Numerical Analysis of Adjacent Metro Tunnel Deformation Induced by Deep Excavation in Soft Soil

Deep excavations in soft soil present a significant threat to adjacent operational metro tunnels. Based on an actual engineering project, field displacement monitoring and analysis were first conducted to establish a validated three-dimensional FLAC3D numerical model. This calibrated model is adopted to carry out systematic single-factor sensitivity analysis encompassing three core design indicators, and further quantifies the influences of groundwater seepage and uncertainties in soil parameters, thereby filling the research gap of integrated multi-index quantitative control criteria for pit–tunnel projects in Nanjing soft soil strata. The sensitivity analysis clarifies tunnel deformation responses concerning pit–tunnel clearance, the number of internal struts, and the retaining pile embedment ratio. Results indicate that tunnel deformation exhibits a ‘middle-large, ends-small’ pattern, with an optimal safe clearance of 8 m, a minimum requirement of three inner struts, and a critical embedment ratio of 0.34. Groundwater seepage increased tunnel deformation by 13.84% horizontally and 11.64% vertically; however, all critical thresholds remained valid. The proposed thresholds also demonstrated robustness under ±20% variation in key soil parameters. These quantitative findings are derived from Nanjing soft soil conditions and provide localized design references for similar metro-adjacent excavations in the region.