Abstract
The collapse of a slope occurred due to heavy rain in Higashihiroshima City’s Shiwa District at about 5:30 a.m. July 25th, 2009. The slope 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. The collapsed soil was liquefied and flowed out downstream 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 form. The history of the geographical change was analyzed by 3-dimentional laser survey, old map, analysis of aerial photographs, and the measurement of thickness of collapsed soil by dynamic cone, and the ground form before the collapse was reconstructed. The stability analysis considering the raise of the ground water level in the slope was made and it is known that the collapse would take place when the ground water level was raised in about 9 m by the water supply through the permeable layer at the bottom of the slope.
