Revisiting Nigawa landslide of the 1995 Kobe earthquake

A simplified pseudo-static limit equilibrium approach is used to analyse a full-scale slope failure that occurred during the 1995 Kobe earthquake. The slope is 28 m high, having a gentle angle of 26·5°, and has a factor of safety as high as 2·43 in the absence of an earthquake. In addition to a factor of safety, the critical failure surface obtained from pseudo-static analysis is compared with that of the actual failure surface. Contradictory to some design codes that use only a horizontal seismic coefficient, this study finds that a rational analysis must take into consideration both horizontal and vertical seismic accelerations. The analysis including vertical acceleration may give a shallower or deeper failure surface than cases where vertical acceleration is ignored. An analysis combining horizontal and upward accelerations results in a deeper failure surface than when only downward or no vertical acceleration is considered. The effects of soil properties on the slope stability are also investigated. The study suggests that for near-fault earthquakes, slopes may be subjected to vertical (upward) seismic coefficient as large as 90% that of horizontal, larger than that specified in existing design codes.