The Instability Mechanism of Shallow Tunnel Entrances in Silty Clay Strata and Corresponding Remediation Strategies

The stability of weak surrounding rock at shallow mountain tunnel entrances poses significant challenges. This study investigates the collapse mechanism of the silty-clay layer at the entrance of the Dajin No. 1 Tunnel using an integrated approach involving field investigation, laboratory tests, numerical simulation, and in situ monitoring. Results indicate that adverse geology combined with long-term rainfall triggers instability by significantly weakening the mechanical properties of the silty-clay. Numerical simulations of parameter degradation reveal that when the elastic modulus, cohesion, and internal friction angle decrease by 33.3%, 17.6%, and 33.1%, respectively, the steel arch stress exceeds the yield strength of Q235 steel, and vault settlement surges to 81.39 mm. A combined strategy of surface grouting and in-tunnel reinforcement was consequently implemented. Post-reinforcement monitoring recorded maximum values of 6.59 mm for vault settlement and 80.50 MPa for steel-arch stress, confirming the strategy's effectiveness. This approach provides a valuable reference for similar projects.