EFFECT OF CLAY AND FIBRE, CONTENT ON DYNAMIC SHEAR STRENGTH OF SAND

Abstract

Various soil improvement methods have been developed to improve the soil properties. Fibre .reinforcement of soil is considered as an effective ground improvement technique because of its cost effectiveness, easy adaptability and reproducibility. Reinforcement of sand using randomly distributed fibres, is known to significantly improve the physical and dynamic properties of the sand. Clay and fibre have been added to the sand as a soil-improvement admixture. The present experimental study investigates the effect of clay content and the effect of randomly distributed fibres on the behaviour of sand under dynamic loading. To determine the shear strength of the sand and the composite soil, direct shear tests were conducted. The sand as well as the composite soil was tested under dynamic loading conditions using an oscillatory shear box. Sand samples having three clay contents of 15%, 20% and 25% and three fibre contents of 0.25%, 0.5% and 1% of the weight of dry sand, were studied in the test programme. A fibre length of 6 mm was used in this study. The tests were conducted at optimum moisture content and maximum dry density of the relevant soil samples. Proctor compaction tests have been conducted on the soil samples to determine the maximum dry density and optimum moisture content. It was observed from compaction tests that (a) With an increase in clay content, maximum dry density increases and optimum moisture content decreases. (b) In fibre reinforced sand, with an increase in fibre content maximum dry density decreases slightly but the optimum moisture content does not exhibit a definite trend. From direct shear tests, it was observed that (a) Both internal friction angle and cohesion, increases with an increase in clay content. (b) Internal friction angle increases marginally with an increase in fibre content, whereas cohesion increases significantly with an increase in fibre content. The results of oscillatory shear tests indicate that (a) Shear strain increases with an increase in the number of cycles of loading. It increases with an increase in the oscillatory shear stress but reduces as normal stress increases. Shear strain decreases significantly with an increase in clay content, while in fibre reinforced sand this reduction is not significant. (b) Dynamic shear modulus increases with an increase in clay content, for any given shear stress. Dynamic shear modulus increases with an increase in fibre content, for any given shear stress, this increase is more significant at higher fibre contents. (c) Dynamic shear modulus increases with increase in clay content for any given normal stress. As fibre content increases the dynamic shear modulus increases, for a given normal stress.