal and Anil, 1994; Hufenus et al., 2006; Leng,2000; Love et al., 1987; Miura et al., 1990; Moghaddas-Nejad andSmall, 1996; Perkins, 1999; Som and Sahu, 1999) which results inincreased service life of the pavement or reduced base thickness tocarry the same number of load repetitions. Benefits of reducing basecourse thickness are realized if the cost of the geosynthetic is lessthan the cost of the reduced base course material. In developingcountries like India cost and availability of geosynthetics are themajor constraining factors for the construction of reinforced soilstructures. High cost of geosynthetics and stringent environmental

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protection requirements make it important to explore alternativenatural products to make the constructions cost efficient and eco-friendly (Sarsby, 2007; Rawal and Anandjiwala, 2007; Chauhanet al., 2008). But deterioration over time limits the use of naturalgeotextiles to temporary applications only. One of such applicationscan be in unpaved road over soft subgrade where the rate of plasticdeformation (rut development) due to repeated traffic loads isfaster during the initial stage and gets stabilized later (Fannin andSigurdsson, 1996). In this case, it is expected that consolidation ofthe soft subgrade soil will make reinforcement unnecessary in thelong-term. Natural fibre geotextiles can be a feasible solution insuch applications where these products are meant to serve onlyduring the initial stage and final strength is attained by soilconsolidation due to passage of vehicles. These natural materialsinclude coir, which is the husk of coconut, a common waste materialwhere coconuts are grown and subsequently processed. Coir fibre isstrong and degrades slowly compared to other natural fibres due tohigh lignin content (Rao and Balan, 2000). The degradation of coirdepends on the medium of embedment and climatic conditions andis found to retain 80% of its tensile strength after 6 months ofembedment in clay (Rao and Balan, 2000). Coir geotextiles arepresently available with wide ranges of properties. Closely wovencoir geotextiles possess high tensile strength and pullout resistance(Subaida et al., 2008) which can be economically utilized fortemporary reinforcement purposes.

In unpaved roads, major functions of geotextile materialsinclude filtration, separation, and reinforcement. Coir geotextiles

0

20

40

60

80

100

0.01 0.1 1 10 100

Particle size (mm)

Fra

ctio

n pa

ssin

g (%

)

Fig. 1. Particle size distribution of base course material.

