This paper describes the mechanism of four deep]seated catastrophic landslides (Kuridaira, Nagatono, Akatani, Tsubonouchi) caused by heavy rainfall during Typhoon Talas (T1112) in Kii Peninsula, Southwest Japan. We conducted a field survey, and examined the physical and mechanical properties of the sliding layers. The survey revealed some common phenomena among the four landslides, such as the strata of the deep]seated catastrophic landslides consisting of heavily weathered cracked bedrock. The crossed two joint planes formed wedge]shaped blocks, such as the upper slope block and lower slope block. This allowed the ground water to deeply infiltrate the planes. The angle of the shearing resistance of the sliding layers were measured to be between 28 and 40‹. Meanwhile, we recognized the occurrence of excess pore water pressure under shear deformation by the stress]controlled ring shear test. This indicates that the upper slope block started sliding and rode on the lower slope block where compression deformation was produced. Consequently, the positive excess pore water pressure occurred in the rock mass, and the deep]seated catastrophic landslide started in a body. In addition, as the excess pore water pressure increased under the shearing, the sliding soil mass gained high speed, and eventually traveled for a long distance. This indicates that excess pore water pressure is an important factor to expound the mechanism of deep]seated catastrophic landslides.

Key wordsFdeep]seated catastrophic landslide, geological factor, soil properties, stress]controlled ring shear test, pore water pressure