Takuya HASEGAWA
Analysis on the occurrence mechanism of the subsidence of the Kashiwazaki Plain after the earthquake
Satoru OHTSUKA
In Niigata prefecture, subsidence of ground continues due to pumping groundwater used in snow removal in urban areas, which is regarded as a problem. However, after the Chuetsu-oki earthquake that occurred in 2007, subsidence in the viscous land plate was confirmed in Kashiwazaki Plain, Niigata Prefecture. It is speculated that ground subsidence occurred due to seismic motion rather than conventional subsidence caused by pumping because ground subsidence after the earthquake was rapid and the groundwater level did not fluctuate. Moreover, the plane distribution of subsidence after the earthquake was not consistent with the plane distribution of the conventional ground subsidence. Therefore, in this study, we investigated the distribution of ground subsidence from the comparison of the geological composition and topography of the Kashiwazaki plain and aimed to clarify the cause of the long term settlement after the earthquake.
From the topographical classification, immediate sinking of sand occurred in the sand dune area. In addition, ground subsidence of viscous soil due to the earthquake occurred in the deltas of the Ugawa basin. However, it did not occur in the valley floor where soft viscous soil is also distributed.
From the physical test and the mechanical test, the Ugawa basin has high compressibility and marine clay with low water permeability. It was also found that the marine clay is a ground material having a high-level structure causing strain softening.
From these results, it was found that marine clay is the cause of ground subsidence after the earthquake. In addition, it was found that the layer thickness of the marine clay found from the drilling results at various points is related to the ground subsidence amount.