Performance evaluation of silty sand subgrade reinforced with fly ash and fibre

In this paper, the effectiveness of fibre reinforcement (coir fibre and synthetic fibre) in subgrade soil has been studied from the point of view of strength. The permanent strain, resilient strain behaviour and resilient modulus of subgrade soil have been determined in the laboratory. A value of 10% (20 mm) strain is taken as the failure criterion for the subgrade for pavement in rural area. A subgrade soil of silty sand mixed with optimum content of fly ash and two different types of fibres varying in their tensile strength and coefficient of frictions were used. Repeated triaxial tests on samples, unreinforced and reinforced at the optimum content of fibre, were carried out at a confining pressure of 25, 50 and 75 kN/m(2) and the stress levels of 153 and 204 kN/m(2), producing six different deviator stresses. It is concluded from this study that both the permanent and resilient strains in all materials decrease with confining pressure but increase with the number of load cycles and deviator stress in reinforced and unreinforced conditions. Further, the resilient modulus decreases with the number of load cycles and deviator stress and increases with the confining pressure. Coir fibre shows better resilient response against synthetic fibre by higher coefficient of friction. Fly ash is also used in this study and for maximum dry density, the 30% fly ash and 70% sand mix is tested for various parameters. (C) 2008 Elsevier Ltd. All rights reserved.