TXT-tool 4.081-1.1: Mechanism of Large-Scale Deep-Seated Landslides Induced by Rainfall on Gravitationally Deformed Slopes: A Case Study of the Kuridaira Landslide in the Kii Peninsula, Japan

Abstract

In September 2011, heavy rainfall brought by Typhoon Talas triggered 72 large-scale deep-seated landslides in Nara and Wakayama Prefectures, the Kii Peninsula, Japan. Most investigated landslides on the gravitationally deformed slopes were preceded by pre-existing small scarps along or near the head of the slopes. This study seeks to clarify the mechanism of the huge rainfall-induced Kuridaira landslide by simulating the increasing of pore water pressure with undrained high-stress dynamic loading ring shear apparatus. The authors also examined how gravitational deformations of upland slopes contribute to the mass movement under shear deformation. Laboratory experiments were conducted on two samples of the sliding plane taken in a site investigation, namely sandstone-dominated materials and shale materials. The pore water pressure control tests and shear displacement control tests clearly indicated that the rapid landslide was initiated due to high excess pore pressure generation and significantly shear strength reduction in the progress of shear displacement. The critical pore pressure ratio (r_u) was about from 0.33 to 0.36 while shear displacement at the starting point of failure (DL) had a threshold value ranging only from 2 to 6 mm. More specifically, the high mobility of the landslide was in tests on shale sample due to a significant loss of shear strength. In addition, the authors observed the landslide occurrence associated with the sliding surface liquefaction behavior for both samples. The evidence of liquefaction phenomena in the tests was in accordance with the findings in the field survey and previous studies.