Analysis of a slope failure triggered by the 2007 Chuetsu Oki Earthquake
The powerful Chuetsu Oki Earthquake with a magnitude 6.6 hit the northwest of Niigata Prefecture, Japan on July 17, 2007. The earthquake caused economic loss as it reportedly destroyed 342 buildings, mostly older wooden structures. A few embankment failures as well as shallow landslides were triggered by the earthquake. The Oumigawa landslide (Fig. 1) seems to have received the most attention from the local government as well as geotechnical community as it blocked the rail track near Oumigawa station JR, suspending the train service in the area for 1 month. A few days after the event, a research group from the University of Tokyo, including the authors, conducted a survey of the site in order to evaluate the structural damage caused by the slide. It was found that the landslide occurred in highly weathered zone of mudstone, which consisted of plastic fine-grained soil with the inclusions of rock fragments. It is interesting to note that, according to the statistics of slope failures triggered by the Niigata Chuetsu earthquake, a strong quake that hit the same prefecture in 2004, seismically-induced slope failures in plastic fine-grained soils are rather rare and significantly outnumbered by slope failures in sandy soils. One of the reasons for this discrepancy is that plastic fine-grained soils have a smaller potential for generation of high pore pressures during cyclic loading and the material retains most of its static undrained strength. Thus, the Oumigawa landslide presents the unique opportunity for geotechnical engineers to study the mechanism of such slides. The procedure used in this study included a series of triaxial tests on soil samples collected from the failure plane, slope stability analysis prior and after the earthquake, and dynamic analysis. On the basis of the obtained results, the mechanism of the Oumigawa landslide was posited. It was also found that only the dynamic analysis provided the location of the most critical sliding surface, which was in a reasonably good agreement with the position of the sliding zone in the slope.
The First World Landslide Forum
Civil Geotechnical Engineering