Investigating Dawanzi tunnel deformation induced by deep-seated gravitational slope deformation in Baihetan reservoir at Jinsha river

Mountain tunnels are usually vulnerable due to the existence of fault fracture zones and various types of slope instability. Existing research primarily focuses on tunnelling through fault zones and structural damage caused by fault movements. The research on the deformation of built tunnels caused by reservoir-induced slope deformation remains limited. This study aims to investigate the deformation mechanism of the Dawanzi tunnel after reservoir filling in the Baihetan hydropower station and analyze the impact of different mechanisms on the governance decision of the tunnel. The integrated methods, including the field investigation, airborne light detection and ranging (LiDAR) survey, and interferometry synthetic aperture radar (InSAR) observation, ensured the detailed interpretation of the geological, geomorphologic and surface deformation characteristics of the slope. A three-dimensional numerical model of a tunnel crossing a fault fracture zone was established to analyze the displacement and stress condition of the lining under reservoir filling. Results show that the fault fracture zone plays a vital role in tunnel damage. It provided favorable geological conditions for the deep-seated gravitational slope deformation (DSGSD) induced by reservoir filling. A local ancient landslide developed on the right front side of the DSGSD with a larger deformation magnitude. Several pieces of evidence, including geomorphic features, deformation characteristics, and drilling, indicate that the DSGSD caused the tunnel damage rather than the landslide movement. No matter the spatial position of lining damage and shear stress distribution, simulated results based on the geological model can well correspond to the actual situation, which verifies the correctness of the proposed tunnel deformation mechanism. The research result can provide helpful information on the supporting design and governance decisions of the Dawanzi tunnel.