More complex geological conditions could be encountered with the construction of urban subway projects. At present, many subway tunnels have been built in composite strata with upper soft and lower hard layers, but the presence of a cavity in the strata increases the risk of collapse during construction. In this paper, a series of model experiments and discrete element methods were conducted to investigate the failure behavior of composite strata with a cavity caused by tunnel excavation disturbance. The influence of the distance between the cavity and vault (h_d) and the distance between the soil–rock interface and vault (h_r) on the collapse of the composite strata are analyzed. The research results indicate that tunnel collapse exhibits progressive failure because of the forming of a collapsed arch in the strata. If the h_d is greater than the tunnel span (D), the arch can be stabilized without other disturbances. Additionally, the thickness of the tunnel rock layer affects the height of the collapsed arch significantly, as it is difficult to form a stable arch when the h_r is less than 2/3 D. Finally, reasonable construction safety distances are proposed based on the possibility of forming a stable arch collapse in the tunnel and determining the range of the collapse.

Operational tunnels are generally accompanied by time-dependent deformation and structural failures due to delayed behaviors, e.g., loading effects from surrounding rock and degradation of the concrete lining. This paper presents an analytical approach to investigate the long-term stability of tunnels considering those delayed behaviors. To quantitatively characterize the degradation process of concrete lining, specific degradation models are adopted according to the identified obstacles in service environments. The viscoelastic Burgers model is selected to recognize the long-term creep properties of the surrounding rock. The time-varying solutions for tunnel deformation and lining stress can be obtained using the displacement compatibility condition between the concrete lining and the surrounding rock. The results find that the long-term stability of tunnels is governed by the interaction between the concrete lining and the surrounding rock. Different degradation models and rates significantly influence mechanical response, with thinner linings showing greater susceptibility. Viscoelastic rock properties further affect system behavior. The amplified effect of degradation under long-term rock loading underscores the necessity of understanding these coupled mechanisms for accurate life predictions. On account of the findings, a long-term performance maintenance method for operation tunnels is proposed and illustrated by a rehabilitation project for tunnel damage. Remediation of structural damage in operation tunnels should consider the surrounding rock condition and support structure performance, significantly improving long-term safety and reducing remediation costs. Overall, the present work provides some insight into the long-term stability assessment of operation tunnels.