Behaviour of Black Cotton Soil Reinforced with Sisal Fibre

Transcription

1 10th National Conference on Technological Trends (NCTT09) 6-7 Nov 2009 Behaviour of Black Cotton Soil Reinforced with Sisal Fibre Santhi Krishna K. M Tech Student Department of Civil Engineering College of Engineering Trivandrum, Kerala sankris86@yahoo.co.in Abstract - A large part of Central India and a portion of South India are covered with black cotton soils. These soils have high swelling and shrinkage characteristics and extremely low CBR value and shear strength. Hence, there is need for improvement of these properties. The practice of using reinforced earth has been well established in soil engineering profession. The concept of reinforcing soil masses with natural fibres like coir fibre, banana fibre, aracanut fibre, sisal fibre etc. is a relatively new development to improve the properties of soil. The present study is aimed at determining the behavior of black cotton soil reinforced with sisal fibre in a random manner. The soil used is a type of black cotton soil collected from Chittur in Palaghat District. The fibres are cut to different lengths (1.5cm, 2.0cm, 2.5cm and 3.0cm) and mixed randomly with soil in varying percentages (0.25%, 0.50%, 0.75% and 1.00%) by dry weight of soil and compacted to maximum dry density at optimum moisture content. The test results indicate a reduction in the maximum dry density and the optimum moisture content of soil due to the addition of sisal fibre. It also indicates an improvement in the CBR value and Unconfined compressive strength of soil due to the addition of sisal fibre. The optimum CBR value and UCC value is obtained for 2.5cm length of fibre with 0.50% fibre content. Keywords - Black Cotton Soil, Sisal Fibre, Reinforcement, Standard Proctor Test, Unsoaked CBR, UCC, Short term durability. I. INTRODUCTION During the last 20years, the reinforcement of soil by high tensile strength materials has become a widespread technique in earthwork construction. Metal strips, plastic grids and fabrics have all been used to increase the tensile strength, shear resistance and stiffness of soils. Tensile strains due to flexure occur in earth structures like embankments, dams and multilayer pavements. Tensile stresses are induced in the sub grade soil of multilayer pavements under traffic loads when the deformation modulii differ in various layers. In earthrockfill dams, tensile failure could result in cracking of the clay core. A variety of inclusions ranging from low modulus polymeric materials to relatively stiff high strength metallic inclusions have been used to reinforce soils. Fibre reinforcement is significant in both nature and engineering practice. The basic principle of reinforced earth is demonstrated in nature by the action of the Sayida M.K. Lecturer Department of Civil Engineering College of Engineering Trivandrum, Kerala sayidamk@rediffmail.com tree roots. Plant roots penetrating through soil contribute to the stability of natural slopes. An earth mass stabilized with discrete randomly distributed fibres resembles traditional earth reinforcement in many of its properties. Discrete fibres are simply added and mixed with soil much in the same way as lime, cement or other additives. One of the main advantages of randomly distributed fibres is the maintenance of strength isotropy and the absence of potential planes of weakness that can develop parallel to the oriented reinforcement. The behaviour of soil due to the introduction of coir fibre and banana fibre have already been studied on a type of red soil collected from Thiruvallam in Trivandrum District. The concept of strengthening soil with added rods or fibres is not new, although a systematic study of reinforced earth started only very recently. In the past, there is evidence of man trying to improve the quality of bricks by adding straw. The practice of building houses on soil reinforced with natural fibres and constructing earth walls with different types of reinforcing inclusions are age old arts to the villagers in tropical Africa and Southern Asia. Rope fibres and bamboo strips were used to strengthen rural road bases. Vertically arranged rectangle grids of bamboo strips and stalks of palm branches were used as central core for mud walls. Such walls are more resistant to crack propagation than plain unreinforced mud walls. The objectives of the present study is to determine the reinforcing effect of randomly distributed, discrete sisal fibres on the behaviour of the black