The microscopic characteristics of Shanghai soft clay and its effect on soil body deformation and land subsidence

Abstract

Compaction of the 40-m-thick, upper Shanghai soft clay contributes to land subsidence in the Shanghai region. The upper clay, a marine deposit, continuously subsides at rate of 3mm/years despite mitigation strategies designed to control land subsidence. These strategies include the artificial injection of water into the subsurface. Data describing the particle-size distribution, pore-size distribution, microstructure, mineralogical composition, pore-solution composition, and cation-exchange capacity indicate that the clay is semidispersed, substable, and susceptible to compaction. An evaluation of soil body deformation caused by artificial injection of dilute water into a marine clay suggests that depression of the electrostatic double 1ayer in the marine clays may be responsible for the observed compaction and resulting subsidence. A decrease in the swelling pressure of the marine clays might result from a reduction in the concentration of dissolved sodium in pore fluids when calcium-bicarbonate water is injected. Thus, ironically, the strategy designed to mitigate land subsidence could be contributing to it. Increasing the concentration of dissolved salts in the injection water may prevent the collapse of the clay structure and halt or reverse the subsidence process. The analysis results indicate the deformation of soft clay by consolidation is subject to its microscopic physical and chemical features, rather than the fluctuation of groundwater level. This continuous compression contributes as a major part to the land subsidence in Shanghai.