Abstract: On November 4～5,1993, Lantau Island with an area of 120 km2 which is the larg est outlying island of Hong Kong Territory experienced an extremely severe loca lized rainstorm, which resulted in over 800 soil slide debris flows on the natu ral terrain there. The majority of the soil slides debris flows occurred in are a with volcanic rock derived residual soil and colluvium. Triaxial tests, incl uding compression tests anisotropically consolidated undrained and drained, und isturbed volcanic rock derived soil specimens at constant shear stress, were pe rformed to investigate the stress strain behaviors of the soil. The test result s are interpreted to suggest that soil slide debris flow initiation occurs as a two stage process which consists of an initial dilation failure under drain co nditions followed by undrained failure as a result of strain softening.