Response Characteristics and Adaptability Analysis of the Benching Method in Grade IV and V Surrounding Rocks in Sandstone–Mudstone Strata

To improve construction efficiency for large-section tunnels in sandstone–mudstone strata, this study investigates the applicability of the two-bench method and the three-bench method for grade IV and grade V surrounding rock, respectively. Based on FLAC3D, numerical simulations of excavation and support for the two benching methods were conducted to analyze deformation responses, including ground settlement, crown settlement, haunch convergence, floor uplift, and face extrusion. The simulation results were then compared and validated against field monitoring data to evaluate the applicability and feasibility of the construction methods. The results show that, for grade IV surrounding rock excavated using the two-bench method, crown settlement, floor uplift, and horizontal convergence converge and stabilize on days 17, 15, and 25, with stable values of 14.0 mm, 10.3 mm, and 13.2 mm, respectively. For grade V surrounding rock excavated using the three-bench method, these indices stabilize on days 24, 22, and 32, with stable values of 26.3 mm, 20.3 mm, and 20.8 mm, respectively. The surrounding rock pressures at the crown and spandrel gradually attenuate after excavation and stabilize at 1–4 MPa after approximately 20–26 days, whereas stress release at the haunch lasts longer and the stabilized stress level remains higher. Meanwhile, the anchor bolt axial force at the haunch is significantly greater than that at the spandrel, indicating that the haunch is a critical zone for support load-bearing and deformation control. The benching method can effectively control surrounding rock deformation under grade IV and V surrounding rock conditions in sandstone–mudstone strata; however, in engineering practice, the haunch should be treated as a key monitoring target, and targeted support and reinforcement measures should be implemented.