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[HDS17-09] A slope failure at Shiretoko-Rausu, Hokkaido, in August 2016
Keywords:slope deposits, periglacial deposits, piping hole, groundwater level, slope failure, heavy rain
The primary source of the collapse consists of poor sorted sediments, silt with angular, on alternation of gravel and mud with low permeability. This is periglacial sediment, which is common slope deposit in Hokkaido, northernmost Japan. This sediment holds much water after heavy rain as a result of loose and high permeability. A vertical hole, 3m in diameter, appeared in the deposits. Groundwater streams concentrate on the base of the sediment. Because piping holes were eroded by the streams, the vertical hole was formed by the collapse of one of the piping holes.
Andesite is distributed behind the collapsing slope deposits, and the andesite is lower permeability than the slope deposits. This causes that the groundwater level of the andesite rises appeared to be delayed. It is likely that the slope failure occurred with a delay due to the supply of the groundwater from the andesite behind. Because the front of the slope sediment has disappeared by the slope failure, groundwater flowed and formed piping falls due to water level difference.