Damage mechanism of a shield tunnel with cavities behind the concrete lining: An insight from a scaled model test

Cavities behind the concrete lining of a shield tunnel may result in apparent damage or even collapse of the tunnel during its operation. It is necessary to predict the damage modes of a shield tunnel with cavities, and accordingly reinforce vulnerable areas of the tunnel. This paper investigates the damage modes of shield-tunnel models with cavities at different locations and sizes behind the concrete lining. The tunnel models used in the test are created using a 3D printing technique, with an aim of simulating the joints between segments. To consider the stratum-structure interaction, the tunnel models are created with grout-layers prefabricated between lining and soil. The 3D point cloud technique is then applied to observe the damage modes of the tunnel linings. The safety status of the shield tunnel is evaluated during the loading process, and categorized into safe, dangerous, and failure stages. Experimental results show that the damage modes of the shield tunnel with cavities contain concrete crack, concrete spalling, segment misalignment, and lining crush. Cavities at the tunnel crown and shoulder impose a substantial impact on the lining structure. Cracks propagating across three or more segments result in mutual compression between segments, forming a crack mesh, and consequently leading to concrete spalling. The tunnel lining undergoes a failure mode of segment misalignment when the cavity angle (size) is greater than 45°. As the volume of the cavity increases, the tunnel lining transitions to a failure mode of lining crush. The results in this study will facilitate the proactive reinforcement of the tunnel by predicting damage modes induced by cavities, ensuring its safe operation to a certain extent.