The role of hydrologic response and soil behavior in the initiation of rainfall-induced debris flows

Conditions leading to rainfall-induced debris flows were investigated by a field study of the hydrologic response and a laboratory testing program of soil strength and behavior. In-situ stresses at a field site were determined from the slope characteristics and the hydrologic response to a storm that generated debris flows on adjacent slopes in 1986. The laboratory test program consisted of direct shear (DS) tests, isotropically consolidated undrained (ICU) triaxial tests, anisotropically consolidated undrained (ACU) triaxial tests and field stress path (FSP) tests. The field stress path tests were performed to imitate the decrease in effective stress under constant shear stress due to an increase in pore pressure during an intense rainstorm.; The drained strength at the in-situ stress level was found to be less than that suggested by strength parameters from other test types at higher confining stresses and the volume change behavior of the soil was found to depend on the consolidation state and the stress path. The results indicate that the field stress path alone will not lead to debris flow mobilization and a flow failure is possible only when the soil is subjected to an undrained increase in shear stress. The field stress path may lead to local drained initiation, and a stress redistribution which causes undrained loading, and debris flow mobilization. To determine a factor of safety (FS) against this type of failure, initiation and mobilization have been analyzed independently and a simple mechanism of stress redistribution has been introduced.; Conditions for initiation were evaluated by an infinite slope effective stress analysis (ESA) and strength parameters from FSP tests at in-situ stresses. Conditions for mobilization were evaluated by a steady-state analysis and an undrained strength analysis (USA). The steady-state analysis proved difficult to use because the steady-state line was non-unique. In contrast, the USA procedure involved undrained strength values which were more easily interpreted and which led to a failure envelope similar in form to the ESA. The ESA and USA results and the stress redistribution mechanism of undrained loading form a basis for determining a FS against debris flow failure.