The influence of impacting orientations on the failure modes of cracked tunnel

Underground tunnels are often subjected to dynamic loadings, and the dynamic loadings may come from various directions. To study the influence of loading orientations on the failure modes of cracked tunnels, laboratory impact experiments were conducted by a drop weight testing apparatus with synchronized measuring system. Green sandstone was selected as the experimental material to fabricate the tunnel samples, a pre-crack emanating from tunnel roof was designed to simulate the natural crack around a tunnel. In the process of dynamic loading tests, strain gauges were employed to determine crack time-to-fracture and crack velocity. Numerical simulation was carried out by the finite element software ABAQUS and the finite difference software AUTODYN. Crack initiation fracture toughness (IFT) was evaluated by experimental-numerical technique and the failure behavior of tunnel model under different orientation impacts was presented in this paper. The studying results show that as impacts come from tunnel roof, tunnel failure mainly occurs near the crack tip and the tunnel bottom (or the tunnel foot); as impact come from tunnel sidewall, the tunnel failure only occurs at the two sides of arch shoulder. The dynamic initiation fracture toughness of the pre-crack in the tunnel models is greatly related to the impact loading orientations.