Numerical Simulation of Viscoelastoplastic Land Subsidence due to Groundwater Overdrafting in Shanghai, China

Abstract

Land subsidence caused by groundwater overpumping in Shanghai is becoming a serious geological hazard. Due to its important economic position, the field data, including compression of individual stratum from extensometer groups and the groundwater levels from observation wells, have been collected over the past 45 years. Considering the fact that different hydrostratigraphic units have different kinds of deformation and that an identical unit may also present different deformation characteristics, such as elasticity, elastoplasticity, and viscoelastoplasticity, at different sites of the cone of depression or in different periods, a nonlinear coupled regional land subsidence model is developed. The coupled model consists of a three-dimensional groundwater flow model and a one-dimensional vertical deformation model, both based on a viscoelastoplastic constitutive laws (called modified Merchant model), and then solved using a multiscale iterative finite-element method. The model is calibrated using 28,184 hydraulic head measurements and 26,732 deformation measurements from 1961 to 2005. The calibrated and evaluated model is then used to assess the future evolution of land subsidence under two groundwater pumping scenarios. Predicted results indicate that restricting groundwater pumping is effective in reducing the annual subsidence rate. Moreover the average cumulative land subsidence from 2006 to 2020 can be controlled less than 40 mm when the groundwater pumping rate is reduced to 25 million cubic meters per year. Although the area affected by land subsidence continues to expand and the cumulative amount of land subsidence still continues to increase because of the deformation delay.