Structural damage mechanism and treatment of a shield tunnel under asymmetric unloading

Investigating the damage behaviors of a shield tunnel response to a nearby excavation is of great significance for structural safety assessment and protection. In this study, the self-developed scaling model tests for shield tunnels under asymmetric unloading were performed, to reveal the structural damage evolution process and assess the effectiveness of steel plate reinforcement. An elaborate finite element model of a shield tunnel was proposed and verified by the model test. Using this validated model, the differences in damage behaviors and reinforcement effects of shield tunnels under different unloading modes were investigated. The results show that the damage process of a tunnel affected by asymmetric unloading can be roughly divided into three stages: elastic stage, damage evolution stage, and severely damaged stage. Based on the control effect on segment deformation, internal force, and concrete cracking of the lining under asymmetric unloading, the recommended reinforcing time is the transverse deformation between 7.4%0 and 13.0%0 of the outer diameter of the segmental ring. The structural bearing capacity loss of a tunnel under asymmetric unloading is mainly caused by segment cracking, whereas under symmetric unloading, it is induced by the formation of plastic hinges. In the case of symmetric unloading, it is recommended to implement steel plate reinforcement before the formation of the initial plastic hinge, resulting in the upper limit of the transverse deformation before reinforcement being set as 10.0%0 of the outer diameter of the segmental ring.