Model tests on influences of cavity defects on mechanical characteristics and failure laws of segment linings of shield tunnelsc

By using the method of model tests and introducing the progressive failure theory into the analysis of disease occurrence and propagation of segment linings of shield tunnels, from the internal force, structural displacement, acoustic emission signals and process of crack occurrence and extension, a comparative study on the influences of cavities behind linings under extreme loading conditions is carried out. The results show that the existence of the cavities has significant influences on the ultimate bearing capacity and failure laws of shield tunnels. First, it decreases the corresponding load at the critical point. Meanwhile the ration of the maximum displacement to the tunnel radius under this load also decreases. Secondly, the load of starting to induce macroscopic crack is lowered. For different cavity locations, the cavities located at spandrel are more likely to result in instability of the structure than those at hance and vault. The cavities may also change the starting location of instability of the structure, leading to that the position of instability is near or in the cavities directly.