Numerical Simulation and Support Scheme Optimization of Thin-roof Tunnel Considering Blast-induced Damage

During tunnel excavation, blasting dynamics cause substantial damage to surrounding rock mass in thin bedrock roof conditions, while roof thickness progressively increases along the advancing direction. Current support schemes lack differentiated designs to accommodate varying roof thicknesses across tunnel sections. Focused on the Qingdao Tangshan Road Tunnel, this study defined a damage variable through dominant frequency attenuation analysis during blasting excavation of the surrounding rock mass. A quantitative relationship between blasting damage and the distance to the blast center was established to assess the severity of roof rock damage. Using 3DEC discrete element software, a three-dimensional numerical simulation of the continuous blasting excavation process for 9 pilot tunnels and 5 sections was conducted, with rock mass mechanical parameters adjusted to generate critical data for support system optimization. The research findings indicate that arch crown excavation in sections 1 and 2 with thinner roof thickness resulted in a 10 mm increase in arch settlement values. These sections remained significantly affected by subsequent excavation activities, ultimately exhibiting final arch settlements exceeding 60 mm. The arch crown anchor cables demonstrate effective anchoring performance, successfully controlling surrounding rock settlement. After reinforcement replacement in sections 1 and 2, arch crown settlement reduced by approximately 10 mm. All support schemes exhibit abrupt changes in curvature in settlement curves at the critical roof thickness of 7.4 m, which is identified as the tunnel safety threshold. Numerical simulations guided optimized support parameters, yielding a combined bolt-anchor cable system for thin roof sections and single-layer initial support for thick roof sections. Both simulation and field monitoring confirm stabilized arch crown settlements at 45 mm across varying roof thicknesses, validating the scheme′s effectiveness in maintaining tunnel stability.