Deformation behaviour of over-consolidated soil in a deep confined aquifer under multi-stage pore pressure changes in Shanghai, China.

Shanghai, a typical coastal soft-soil area, is characterised by thick soft layers with high compressibility and low permeability. Together with five confined aquifers, these layers form a complex multi-aquifer system. Historically, excessive deep groundwater extraction caused severe land subsidence. However, after years of extraction restrictions and artificial recharge, subsidence has been controlled and groundwater levels have risen, creating possibilities for groundwater extraction in emergency situations. Focusing on Shanghai's third confined aquifer (Layer ⑪) and the adjacent aquitard (Layer ⑫), this study first employed high-pressure consolidation tests to verify the over-consolidated state of the deep soils, followed by high-pressure triaxial tests and numerical simulations to investigate their deformation behaviour under staged pore pressure changes. The triaxial tests indicated that during staged dewatering, the deformation modulus (E) dropped sharply once the effective stress exceeded the preconsolidation stress, falling to 1/2 to 1/4 of its initial value and marking the elastic-elastoplastic transition. Moreover, the void ratio change was strongly negatively correlated with the over-consolidation ratio (OCR), and a higher OCR significantly suppressed compression during pore pressure changes. Numerical simulations based on the Modified Cam-Clay model agreed well with the experiments. These findings provide a scientific basis for predicting safe drawdown thresholds in emergency groundwater extraction.