Flexural Behavior of Curved-Bolt Jointed Tunnel Segments under Combined Axial Force and Bending Moments

Abstract Precast segmental tunnel linings are structurally vulnerable due to the presence of mechanical joints between segments. However, the lack of detailed design guidelines for these joints has led to limited recognition of the need to evaluate their structural performance. Consequently, most previous studies have focused primarily on the behavior of single segments. Although some research has addressed jointed segments, the majority employed straight-type specimens that fail to capture the actual curvature of tunnel linings. As a result, the influence of joint and segment geometry on the structural response of tunnel linings remains insufficiently quantified, and ultimate failure modes—essential for ultimate limit state design—have not been adequately discussed. To fill this gap, this study conducted full-scale experiments on both single and jointed segments, including arch-type and straight-type geometries. The objective was to examine how segment joints and geometries affect structural responses and to identify the ultimate failure modes of jointed segments. The results revealed that jointed segments exhibited distinct behaviors compared with single segments, governed by joint rotation and local stress concentration. Their response progressed through three stages—bolt contact, joint surface contact, and concrete crushing. The ultimate failure pattern varied with moment direction, with cone-shaped failure developing under sagging, whereas only intrados crushing occurred under hogging. Axial force and geometry influenced stiffness and post-cracking performance, with arch-type segments showing stiffness recovery after cracking and demonstrating superior performance to straight-type segments. These findings contribute to a more comprehensive understanding of tunnel segment behavior.