Experimental and numerical research on the tensile performance of a novel three-layer ring spring flexible joint for shield tunnels

In recent years, prefabricated segmental shield tunnels have been increasingly adopted for their construction efficiency. However, in zones with abrupt geological changes, shield tunnel joints are prone to tensile failure under longitudinal ground deformation. In this context, flexible joints have been proposed to enhance deformation accommodation by reducing joint stiffness. This study proposes a novel tunnel flexible joint based on a three-layer ring spring (TRS). The TRS can be implemented without altering the original segment design, provides additional energy-dissipation capacity, and incorporates a displacement-limiting mechanism to prevent waterproofing failure caused by excessive deformation in conventional flexible joints. A design method for the TRS flexible joint is established. Full-scale local axial tensile tests on tunnel segments and finite-element simulations are then conducted to compare the mechanical performance and failure processes of TRS flexible joint specimens and ordinary bolt joint specimens. The results show that the TRS reduces joint stiffness and provides favourable energy-dissipation and self-centering characteristics. Once the joint deformation reaches the design target, the TRS transitions from a flexible state to a rigid-body state, thereby restraining further deformation and preventing the joint opening within the waterproofing limit. In addition, the TRS markedly reduces strain in the segment handhole region and effectively alleviates tensile damage. Under failure loading, segments equipped with the TRS enter the plastic state and failure stage at higher load levels, indicating an improved safety factor. Overall, this study proposes a novel solution for tunnel protection and elucidates its working mechanism and performance advantages.