MOTOKO FUJITA

Numerical study on post-earthquake land subsidence of cohesive ground

SATORU OHTSUKA

The Niigata-ken Chuetsu-oki Earthquake recorded a magnitude of 6.8 and a maximum intensity of just over 6 on July 16, 2007, causing serious damage to various parts of the country. Damage to the sandy ground due to liquefaction was reported in the dune area and the old river channel, but the earthquake damage was caused not only by the liquefaction due to sandy soil, but also after the earthquake in the Kashiwazaki Plain composed of soft clay soil. A long-term settlement of the ground was confirmed. Regarding land subsidence after the earthquake, we are investigating the relationship between land subsidence distribution and topography and geological composition, and this study is to investigate by numerical analysis. Since the land subsidence has already converged, the purpose of the study by numerical analysis is from prediction to post-mortem analysis. Analytical consideration is given to the factors of land subsidence after the earthquake.
In the Kashiwazaki Plain, sand dunes develop along the coast, behind which deltas and valley bottom plains are distributed. In deltas and valleys, soft clayey soils are deposited up to a depth of 50m, with terrestrial clay near the surface and marine clay below. It has been measured by a stratified subsidence meter that the cohesive soil caused land subsidence after the earthquake.A study of the relationship between land subsidence and topographic classification suggests that marine clay caused larger land subsidence. Is performed. Marine clay in the Kashiwazaki Plain is a highly plastic clay with a high porosity and a high natural water content, based on tests of local sampling samples. Since the liquid limit is also large, it shows high compressibility and is considered to have a high-order structure at the beginning. When disturbing or shearing is applied, there is a property that the strength is reduced as the structure is lowered.
In this study, we simulated the subsidence of soft ground caused by earthquake using SYS Camclay model. An analysis was conducted taking into account the drainage boundary due to the sand layer between the terrestrial clay and the marine clay, but it was found that the settlement would be extremely long-term, even though the settlement is expected to end early due to the drainage effect. Was. The reason is that the consolidation settlement of the cohesive soil in the deep layer occurs. In general, numerical analysis often models shallow ground, and it is necessary to pay attention to the fact that cohesive soil in deep layers is often neglected.
In the measurement of land subsidence in the Kashiwazaki Plain, it seems that the land subsidence has converged after the earthquake, but the ground subsidence due to groundwater pumping and the geological structural subsidence of the basement are mixed, It is difficult to define clearly. In the numerical analysis, the effect of land subsidence after the earthquake was obtained for a long time, so it is necessary to pay attention to the progress of land subsidence.