Hybrid arch-forepoling synergy for construction-stage deformation control in shallow swelling soft-rock tunnels under asymmetric loading

Shallow tunnels in swelling soft rock under asymmetric loading experience severe deformation from coupled swelling-squeezing mechanisms. Swelling pressure ranges from 0.2 to 0.8 MPa, while rock strength remains low (c = 100–200 kPa and φ = 20–25∘). Conventional single-type supports inadequately control progressive deformation due to time-dependent ground-support interaction. This study develops a staged ground-support interaction framework that coordinates hybrid steel-lattice arches, dense-array pipe roofs and multi-stage chemical grouting. Different from conventional combined support schemes that mainly assemble individual measures, the proposed framework assigns distinct spatial, stiffness, and temporal functions to each component: early confinement by steel arches, delayed composite action by lattice girders and hardening shotcrete, and advance pre-confinement by densified forepoling and controlled grouting. Three-dimensional finite element analyses incorporating time-dependent shotcrete hardening were conducted to evaluate system performance. The hybrid arch configuration (I25b/Φ28, 0.6 m spacing) reduces deformation by 10–22% and increases the axial–bending safety index from 1.08 to 1.35. Dense-array forepoling (20–25 cm circumferential spacing) achieves 15% settlement reduction and 58% shotcrete stress mitigation. Field validation at Guantian Tunnel through 15 monitoring sections confirms 30–40% deformation reduction, 70–80% crack mitigation and prevention of three potential collapses with 15% cost savings. The framework demonstrates robust applicability to Grade V-VI swelling ground under asymmetric loading, with successful adoption in 11 additional projects.Highlights•Proposes a construction-stage deformation control scheme for shallow swelling soft-rock tunnels under asymmetric loading.•Develops hybrid arch sets to couple early confinement with long-term composite action.•Applies densified fore poling for enhanced advance confinement and reduced crown settlement/lining stress.•Quantifies arch–fore poling synergy via 3D analyses with time-dependent shotcrete hardening.•Delivers practical parameter ranges and implementation guidance for Grade V–VI swelling ground.