Analytical method regarding compression-bending capacity of segmental joints: Theoretical model and verification

Abstract

The flexural bearing capacity of a segmental lining structure, mainly comprising the flexural bearing capacities of segments and the compression-bending capacities of joints, is an important index for evaluating the mechanical performance of a shield tunnel. As indicated by this study, the flexural bearing capacity of a segment has a theoretical basis for the calculation, whereas that of a joint does not. Combined with the deformation characteristics of a segmental joint under compression-bending loads, concrete yielding at the joint surface is taken as the criterion of the ultimate bearing state of a segmental joint, upon which a set of theories for calculating the compression-bending capacity of the segmental joint is established in terms of the mechanical model of the segmental joint. Therefore, according to an analytical solution of compression-bending capacity of a segmental joint, the accuracy and correctness of the designed algorithm are verified through a full-scale compression-bending test of a segmental joint, and the proposed method is then applied to prototype tests of the segmental lining structure for interpretation of the test results. A comparison with the results of full-scale segmental joint tests indicates that the relative errors of the joint bending moments between the test results and the theoretically calculated results are 3% and 5.6% for positive and negative bending, respectively, which not only confirms the accuracy of the proposed theoretical model but also rationalizes the assumption that the yielding of the concrete joint surface is a sign of the joint reaching its ultimate limit state. Using the proposed method, the prototype test results of the segmental rings are interpreted and indicate that the structural failure of a straight-jointed assembly structure (STRS) is mainly caused by joint failure because most of the joints have already exceeded their ultimate bearing capacity, whereas none of the segments have reached their flexural bearing capacity. In the test results for a staggered-jointed assembly structure (STGS), both the segments and joints have reached the ultimate joint bearing capacity or are nearly yielding at the same time. The interaction between the segmental rings of an STGS not only allows both the bearing capacity of the segment and the joint to be fully exerted while the structure bears a load but also generates yielding of the two-part structure almost concurrently; thus, an STGS has better bearing performance than an STRS.