Groundwater Seepage and Source Head Collapse Mechanism of Debris flow in heavy Rain using Finite Element Method and Full Scale Failure Experiment

In the heavy rainfall in the district of Chugoku, north Kyushu,Japan on July 21, 2009, a lot of debris flows occurred around the boundary between Yamaguchi City and Hofu City. Observing such debris flow occurrence slopes, granite hard rock layer exposed, and marsified weathered residual soil is thinly distributed at source heads and the difference in permeability between the granite hard rock layer and the weathered residual soil was observed. In this study, we performed FEM analysis that reproduced these characteristics. In addition, a full-size experimental model slope was created, and a rainfall experiment was conducted under the conditions where a bare ground surface slope and the multi -function filter were laid. A comprehensive examination of the two results shows that the presence of a high permeable coarse grained layer between the weathering granite layer and impermeable bed rock layer causes a sharp rise in pore water pressure at lower part of the slope due to the effect of underground penetration and groundwater funnel flow. The flow has become clear that the occurrence of boiling causes the collapse. Moreover, the expected effect of multi function filter which control the seepage in heavy rain was confirmed.