2019 年 72 巻 3 号 p. 11-18
The Kumamoto earthquake in Japan on April 16, 2016, triggered numerous landslides on the hillslopes of the post-caldera central cones of Aso Volcano in Kyushu, Japan. The hillslopes are mantled by ≥ 10-m thick multiple tephra layers produced by repeated eruptions. In this study, we examined the tephra stratigraphy and measured the soil strength of the tephra layers around a sliding surface in two selected landslides: the Takanodai landslide, with a depth of 10 m and that traveled a long distance, killing five people, and the San'oudani landslide, with a depth of 10 m and caused a debris flow running downstream along the San'oudani Creek. The stratigraphic surveys of these landslides demonstrated that the sliding surfaces were formed in a layer of weathered pumice that turned into soft and easily crushable grains. We identified the pumice as the Kusasenrigahama fallout pumice, approximately 30,000 years BP. The weathered pumice layer where the sliding surface was formed showed extremely high water content. We measured the soil hardness using a Yamanaka-type tester, the soil strength by cone penetration tests, and the soil cohesion and internal friction angle by vane cone shear tests. All the properties measured of the weathered pumice layer were lower than those of the upper and lower layers, suggesting that the weathered pumice layer is remarkably weak in all the tephra layers of the soil profile. A probable initiation mechanism of the landslides is as follows: first, shear failure occurs within the weathered pumice layer because of earthquake motion; next, soft pumice grains are crushed along the sliding surface as the shear displacement increases, resulting in a reduction in volume because of earth pressure; and finally, an excess pore water pressure is generated under an undrained condition, and soil shear resistance rapidly decreases.
