Shear strength characteristics of unsaturated residual soils

Abstract

The tropical climate conditions cause the residual soils near ground water level to be in an unsaturated state generally. Matric suction enhance the shear strength of the soil. In tropical countries like Sri Lanka where the soil formation is mainly residual, this added shear strength is no longer reliable due to frequent rainfall periods. With rainfall infiltration, the matric suction reduces and the shear strength diminishes making the slopes vulnerable. Therefore, it is necessary to study how the moisture content of soil is affected after the rainfall and how it can be related to shear strength. As an initial step, the variation of apparent cohesion with degree of saturation was investigated in the present study for residual soil obtained from the site of the failed slope at Southern Transport Development Project in Welipenna. Direct shear tests was preferred in the study over triaxial test because of the shorter time taken for the pore water pressure to reach equilibrium.