Mechanical behavior and failure mechanisms of shield tunnel linings reinforced with lightweight epoxy bonded-bolted steel plates

The epoxy bonded-bolted steel plate reinforcement technique has been extensively employed to enhance the stiffness and load-bearing capacity of deteriorated shield tunnel linings. However, traditional reinforcement structures suffer from high construction complexity, large component size, and inefficient material utilization issues, limiting their widespread application, especially in constrained tunnel internal space. Moreover, prior investigations rarely integrate full-scale prototype testing with elaborate numerical analysis to comprehensively reveal the underlying failure mechanisms of reinforced structures. To address these challenges, this study proposes an improved lightweight epoxy bonded-bolted steel plate reinforcement structure by optimizing the steel plate dimensions, steel plate segmentation, and chemical anchor configurations, achieving a synergistic improvement in structural performance, constructability, and economic efficiency. To validate the effectiveness of the proposed reinforcement method, full-scale prototype tests are first conducted, revealing the two-stage mechanical behavior of the structure (before and after reinforcement). Thereafter, a detailed three-dimensional nonlinear finite element model incorporating joint-level details and reinforcement loading processes is developed, enabling in-depth analysis of the load transfer mechanisms and failure processes of the reinforced structure. Parametric analysis is conducted to identify the influence of various reinforcement component parameters on the structural mechanical performance and to determine the key influencing factors. Experimental and numerical results indicate that the lightweight reinforcement improves the ultimate load-bearing capacity of the damaged lining by 21%, exhibiting favorable ductile failure characteristics. The steel plates inhibit further damage and enhance the overall structural stiffness by changing the internal load transfer mechanisms. Failure of the epoxy adhesive interface initiates at the longitudinal joints at the shoulders and toe regions of the tunnel, where shear stress concentrations accelerate the damage evolution process. Throughout the loading process, the interface stresses maintain a strong correlation with either the axial stresses in the steel plates or the rate of their change with respect to the circumferential angle. The chemical anchors contribute to delaying ultimate failure by converting local load-bearing mechanisms and facilitating the redistribution of internal forces within the tunnel lining. Regarding the overall structural failure mechanism, although multiple generalized plastic hinges are observed during the deformation process, the decisive failure is ultimately governed by the loss of structural stability due to increased degrees of freedom associated with combined compression-bending-shear failure modes localized at two longitudinal joints. These findings not only offer a reliable basis for designing more efficient reinforcement schemes but also contribute to advancing preventive maintenance strategies for aging shield tunnels, enabling earlier intervention and improved lifecycle performance of subsurface infrastructures.