E.A. Subaida et al. / Geotextiles and Geomembranes 27 (2009) 204–210 205

were reported to possess good filtration and drainage properties(Ramanatha Ayyar et al., 2002; Lekha and Kavitha, 2006; Babu,2007). The benefits of using reinforcements in flexible pavementsdepend largely on the quality and thickness of the granular baseand location of the geosynthetics within the pavement structure(Chan et al., 1989) along with other factors such as mechanicalproperties of reinforcement material (Perkins, 1999), subgradestrength (Cancelli et al., 1997), nature of interaction between soiland geosynthetics (Ghosh and Madhav, 1994) and applied loadmagnitude. The reinforcement mechanisms in geosynthetic rein-forced pavement include base course lateral restraint, increase instiffness of the base course aggregate layer (Bender and Barenberg,1978), reduction of shear stress in the subgrade soil (Love et al.,1987), improved vertical stress distribution on the subgrade (Mil-ligan et al., 1989) and tensile membrane action (Giroud and Noiray,1981). Significant rut depth and high stiffness of the geosyntheticmust be provided to initiate the membrane effect and thus toenhance the bearing capacity of the subgrade (Som and Sahu, 1999;Gobel et al., 1994). The placement position of reinforcement is themain factor affecting the bearing capacity of reinforced granularsoil and higher bearing capacity has been observed when the depthof placement of reinforcement is decreased (Akinmusuru andAkinbolade, 1981; Fragazy and Lawton, 1984; Sankariah and Nar-ahari, 1988; Reymond, 1992). The optimal position was reported tolie at the base of the fill with a very soft subgrade and a fill thicknessless than 0.4 m (Cancelli and Montanelli, 1999; Haas et al., 1988;Miura et al., 1990). Babu (2007) reported increased bearing capacitywhen woven and non-woven coir geotextiles were used at theinterface of silty clay subgrade and granular base course of 150 mmthickness. It has been found that the membrane effect of rein-forcement diminishes with an increase in the thickness of the roadaggregate layer (Hufenus et al., 2006; Kinney et al., 1998). Withhigher fills, the depth effect of a wheel load is generally too small tomobilize a noticeable tensile force within the reinforcement whenplaced just above the subgrade. At small deformations an efficientmobilization of tensile strength of reinforcement is dependent onboth interlock and stiffness (Fannin and Sigurdsson, 1996) in whichcase the effective location appears to depend on both the qualityand thickness of the granular material in which the geotextile isinstalled and the magnitude of the applied loads. Also the role ofgeotextile/geogrid used as aggregate reinforcement is purelystructural, and no separation benefit should be expected. In thiscase it is not placed directly at the interface, but rather at anoptimum depth within the granular base (Ashmawy and Bourdeau,1995). The interaction between soil and inclusion depends uponthe limiting friction or adhesion at their interface (Ghosh andMadhav, 1994). Reinforcement placed high up in the granular layerhinders lateral movement of the aggregate due to frictional inter-action and interlocking between the fill material and the rein-forcement which raises the apparent load-spreading ability of theaggregate and reduces the necessary fill thickness (Chan et al.,1989; Gobel et al., 1994; Miura et al., 1990; Moghaddas-Nejad andSmall, 1996; Perkins, 1999). Coir geotextile develops good interfacefriction with granular fill (Ajitha and Jayadeep, 1997; Subaida et al.,2008) which can induce tensile stress in the reinforcement whenembedded within the fill material. Such minor changes in hori-zontal stress distribution can cause significant changes in systemperformance. Hence, when used as reinforcement in unpavedroads, laying of coir geotextile must be carried out so as to take fulladvantage of this biodegradable material during the early period ofconstruction when much of the working of membrane actioncannot be expected.

No significant study has been reported on the use of coir geo-textiles as aggregate reinforcement in unpaved road sections.Hence a detailed experimental study has been planned to investi-gate the reinforcing benefits of woven coir geotextiles in

a laboratory two-layer pavement section and the present paperdescribes the results so obtained. Two types of woven coir geo-textiles and two base course thicknesses were adopted in the study.The effectiveness of such applications was investigated througha series of monotonic and repeated loading tests conducted underwell-controlled testing conditions.

2. Materials used for the study

The subgrade of test sections consisted of clay having a liquidlimit of 60% and plastic limit of 25%. The clay is classified as CH (asper Indian Standards) and had a specific gravity of 2.47. Optimummoisture content and maximum dry density were obtained as 25%and 15 kN/m3 respectively in standard proctor test. To prepare thetest sections clay was compacted to a dry density of 12 kN/m3 ata water content of 46%, to simulate the natural condition of the claydeposit during the time of collection. The CBR value obtained at thiswater content and density was 1.2%.

The base course aggregate was a crushed stone with the particlesize distribution shown in Fig. 1. The material is classified as GW asper Indian Standards and had a specific gravity of 2.67. Maximumdry density obtained was 20 kN/m3 at a water content of 5.5%. Thematerial was compacted to 90% of maximum dry density ata moisture content of 5% to make the base course in all tests. Directshear tests performed at stress levels ranging from 100 to 300 kParesulted in a friction angle of 48.3�.

Two types of woven coir geotextiles designated as MMA2 andMMA3 were used as reinforcements in the study. Woven coirgeotextile is designated as mesh matting based on the type of warpyarn. Fig. 2 shows photographs of these two types of geotextiles.The properties of geotextiles used are presented in Table 1.

