DETERMINATION OF SHEAR-STRENGTH OF HAWAIIAN RESIDUAL SOIL SUBJECTED TO RAINFALL-INDUCED LANDSLIDES

The determination of shear strength parameters for the stability analy sis of rainfall-induced landslides requires both a knowledge of the st ress history and a laboratory simulation of the failure stress path of the soil involved. Such soils exist on steep slopes and have experien ced st highly anisotropic stress action through which they have been c onsolidated. A series of tests has been carried out on residual soil f rom a steeply sloping site in Hawaii, typical of locations subject to rainfall-induced landslides. Two types of tests were carried out: anis otropic-consolidation-undrained (ACU) tests, which mimic field consoli dation stress paths of the soil, and constant-shear-stress-drained (CS D) tests, which best simulate the failure stress path for rainfall-ind uced landslides. Substantial differences of soil behaviour in CSD test s and ACU tests are recognized. The CSD strength parameters are consid ered most appropriate for analysing the rainfall-induced landslides. T ypical 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 k Pa for the large-strain state.