Investigation of landslide calamity due to torrential rainfall in Shobara City, Japan

On 16th July 2010, a series of slope failures occurred in the city of Shobara, Japan due to a torrential rainfall. The slope failures were found to have occurred at about 200 locations in a narrow area of 16 km2. Most of the failures in this catastrophic disaster were due to the collapse of the middle portion of a mountainous slope. The primary purpose of this study was to present the landslide event that took place in Shobara City and to explore the mechanism of the multiple planar slope failures in the region. A series of field tests was carried out using a recently developed lightweight dynamic cone penetrometer in the middle portion of the selected planar slopes. The results revealed that the slopes were composed of two soil layers, weathered Rhyolite and Kuroboku. The failures took place at the boundary of these two soil layers. Laboratory tests were conducted to find the relevant parameters for the stability analysis, and laboratory scale model tests were performed to understand the mechanism of the slope failures in this region. The results of the model tests revealed that the Kuroboku layer led to a rise in the water table in the middle portion of the slope, while preventing the downward flow of groundwater. As a result, the failures are found to have initiated and occurred in the middle portion of the mountainous slope. The factor of safety, evaluated based on a slope stability analysis, was found to be close to 1, when the groundwater level rose to the ground surface and the apparent cohesion of the weathered Rhyolite soil over the Kuroboku layer decreased to 1.3 kPa.