Micro-, Meso- and Unit-Scale Characterization of Shanghai Soft Soil Under Groundwater Level Fluctuations

Coastal deltas worldwide are among the regions most vulnerable to land subsidence caused by intensive groundwater extraction, posing serious challenges to urban infrastructure and environmental sustainability. Shanghai exemplifies a typical coastal city experiencing severe land subsidence. Although groundwater abstraction has been strictly regulated and largely prohibited for most purposes, dewatering during deep excavations has become the primary cause of groundwater level fluctuations and the associated soil deformation. This study investigates the deformation mechanisms of Shanghai’s soft soil strata (layers ④ and ⑤3) under complex groundwater level fluctuations, using a multi-scale experimental approach. Stress-path-controlled triaxial tests were conducted to simulate staged dewatering, repeated dewatering, and artificial recharge processes. The results reveal an approximately linear stress–strain relationship during dewatering, with cumulative irreversible compression observed after cyclic loading. Partial deformation recovery occurred upon recharge, but only at relatively low pressure levels; once the recharge pressure exceeded a certain threshold, considerable axial compression was induced even as radial expansion continued. Microstructural analyses using an Environmental Scanning Electron Microscope (ESEM) and Mercury Intrusion Porosimetry (MIP) show that cyclic effective stress led to the fragmentation of clay aggregates and pore collapse, reducing total pore volume by approximately 10–17%. The deformation mechanism is attributed to the slippage and reorientation of clay particles under face-to-face (F–F) and line-to-face (L–F) contacts. These findings provide critical insights into the multi-scale behavior of soft soils under hydraulic stress and support the development of more sustainable land subsidence mitigation strategies in urban environments.