Seepage of a deep-buried tunnel in large-scale fault zones: Effects of fault heterogeneity and excavation process

Tunnel excavation through large-scale fault zones poses significant risks of water inrush; however, the evolution of unsteady seepage under the coupled effects of progressive excavation and structural heterogeneity (e.g., internal sub-layers) remains insufficiently understood. This study investigates a tunnel section intersecting a major fault zone in the Yangtze-to-Han River Water Diversion Project. A numerical modeling framework integrating the finite element method with the Parabolic Variational Inequality (PVI) approach is developed to simulate transient seepage, explicitly incorporating progressive excavation sequences and seepage control measures according to regional hydrogeological conditions and engineering design specifications. Three-dimensional transient simulations are conducted to characterize groundwater flow evolution and evaluate seepage stability throughout excavation. The results show that excavation alters seepage boundaries and significantly increases hydraulic gradients near the tunnel. As tunneling approaches the fault zone, groundwater disturbance intensifies. The combined application of pre-grouting and advanced drainage effectively reduces the peak tunnel inflow from 26,300 m 3 /d to 11,397 m 3 /d (a 56.7% decrease) and decreases the maximum hydraulic gradient from 59.8, initially concentrated within the fault gouge sub-layers and near the tunnel face, to 13 (a 78.2% reduction). Sensitivity analyses further reveal that tunnel inflow increases with the permeability of the grouting zone and with the number of drainage holes, whereas hydraulic gradients decrease correspondingly. These findings highlight the importance of explicitly incorporating internal fault-zone sub-layer structures into transient seepage modeling to accurately identify excavation stages most susceptible to seepage instability. Such consideration enhances hydrogeological risk assessment for deep-buried tunnels crossing large-scale faults with complex internal structures.