Full-scale experimental study on failure characteristics of the key segment in shield tunnel with super-large cross-section

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Due to the size effect and the change of joint design, the mechanical behavior of the shield tunnel with super-large cross-section is different from that of the conventional medium and large section tunnel. In addition, the mechanical and failure characteristics of the key segment are different from those of other segments in shield tunnels due to its unique shape and weaker stiffness. In this paper, a series of full-scale experiments were carried out to investigate the failure characteristics of the key segment in the shield tunnel with a super-large cross-section under general loading. The safety state of the whole structure was quantitatively evaluated based on the obtained bearing capacity, deformation characteristics, and progressive damage features. The testing procedure was introduced in detail, including the examined specimen, measurements, and loading conditions based on a revisit to the prototype. A multi-functional three-dimensional loading system was adopted to apply a compression-bending load similar to that in practice on the testing specimen. In-depth discussions were conducted on the displacement, joint opening, joint dislocation, and the failure characteristics of the segments directly recorded and observed during the tests. The results show that the longitudinal force acting along the tunnel axis significantly limits the inward convergence of segments, the opening of circumferential joints, and the dislocation of longitudinal and circumferential joints in a specific range. Under general loading conditions, when the load reaches the most unfavorable load combination, the testing segments are still in a safe state, whose deformation does not exceed the design requirements, and no damage occurs. After reaching the destructive load, distinct crack propagation behavior is found in different stages, which can be divided into four steps according to the observed distribution characteristics. The final failure of the segments begins with the collapse of the longitudinal joint compression zone. Additionally, the reverse shear circumferential joint is seriously damaged, whereas the forward shear circumferential joint is only slightly damaged. The results can provide novel insights into the failure characteristics of the key segment in the super-large cross-section tunnel.