Abstract: Fractured-vuggy formations are characterized by well-developed fractures and karst features, where the non-Darcy behavior of groundwater flow becomes significant under high hydraulic pressure. Multiple discontinuous media and complex flow regimes make accurate analysis of seepage stability in deep-buried diversion tunnels challenging. Based on the Forchheimer seepage equation, a hydraulic model coupling free flow and nonlinear seepage is established for deep-buried tunnels in fractured-vuggy formations containing karst conduits. The Navier-Stokes equations are solved to characterize the free flow patterns within karst caves, while the Beavers-Joseph-Saffman boundary condition is employed to facilitate fluid exchange between the karst caves and the surrounding media. The proposed seepage model is validated by comparison with the analytical solutions of the Beavers-Joseph model, the combined fracture-cave model, and the nonlinear seepage model. Using Section 5+880 of the No.2 diversion tunnel at Jinping Ⅱ Hydropower Station as a case study, a steady-state model of the diversion tunnel and its surrounding fractured-vuggy rock is developed. The evolution mechanisms of the tunnel stress-seepage field are revealed under various scenarios, including karst caves connected to fractures, cave penetration, and different karst distribution characteristics. The results indicate the following: (1) When a karst cave is connected to the lining via fractures, the external water pressure on the lining is primarily governed by the permeability of the connecting fractures. However, when the karst cave is connected to the lining through a karst conduit, the external water pressure on the lining reaches 1 259.3 kPa, even during the dry season. Therefore, grouting should be implemented to reduce the permeability of fractures in the surrounding rock, while sufficient lining permeability should be maintained to ensure stability. (2) The spatial distribution of the karst cave relative to the tunnel significantly influences the external water pressure on the lining. The external water pressure varies only slightly when the karst cave is located beside the tunnel sidewall or beneath the invert arch. However, when the karst cave is located above the tunnel crown, the external water pressure is significantly lower than in the previous two scenarios, with a maximum difference of 362.9 kPa.

Key words: fracture-vug network, nonlinear seepage, seepage stability, fractured rock mass, water-diversion tunnel; fluid-structure interaction model