Analysis and treatment of invert uplift in a long span tunnel in weak mudstone

To address the invert heave issue in large-span tunnels traversing fully weathered, water-rich cretaceous mudstone in western China's complex geological zones, the asymmetric invert heave mechanism was investigated and corresponding treatment measures were proposed using the reservoir-underpass section of the Shaofang Yakou Tunnel as a case study. A combined approach of field monitoring, geological investigation, and finite element numerical simulation was employed. The results indicate that the tunnel's flat structural geometry induces stress concentration at the arch foot. The bearing capacity of the fully weathered mudstone at the tunnel base decreases to 50 kPa upon water-induced softening, compounded by a hydraulic head pressure of 84 m, leading to a maximum left invert heave of 174 mm. An integrated treatment strategy of "drainage holes for pressure relief + grouted steel pipe reinforcement + reinforced concrete connecting beams" was developed to mitigate this. Post-implementation monitoring confirmed stabilized tunnel deformation, with residual heave reduced to less than 3 mm. The proposed technology effectively suppresses floor heave progression and offers a reference for analogous water-rich soft rock tunnel projects.