Research on cavity formation below Ohkozu old movable weir in the Shinano River

Abstract

Ohkozu old movable weir of the Shinano River at Nagaoka city in Niigata Prefecture was demolished due to antiquated equipment in accordance with the construction of a new movable weir. Based on the results of boring exploration conducted during the construction, continuous cavity below the old movable weir was observed. Its continuous cavity is present over the entire area of the movable weir. The cavity of up to 16 cm has been confirmed. The following reason have been considered as the cause; 1) erosion by river water seepage flow due to big water head difference, 2) settlement of the surrounding ground due to liquefaction. Even if the pile foundation supporting the weir is robust after an earthquake, the bearing capacity of pile foundation is reduced by the liquefaction-induced ground settlement and the weir structure becomes unstable. Accordingly, risk of accelerating the erosion by the river water seepage flow is also increased. In this research, the cavity generating mechanism due to the liquefaction-induced ground settlement is focused on. 1-G shaking table model tests are conducted to clarify the influence of the presence of pile and weir pillar, number of piles to deformation and liquefaction of the surrounding ground.