Case study on slope failure caused by heavy rainfall in Niigata area in July 2004
An unprecedented local downpour occurred in Niigata Tyuetu region on 13th July 2004. The heavy rainfall caused serious damage to residents or public infrastructures. As over three hundred landslides also occurred, a detailed investigation was carried out on a slope failure at Nakayama area of Izumozaki to clarify the failure mechanisms.
The scale of slope failure was about 60m long, 35m wide and an inclination of 25 degrees. A discontinuity surface of mudstone (a dip slope) appeared in the slope failure and striations remained on the mudstone. The mudstone, colluvial deposit (sand) and collapsed deposit on the mudstone layer were extracted for consolidation, box shear and ring shear tests. Shear surface was clearly formed and striations were confirmed on the surface as well as the field in ring shear tests on mudstone. Residual strength between colluvial deposit (sand) and mudstone was small and equal to that of mudstone. From the observation after the tests, it was obvious that a thin-layer of mudstone was pasted on the surface of the sand. Therefore, residual strength of mudstone should practically be used as a minimum strength of the discontinuity surface between colluvial deposit and mudstone.
Finally a slope stability analysis based on a slice method was conducted using the strength parameters from the experiments. As a result, it became clear that safety factor became less than one, when a water level of the slope went up.