COIR GEOTEXTILES AS SEPARATION AND FILTRATION LAYER FOR LOW INTENSITY ROAD BASES

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1 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 , India. S. Ramakrishna Formerly M.S. Research Scholar, Department of Civil Engineering, IIT Madras, Chennai , 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 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 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/m 3 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 941

2 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/m 2 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 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 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

3 number number weight height of of blows of of drop of per layer layers hammer hammer E= volume of soil layer (1) in which E is the standard compaction energy ( kj/m 3 ) 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. In Types 2 4, six sub-base layer thickness (h) values of 100, 150, 200, 250, 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 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, 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 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 was continued until a total plate settlement of 150 mm has occurred which took approximately 6 days. 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. 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

4 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). 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 Unreinforced subbase layer One layer of reinforcement Two layers of reinforcement 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. The above data is presented in a different form in Figure 3 for Indian Road Congress (IRC) standard wheel load of kn. Similar curves have also been developed for other 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. Fig. 2: BCR for a Rut Depth of 75 mm Fig. 3: Design Chart for Wheel Load of 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. required ultimate bearing pressure on subbase course BCR = ultimate bearing capacity of subgrade 944

5 In the present example, BCR = = 20 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 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. Design case 1. r = 25 mm P = 28 kn P = 48 kn 2. r = 50 mm P = 28 kn P = 48 kn 3. r = 75 mm P = 28 kn P = 48 kn Table 2: Design of Subbase Thickness over Soft Subgrades gravel alone (A) Design base course thickness (mm) gravel + one layer reinf. (B) gravel + two layer reinf. (C) Savings in thickness (mm) Savings for B over A Savings for C over A

6 Four naturally occurring chemical compounds, Calcium Chloride (CaCl 2 ), Magnesium Sulphate (MgSO 4 ), Sodium Hydroxide (NaOH), and Potassium Carbonate (K 2 CO 3 ), 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 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 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 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. REFERENCES Box, G. and Hunter, J. (1957). Regression Analysis, J. Ana. Math. Statistics, Vol. 28, Datye, K.R. (1988). Natural Materials for Soil Reinforcement, Int. Symp. on Theory and Practice of Earth Reinforcement, Fukuoka, Japan, Douglas, R.A. and Valsangakar, A.J. (1992). Unpaved Geosynthetic-Built Resource Access Roads: Stiffness Rather than Rut Depth as the Key Design Criterion, Journal of Geotextiles and Geomembranes, 11, pp Kulkarni, A.G., Cherian, K.A., Satyanarayana, K.G. and Rohatgi, P.K. (1983). Studies on Moisture Sorption of Coir Fibres (Cocos Nucifera L), J. Applied Polymer Sciences, Vol. 28, Kuntiwattanakul, P., Towhata, I, Oshishi, K. and Seko, I. (1995). Temperature Effects on Undrained Shear Characteristics of Clay, J. Soils and Foundations, 35, pp Prasad, S.V., Pavitran, C. and Rohatgi, P.K. (1983). Alkali Treatment of Coir Fibres for Coir-Polyester Composites, J. of Mat. Science, Vol. 18, Ramakrishna, S. (1996). Investigation on Applications of Coir Reinforcement in Geotechnical Engineering, Thesis Submitted for Award of Master of Science Degree, Indian Institute of Technology Madras, Chennai. Rao, G.V. and Balan. K. (1994). Coir Geotextiles A Perspective, Proc. of 2nd Int. Workshop on Geotextiles, CBIP, New Delhi, India, pp Uma, L., Kalaiselvi, R. and Subramanian, G. (1994). Isolation of a Lignolytic Bacterium for the Degradation and Possible Utilisation of Coir Waste, J. of Biotechnology Letters, Vol. 16,