2012 Volume 7 Issue 1 Pages 295-309
From 3 p.m. to 6 p.m. on July 16th, 2010, a heavy rainstorm of 173 mm (at Oto precipitation station) suddenly attacked in Shobara city, Hiroshima Prefecture. The landslides and the mud flows over 200 sites were caused in a narrow mountainous area of 4 kilometers square by the concentrated heavy rain. It was found that, by the field survey, a typical pattern of slope failure in this disaster is a collapse at the middle portion of plane slope with any trace of valley. In this study, a site investigation including a lightweight dynamic cone penetrometer was carried out at a site of collapse at the middle portion of plane slope. parallel slope. Further, the laboratory tests of soil samples, slope model experiment and stability analysis were conducted to explain the collapse. It is known that there is Kuroboku layer which has a low permeability on the weathered rhyolite layer at the middle point of the slopes, and the slope failures took plane at the boundary of two layers. The result of model slope experiment showed that Kuroboku layer prevented the groundwater flow making the groundwater level rise at the middle of the slope. The safety factor obtained by the slope stability analysis was close to 1.00, when the groundwater level rises to the ground surface.
