Experimental and Numerical Optimization of Prestressed Anchor Cable Support for In-Situ Large-Span Tunnel Expansion with an Energy Balance Framework

In-situ enlargement of super-large-span tunnels can intensify excavation-induced unloading in the surrounding rock, increasing deformation demand and failure risk during construction. This study combines laboratory model tests with FLAC3D simulations to evaluate the stabilizing role of prestressed anchor cables and to establish an energy-balance framework for support optimization. Comparative model tests of existing and enlarged tunnel sections, with and without anchors, show that reinforcement increases load-carrying capacity, reduces displacement, and confines damage to more localized zones. The numerical simulations reproduce displacement fields, shear-strain localization, and plastic-zone evolution with good agreement against the experimental observations. The energy framework is implemented in the in-situ simulations by quantifying unloading-related energy release in the rock mass and reinforcement work contributed by the anchors, and by introducing an energy release–reinforcement ratio as a stability indicator. Parametric analyses indicate that anchor length, spacing, and prestress influence stability in a nonlinear manner, with diminishing returns once reinforcement extends beyond the mechanically dominant deformation zone. An efficient parameter window is identified that improves deformation and yielding control while avoiding unnecessary reinforcement. The results provide an energy-consistent and design-oriented basis for prestressed anchorage selection in large-span tunnel expansion.