Failure tests and bearing performance of prototype segmental linings of shield tunnel under high water pressure

The occurrence and characteristics of structural failure under high water pressure and the evaluation of structural bearing capacity are of great significance in the rational design of shield tunnels. In this study, the failure process of two types of segmental lining structures used in the Shiziyang Tunnel in China, namely, the straight joint assembly structure (STRS) and staggered joint assembly structure (STGS), are investigated and summarized with prototype tests. The differences in the failure characteristics between the two types of lining structures are observed and analyzed, and two indexes are proposed to evaluate the damage status of the entire structure and local structure by analyzing and interpreting the failure phenomena during the tests. The results show that (1) The failure processes of both STRS and STGS under high water pressure perform as: the accumulation of previous occurred strength damage of segments ultimately leads to the displacement instability failure of structure. However, the difference is that besides the occurrence of large number longitudinal penetrating cracks at segments, there are local crushing and shear cracks occurred at the joints in STRS. (2) The failure processes of STRS and STGS under high water pressure can be divided into three stages: normal deformation stage, strength damage stage, and displacement instability stage. (3) With the proposed two indexes, the failure phenomena and structural bearing capacities at various stages of the failure process can be theoretically analyzed and quantitatively evaluated. The effective stiffness coefficient beta(e) (specifically, beta(te) for STRS and beta(ce) for STGS) is recommended to assess the structural bearing status, and the residual bearing capacity coefficient R-bc is suggested to evaluate the residual structural bearing capacity. (4) For STRS, when obvious cracks appear, beta(te) is 0.83 and R-bc is 0.71; when significant deformation occurs, beta(te) is 0.65 and R-bc is 0.52; and when the structure fails, beta(te) is 0.4 and R-bc is 0.35. For STGS, when microcracks appear, beta(ce) is 0.67 and R-bc is 0.58; when the structure is significantly deformed, beta(ce) is 0.49 and R-bc is 0.43; and when the structure fails, beta(ce) is 0.18 and R-bc is 0.27. (5) In the loading process, beta(e) and R-bc of STRS and STGS decrease constantly. For STRS, the two indexes rapidly decline after visible cracks appear, but for STGS, the decreasing amplitude is relatively small.