cotton soil collected from Chittur in Palaghat District. The study focuses on the effect of change of length and percentage fibre content on the engineering properties of compacted soil. II. LITERATURE REVIEW Ingold (1983) reported, on the basis of experimental studies in triaxial tests, that there is loss of strength when impermeable reinforcement was used with saturated clay during undrained loading. However, when permeable reinforcement layers were used at closer spacing and connected by a drainage layer, there was an increase in strength even in undrained loading. The study conducted by Dean (1986) gives some idea about the effect of College of Engineering Trivandrum 88

2 fibres on the strength of compacted Lean Sandy Clay. The three different synthetic fibres used as reinforcement were a spun nylon string, polypropylene rope fibre and a polypropylene olefin. The approach used in this study was to compare the unconfined compressive strength of plain and reinforced specimens compacted over a wide range of water contents. The results showed that the strength of reinforced soil compacted near and wet of optimum was greater than for plain soil at the same water content. Investigation previously done by Susan (1986) showed that the coir fibre reinforcement improved the flexural strength, unconfined compressive strength, permeability characteristics, shear strength and consolidation characteristics of reinforced compacted soil specimens. The coir fibre cut to a small length of 2.5cm and mixed randomly with soil at different percentages of fibre content. The result showed that the maximum flexural strength of soil increased upto 0.75% fibre content and decreased with further increase in fibre content. The unconfined compressive strength was obtained at 1% fibre reinforcement and decreased with further addition of fibre content. The least permeability was obtained at 0.25% of fibre reinforcement. The use of reinforcement to control the heave of expansive soils has been a subject matter of recent studies. Srinivasamurthy (1987) have dealt with this problem. The mechanism for the control of heave was to arrest the movement of top surface of soil by introducing reinforcement in the direction of the heave. The tendency to heave will induce a pullout tension in the reinforcement which will be resisted by the reinforcement either through friction or passive resistance of anchors. Experiments conducted in black cotton soils with various types of reinforcements like plain M.S. bars and Tor steel have shown that the control of heave depends up on the surface characteristics of the reinforcements and internal friction. Ajitha (1989) after conducting triaxial and consolidation tests on reinforced compacted soils, concluded that axial stress at failure in undrained triaxial tests increased with percentage fibre. Failure strain was found to increase, so did cohesion and angle of internal friction of the soil. Coefficient of compressibility, coefficient of volume compressibility, compression index and coefficient of consolidation were found to be optimum at 0.5% fibre content for all consolidation pressures. Sivanna (1990) conducted studies on the effect of length of fibre on coir fibre reinforced compacted soil and concluded that flexural strength and tensile strain at failure increased with the length of fibre. Unconfined compressive strength attained a peak value at 1% fibre content for all length of fibres. As length of fibre increased from 2cm to 3cm, peak value of unconfined compressive strength increases considerably. Mini (1993) conducted experiments to find out the effect of banana fibre reinforcement on the consolidation characteristics of compacted soil. The results showed that coefficient of compressibility, volume compressibility, compression index and coefficient of consolidation attained maximum values at 0.5% fibre content. Investigations were carried out by Suresh (1993) to find out the effect of banana fibre reinforcement on flexural strength, shear strength and permeability characteristics. The results show that flexural strength increases with increase in percentage fibre and reaches an optimum value at 0.75% and then it decreases. There is a rapid gain in strength when fibre content is increased from 0.5to 0.75%. Aziz (1994) conducted experiments to find out the effect of coir fibre as reinforcement to sand for preparing blocks, to be used as a building material. In this study, the soil is prepared by mixing 15% of clay by weight to sand. From the results