Tensile and shear performance of new-to-old concrete interface in reinforced double-layer linings of subsea shield tunnels

The interfacial mechanical behavior of double-layer lining concrete in shield tunnels is a critical factor that ensures the synergistic load-bearing performance of the overall structure. In this research, laboratory tests, including shear and axial tensile tests, were conducted on new-to-old concrete interfaces under diverse working conditions. Taking into account the effects of interface treatment approaches, anchor bar parameters, and normal compressive stress, the variation rules for interfacial shear strength, tensile strength, and bond stiffness were quantified, and the core control factors affecting interfacial mechanical performance were identified. Afterwards, a refined numerical model targeting new-to-old concrete was built in ABAQUS to explore the damage evolution law and mechanical response features at the contact interface. Eventually, relying on the engineering background of Jintang Highway Subsea Shield Tunnel, the bearing performance of the double-layer superimposed lining under eccentric compression was analyzed, with emphasis on the impacts of interfacial mechanical parameters on the collaborative working efficiency of the lining structure. The findings demonstrate that the mechanical characteristics of new-to-old concrete interfaces are mainly controlled by interface treatment schemes, anchor bar parameters, and normal pressure. The simulated shear strength falls within the range of 2.804–5.197 MPa, and the tensile strength ranges from 1.543 MPa to 1.918 MPa, both of which are lower than the corresponding strength indexes of C40 concrete. The flexural bearing capacity of the double-layer superimposed lining is remarkably superior to that of the single-layer segment structure, and presents a positive correlation with the interface deterioration coefficient (λ). Under high axial compression, reinforcing the interfacial mechanical properties can prominently elevate the ultimate structural capacity of the tunnel lining; conversely, the lifting effect of interface performance optimization is relatively limited under low axial compression. Accordingly, the layout of anchor bars can be rationally optimized while ensuring interface roughness and the construction quality of the secondary lining.