Spatio-temporal evolution analysis of geological parameters and land subsidence prevention and control in coastal soft soil megacities

The soft soils in coastal megacities are under dual control of depositional environments and engineering disturbances. Long-term construction activities may potentially trigger the temporal evolution of their physical and mechanical parameters, thereby influencing the land subsidence process. To explore the evolution characteristics of soft soil parameters under different construction histories and intensities, and their impacts on land subsidence, this study introduces the concepts of "narrowly defined built-up areas" and "newly built areas", taking Shanghai City as a case study and selecting the Lujiazui area (built-up area) and the Lingang New Area (newly built area) as research subjects. The LSTM-AM model and Mann-Kendall test are employed to analyze the temporal evolution characteristics of soft soil parameters in the two regions. The results show that, in the Lujiazui built-up area, soft soils exhibit trends of densification, increased stiffness, and reduced compressibility under multiple engineering disturbances, while the soft soils in the Lingang New Area primarily exhibit slow changes due to natural consolidation. Correlation analysis further reveals that the relationships between void ratio and compression coefficient/compression modulus in the built-up and newly built areas show different evolutionary characteristics, indicating that under the same additional load the soft soils in the built-up area are less sensitive to subsidence, while those in the newly built area are more sensitive. Based on these findings, the study suggests incorporating the distinction between built-up and newly built areas into land subsidence zoning control and proposes dynamic threshold updates based on the soft soil parameters-subsidence relationship. The research findings can provide technical references for differentiated land subsidence management in coastal soft soil megacities.