Numerical Study on Large Deformation Characteristics of Tunnels Excavated in Strain-Softening Time-Dependent Rock Masses

Large deformation is a kind of geological hazard in the construction of soft rock tunnels, which hinders construction, threatens workers’ safety, and raises project costs. Accurately obtaining the large deformation characteristics and patterns of tunnels is the prerequisite for taking targeted support measures. First, a viscoelastic–plastic model that can simulate both the short-term strain-softening effect and the long-term creep effect was proposed to investigate the large deformation features of soft rock tunnels. Then, a sophisticated model for creep parameter inversion was developed using the support vector machine, genetic algorithm, and particle swarm optimization. Finally, the deformation, stress, plastic zone, and internal forces in the lining of a large deformation tunnel were determined using the proposed constitutive model and the creep parameters obtained by inversion. As the creep displacement only makes up 14.4–23.2% of the total displacement, the results demonstrate that the elastic–plastic displacement is much more than the creep displacement. Notably, the connection between the top and middle benches has the most pronounced horizontal movement, accompanied by a significant strain-softening effect, which ultimately becomes the weak point in the support system. The excavation disturbs the surrounding rock, causing a high-stress zone and a low-stress zone, with the interface located around the junction of the elastic and plastic zones. The bending moment is positive at the wall waist and negative at the vault and arch waist. In addition, there is a progressive rise in the bending moment from negative to positive at the arch foot.