3. Test set-up

The experiments were conducted in a concrete tank of size 1.5 mlength, 1 m width and 1 m depth. A reaction frame was fabricatedusing steel channels and plates to take up the loading and to holdthe loading devices to be placed at the centre of the tank. Load wasapplied through a circular plate, 200 mm in diameter and 25 mmthick. The vertical load was applied on the footing through a steelshaft using a mechanical device based on the principle of screwmotion that was measured using a proving ring of 50 kN capacity.Load was transferred to the plate through a steel ball kept ina groove which was made at the centre of the footing to ensure theapplied load to be vertical. The settlement of the plate was

Fig. 2. Photographs of woven coir geotextiles used for the study.

Fig. 3. Experimental set-up. (a) Loading arrangement. (b) Arrangement of dial gauges.

Table 2Test-section variables.

Testsection

Basethickness(mm)

Subgradesoil

Type ofGeotextile

Type ofloading

Position of Geotextile

A 0 Clay – Monotonic N/AB 167 Clay Control Monotonic N/A

E.A. Subaida et al. / Geotextiles and Geomembranes 27 (2009) 204–210206

measured using two dial gauges, fitted on the plate on either side ofthe loading shaft. Three more dial gauges were placed at radialdistances of 200, 300 and 400 mm from the centre of the plate tomeasure the surface deformations. Fig. 3 shows the schematicdiagram of the test set-up.

4. Test section construction

Table 2 provides information on the types of test sections con-structed and the loading applied. Variables included geotextiletype, geotextile placement position in the base layer, and basecourse thickness. Subgrade was prepared by compacting clay inlayers of 50 mm thickness. Pulverized clay required for each layerwas weighed and mixed with the desired quantity of water. Thelayer was compacted to the required thickness using a hammer of4.5 kg. Uniformity of water content and density were checked bytaking samples using cylindrical tubes at different locations within

Table 1Properties of woven coir geotextiles used for the study.

Particulars MMA2 MMA3

Mass/unit area (g/m2) 1286.56 710.50Thickness (mm) 8.39 8.47Number of ends/decimeter 18.90 8.90Number of picks/decimeter 10.90 6.50

Tensile strength (kN/m)Warp 36.00 12.60Weft 20.70 9.12

Failure strain (%)Warp 36.12 20.70Weft 28.45 23.00

C 167 Clay MMA2 Monotonic Base-subgrade interfaceD 167 Clay MMA2 Monotonic Mid depth of baseE 267 Clay Control Monotonic N/AF 267 Clay MMA2 Monotonic Base-subgrade interfaceG 267 Clay MMA2 Monotonic Mid depth of baseH 167 Clay Control Repeated N/AI 167 Clay MMA2 Repeated Base-subgrade interfaceJ 167 Clay MMA2 Repeated Mid depth of baseK 167 Clay MMA3 Repeated Base-subgrade interfaceL 167 Clay MMA3 Repeated Mid depth of baseM 267 Clay Control Repeated N/AN 267 Clay MMA2 Repeated Within base(U/D ¼ 0.2)O 267 Clay MMA2 Repeated Within base(U/D ¼ 0.3)P 267 Clay MMA2 Repeated Within base(U/D ¼ 0.6)Q 267 Clay MMA2 Repeated Within base (U/D ¼ 1)R 267 Clay MMA2 Repeated Base-subgrade interfaceS 267 Clay MMA3 Repeated Within base(U/D ¼ 0.3)

Table 3Subgrade properties for different test sections.

Test section Before test After test

Water content (%) Dry unit weight (kN/m3) Water content (%) Dry unit weight (kN/m3)

