Study of Surface Dewatering Effects and Deformation Control of Tunnels in Water-Rich Completely Weathered Granite Strata

Tunnel construction in completely weathered granite strata with rich water storage is potentially risky and can lead to catastrophic hazards without proper engineering measures. This study investigates the mechanisms of key surface dewatering parameters—well spacing, dewatering depth, and well–tunnel distance—on the groundwater-level field, stress field, and displacement field, as well as their control efficacy with respect to tunneling-induced deformation, based on three-dimensional finite element analysis and field monitoring. The results demonstrate that reducing well spacing (from 22 m to 7 m) promotes a progressively continuous and gentle drawdown funnel. The maximum surface settlement induced by dewatering to 20 m below the ground surface increases from 3.5 cm to 5.0 cm when the well spacing changes from 22 m to 7 m, while excavation-caused deformation decreases and the surface settlement trough evolves from discrete “crest-shaped” patterns to a continuous “U-shaped” profile, accompanied by an increase in total settlement. Increased dewatering depth enhances the effective stress in the soil but also significantly expands the extent of consolidation settlement and shear strain. The well–tunnel distance critically influences the effectiveness of dewatering in controlling the tunnel-adjacent zone. Surface dewatering, through a synergistic “pressure relief–consolidation” mechanism, systematically improves the geotechnical properties of strata by both reducing hydrostatic loads and enhancing soil resistance. Based on derivations from discrete simulated cases, a recommended dewatering design is proposed for similar water-rich weak-stratum tunnel projects, with well spacings of 11 m to 15 m and dewatering depths extending to approximately the tunnel invert level. These recommendations provide a reference for dewatering design and construction control in related engineering contexts comparable to the present case study.