Influence of Seepage Control Measures During Deep-Burial Tunnel Construction on External Water Pressure on Tunnel Lining

[Objective] The Xianglushan Tunnel is a challenging and key control project of the Central Yunnan Water Diversion Project. Branch Tunnel No.7 of the Xianglushan Tunnel, located in Songgui Town, Heqing County, Dali Prefecture, Yunnan Province, generally lies at a depth of 600-1 300 m, with a maximum burial depth of 1 415 m. High external water pressure is regarded as a major threat to its safety. Existing studies on tunnel external water pressure have mostly assumed homogeneous strata, paying relatively little attention to geological structures. The influence of geological structures—particularly the presence of aquitard layers above the tunnel—remains to be investigated in depth.[Methods] A typical deep-burial section of the Central Yunnan Water Diversion Project was investigated through field observations and numerical simulation to study the external water pressure acting on the tunnel lining. The effects of geological structures and seepage control measures on the external water pressure were analyzed to provide a reference for the design and construction of deep-burial tunnels.[Results] During in-situ drilling, groundwater in the tunnel was identified as fissure groundwater. Obvious water inflow occurred if a borehole intersected water-conducting fissures; otherwise, the boreholes remained essentially dry. Monitoring data from five piezometers installed in the tunnel over nearly one year indicated that the external water pressure around the unlined tunnel during construction was relatively low, only several meters of water head. Numerical simulation of the seepage field revealed that higher rock permeability and shorter distance between the tunnel and water-conducting structures increased both external water pressure and seepage discharge. Impermeable linings, while blocking water, caused an increase in external water pressure. Drainage holes, while reducing external water pressure, resulted in an increase in tunnel seepage discharge. Under specific geological structures and seepage control measures, tunnel excavation and drainage may only cause local groundwater drawdown around the tunnel, without affecting the regional phreatic surface.[Conclusion] In the model of this study, an aquitard layer with relatively low permeability exists above the tunnel, which limits the influence range of tunnel drainage. As a result, drainage only forms a localized desaturation zone between the tunnel and the aquitard, exerting minimal effect on groundwater above the aquitard. This localized desaturation explains the phenomenon observed in tunnel projects in water-rich areas, where the regional phreatic surface is high while the external water pressure acting on the tunnel remains relatively low. Near the tunnel face, equipotential lines are densely spaced, and the hydraulic gradient is relatively large, whereas a smaller gradient prevails behind the face. This indicates that greater seepage pressure is imposed near the tunnel face, explaining why seepage-induced failures frequently occur in this area.