obtained, he concluded that the compressive strength of the blocks increased only upto a particular fibre content, and after that it decreased. The rate of this decrease reduced with increase in cement content. The maximum value of compressive strength of blocks was obtained at a fibre content of 0.45% by weight for blocks with no cement, and at 0.55% by weight for a cement content of 2% and 4%. Ranjan (1998) conducted studies on the strength characteristics of fibre reinforced sand and concluded that the increase in shear strength of fibre reinforced sand depends on concentration, modulus, initial orientation of fibres and confining stress. Roy (1999) conducted experiments to find out the effect of fibre reinforcement on the penetration resistance of compacted soil and concluded that the CBR value of soil is found to increase with the addition of fibre to the soil. The peak value of CBR is obtained at 0.5% fibre content for all types of reinforced soils. Prabakar et.al. (2002) conducted studies on the effect of random inclusion of sisal fibre on the strength behavior of soil and concluded that the increase in fibre length and fibre content reduces the dry density and optimum moisture content. And the shear strength and cohesion also increases with the addition of sisal fibre. Sivakumar et.al. (2008) conducted a series of triaxial test, UCC test and consolidation test on coir fibre reinforced compacted black cotton soil. Their results showed that the failure stress increases with the confining pressure, fibre content and diameter of fibre. And the percentage swell and compression index decreases with the inclusion of coir fibre. III. SCOPE OF THE STUDY Previous studies have shown that coir fibre and banana fibre reinforcements increased the unconfined compressive strength, flexural strength and permeability characteristics of compacted soils. Short term durability of reinforced soil samples has also been checked. The shear strength and consolidation characteristics of coir and banana fibre reinforced soil have also been studied. The results have shown improvements in the value of cohesion, angle of internal friction, coefficient of compressibility and coefficient of consolidation. The penetration resistance of coir and banana fibre reinforced soil has also been studied. The results have shown improvements in CBR value. This study is intended to find out the effect of sisal fibre on the compaction characteristics, penetration resistance and unconfined College of Engineering Trivandrum 89

3 compressive strength of soil. For the study, black cotton soil collected from Chittur in Palaghat district is used. The fibres are cut to different lengths (1.5cm, 2.0cm, 2.5cm and 3.0cm) and mixed randomly with soil at different percentages of fibre content (0.25%, 0.5%, 0.75% and 1%) by dry weight of soil. The soil fibre mixture is prepared at optimum moisture content for each percentage of fibre to attain maximum dry density. A. Soil IV. MATERIALS FOR STUDY In this study, a type of black cotton soil collected from Chittur in Palaghat district is used. The properties of soil are determined by standard test procedures and tabulated in Table I. The grain size distribution is found out by conducting wet sieve analysis and hydrometer analysis and shown in Fig 1. B. Reinforcement Used For the present study, sisal fibre is obtained from Alappey. The fibres are cut to pieces of different length (1.5cm, 2cm, 2.5cm and 3cm) and are randomly mixed with soil in varying percentages (0.25%, 0.5%, 0.75% and 1%) by dry weight of soil. The diameter of the fibre is determined using Brinnel microscope and the tensile strength of the fibre is determined using Spring Testing machine. The properties of sisal fibre shown in Table II. Fig. 1 Grain Size Distribution Curve of Black Cotton Soil TABLE I INDEX PROPERTIES OF SOIL Property Specific Gravity Gravel (%) Sand (%) Silt &Clay (%) Liquid Limit (%) Plastic Limit (%) Shrinkage Limit (%) Plasticity Index (%) Classification Maximum Dry Density (kn/m 3 ) Optimum Moisture Content (%) CBR Value (%) Unconfined Compressive Strength (kn/m 2 ) Property TABLE II PROPERTIES OF FIBRE USED Average Diameter (mm) Average Tensile Strength (kg/cm 2 ) Density (g/cc) Value CH Sisal Fibre Fig. 2 Photograph of Sisal Fibre V. EXPERIMENTAL PROGRAMME A. Moisture-Density Relationships The maximum dry densities and optimum moisture content of the unreinforced and reinforced soils are determined by standard proctor compaction tests. About 3kg of dry soil passing through 20mm IS sieve is taken. For compaction of soil-fibre mix, the required amount of fibre is mixed with the dry soil before adding water. Water is then added and thoroughly mixed before compacting in the mould. Standard proctor compaction tests are done to determine the Maximum Dry Density (MDD) and the Optimum Moisture Content (OMC) in each case. College of Engineering Trivandrum 90

