Study on long-term subsidence of soft clay due to Niigata-ken Chuetsu-oki earthquake of 2007

Abstract

In the Niigata-ken Chuetsu-oki Earthquake of 2007, ground liquefaction was outstanding at the foot of a sand dune and in old river channels. Although no distinct disaster was found in the clayey ground after the earthquake, the long-term settlement of the ground was observed after the earthquake in the Shinbashi district of Kashiwazaki City. At one observation site, the cumulated ground subsidence of the layers from the ground surface to a depth of 23 m had reached 71 mm 14 years after the earthquake. In order to study the mechanism of the deformation during the earthquake and the long-term settlement after the earthquake, ground investigations, such as a boring survey at the observation site and indoor element tests on sampled soil, were conducted in this study. The results showed that the sampled soil was very soft, strongly compressible, and relatively highly structured. Subsequently, the transformation stress-cyclic mobility (TS-CM) constitutive model, developed by Zhang et al. (2007), was used to simulate the results of the indoor element tests, and the soil parameters were determined based on the results of these tests. The TS-CM model contains the concepts of subloading, described by Hashiguchi (1977), and superloading, described by Asaoka et al. (2002). Therefore, the subsidence behavior of the ground was simulated by a soil–water coupling elasto-plastic finite element (FE) analysis using the TS-CM constitutive model and the determined parameters. The FE simulation results agreed well with the actual site subsidence observation data. Based on the simulation results, the post-earthquake behavior of the soft clay and its mechanism were discussed, and the successive subsidence was predicted forward. According to the simulation results, the relatively highly structured susceptible clay at this site was found to have greater potential in terms of long-term consolidation than relatively less structured susceptible clay due to the large excess pore water pressure generation during the ground motion and the consolidation process after the earthquake. This conclusion was verified by consolidation tests on two types of clay.