Monotonic and Cyclic Behaviour of Tephra Layer Landslide at Takanodai from the 2016 Kumamoto Earthquake

The 2016 Kumamoto earthquakes, Japan, caused a large numbers of slope failures, especially a large—scale flow—type landslide was induced in the area near the Aso Volcanological Laboratory of Kyoto University in Takanodai, Minamiaso Village Kumamoto, Japan. From the field observation, sliding surface was the tephra layer of Kusasenrigahama fallen pumice (Kpfa) which was deposited by the volcanic activity (around 30,000 years ago, Miyabuchi et al. in VSJ 48(2):195–214, 2003). This tephra layer (Kpfa) was a key soil to understand the mechanism of this large-scale flow-type landslide, then some soils from this tephra layer were retrieved from the sliding surface in the field. This study is focused on the physical properties, monotonic and cyclic direct shear behaviour for the tephra layer (Kpfa) to understand the mechanism of this landslide during earthquake. Constant vertical stress and constant volume cyclic direct shear tests were performed to study the cyclic behaviour of the tephra layer (Kpfa). From the constant vertical stress test, strain hardening behaviour was observed. From the constant vertical stress test, shear displacement increased largely during cyclic loading, despite the tephra layer being in an unsaturated state, which were similar to the behaviour of saturated sand during liquefaction. In the future, this research will be developed considering the study of particle breakage on Kpfa.