Top 200mm layer

Middle 200mm layer

Bottom 200mm layer

Top 200mm layer

Middle 200mm layer

Bottom 200mm layer

Top 200mm layer

Middle 200mm layer

Bottom 200mm layer

Top 200mm layer

Middle 200mm layer

Bottom 200mm layer

A 45.9 46.0 46.2 12.15 12.04 12.07 45 44..8 45.1 12.4 12..20 12.26B 45.6 45..8 45.8 12.06 11.87 11.86 44.9 44.6 45.0 12.2 12.10 12.04C 45.8 46.1 46.0 11.81 11.81 12.04 45.2 45.0 45.1 11.7 11.81 11.79D 45.3 45.5 45.5 12.23 12.00 11.95 45.6 45.8 45.6 12.3 12.02 12.25E 46.1 46.3 46.1 11.71 11.72 11.63 45.2 45.1 45.3 11.9 11.92 11.86F 45.7 45.6 45.8 11.84 11.80 11.91 45.4 45.0 45.3 11.7 11.75 11.67G 45.2 45.4 45.4 12.22 12.14 12.30 45.0 45.8 45.6 12.1 12.20 12.08H 45.5 45.4 45.2 12.131 12.08 12.23 45.3 45.6 45.5 11.8 11.92 11.90I 45.5 45.7 45.8 11.72 11.65 11.80 45.2 44.9 44.8 12.1 11.97 12.03J 45.6 45.5 45.6 11.51 11.53 11.50 45.5 45.3 45.2 11.6 11.53 11.56K 45.7 45.8 45.9 12.32 12.30 12.06 45.4 45.3 45.1 12.2 12.18 12.21L 45.3 45.6 45.4 12.15 12.13 12.16 45.3 45.1 45.2 12.2 12.23 12.20M 45.8 45.5 45.6 12.14 12.08 12.16 45.6 45.3 45.5 12.4 12.37 12.41N 45.7 45.5 45.7 12.21 12.26 12.19 45.4 45.5 45.4 12.3 12.26 12.21O 45.5 45.3 45.3 11.92 12.06 11.96 45.3 44.9 44.8 12.1 12.31 12.2P 46.0 46.2 46.0 12.34 12.41 12.37 45.7 46.0 46.1 11.9 12.04 12.01Q 45.9 45.8 45.7 12.12 12.15 12.14 45.6 45.2 44.9 11.7 11.69 11.67R 45.9 46.1 46.1 12.23 12.20 12.22 45.5 45.4 45.2 12.1 12.16 12.09S 46.1 46.3 46.1 12.3 12.17 12.28 45.4 45.1 45.5 11.8 11.85 11.78

E.A. Subaida et al. / Geotextiles and Geomembranes 27 (2009) 204–210 207

the tank. To check the vertical uniformity of the bed, one samplewas taken from every 200 mm thickness of bed layer at the samelocation. Table 3 shows the mean values of water content and dryunit weight of subgrade measured at different locations fordifferent tests.

The base course aggregate was compacted using a hammer of10 kg falling from a height of 200 mm. The plate diameter,thickness of layers and load intensity were scaled to a factor of 2/3with respect to a full scale wheel load size of 300 mm. Sectionswere constructed with two base course thicknesses of 167 and267 mm to represent thin and thick pavement sections of 250 and400 mm respectively. The thicknesses of base course layers forcompaction were conveniently decided based on the location ofgeotextile in each case. Uniformity in water content and density ofbase course was achieved for the different tests. To preparereinforced sections geotextile was placed at the base/subgradeinterface or within the base at the desired location. Since thewidth of the geotextile roll was equal to the internal width of thetest tank a single piece of geotextile was used to cover the entiresurface of the test section.

0

100

200

300

400

500

0 20 40 60 80 100

Settlement (mm)

Bea

ring

pre

esur

e (k

Pa)

Test A

Test B

Test C

Test D

Fig. 4. Bearing pressure-settlement of plate curves for 167 mm thick base undermonotonic load tests.

5. Test procedure

Monotonic load tests were conducted on reinforced and unre-inforced sections as per Indian Standards (IS 1888, 1982). The testbed was levelled and the footing was placed centrally over it. Loadwas applied by rotating the wheel of the screw jack manually. Aseating pressure of 7 kPa was applied. Dial gauge readings weretaken at every load increments of 10 kN. Each load increment wasapplied when the settlement became less than 0.025 mm/min.Effect of reinforcement location on bearing capacity was studied bykeeping the geotextile at the base/subgrade interface and at middepth of base course. Tests were conducted on two types of testsections with base thicknesses of 167 and 267 mm. Only MMA2 wasused as reinforcement in these tests.

