Study of Impact Load Directions on Tunnel Stability in a Cracked Rock Mass

During tunnel construction and service, the rock surrounding tunnel is often subjected to multiple factors that influence its behaviour, such as dynamic disturbances (explosions, mechanical excavations, etc.) and existing cracks. These factors can readily induce safety issues, such as rock bursts and collapses. To investigate the effect of the loading direction on the failure modes of fractured tunnels, this study numerically investigated the destructive behaviour of tunnel models under the coupling effect of dynamic disturbance loads and external cracks using the finite-difference method (FDM). Additionally, a physical tunnel model with prefabricated cracks was created using green sandstone. A drop weight impact testing device (DWITD) was employed as the dynamic disturbance loading apparatus, while the relative azimuth angle between the tunnel and the cracks was varied. The crack initiation, arrest time, and extension rate were obtained using a crack fracture tester (CFT). The research results indicated that the preexisting cracks propagated continuously and eventually connected with the tunnel on the incident side under impact loads. The failure area of the tunnel was primarily controlled by the loading direction, exhibiting different modes of failure, often occurring at the bottom and arch of the tunnel. New cracks on the transmitted side of the tunnel appeared at different locations for different impact angles. The presence of cracks around the tunnel had a significant impact on the dynamic stress concentration factor (DSCF) of the rock surrounding the tunnel. The findings of this research can provide valuable guidance for tunnel stability analysis and the optimization of support schemes.