Effective Use of Coir Products in Ground Improvement
EFFECTIVE USE OF COIR PRODUCTS IN GROUND IMPROVEMENT
P. Vinod
Assistant Professor, Department of Civil Engineering, College of Engineering, Trivandrum–695 016, India.
E-mail:dr_pvinod @ rediffmail .com
Ajitha B. Bhaskar
Assistant Professor, Department of Civil Engineering, College of Engineering, Trivandrum-695 016, India.
E-mail:ajithabhaskar @ yahoo.com
ABSTRACT: The technique of soil reinforcing using natural materials in place of synthetic tensile resisting elements is gaining importance in geotechnical engineering practice, particularly in countries where the availability and cost of the reinforcing materials are the major constraining factors. Amongst the natural materials serving human utilities, coir, which is the processed husk of ripe coconuts, is reputed to be the strongest and most durable. Coir is a cheaply and abundantly available waste material in India, Indonesia, Brazil, Sri Lanka and in some other Asian countries where coconuts are grown and subsequently processed. Unlike synthetic reinforcing materials, coir is biodegradable; however, due to its high lignin content (about 40–46%), degradation of coir takes place much more slowly than that in the case of other natural materials in an earth context. The research on use of coir as a soil reinforcing material started in the 1990s. A review of literature indicates that coir has great potential as a soil reinforcing material in field applications. This paper examines the feasibility of coir products of different configurations/types in improving the bearing capacity of loose soil deposits. Coir in the form of geotextile as well as braided rope have been used for this purpose. Plate load tests were conducted on model test tanks for a combination of reinforcement parameters and configurations. The test results indicate that about three to four-fold improvement in strength and more than eighty percent reduction in settlement can be achieved through the use of these coir products. It was also found that rate of improvement in strength increases with increasing values of settlement.
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Effective Use of Coir Products in Ground Improvement
1. INTRODUCTION
The technique of soil reinforcing using synthetic tensile resisting elements is widely used nowadays in geotechnical engineering practice and is fast replacing the conventional ground improvement techniques (e.g., Dash et al. 2001). But, these synthetic materials and products are generally highly expensive. In countries where the availability and cost of the reinforcing materials are the major constraining factors, the potential of natural materials such as coir and jute as soil-reinforcing elements is worth researching. Coir, which is reputed to be the strongest and most durable natural material (Ramanatha Ayyar et al. 2002; Sivakumar Babu et al. 2008) is cheaply and abundantly available in India and in a few other Asian countries where coconuts are grown and subsequently processed. Unlike synthetic reinforcing materials, coir is biodegradable; however, due to its high lignin content (about 46%), degradation of coir takes place much more slowly than that in the case of other natural materials. Increasing awareness and demand for environment friendly engineering solutions give coir an edge over synthetic reinforcing materials.
A review of literature indicates that coir in the form of fiber has great potential as a soil reinforcing material (Rao and Balan 1997; Vinod et al. 2007; Sivakumar Babu et al. 2008) However, the effect of inclusion of horizontally laid coir products in ground improvement has not so far been reported. The present study is an attempt in this direction. This paper presents a few model load test results on loose sand bed reinforced with coir geotextile well as braided coir rope.
2. MATERIALS AND METHODS
The soil used in the present investigation to simulate the weak deposit which needs stabilization was dry sand. The basic and index properties of the sand, determined from laboratory experiments are shown in Table 1 while Table 2 presents the properties the braided rope and the coir geotextile used.
Table 1: Properties of Sand
Effective size, D10 (mm) / 0.31Uniformity coefficient (Cu) / 1.58
Coefficient of curvature (Cc) / 1.00
Dry unit weight of sand in experiments (kN/m3) / 16.90
Relative density of sand in experiments (percent) / 25
Angle of internal friction (degree) / 27
Table 2: Physical Properties of Braided Coir Rope
and Coir Geotextile
Width / 21.4 mm / Mesh opening / 20 mm × 20 mm
Thickness / 11.5 mm / Thickness / 7.02 mm
Breaking load / 1.12 kN / Wide-width strip tensile strength / 4.95
Mass per length / 43.67 gm/m / Mass per unit area / 365 g/m2
Model tests were conducted in the laboratory in a steel tank of size 600 mm × 600 mm × 500 mm. Raining technique was used for pouring sand inside the model test tank. At the prescribed depths of reinforcement, the raining of sand was temporarily stopped and the braided rope/geotextile was placed on the surface of sand symmetrically beneath the footing location. This was then followed by pouring of sand through raining technique up to the required height. The model footing (100 mm × 100 mm) was placed on the leveled surface of sand on the predetermined alignment and loaded in small increments until footing settlement reached at least 30 mm. All the experiments were repeated to verify the consistency of the test data.
