Ground Settlement Analysis of Twin Shield Tunnels Under River-Crossing Conditions with Fluid–Solid Coupling

Twin shield tunneling in river-crossing soft soils faces increased risks like face instability, largely due to unclear mechanisms linking water level fluctuations to ground settlement. To address this issue, a 3D fluid–solid-coupled FLAC3D model is developed based on the Zhanqiaogang section of Hangzhou Metro Line 15. Specifically, the model simulates four hydrological conditions: low water, normal water level, high water level, and flood level. It examines their effects on ground settlement, stress distribution, and pore water pressure during twin-tunnel excavation. The results indicate that the maximum ground surface settlement on the left alignment under flood-level conditions increased by 41.58% compared with that under normal water levels. On the right alignment, surface settlement increased from 6.54 mm under normal water levels to 8.54 mm under flood conditions, representing a 30.6% increase. A sensitivity analysis is also conducted on pore water pressure, soil internal friction angle, and support stiffness. Results show that ground settlement increases with rising river levels, mainly due to elevated pore pressure reducing soil strength. Sensitivity analysis confirms pore pressure as the key factor influencing deformation. Numerical results align well with field data, highlighting the critical role of hydraulic boundaries. Based on these results, control measures are proposed to provide practical guidance for mitigating settlement in similar river-crossing tunnel projects.