COIR GEOTEXTILES IN RURAL ROADS
Many types of coir geotextiles are now being manufactured in our country. They have a great potential for application in rural roads. Model testing was carried out using 8 different types of coir geotextiles on soft clay with sand course (representing type unpaved roads). Results of monotonic and cyclic load tests are presented in this paper. The paper highlights the significant behavioral changes obtained by using the coir products and demonstrates the potential for application in rural roads.
At locations with adequate subgrade bearing capacity/California Bearing Ratio (CBR), a layer of suitable granular material can improve the bearing capacity to carry the expected traffic load. But at sites where subgrade is of poor quality, i.e., soft clays, with CBR less than 2 %, problems of shear failure and excessive rutting are often encountered. The ground improvement alternatives such as excavation and replacement of the unsuitable material, deep compaction, chemical stabilization, preloading and polymeric geosynthetics, etc are often used at such sites. The cost of these processes as well as virgin materials involved is usually high and as such they are yet to be commonly used in developing nations like India. In this context natural fibre products hold promise for rural road construction over soft clays.
India is the first largest country (66% of world production), producing coir fibre from the husk of coconut fruit. The coir fibres (50 to 150 mm long and 0.2 to 0.6 mm diameter) till recently were being spun into coir yam and then woven to obtain woven nettings.
The fibres-are also now a days being air laid, needle punched or adhesive bonded to obtain non-woven products or blankets. Like their polymeric counterparts geotextiles can be synthesized for specific applications in civil engineering like erosion control, ground improvement etc. Most of the present day products are being developed with an eye on erosion control applications (for vegetative growth), particularly, because among naturals they have much longer life. Their biodegradability has not encouraged users for more permanent applications. Studies in this direction were initiated at Indian Institute of Technology, Delhi (Balan 1995 and Venkatappa Rao 1997). These studies have broadly indicated that their biodegradability can be used to advantage and the^ coir based geotextiles have potential of being used for rural road construction over soft clays.
The paper presents the results of monotonic and cyclic load tests carried in a model test tank simulating rural roads with coir geotextiles. The results are encouraging, for use in developing countries (like India) in rural roads that are yet to be developed to connect as many as 0.2 million villages (with a population less than 1,000) as most of these roads happen to be on soft clays. It is also pertinent to mention at this juncture that the Rural Road Manual recently brought out by the Indian Roads Congress also suggests use of coir geotextiles. However, no design methodologies, construction guidelines and product specification were mentioned.
AMa Apparent resilient modulus
Coir is classified into two varieties, viz, ‘white coir’ and brown coir’ depending on the process of extracting the fibres from the husks. In India white coir is produced from husks of mature green coconuts by subjecting the husk to a retting process of 6 to 8 months in back waters, followed by manual separation of fibre from the pith surrounding it. On the other hand, brown coir is produced from dry/semi dry coconut husks by mechanical process. White coir is utilized for production of more durable value added goods (like carpets etc.) whereas brown coir which is inexpensive is used in the manufacturing of geotextiles. Coir industry in India in Southern region is well developed (Kerala being the foremost, followed by Tamil Nadu, Andhra Pradesh and Kamataka), the overall production being around 200,000 tonnes/annum.
Balan (1995) and Venkatappa Rao (1997) have developed methodologies for engineering characterization and evaluation of coir geotextiles.
In one of the earliest studies, Schurholz (1991) reported the durability of coir geotextile. According to him the coir geotextiles retained 20 % of their original tensile strength after one year in incubator tests with high fertile soil. He further observed that when natural fabrics were put in a shower room and kept wet for 167 days with conditions to simulate the traction effect while flooding, coir had almost no damage.
While studying the performance of model retaining wall reinforced with jute and coir ropes Sarsby et al. (1992) reported the loss in strength of jute and coir rope after 10 months in pulverized ash was 80 % and 20 % respectively.
Balan (1995) reported that coir degrades at a faster rate in sand with high organic content followed by clay with high organic content/burial, sand finally saturated soft clay, where the degradation is the least. He further reported that the overall life of coir is more than two/three years and brown coir degrades (about 20 % in 7 months) at a faster rate than white coir (about 10 % in 7 months).
Sheeba et al. (2000) performed CBR tests on kaolinite clay bed, and reported marked improvement with non-woven coir geotextiles.
Thus from the literature presented above, it is evident that no systematic study for the use of coir geotextiles in rural road construction is reported.
Woven coir geotextiles
Four different varieties of woven coir geotextiles designated as A, B, C and D were used in the present study. The physical and engineering properties of these coir products are presented in Table 1. The woven coir geotextiles Types A, B, C and D are netting composed of 100 % coir fibre spun into yam and woven in conventional flat bed looms in widths of 1, 2 or 4 m.
Non-woven coir geotextiles
Four different varieties of non-woven coir geotextiles designated as Type E, F, G and H were used in the present study and their physical properties are presented in Table 2. The Type E is composed of 100% de-curled coir fibre web of 400 g/m2 encased over top and bottom with brown PP netting. The mass per unit area of top and bottom netting is 7.1 g/m2 and 4.8 g/m2. The matrix is stitched together on 50 mm centres
with white PP thread dipped in black natural glue. Type F is similar to Type E except that the coir fibre web is 750 g/m2. The nonwoven coir geotextile Type G consists of 100% de-curled coir fibre web of 650 g/m2 encased over top and bottom with stable woven heavy jute netting. The matrix is stitched together on 50 mm centres with 2-ply jute yam. The mass per unit area of the top and bottom jute netting is 100 g/m2 each. The Type H comprises 100% de-curled coir web of 390 g/m2 encased over the top with heavy duty woven coir netting of 700 g/m2 and at the bottom with brown UV stabilized PP netting of 4.8 g/m2. The matrix is stitched together on 50 mm centres with heavy 2 ply jute thread. More details on the coir geotextiles are available from Dutta (2002).
