Response of unsaturated sandy soils under constant shear stress drained condition

In order to understand the mechanism and conditions leading to failure of sandy slopes due to the infiltration of rainwater, constant shear stress drained triaxial tests were conducted on three sets of soil samples-normal sand, gravelly sand and silty sand-taken from natural slopes where large-scale landslides have occurred in the past. Water was infiltrated from the bottom of an initially unsaturated soil specimen under constant shear stress drained condition until failure occurred. Such a loading pattern simulated the stress path followed by a soil element on a potential failure plane on a slope subjected to rainwater infiltration. The effects of various parameters reflecting the initial condition, such as relative density, principal stress ratio, degree of saturation and infiltration rate, on the development of deformation during the infiltration process were investigated. The test results confirmed that the development of pore water pressure within the soil is the main reason for the failure of slopes during heavy rainfall. The results obtained can serve as guidelines in developing warning systems against impending rainfall-induced slope failures.