To investigate the influence of coir geotextile reinforcement onthe behaviour of the pavement section under repeated loading, 100cycles of load were applied on sections of 167 mm base thickness intests H, I, J, K and L with a stress intensity of 260 kPa. The stress levelcorresponded to 60% of the strength of reinforced pavement usingMMA2 under monotonic load tests. In tests M, N, O, P, Q, R and S,

1000 cycles of load intensity of 400 kPa (equivalent to a tyrepressure of 600 kPa) was applied on pavement sections of 267 mmthickness. Pressure on the footing is increased from zero to therequired level by rotating the wheel. The maximum desired pres-sure is applied for 1 s and then load is released to zero. The periodof rest for the section was also maintained as 1 s in all the tests. Twotypes of geotextiles MMA2 and MMA3 were used in these tests.

6. Results and discussion

6.1. Monotonic load tests

Monotonic load tests were conducted on sections of 167 and267 mm base thicknesses. Fig. 4 shows the relationship betweenthe bearing stress and settlement of the footing with and withoutcoir geotextile reinforcement in a pavement of 167 mm basethickness. The unreinforced pavement failed at strength of 247 kPa.The ultimate bearing capacity increased to 366 kPa when thegeotextile MMA2 was placed at the interface of subgrade and basecourse and to 433 kPa when the same geotextile was placed withinthe base course at mid depth. In the reinforced pavement, defor-mation is restrained because of the presence of geotextile. Theincreases in bearing capacity observed are 45 and 75% respectively

Table 4Load capacity ratio at different settlements under monotonic loading (167 mm base).

Settlement (mm) Load capacity ratio

MMA2 at interface MMA2 at mid depth of base

20 1.23 1.8750 1.47 1.86

-1

1

3

5

7

0 10 20 30 40 50 60

Settlement of plate (mm)

Hea

ve (

mm

)

unreinforced

MMA2 at mid depth of base

MMA2 at interface

`

Fig. 6. Heave at 300 mm from footing centre under monotonic loading (167 mm base).

E.A. Subaida et al. / Geotextiles and Geomembranes 27 (2009) 204–210208

when the geotextile is placed at the interface and within the baserespectively. Table 4 illustrates the ratio of load required to producethe specified settlement (load capacity ratio) for reinforced andunreinforced sections. Load capacity ratios corresponding tosettlements of 20 and 50 mm have been shown to represent rela-tively smaller and higher settlements. When MMA2 is placed at thebase/subgrade interface, a significant increase in the load isobserved only at higher settlements which may be due tomembrane action. Whereas, placing the geotextile within the baseresulted in a considerable increase in load at small as well as largesettlements.

For pavement section with 267 mm base thickness ultimatebearing capacity increased by 11.8% when the geotextile MMA2 isplaced at the interface of subgrade and base course (Fig. 5). Shiftingof the position of geotextile to the middle of the base courseresulted in an increase in the ultimate bearing capacity to 17.9% ofthat of the unreinforced section. The results indicate that thepercentage increase in the bearing capacity due to coir geotextilereinforcement is significant only for pavement sections with thinbase courses.

Fig. 6 shows the surface deformation at a distance of 300 mmfrom the footing centre for reinforced and unreinforced sections of167 mm base thickness. In the case of unreinforced section heavingof surface takes place after a footing settlement of 5.41 mm. WhenMMA2 is placed at the interface of base and subgrade, the fillsurface is found to undergo settlement in the initial stages ofloading up to a footing penetration of 22 mm followed by slightheaving at relatively larger settlement of the footing. In the case ofreinforced section with MMA2 placed within the base course,heaving starts at a footing settlement of 11.24 mm. The results showthat geotextile kept at the interface inhibits the development ofrupture planes in the soil bed by enhancing the subgradeconfinement which gives rise to reduction of surface heaving.Geotextile placed within the base course is found to be less effectivein reducing the heaving due to lack of subgrade confinement.

In the case of 267 mm base thickness the heaving of thesurface was not considerable both for reinforced and unrein-forced sections under monotonic loading due to lower footingpenetration.

