北川で繰返し発生した噴砂による堤内・裏法尻箇所のゆるみ調査

抄録

A large number of significant sand boils have been repeatedly appearing during flooding events in recent years in the left hinterland of Kita River at around 13 km from the river mouth. Versatile in-situ investigations, including topography survey, penetration tests, and trench excavations, were conducted to understand the influences of sand boils on the loosening of soils in the hinterland and around the back slope toe and on the damage to the river with seepage analysis using FEM. Results confirmed that the loosening of soil was from the hinterland and around the back slope toe. The thickness, fine-grained fraction content, and permeability of the sandy soil layer overlaid on the gravel layer with high permeability determined the occurrence of soil boiling. Moreover, the sheet pile constructed as countermeasure against piping failure would need to be effective to restrain the rapid propagation of piping.