Investigation of the mechanical properties of double lining structure of shield tunnel with different joint surface

For the shield tunnel with double-layer lining, the type the joint surface between the segment and secondary lining has significant influence. Depending on the smoothness of the border and the transmit patterns of the forces between the segment and the secondary lining, the lining structure is divided into double-shell and composite structures. For the double-shell structure, only the axial force is transmitted through the border, while the axial and shear forces are transmitted through the border for the composite structure. In this paper, based on the similarity model test and the engineering background of the Shiziyang Tunnel, the influence of the joint surface type on the mechanical behaviors and failure characteristics of the lining structure is studied under the condition that the segment and secondary lining of the shield tunnel double-lining structure share the overload together. The result shows that (1) the differences in the bending moment values between the double-shell and composite structures are small, but the axial force values and the changing trends are obviously different; the load bore by the secondary lining and its ratio to the external load increases with the external load; further, the segment is still the main bearing structure; (2) after the macroscopic failure of the segmental lining structure, the double-shell lining loses stability rapidly, the bearing capacity loss is abrupt, and the damage of lining structure is more serious; when the composite structure loses stability, the failure process of the composite structure is longer, and the structural failure is ductile; (3) with the same load, the ratio of maximum displacement to the tunnel radius of the double-shell structure is larger, and the load level of the lining structure when failure occurs is lower. Generally speaking, the composite structure improves the overall stiffness of the double-lining structure, which is in favor of the joint load carrying of the segment and secondary lining to control the structural deformation; additionally, the bearing capacity of the composite structure is obviously better than that of the double-shell structure; therefore, it is recommended in the double-lining design of the shield tunnel.