The long-term safety of a deeply buried soft rock tunnel lining under inside-to-outside seepage conditions

For deeply-buried long tunnels, which are constructed in environments with high stress and soft rock, stability is definitely an important engineering problem. In this study, the buried depth of the chlorite schist section of a headrace tunnel in the Jinping II Hydropower Station was between 1550 and 1850 m. In this project, rheological problem, the wetting-induced softening problem and inside-to-outside seepage problem were all prominent, which posed serious threats to the long-term stability of the tunnel lining. This study detailed inspected the engineering geological conditions of the chlorite schist sections, the extrusion deformations of the surrounding rock mass following the excavation, and the surrounding rock support and reinforcement after the expansion excavations. In addition, the characteristics of rheological mechanical and wetting-induced softening for the chlorite schist were determined through laboratory testing and field monitoring. The rheological mechanical characteristics of the chlorite schist were not obvious when the stress was low. However, they were quite obvious when the stress was high. Based on the results, the rheological mechanical behaviors of the chlorite schist were described by a visco-elastoplastic rheological model (CVISC). Then, this study verified the rationality of the existing reinforcement scheme and obtained the final deformation stability time of the surrounding rock through a numerical simulation of the support reinforcement and the secondary lining during the operational period. Furthermore, the safety of the lining structure during the operational period was evaluated. These results may potentially play an important role in the guidance of future engineering designs and construction and may potentially be used as a reference for the support designs of similar deeply buried soft rock projects.