Effect of dynamic loading orientation on fracture properties of surrounding rocks in twin tunnels

For expedited transportation, vehicular tunnels are often designed as two adjacent tunnels, which frequently experience dynamic stress waves from various orientations during blasting excavation. To analyze the impact of dynamic loading orientation on the stability of the twin-tunnel, a split Hopkinson pressure bar (SHPB) apparatus was used to conduct a dynamic test on the twin-tunnel specimens. The two tunnels were rotated around the specimen’s center to consider the effect of dynamic loading orientation. LS-DYNA software was used for numerical simulation to reveal the failure properties and stress wave propagation law of the twin-tunnel specimens. The findings indicate that, for a twin-tunnel exposed to a dynamic load from different orientations, the crack initiation position appears most often at the tunnel corner, tunnel spandrel, and tunnel floor. As the impact direction is created by a certain angle (30°, 45°, 60°, 120°, 135°, and 150°), the fractures are produced in the middle of the line between the left tunnel corner and the right tunnel spandrel. As the impact loading angle (α) is 90°, the tunnel sustains minimal damage, and only tensile fractures form in the surrounding rocks. The orientation of the impact load could change the stress distribution in the twin-tunnel, and major fractures are more likely to form in areas where the tensile stress is concentrated.