Individual landslide hazard assessment of natural valleys and slopes based on geotechnical investigation and analysis

Slope failures caused by rainfall on weathered granite are common occurrences in many parts of the world and it is more severe in Japan where there are a significant number of slopes susceptible to failure. At present, about 31,987 natural, and valley slopes are found to be susceptible to landslide disasters in Hiroshima Prefecture, the highest number of any prefecture in Japan. In Hiroshima Prefecture, the prefectural land is divided into 350 units of 5 km2, and the hazard of each unit was assessed based on the rainfall data and past records of failures. However, the existing hazard assessment system was found not to be adequate of making better predictions of individual failures in the region, and moreover the system was developed without carrying out any reliable slope stability analyses. In this study, an attempt has been made to understand the mechanism of individual slope failure through seepage analyses and stability analyses. A series of laboratory model tests were conducted to develop the relationships among volumetric water content, degree of saturation, suction, and coefficient of hydraulic conductivity under unsaturated conditions. Field investigation and laboratory tests were conducted based on the four valleys which were identified as potentially hazardous zones in Higashi-Hiroshima city in Hiroshima Prefecture. Graphical relationships were developed among the volumetric water content, suction, rainfall intensity, coefficient of hydraulic conductivity, and degree of saturation for Masado soils. Based on the relationships developed, suction was calculated along the soil profile of natural valleys with the lapse of time. Thus, shear strength parameters were determined based on the degree of saturation calculated from the data of volumetric water contents. The stability analyses revealed that the factor of safety gradually decreases with the formation of High Moisture Content Belt (HMCB) and drastically reduces with the formation of the water table. The susceptibility of failure for each of the four valleys is given based on the hazard assessment system, and is compared and discussed.