ANALYSIS OF RAINFALL-INDUCED DEBRIS FLOWS

Rainfall-induced debris flows are flow failures in residual and colluvial soils initiated by a reduction of confining stress as a result of pore-water pressure rise during or following periods of intense rainfall. To establish appropriate stability analysis procedures, the behavior of soil from a debris flow source area is investigated. Constant-shear-drained triaxial tests, tests that mimic the field stress path, and anisotropically consolidated undrained triaxial tests performed at the in-situ stress level indicate that undrained loading is a prerequisite for flow failure. Mechanisms of stress transfer are proposed to explain how the initially drained deformation that occurs along the field stress path can lead to undrained mobilization. Because a debris flow involves both drained initiation and undrained mobilization, a complete stability analysis includes an effective stress analysis with constant-shear-drained parameters to determine the potential for failure initiation, and an undrained strength analysis using undrained residual strength to determine the potential for debris flow mobilization.