Effects of cavity locations and numbers on the failure mechanisms of 3D printing horseshoe-shaped tunnel specimens: Experiments and numerical simulations

The presence of voids behind the tunnel lining holds crucial significance for the long-term safe and steady operation of tunnels. To explore this impact comprehensively, the present research utilizes sand 3D printing technology to fabricate horseshoe-shaped tunnel specimens featuring diverse void configurations (namely, single void, double voids, and no voids). Subsequently, uniaxial compression tests are carried out to examine how these voids influence the mechanical characteristics and failure modes of the tunnels. Surface strain distribution is meticulously recorded through Digital Image Correlation (DIC). The Discrete Element Method (PFC2D) was employed to simulate specimen failure processes and analyze damage mechanisms. The stress–strain curves of rock specimens exhibit four distinct phases: the compaction stage, the elastic deformation stage, the plastic deformation stage, and the specimen’s terminal failure stage. It is substantiated that the position of voids exerts a substantial impact on the peak strength of the tunnel models. With regard to the single void specimen, when the angle α ranges from 0° to 180°, the peak strength displays a tendency of increasing initially, then declining, and finally increasing. An analogous pattern is noticed in the double voids specimen when the angle β varies from 45° to 180°. Herein, the peak strength displays a consecutive pattern of first rising, then decreasing, and ultimately rising. Cracks were observed to emerge at the crown of the arch as well as along the perimeter of the tunnel’s base in the specimen without voids. Moreover, it was discovered that the paths along which cracks extended of both single-void and double-void specimens were restricted by the alignment of the voids. This led to the emergence of diverse crack configurations, namely: the Top main crack, the Underside main crack, the Side crack, and the Corner crack. It has been demonstrated that the numerical simulation of the specimen cracking process is highly consistent with the experimental findings. Ultimately, the crack initiation mechanisms are detailly discussed. This research offers a valuable benchmark for grasping the influence exerted by voids behind tunnel linings on the operational safety of tunnels. Moreover, it serves as a directional compass for damage anticipation and disaster prevention as well as mitigation within the realm of underground engineering.