Numerical analysis and optimization of the impact of complex foundation pit construction on the deformation of adjacent subway structures

The expansion of urban underground infrastructure frequently necessitates deep foundation pit construction adjacent to operating subway tunnels, posing significant risks to structural serviceability due to excavation-induced deformation. However, the coupled influence of internal structure construction (e.g., culverts) and excavation sequencing on tunnel response has not yet been systematically investigated under complex coupled construction conditions. This study aims to identify the optimal construction sequence to minimize deformation in adjacent subway structures under complex spatial constraints. A high-fidelity three-dimensional finite element model was established using MIDAS GTS NX software. The Mohr-Coulomb constitutive model was employed to simulate the elastoplastic behavior of the soil, with material parameters calibrated based on detailed site investigation data from the Fengtai Railway Station project. Ten representative scenarios, integrating different culvert construction directions (forward/reverse) with various excavation sequences (e.g., unidirectional, bilateral, staggered), were systematically simulated. Quantitative comparative analysis reveals that Scenario 3 is the optimal strategy, resulting in a minimum maximum tunnel settlement of 1.875 mm. In contrast, the most unfavorable strategy (Scenario 7) induced a settlement of 1.961 mm. Adopting the optimal sequence achieved a 4.4% reduction in maximum settlement compared to the worst-case scenario. A limitation of this study lies in the use of the Mohr-Coulomb model, which may not fully capture the complex small-strain stiffness degradation of soils compared to more advanced constitutive models. Nevertheless, these findings provide a scientific basis and quantifiable criteria for optimizing construction sequences in similar complex geotechnical projects.