Abstract: To investigate the stress strain behavior of a loosely compacted volcanic derived soil and the mechanism of rainfall induced fill slope failures, triaxial tests, including isotropically consolidated drained and undrained compression and anisotropically drained tests at constant deviatoric stresses, are performed on loosely compacted specimens prepared by moist tamping The critical state line is obtained in the e log p ′ and q p′ plots The soil is of perfect plastic behavior after peak strength in the stress strain relations, and the drained tests at constant deviatoric stresses indicate that very small strains are needed for initiation of failure triggered by pore water pressure rise The mechanism of fill slope failures induced by infiltration of rainwater is analyzed based on laboratory test results