The investigation was carried out on locally available Badarpur sand which is medium grained uniform quarry sand having sub-angular particles of weathered quartzite. The sand has a uniformity coefficient of 2.11 and a coefficient of curvature of 0.96. The placement dry unit weight of sand in the test tank was 14.95 kN/m3.
The kaolinite clay used in this study was of commercial grade. The clay is classified as CH. The placement dry unit weight of kaolinite in the test tank was 12.35 kN/m3.
Model tests were carried out in a tank made of 10 nun thick perspex side plates fitted inside a rigid aluminum frame. The internal dimensions of the tank were 350 mm x 350 mm in plan and 420 mm in depth. A view of the model tank is shown in Fig. 1. The outer dimensions of this’ model tank were such that it can be accommodated on the Hounsfield Universal Testing Machine, a micro processor controlled universal testing machine of 50 kN static capacity and 25 kN capacity for cyclic loading, with provision for different cross head speeds.
Effect of aperture size and tensile strength
The influence of the aperture size of the coir geotextiles on the behaviour of the model pavement is shown in Fig. 2. The influence of the aperture size is evident from this figure. It is imperative that with aperture the interlocking mechanism becomes increasingly dominant. The variation of the bearing pressure with tensile strength is also shown in the same figure. The trend is as expected, showing better improvement with strength.
Models reinforced with non-woven coir geotextiles
The bearing pressure versus deformation plots obtained in model tests with non-woven coir geotextiles Types E, F, G and H are shown in Fig. 3. It may be noted Hf represents the case of geotextile H where the woven coir scrim is in contact with the subgrade and is the reverse case for Hb. It is evident from the figure that there is an improvement in the bearing pressure on introduction of geotextile at the interface similar to that observed in case of woven coir geotextile. At a deformation of 20 mm, the non-woven geotextile Type G reinforced case exhibits an overall best performance than the other non-wovens. The bearing pressures of unreinforced model being 54.03 kPa, improved for reinforced cases with Type E, F, G, Hf and Hb to 63.71 kPa, 74.01 kPa, 72.58 kPa, 71.84 kPa, 63.13 kPa respectively. In the same sequence the reinforced models thus exhibit an improvement of 18 %, 37 %, 34 %, 33 % and 17%.
Figure 4 presents the results with all non-woven and woven geotextiles It is observed that model reinforced with Type D exhibits the best performance than the rest. This, as earlier stated, could be attributed to highest modulus and highest tensile strength of this product. It is also interesting to note that the models with non-wovens show better performance than models reinforced with Type C, B and Type A. It could perhaps due to the greater direct A, contact of the non-woven geotextiles with the soft subgrade, thereby perhaps leading to better interface friction. The variation in bearing capacity ratios (BCR, defined as the ratio of bearing pressure of reinforced model to that of unreinforced model) of these models with respect to deformations are shown in Fig. 5. It may be seen that BCR values for model using wovens and non-wovens have an increasing trend upto 60 mm deformation.
Apparent resilient modulus
The variation of apparent resilient modulus AMR (ratio of applied repeated stress to recoverable deformation) of different test models with number of load repetitions is presented in Fig. 7. From this figure, it can be seen that for unreinforced model under a repeated load of 17.94 kPa, the AMp has decreased from 16 kPa/mm for N = 1 to 10.2 kPa/mm for N = 100. Similarly, under a repeated load of 35.88 kPa, the apparent resilient modulus decreases from 18 kPa/mm at N = 1 to 12.3 kPa/mm at N = 100 at which the unreinforced model failed. This figure also shows that for the reinforced models, there is a distinct decrease in the apparent resilient modulus with the number of load repetitions. Similarly, for the model reinforced with non-woven geotextile Type E, the apparent resilient modulus decreased from 24 kPa/mm at N = 1 to 18 kPa/mm at N = 100. Whereas, the trend in apparent resilient modulus for the model reinforced with woven geotextile Type C was some what similar and it varies from 20.5 kPa/mm at N = 1 to 15.1 kPa/mm at N = 100. Further, it can also be seen from Fig. 1 that as the intensity of repeated loading is doubled, the apparent resilient modulus is marginally increasing.
On the basis of results and discussion presented in this paper, it is evident that coir geotextiles serve a useful function in rural roads on soft clays. Whereas the previous studies show that the life of coir could be two to three years, coir geotextiles are expected to improve the behaviour of the pavements in course of time, possibly through filteration and drainage as well as separation. As the products are being manufactured in rural areas, their use in rural roads may provide a great impetus in construction and maintenance.
Thanks are due to The Kerala State Coir Corporation Ltd. Alappuzha, Kerala, India for providing the coir geotextile samples. Special thanks are due to Dr. K. Balan , MD for their co-operation.
1. Balan, K (1995),”Studies on engineering behavior and uses of geotextiles with natural fibres”, -unpublished Ph.D Thesis submitted to Indian Institute of Technology Delhi (India).