Mechanism and future risk of slope instability induced by extreme rainfall event in Izu Oshima Island, Japan

A volcanic slope in Izu Oshima Island in Japan experienced a profound rain-induced disaster in October 2013. Since this slope had been stable for centuries except for minor failures, a special investigation was carried out on the cause. Because of its volcanic origin, the failed slope consisted of layers of ash, sand and lava. While the investigation concerned many disciplines, the present paper addresses one part of the geotechnical studies. The particular emphasis was put on the reason why some part of the slope "did not" fail because the post-disaster construction of infrastructures in the affected area relied on the future stability of the affected mountain slope. In line with this, another focus of the study was on the geohydrological feature of the underlying lava layer that possibly controlled the instability of the volcanic slope. It was concluded finally that the lava layer is pervious and allows drainage of infiltrated rainwater and that, only during extremely heavy rain, the subsurface lava topography triggered slope failure only in its "valley" parts. For 7 years after the disaster, the remaining parts of the slope have been stable as judged and many local infrastructures have been reconstructed.