Insights into mechanisms of pumping-induced land subsidence through multiple-method investigation

Land subsidence is a worldwide geological disaster and the classical effective stress principle cannot explain all related field phenomena well. To comprehensively reveal the mechanism of pumping-induced land subsidence, a multi-method investigation was conducted in this study, which integrated physical model tests, laboratory soil mechanical and micro-structure experiments, and numerical modeling. The results show that the change of the pore water pressure in an aquifer is consistent with the pumping and recharging. The lagging of land subsidence behind the change of pore water pressure can be attributed to soil creep. When the current effective stress in aquifer sand is greater than the historically greatest one, sand exhibits clear creep behavior. Clay has greater creep deformation than sand, and its creep capacity lowers with increasing load level and number of cycles when no pre-consolidation load is applied. After unloading, the change in the void ratio and porosity per unit area are only restored to one-fifth of the variation induced by compression, and the number of pores in clay increases by 1.6 times. The clay structure undergoes reorientation and the pore shape flattens, causing it to deform irrecoverably, which explains the recharging-induced rebound in the aquitard fails to fully compensate for the subsidence as the aquifer does. Meanwhile, the evolution of pore volume in clay under various compression pressures is characterized by a transformation from medium pores (1–5 μm) to small pores (0.1–1 μm). The microstructural evolution and creep behaviors of soil reveal the mechanisms of land subsidence during pumping and recharging more effectively.