Effects of rainfall on stability of cut slopes in residual soils

Rain induced slope failures are a common geotechnical problem in tropics where residual soils are abundant. Residual soils are characterized by significant differences in the levels of weathering within short distances. Two typical cut slopes from the Southern Transport Development Project were used in theanalysis. The complex geological situation in an unsaturated cut slope is idealized by two cases; a uniform soil and a soil overlying moderately weathered rock. The changes to the pore pressure regime with the progression of the rainfall and consequent reduction of the safety margin of the slope was modeled with SEEP/W and SLOPE/W software. Both the deterministic approach and the probabilistic approach were used in the evaluation of stability. With the continued rainfall, matric suctions depleted, perched water table developed and the safety margins reduced. The presence of a moderately weathered rock layer at a shallow depth was found to be more critical