Environmental effects of dewatering procedure when subway's deep excavation in marine continental sedimentary soil

Since the Quaternary Pleistocene, the estuary region of the Yangtze River in China has undergone extensive sedimentation due to the combined effects of tectonic movements, ancient river systems, paleo-marine environments, and other geological factors, resulting in a thick sequence of Quaternary deposits exceeding hundreds of meters in depth. In engineering practice, the complex interaction between groundwater conditions and soil has led to incidents of foundation pit instability. Previous research on ensuring the safety of foundation pit construction under such geotechnical conditions has primarily focused on catastrophic phenomena such as soil piping and boiling; however, these studies often fail to adequately explain certain accidents that occur without evident signs of such failures. This study investigates the micro-scale migration and structural reorganization of soil particles induced by groundwater seepage, using the foundation pit project of Nantong Metro as a case study. A combination of physical model testing and discrete element method (DEM) simulations is employed to analyze the underlying mechanisms. The results indicate that soil settlement resulting from pore water pressure dissipation due to groundwater level fluctuations is significantly smaller than the differential settlement caused by seepage forces. The formation of a "sand-clay" dual structure in the soil is attributed to the combined influence of marine and fluvial sedimentation processes. The loss of clay particles induces compression of the sand skeleton, which constitutes the primary mechanism responsible for macroscopic soil mass settlement. Specifically, localized leakage at weak zones of the waterproof curtain can trigger fine particle loss and progressive weakening of the silt layer structure, leading to uneven settlement, lateral displacement, or even instability of the retaining system—posing significant risks to foundation pit safety.