Landslide and mudflow disaster in disposal site of surplus soil at Higashi-Hiroshima due to heavy rainfall in 2009

The collapse of a filling occurred due to heavy rain in Higashi-Hiroshima City׳s Shiwa District at about 5:30 am July 25th, 2009. The filling was made of surplus soils, and it contained a mass of water supplied from rainfall and ground water flow of a permeable layer at the bottom of the filling. The collapsed soil flowed down and destroyed a house. In this paper, the cause of this disaster is discussed. The site of the disaster was used as the dumping site of surplus soils, after several changes of ground formation. The history of the geographical change was reconstructed by the image processing of old map, aerial photographs, result of 3-dimentional laser survey carried out after collapse and the measurement of thickness of collapsed soil by dynamic cone penetration test. According to the result of processing, the shape and the size of the filling before collapse was reconstructed. The relationship between the rainfall and the groundwater in the river sediments layer over which the filling was constructed was determined. A stability analysis of the filling was conducted considering the rise of the groundwater level in the filling and the laboratory measured strength parameters. The results of the stability analysis showed that the collapse would have taken place when the groundwater level rose by about 9 m due to the supply of groundwater through the river sediments layer.