Mechanism of progressive collapse induced by partial failure of shield tunnels in sandy soil

The shield tunnel is generally composed of concrete segments, and there are many risks during both construction and operation because of weak joint parts. Both in China and other countries, the disasters that partial failure of tunnel linings causes a successive failure and even large ground surface subsidence happens in tunneling engineering are not very rare. According to the analysis of joint stresses, an ultimate bearing capacity envelope is established to address the failure criteria for the joints under different internal force conditions, and the failure criteria for both concrete segments and joints are implemented in the discrete element software PFC by using the FISH language. The progressive failure mechanism of the tunnel linings induced by partial failure is discussed. The results show that the partial failure at the springline reduces the resistance of the soil, and the soil arch formed above loose soil area falls on the top of the tunnel, both resulting in a sharp increase in the internal forces of the tunnel, which exceed the ultimate bearing capacity envelope and then induce an extension of partial failure. The failure rings with large transverse deformation cause the adjacent rings to be damaged because of the bolts between the rings. Finally, preliminary prevention measures are proposed and analyzed.