4 B.CBR Test The experimental study involved performing a series of laboratory CBR tests on the unreinforced and randomly distributed fibre reinforced soil specimens. The tests were conducted inside a modified proctor mould at unsoaked state per ASTM D soil, and a further increase in fibre content reduced the OMC. But in all cases OMC is greater than that of raw soil. C.Unconfined Compression Test Unconfined confined compression tests were carried out on cylindrical specimens of 38mm diameter and 76mm height. These specimens are of maximum dry unit weight and optimum moisture content state, prepared by static compaction. For making each specimen, the dry soil and the fibres were laid to a resolution of 0.1gm and 0.01gm respectively and were laid in separate containers. To ensure that effective mixing between the soil and fibres was achieved the process was staged. Initially all of the soil and half of the water and fibres were mixed, after which the proportions of water and fibre were gradually increased upto optimal water content and the prescribed fibre percentage. The mixed samples were tested by unconfined compression test equipment. Fig. 3 Variation of MDD with respect to Length of Sisal Fibre D. Short Term Durability Test The durability of sisal fibres when permanently immersed in normal water and clay was also studied. For this purpose, naturally dried fibres, soaked in normal water or embedded in clay and tested at one week interval for up to one month. VI. RESULTS AND DISCUSSIONS Various tests are carried out in accordance with the standard procedure to determine the compaction characteristics, penetration resistance and unconfined compressive strength of fibre reinforced compacted soil. The test results are as follows. A. Influence of Fibre Inclusion on MDD and OMC The variation of MDD and OMC with respect to the length of sisal fibre is plotted in Fig. 3 and 4. In the case of sisal fibre reinforced soil it is observed that, as the fibre length and fibre content increases, the optimum moisture content and the maximum dry density decreases. The variation of maximum dry density with respect to fibre length as well as fibre content is more or less parallel. The decrease in maximum dry density can be attributed to the lower density of fibre compared to that of soil. Addition of 1.5cm fibre has led to an increase in OMC compared to that of raw soil. And further increase in fibre length causes a decrease in the OMC. The initial inclusion of fibre at 0.25% caused a sudden hike in the OMC than that of the ordinary Fig. 4 Variation of OMC with respect to Length of Sisal Fibre The decrease in optimum moisture content may be due to the presence of water content in sisal fibre. This may be the reason for the initial increase in OMC when fibre is included in the soil. B.CBR Value of Reinforced Soils Fig. 5 shows the variation of CBR value against the fibre contents for the soil reinforced with sisal fibre. The CBR value is highest at 0.50% fibre content for all the cases of sisal fibre reinforced soils. In case of sisal fibre, when the fibre content is College of Engineering Trivandrum 91

5 0.50%, the CBR value increased by 1.85 times for 1.5cm fibre length, 1.96 times for 2.0cm fibre length, 2.74 times for 2.5cm fibre length and 2.02 times for 3.0cm fibre length over plain soil. The optimum CBR value of 14.21% obtained for 2.5cm length of fibre with 0.50% fibre content for sisal fibre. There is a sharp decrease in the CBR value, when the fibre content increases from 0.50% to 0.75% for sisal fibre. Since, there is no marked difference in the values of optimum moisture content and maximum dry density for each case, the drop in the penetration resistance may be attributed to the loss of friction between the fibre and the soil. C. Unconfined Compressive Strength of Reinforced Soils The variation of Unconfined Compressive Strength against the fibre contents for the soil reinforced with sisal fibre is shown in Fig. 6. The Unconfined Compressive Strength is highest at 0.50% fibre content for all the cases of sisal fibre reinforced soils. In case of sisal fibre, when the fibre content