Establishment of fundamental characteristics of some unsaturated Sri Lankan residual soils

Abstract

Slope failure in tropical climates frequently occurs due to excessive rainfall. Heavy infiltration causes destruction of matric suctions, development of perched water table conditions and rise of ground water table. Severe erosion and surface destruction will also be caused by the heavy prolonged rainfall. In order to understand the threshold values of rainfall leading to instability it is necessary to model this process with a reasonable accuracy. Sri Lankan residual soil formations are formed by weathering of the metamorphic parent rock and have inherited significant abrupt variations in engineering characteristics. Basic characteristics of these soil formations such as soil water characteristic curves (SWCCs), variation of permeability with water content and unsaturated shear strength parameters are essential parameters in these analyses. These characteristics have not been established for typical residual soils forming slopes in Sri Lanka. This thesis highlights the need for detailed experimental studies and presents comprehensive studies that have been conducted at the University of Moratuwa and National Building Research Organization (NBRO) laboratories to establish the fundamental characteristics of unsaturated Sri Lankan residual soils. Undisturbed samples of soil obtained from the failed slope at Welipenna in the Southern Expressway were used in this study. Direct shear tests were done by modifying the conventional apparatus by incorporating a miniature tensiometer which allows for the simple and direct measurement of soil matric suction during shearing. Soil water characteristic curves (SWCCs) were also established using these apparatus. Alternatively, pressure plate apparatus was also used for this purpose. In addition to that, soil water characteristic curves (SWCCs) were developed from gradation curve also.Permeability of an unsaturated soil varies considerably with the level of saturation and will make a very significance influence on the infiltration process. Permeability function which defines the variation of permeability with matric suction was investigated on undisturbed samples. The method is based on continuously drying and wetting the soil sample while continuously monitoring the suction gradient and the change in soil mass. The thesis highlights the importance of these studies and presents the procedures that are being used.