0

200

400

600

800

1000

0 10 20 30 40

Settlement (mm)

Bea

ring

pre

ssur

e (k

Pa)

Unreinforced

MMA2-subgrade/base interface

MMA2-mid depth of base

Fig. 5. Bearing pressure-settlement of plate curves for 267 mm thick base undermonotonic load tests.

6.2. Repeated load test

The relationship between number of load applications andpermanent surface deformation in sections of 167 mm thick basesare shown in Figs. 7 and 8 respectively. Plastic deformation is foundto be reduced due to the introduction of coir geotextile reinforce-ment. The performance of sections in which coir geotextile is placedwithin the base is superior to the section with geotextile placed atthe interface. Lateral movement of the base allows for verticalstrains to develop leading to permanent surface deformation of theloaded area. Placement of a geotextile layer in the base courseallows for shear interaction to develop between the aggregate andthe geotextile as the base attempts to spread laterally. Shear load istransmitted from the base aggregate to the geotextile and placesthe geotextile in tension which retards the development of lateraltensile strain in the base adjacent to the geotextile. Lower lateralstrain in the base results in less vertical deformation of the surface.Smooth interface between soft subgrade and geotextile placed atthe interface of base and subgrade results in a decrease in thepercentage reduction of plastic deformation. Reduction in perma-nent deformation is more due to inclusion of MMA2, compared toMMA3, because of higher stiffness which suggests that, with allother factors being the same, an increase in reinforcement stiffnessand strength results in superior pavement performance. Rut depthis 68 and 44% that of unreinforced section when MMA2 is placed atthe interface and mid depth of the base respectively. Use of MMA3at the interface and mid depth of base resulted in respective plasticsurface deformations of 86 and 74% that of unreinforced section.Improved performance with respect to bearing capacity andresistance to plastic deformation is observed when coir geotextilesare placed within the base course.

0

20

40

60

80

100

0 20 40 60 80 100 120

Number of load applications

Pla

stic

def

orm

atio

n (m

m)

Unreinforced

MMA3

MMA2

Fig. 7. Plastic surface deformation of 167 mm thick base (Geotextile at interface).

0

20

40

60

80

100

0 20 40 60 80 100 120

Number of load applications

Pla

stic

def

orm

atio

n (m

m)

Un reinforced

MMA3

MMA2

Fig. 8. Plastic surface deformation of 167 mm thick base (Geotextile at mid depth ofbase).

0

20

40

60

80

0 200 400 600 800 1000

Number of load applications

Pla

stic

def

orm

atio

n (m

m)

Unreinforced

MMA3

MMA2

Fig. 10. Plastic surface deformation of 267 mm thick base (U/D ¼ 0.3).

E.A. Subaida et al. / Geotextiles and Geomembranes 27 (2009) 204–210 209

Repeated load tests were conducted on 267 mm thick sectionskeeping the geotextile at different positions within the base tolocate the optimum placement depth of coir geotextile within thebase. The variation of rut depth with the number of load applica-tions for different placement locations of reinforcement is illus-trated in Fig. 9. All of the curves exhibit a transition to a stableresponse at large numbers of load applications. The generalresponses of reinforced sections are similar to that of unreinforcedsection. However the curves indicate improvement in the perfor-mances of reinforced sections over the unreinforced section. Testresults indicate that plastic surface deformation is a minimumwhen the geotextile is placed at a depth of one-third of the diam-eter of the plate (U/D ¼ 0.3, where U is the depth of placement ofcoir geotextile below the surface and D is the diameter of plate)among the different locations adopted in this study. A coir geo-textile layer is able to mobilize stresses within the reinforcedsections due to high interface friction, preventing local shear failureand deformations. When the placement depth of geotextile is muchless (U/D ¼ 0.2) the overburden is not sufficient to develop fric-tional resistance at the interface of the reinforcement and the fillresulting in lower reduction in plastic surface deformation. Whenthe placement location is increased beyond U/D ¼ 0.3, reductions inplastic deformation continue to decrease. Complete restraining oflateral spreading of base course material due to geotextile may nottake place when kept at higher depths because of increased fillthickness and hence the reinforcement effect may be retarded.These results indicate that the performance of the unpaved road

