Investigation of rainfall-induced shallow landslides on the northeastern rim of Aso caldera, Japan, in July 2012

Abstract

High-intensity rainfall struck the Northern Kyushu area of Japan between 11 and 14 July 2012. This heavy rainfall triggered many shallow landslides, especially on the northeastern rim of Aso caldera, leading to significant loss of life and damage to many villages. One landslide site at Ichinomiya (Kumamoto Prefecture) was selected for detailed study. Field and laboratory investigations were conducted to identify the initiation mechanism of the shallow landslides during heavy rainfall. The thickness of the soil layer was determined using portable dynamic cone penetration tests. The soils became thinner (about 1 m) from the upper to the lower slope. In-situ infiltration tests indicate that hydraulic conductivity of this thin soil layer is low. During heavy rainfall events, most rainfall will transform into surface runoff instead of infiltration. The results of consolidated-undrained triaxial compression tests show that the effective friction angle and cohesion of the soil are 36.9° and 6.3 kPa, respectively. Soil behavior in response to increase in pore-water pressure was evaluated using a pore-water pressure controlled triaxial test. The results indicate that the steep slopes on the northeastern rim of Aso caldera will remain stable under normal rainfall conditions, due to the high shear strength of the soil. The probable initiation mechanism for the shallow landslides on the northeastern rim of Aso caldera suggests that the initial failure process begins with toe erosion caused by surface runoff during heavy rainfall. Due to the loss of the resistance force at the toe of slope, the slope will reach a critical condition when pore-water pressure in the slope also increases.