Determination of shear strength of Hawaiian residual soil subjected to rainfall-induced landslides

The determination of shear strength parameters for the stability analysis of rainfall-induced landslides requires both a knowledge of the stress history and a laboratory simulation of the failure stress path of the soil involved. Such soils exist on steep slopes and have experienced st highly anisotropic stress action through which they have been consolidated. A series of tests has been carried out on residual soil from a steeply sloping site in Hawaii, typical of locations subject to rainfall-induced landslides. Two types of tests were carried out: anisotropic-consolidation-undrained (ACU) tests, which mimic field consolidation stress paths of the soil, and constant-shear-stress-drained (CSD) tests, which best simulate the failure stress path for rainfall-induced landslides. Substantial differences of soil behaviour in CSD tests and ACU tests are recognized. The CSD strength parameters are considered most appropriate for analysing the rainfall-induced landslides. Typical CSD derived parameters are phi(y)' = 32 degrees and c(y)' = 6.9 kPa for the yield state, and phi(ls)' = 35 degrees and c(ls)' = 4.2 kPa for the large-strain state.