Deformation Control and Structural Performance of a Double- Sidewall Pilot Tunnelling Method with Reserved Rock Walls: A Case Study of a Large-Span Tunnel in Grade V Weak Rock

The conventional double-sidewall pilot tunnelling method (CDWM) has been widely applied in the construction of large-span tunnels. However, when applied to shallow-buried tunnels in weak surrounding rock, the method often suffers from excessive deformation and complex support conversion. To address these limitations, this study proposes an optimized excavation scheme, namely the double-sidewall pilot tunnelling method with reserved rock walls (DRWM). The Yangjiashan Tunnel in Zhejiang Province, China, was selected as the engineering case for investigation. Field monitoring data and numerical simulations were integrated to evaluate the deformation behaviour and structural response of the tunnel during excavation. The results indicate that DRWM significantly improves deformation control compared with the conventional CDWM. The maximum crown settlement and horizontal convergence were effectively reduced, and stress concentration in the initial support structure was mitigated. Furthermore, a sensitivity analysis was conducted to investigate the influence of the lateral distance between the temporary support and the reserved rock wall. Within the investigated single-parameter analysis, a spacing of 0.4–0.5 m showed a relatively balanced response in terms of crown settlement, horizontal convergence, support stress, and construction operability. The findings demonstrate that, under the investigated Grade V weak rock conditions, the DRWM showed improved deformation control compared with the CDWM in the numerical comparison, highlighting its potential applicability for optimization in comparable engineering settings.