is 0.50%, the Unconfined Compressive Strength increased by 2.50 times for 1.5cm fibre length, 3.02 times for 2.0cm fibre length, 7.98 times for 2.5cm fibre length and 5.55 times for 3.0cm fibre length over plain soil. The maximum unconfined compressive strength of kN/m 2 for soils reinforced with sisal fibre was at 2.5cm fibre length of 0.5% fibre content. Fig. 6 Variation of UCC Value with respect to Sisal Fibre Content D. Durability Test Results The average values of obtained results for sisal fibres are presented in Fig.7. The results showed that the percentage decrease in tensile strength is large when the fibre embedded in clay as compared to that of normal water. And in both cases, as the time period increases, the percentage decrease in tensile strength increases. By the end of one month, the percentage decrease in tensile strength of sisal fibre in fresh water and clay are 3.12% and 8.98% respectively. Fig.7 % Decrease in Tensile Strength of Sisal Fibre with respect to Time Period VII. CONCLUSIONS Fig. 5 Variation of CBR Value with respect to Sisal Fibre Content This paper evaluated the effect of sisal fibre on the engineering properties of black cotton soil taken from the Chittur Talluk near Palaghat. In the case of sisal fibre reinforced soil, as the fibre content and fibre length increases, the maximum dry density and the optimum moisture content decreases. And the CBR value of soil is found to increase with the addition of sisal fibres to the soil. The peak value of CBR is obtained at 0.50% fibre content for all cases of sisal fibre reinforced soils. The optimum CBR value of 14.21% is obtained for 2.5cm length of fibre with 0.50% College of Engineering Trivandrum 92

6 fibre content for sisal fibre reinforced soil which is 2.74 times over plain soil. The Unconfined Compressive strength of soil is found to increase with the addition of sisal fibres to the soil. The optimum Unconfined Compressive Strength of kN/m 2 is obtained for 2.5cm length of fibre with 0.50% fibre content for sisal fibre reinforced soil which is 7.98 times over plain soil. The short term durability study of sisal fibres was also conducted. The durability test results showed that the percentage decrease in tensile strength is large when the fibre embedded in clay as compared to that of normal water. And by the end of one month, the percentage decrease in tensile strength of sisal fibre in fresh water and clay are 3.12% and 8.98% respectively. REFERENCES [1] Abdul,A.K.,(1994), Strength Characteristics of Fibre Reinforced Sand Blocks Stabilized with Clay and Cement, M.Tech.Thesis, University of Kerala, [2] Ajitha,B.B.,(1989), Shear Strength and Consolidation Characteristics of Fibre Reinforced Compacted Soil, M.Tech.Thesis, University of Kerala, [3] Anitha,S.,(1995), A Study on Effective Strength Parameters of Fibre Reinforced Compacted Soil, M Tech Thesis, University of Kerala, [4] Babu,G.L.S.,Vasudevan,A.K.,Sayida,M.K., (2008), Use of Coir Fibres for Improving the Engineering Properties of Expansive Soils, Journal of Natural Fibres, Vol.5(1). [5] Dean,R.F.,(1986), Soil Randomly Reinforced with Fibres, Journal of Geotechnical Engineering, ASCE, Vol. 112, No. 8, pp [6] Gopal,R.,Charan,H.D.,(1998), Randomly Distributed Fibre Reinforced Soil-A State of the Art Technology, Journal of Institute of Engineering, Vol. 74, pp [7] Ingold,T.S.,(1983), Reinforced Clay Subject to Undrained Triaxial Loading, Journal of Geotechnical Engineering, ASCE, Vol. 109, No. 5, pp [8] Mini,T.N.,(1993), Properties of Coir Fibre and Banana Fibre Reinforced Compacted Soils-A Comparison, M.Tech.Thesis, University of Kerala, [9] Prabakar,J.,Sridhar,R.S.,(2002), Effect of Random Inclusion of Sisal Fibre on the Strength Behavior of Soil, Construction and Building Materials, Vol.16, pp [10] Roy,M.,(1999), A Study on the Penetration Resistance of Fibre Reinforced Compacted Soils, M.Tech.Thesis, University of Kerala, [11] Sivanna,D.,(1990), Effect of Length of Fibre on Properties of Fibre Reinforced Compacted Soil, M.Tech.Thesis, University of Kerala, [12] Srinivasamurthy,B.R.,(1987), Soil Reinforcement Technique for Control of Heave, Proc. International Conference of Expansive Soils, New Delhi. [13] Suresh,K.D.S.,(1993), Study of Shear Strength and Flexural Strength Characteristics of Fibre Reinforced Compacted Soil, M.Tech.Thesis, University of Kerala, [14] Susan,M.,(1986), Flexural Behaviour of Fibre Reinforced Compacted Soil Beams, M.Tech.Thesis, University of Kerala, College of Engineering Trivandrum 93