Strength characteristics of gravitationally deformed slope deposits of tephra and kuroboku soils in the Aso caldera, Japan －Application of revised vane-shear-cone test for estimating shear strength－

Large-scale sediment disasters, induced by heavy rains in 1990, 2001, and 2012, occurred in the Aso caldera in Kumamoto prefecture, Japan. These natural disasters were caused by shallow landslides of unconsolidated slope deposits. Geomorphological and geological surveys of the Mt. Takadake slope in the Aso caldera showed an outcrop that exhibited characteristics of the initial stage of a shallow landslide (Sato et al. 2017). These deposits consist of tephra and “Kuroboku” (humus andosols) layers, deformed by a flow-type gravitational deformation. Sato et al. (2017) indicated that the deformed slope deposits exhibited significant risk of future shallow landslides. Therefore, it is necessary to investigate the ground strength of the deformed layers and evaluate the risk of future landslides. In this study, the ground strength of soil layers is measured using a vane shear cone test and a number of soil tests. The results of these tests reveal that the strength of the deformed layers is lower than that of the un-deformed layers.