Experimental Study on the Bending Performance of Segment Joints in Underwater Gas Transmission Shield Tunnels

The segment joints of shield tunnels are often the most easily damaged parts of tunnels, and when an underwater gas transmission shield tunnel crosses a high water pressure stratum over a long distance, bending failure of these joints poses a significant safety risk. In contrast to the extensive research on shield tunnel joints for large-cross-section, high-water-pressure railway/highway tunnels and for small-cross-section metro tunnels under normal water pressure, the study of underwater gas transmission shield tunnels possessing both high water pressure and a small cross-section is still notably lacking. Therefore, this paper investigates the bending performance and failure mechanism of segment joints in underwater gas transmission shield tunnels utilizing full-scale compression-bending tests integrated with acoustic emission (AE) monitoring technology. The research findings indicate that axial force is beneficial to the bending performance of segment joints in underwater gas transmission shield tunnels, whereas bending moment has a detrimental effect on their bending performance. The influence of increasing axial force on the bending performance is more pronounced under low axial force conditions. The bending performance of segment joints under positive bending conditions is superior to that under negative bending conditions. In the early load-bearing stage, the bending stiffness of the segment joint is reduced by more than 50%. Based on the evolution of AE signals, the failure process of segment joints can be divided into three stages: the concrete load-bearing stage, the concrete and bolts co-load-bearing stage, and the joint failure stage. The damage on the compression side of the concrete is more severe than on the tension side, manifesting as local crushing and spalling. These research conclusions provide a valuable reference for the design and construction of underwater gas transmission shield tunnels subject to high water pressure and a small cross-section.