Subsidence of clayey ground after an earthquake occurred in Iwaki City, Fukushima Prefecture and its generation mechanism

Abstract

Due to a lowering of water table and a man-made fill with urban development, subsidence of soft clayey ground occurs in Taira, Iwaki City, Fukushima Prefecture, Japan. Subsidence also occurred in the area as a result of the 2011 Tohoku earthquake that occurred off the Pacific Coast. In this study, the change in the subsidence was measured by leveling, and it was revealed that 8 cm of subsidence occurred for two years after the earthquake struck, and subsequently, a little less than 1 cm of subsidence per year continues to occur. From the geotechnical investigation and laboratory tests conducted at this site, it was found that the subsidence occurred on a normally consolidated slit layer. Furthermore, undrained cyclic shear tests were performed for clay blending specimens, and reconsolidation tests were carried out following the shearing. It is concluded that the subsidence decreases with an increase in the overconsolidation ratio. Additionally, the subsidence increases with excess pore-water pressure ratio and shear strain amplitude.