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 longterm 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. (c) 2024 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY- NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Due to a build-up of excess pore water pressure and a reduction in the effective stress, saturated clay is characterized by nonlinearity, hysteresis of stress-strain relationship, and degradation of stiffness and strength during undrained cyclic loading. This paper illustrates that the modified mixed hardening model considering stiffness and strength degradation is more reasonable by force of contrast. And the data obtained from dynamic tri-axial tests and published studies are compared to investigate various modulus degradation laws. It is verified that different laws adopted according to the clay property can capture complex behaviors more accurately.