Mechanism and prevention of “Closed Door” collapse in tunnel construction: A case study

Abstract

The prevention of “closed door” collapse is crucial for tunnel construction safety and progress assurance. To address the challenge of preventing “closed door” collapse, a study was conducted based on a case of tunnel collapse. This tunnel passes through sections of fractured rock with groundwater development. The main research methods are on-site monitoring and mechanical model calculations. The research findings indicate that weak fractured rock, groundwater, initial support quality issues, long trailing distances, and extensive excavation footage all contributed to the “closed door” collapse. On-site monitoring in the fractured rock section showed the tunnel deformation is high sensitivity to construction disturbances. Even without disturbances, tunnel deformation gradually increases. This contributes about 20% of the total deformation, worsening surrounding rock damage and loosening. Additionally, the initial support exhibits cantilever arch mechanical response characteristics due to excavation of the surrounding rock on one side of its lower part. The mechanical model calculation has verified that under a cantilever arch state, the initial support lacks the bearing capacity to withstand loose load. The continuous deformation of the surrounding rock damages the collapsed body, generating loose load. Excavation of the surrounding rock beneath the initial support arch foot changed the structure from a fixed arch to a cantilever arch, ultimately leading to the joint failure of the surrounding rock-support structure system. This is the mechanism behind the collapse. By combining the convergence confinement method, the influence mechanism of factors such as excavation footage and trailing distance on the “closed door” collapse was further elucidated. Finally, according to the stability evaluation of the collapsed cavity, a strategy of “supporting first and then grouting” is adopted for collapse treatment. Drawing from the understanding of the “closed door” collapse mechanism, the following principles for collapse prevention are proposed: “small excavation footage, ensuring the quality of initial support, short trailing distance, quick support, frequent measurement.”