Study on Evolution Laws of Lining Mechanical Behavior in Mountain Tunnels Under Heavy Rainfall Conditions

This study reveals the evolution of the mechanical behavior of tunnel lining under the influence of heavy rainfall through field monitoring and coupled fluid-solid numerical simulations. Field monitoring shows that after 14 h of rainfall, the maximum vertical tensile stress increment at monitoring point 2 reached 0.55 MPa. The simulation results indicate that when the rainfall intensity is 1.11 × 10−5 m/s and the rainfall duration lasts for 36 h, the principal stress increment at the sidewall monitoring point is 2.34 MPa (exceeding the tensile strength of C30 concrete of 1.43 MPa). Based on these findings, the suggested threshold for rainfall-induced checks is a single-day rainfall of ≥80 mm or continuous rainfall of ≥10 h. It is recommended to monitor once every 3 days during normal conditions and once every 2 h during heavy rainfall. When the permeability coefficient of the loosened zones increases from 2.05 × 10−6 m/s to 6.48 × 10−5 m/s, the principal stress at the sidewall decreases by 41%. It is suggested to reduce the blind drainage spacing on the sidewalls to 3–4 m. The model reproduces the observed stress increment within a 10% error margin. These results may provide a valuable reference for tunnel design, monitoring, and reinforcement in regions prone to heavy rainfall.