Instability Mechanism of Shield Tunnel Face Induced by Seepage and Soil Softening in Water-Rich Silty Sand: Case Study of Jingu-Haihe Tunnel

The coupling mechanism involving high-pressure seepage and soil degradation regarding the face stability in water-rich silty sand environment remains to be comprehensively elucidated. This paper employs 3D fluid–solid coupling simulations to investigate these interactions taking the Jingu-Haihe Tunnel as a case study, and the dry and saturated hydraulic environments alongside three softening scenarios are set. Results indicate that hydro-mechanical coupling significantly compromises face stability, elevating the limit support pressure from 140 kPa in dry mechanical state to 231 kPa. The failure mechanism transitions from localized “horn-like” shear bands in dry states to global quasi-symmetric “bulb-like” visco-plastic diffusion in saturated seepage field scenarios. Softening effects cause stress-dependent stiffness degradation, increasing the deformation rate by 53.8% under low support pressure, and inducing uneven deformation where the crown displacement increases by 32.8 times, exceeding the 11.8-fold increase at the center as the support pressure drops from 600 kPa to 100 kPa. Moreover, the fluid–solid coupling effect amplifies the stratum’s sensitivity to shear strength parameters by up to 26 times at the face center compared to the dry condition. These findings may offer theoretical insights for optimizing support pressure determination in deep-buried saturated excavations.