Influence of impact loading orientations on the mechanical behaviour of rocks around a tunnel

The surrounding rock of tunnels may suffer from dynamic disturbances caused by impact loading or blasting at different locations nearby, leading to the instability of fractured rock masses. To investigate the mechanical behaviour of the surrounding rock subjected to various impact loading directions, an improved drop weight system is introduced for experimental study. To better understand the mechanisms in fractured rock mass, the extended peridynamic (XPD) method is adopted for auxiliary analysis. Both the numerical and experimental results consistently show that the initiation time, initiation toughness and fracture toughness (KI and KII) of precracks are notably affected by the loading directions. As the crack propagates towards the tunnel, the crack growth speed gradually decreases and is inversely proportional to the fracture toughness. The average errors between the test results and numerical results for crack fracture time, average propagation toughness and crack propagation velocity are 4.85%, 3.76% (KI), 4.08% (KII) and 6.79%, respectively, demonstrating the effectiveness of the XPD model in experimental prediction. Moreover, by using the XPD model, the distribution of deformation is analysed and the failure trend of tunnels on the shadow side can be identified. The precracks exhibit a tensile failure mode shortly after initiation, but then change to a mixed tensile–shear failure mode because of the influence of different loading directions. When the cracks break through the upper side of the tunnel, the crack initiation points of the shadow side are notably different for different loading directions, mainly occurring at the corner (E2), roof (A), spandrel (C2) and tunnel arc-side 45° (B2), providing a valuable reference for tunnel engineering.