Stability Evaluation of a Hydraulic Tunnel Subjected to Partial Lining Removal and Variable Cross‐Section Excavation: Implication From Numerical Modelling

ABSTRACT Tunnel expansion in hydraulic projects often involves partial removal of existing linings and construction of variable cross‐section tunnels, which induces significant stress redistribution and may compromise the stability of surrounding rock masses. This study investigates the variable cross‐section tunnel of a hydropower station, where a partial lining demolition and staged excavation approach was adopted in a geologically complex and water‐bearing environment. A refined three‐dimensional numerical model was developed to simulate the coupled excavation‐support process, incorporating structural joints, water pressure, and support elements including sprayed concrete, steel arches, and rockbolts. And the coupling support scheme was used to reinforce the variable cross‐section tunnels, and the reinforcement effect was monitored in situ. The effectiveness of preliminary support and reinforcement measures was also assessed. The simulation results reveal that partial lining demolition induces localized tensile stress (3 MPa), tunnel face displacement (145 mm), and deep plastic zone development (12 m). The implementation of a prestressed anchor reinforcement system significantly constrained deformation, improved the safety factor from 1.88 to over 2.3, and redistributed loads within the remaining lining. Field monitoring data, including axial anchor forces and convergence, validated the numerical predictions. The findings offer practical guidance for support design and safety control in complex tunnel expansions under high water pressure and poor geological conditions.