Properties of an Unsaturated Residual Soil behind a Failed Slope in Sri Lanka

Abstract

Rain-induced slope failures are common in Sri Lanka. With the infiltration of rainfall, the matric suction at upper levels of the slopes reduces, perched water table develops, ground water table rises and the shear strength diminishes making the slopes vulnerable. Basic characteristics such as; Soil Water Characteristic Curves (SWCC), permeability function and unsaturated shear strength parameters have not been established for typical residual soil slopes in Sri Lanka. Undisturbed samples of soil obtained behind the scar at the location of a failed slope at Welipenna in the Southern Expressway were used in this study. Direct shear tests and permeability tests were done and matric suctions were measured with a miniature tensiometer. Pressure plate apparatus was used to obtain the SWCC. The SWCC obtained by alternate tech-niques and empirical procedures were in reasonably good agreement. Contribution of matric suction to the shear strength found to be in agree-ment with finding of other researchers