Causative mechanisms of rainfall-induced fill slope failures

Slope failures in fill slopes formed by loosely compacted, completely decomposed granite in Hong Kong occur commonly during intense tropical rainstorms. The stress path greatly influences the shear strength of the soil mass, and is therefore crucial to the identification of slope-failure mechanisms. The soil mass in this case is largely unsaturated. In situ hydrologic response to rainstorms indicates that soil suction is reduced by rainfall infiltration, which often becomes the triggering factor in initiating slope instability. The constant dead-load tests on unsaturated, loosely compacted, completely decomposed granite appropriately simulate the field stress path of rainfall-induced fill-slope failure by reducing suction. The tests indicate that matric suction contributes to the dilative or contractive behavior of the unsaturated soils. The anisotropically consolidated undrained triaxial tests demonstrate the consistently contractive behavior of the specimens. On this basis, we delineate the in situ stress conditions leading to the initiation of rainfall-induced fill-slope failure, and the stress paths of the transformation from local failures to flowage. Based on a systematic study of fill-slope case records in Hong Kong, implications of such mechanisms on fill-slope stability are given.