Stress and deformation behaviour of surrounding rock under the five-step single-side drift method in large-section loess tunnels

Large-span loess tunnels present significant challenges due to wide excavation spans and the high deformability of loess, making excavation method selection critical for deformation control. Taking the Fenghuangshan Tunnel in Lanzhou as a case study, this study combines laboratory model tests and numerical simulations to examine seven scenarios under three excavation methods: the optimised five-step single-side drift method (FSSDM), the double-side drift method (DSDM), and the traditional single-side drift method (SDM). Results show that surrounding rock stress increases rapidly during excavation but decreases and stabilises after support installation, accompanied by the formation of longitudinal soil arches. Stress relief is mainly concentrated within 0.4 m of the tunnel boundary. Crown settlement follows a “slow growth–acceleration–stabilisation” pattern, while surface settlement is symmetrically distributed with a maximum at the tunnel centerline. The left arch haunch convergence exhibits an “increase–decrease–stabilise” trend. Among all scenarios, Case 6 provides the most effective deformation control. Field application confirms that the FSSDM improves both construction efficiency and deformation control, demonstrating its practical engineering value.