Experimental study on the mechanical behaviour of shape memory alloy flexible circumferential joint for seismic mitigation in shield tunnel

Shield tunnel lining has a large number of segmental joints, which are prone to damage under strong earthquakes, affecting the tunnel safety. This study applies the SMA flexible circumferential joint consisting of shape memory alloy (SMA) ring springs and circumferential joint bolts for longitudinal seismic mitigation in tunnels. The mechanical behaviour of SMA flexible circumferential joint is investigated by tests. Two forms of circumferential joints with straight and bent bolts are used to analyse the mechanical properties of SMA joints under varying force conditions. A numerical study is also conducted to compare and verify the accuracy of the tests. The test results indicate that both SMA-straight bolt joint and SMA-bent bolt joint can reduce the joint stiffness under axial tension and bending moment, and have energy dissipation effect. Compared with ordinary joints, the tensile and bending stiffness of SMA joints are reduced by about 80%, effectively enhancing the deformation capacity of the joints. Under combined axial pressure and bending moment, the bending deformation pattern of the SMA joint changes and the bending stiffness increases, but it still shows good performance. The SMA joints show excellent performance of reducing joint stiffness and energy dissipation during multiple cyclic loading, indicating the stability and reliability of SMA joints, and proving that SMA joints are suitable for seismic mitigation in shield tunnels. The numerical results are basically consistent with the tests, verifying that SMA joints can reduce joint stiffness and dissipate energy under various working conditions.