3. RESULTS AND DISCUSSION
Different series of experiments were conducted by varying the parameters such as embedment depth, length of reinforcement and the reinforcement type (i.e., geotextile as well as braided rope). Typical results are presented in Figure 1. It gives the pressure versus settlement behaviour of the model footing underlain by coir geotextile reinforced sand bed as well as braided rope reinforced sand bed along with the result obtained for the unreinforced sand bed. Each of the pressure versus settlement curves presented is the average of two plate load test results under identical conditions. The pressure versus settlement response of reinforced sand bed, for both reinforcement types, is seen to be appreciably better than that for the unreinforced case. This can be attributed to the frictional interaction existing at the interface of the sand and the geotextile/braided rope. Mobilization of some limited passive resistance mechanism within the cells of the geotextile/braided rope would also have resulted in soil improvement. It is also seen that no bearing capacity failure has taken place even at a settlement equal to 30% of the plate size in case of reinforced sand bed.
The extent of soil improvement can be represented using a non-dimensional strength improvement ratio [ratio of footing pressures for the reinforced case to the corresponding pressure on unreinforced sand at the same settlement]. In the present study, the pressures corresponding to normalized settlements [(settlement/width of plate) × 100] of 2.5, 5, 10, 15, 20, 25, 30% have been compared for both reinforced and unreinforced conditions and typical values of strength improvement ratio are presented in Figure 2. It is seen that about three-fold improvement in strength is achievable through the use of these coir products.
Fig.1: Pressure versus Normalized Settlement Behaviour of
Unreinforced and Reinforced Soil System
Fig. 2: Normalized Settlement versus Strength Improvement Ratio
Further, use of braided coir rope results in appreciable strength improvement even at low values of normalized settlement. The improvement due to the provision of braided coir rope/coir geotextile reinforcement layer in the sand bed in terms of reduction in footing settlement can be quantified through the parameter settlement reduction factor which is defined as [(s0–sr ) / s0] wherein, s0 is the settlement of the unreinforced sand bed at a given pressure and sr is the settlement of the reinforced sand bed at the same pressure. It is seen that more than eighty percent reduction in settlement can be achieved through the use of the proposed method.
4. CONCLUSIONS
Plate load tests were carried out to investigate the beneficial effect of different varieties of coir products in improving the pressure versus settlement response of a model footing on loose sand bed. Two varieties of coir products ,namely geotextile and braided rope were used for this purpose. The following conclusions are drawn from the study:
(a) Coir geotextile as well as braided coir rope are appropriate materials for subsoil improvement.
(b) Strength improvement as high as about three to four-fold and the reduction in settlement of about 80 percent is achievable through the use of these coir products.
REFERENCES
Dash S.K., Krishnaswamy N.R. and Rajagopal K. (2001). “Bearing Capacity of Strip Footing Supported on Geocell Reinforced Sand”, Geotextiles and Geomembranes, 19, 235–256.
Ramanatha Ayyar T.S., Ramachandran Nair C.G. and Balakrishnan Nair N. (2002). “Comprehensive reference book on Coir Geotextiles”, Published by Centre for Development of Coir Technology (C-DOCT), Trivandrum, India.
Rao G.V. and Balan K. (1997). “Reinforcing Sand with coir Fibre”, Geosynthetic Asia 97, Oxford and IBH Publishing Company Pvt. Ltd., Bangalore, India.
Sivakumar Babu G.L., Vasudevan A.K. and Sayida, M.K. (2008). “Use of Coir Fibers for Improving the Engineering Properties of Expansive Soils”, Journal of Natural Fibers, 5, No.1, 1–15.
Vinod P., Ajitha Bhaskar and Lekshmi C.S. (2007). “Triaxial Compression of Clay Reinforced with Sand-coir Fiber Core”, Geotechnical Testing Journal, ASTM 30, No. 4, 333–336.
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