0

20

40

60

80

100

0 200 400 600 800 1000

Number of load applications

Pla

stic

def

orm

atio

n (m

m)

Unreinforced

U/D-0.2

U/D-0.3

U/D-0.6

U/D=1

Base-subgrade interface

Fig. 9. Plastic surface deformations of 267 mm base for different positions of MMA2.

system can be improved by keeping coir geotextile at a depthsufficient to mobilize the reinforcement within the base course. Forthe load intensity adopted in the study the optimum locationappears to lie within the base at a depth of one-third of the platediameter below the surface. Fig. 10 shows the influence of geo-textile strength in reducing the rut formation in 267 mm thick base.Reinforcement using MMA2 resulted in better performance thanMMA3 in reducing the rut formation as in the case of 167 mm thicksection. When placed within the base course, both types of geo-textile exhibited good performance with respect to plastic defor-mation irrespective of base thickness.

The test results obtained in the present study regarding thelocation of reinforcement may be comparable with the findings ofprevious researchers using geosynthetics. Brown et al. (1985),through wheel tracking of geogrid reinforced asphalt sections,reported significant reductions in surface rutting for a U/D of 0.3,while there was negligible benefit for a U/D of 1. Perkins (1999) alsoreported enhanced performance of asphalt section under dynamicloading by raising the position of geogrid above subgrade/baseinterface. Raymond and Ismail (2003) based on experimentscarried out at one-tenth scale concluded that the geogrids shouldbe placed as close as possible to the underside of the sleeper withinthe ballast. In contrast to these, test results by Brown et al. (2007)indicated that geogrid placed at the mid depth of ballast showed nobetter performance than that placed at the bottom of the ballastlayer.

7. Conclusions

The following conclusions can be drawn from the test results ofthe present investigation:

� The bearing capacity of thin unpaved road section can beincreased by the inclusion of woven coir geotextile. Significantincreases in ultimate bearing capacity and bearing capacity atany settlement were obtained when coir geotextile was placedat mid depth of the base course. For thicker sections theimprovement in bearing capacity due to inclusion of coir geo-textile was only marginal. Coir geotextile placed at the base/subgrade interface enhanced subgrade confinement andreduced heaving considerably.� Woven coir geotextile significantly decreased the permanent

vertical deformation over the loaded area of the pavementunder repeated loading by restraining the lateral spreading ofbase material. Location and stiffness of geotextile greatlyinfluenced the performance of reinforced sections. Enhancedperformance was obtained by keeping the coir geotextile

E.A. Subaida et al. / Geotextiles and Geomembranes 27 (2009) 204–210210

within the base for thin as well as thick sections. The reduc-tions in plastic deformation were of the order of 30 and 14%when MMA2 and MMA3 were placed at the base/subgradeinterface of 167 mm section. Placing of geotextile at the middepth of base resulted in reductions in vertical deformations ofthe order of 50 and 25% for MMA2 and MMA3 respectively.� Placement location of coir geotextile within the base course

significantly affected the performance of thick sections. For thetest conditions adopted in the study the optimum depth ofplacement of woven coir geotextile was obtained as one-thirdof the plate diameter below the surface.� To get satisfactory performance of reinforced systems, woven

coir geotextiles may be placed within the base course ofunpaved roads keeping sufficient fill thickness above the geo-textile layer to mobilize frictional resistance at the interface ofreinforcement and fill which will avoid damage to geotextiledue to traffic also.

The results presented in this paper should be interpreted in thelight of the small plate diameter and lower pressure used. The smallscale model test results provide insight into the load deformationbehaviour of the coir geotextile reinforced unpaved sections.Further study is required to interpret the performance of large scalesections from small scale laboratory tests.

Acknowledgements

The financial support of DOCT, Government of Kerala, India forconducting the experimental investigation is gratefully acknowl-edged by the authors.

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