Abstract: To mitigate tunnel damage under dynamic loads, and address the challenges of complex mechanical responses and unclear crack propagation behavior of tunnel surrounding rock in mix ground, an impact test is conducted using a split Hopkinson pressure bar (SHPB) system. This test is conducted based on a soft and hard rock composite tunnel model with a circular cavity. The digital image correlation method is employed to observe the strain evolution around the tunnel, crack initiation, crack development, and failure modes of the tunnel surrounding rocks under different dip angles of the interface between the soft and hard rock layers. The dip angle refers to the angle between the interface and the incident direction of the incoming stress wave. The results reveal the following: (1) The dynamic compressive strength of the sample changes with the dip angle, showing a pattern of "increase, decrease, and then increase again". The lowest dynamic compressive strength occurs at a dip angle of 60°. (2) The dip angle at the interface between the soft and hard rock layers significantly affects the fracture behavior of the tunnel surrounding rocks. Four main types of cracks are identified at different dip angles: cracks extending along the interface between the soft and hard rock layers, cracks starting from the left side of the tunnel and spreading in the direction of the stress wave，cracks initiating from the tunnel floor and extending along the direction of the stress wave, and cracks overlapping with the fracture propagating along the interface. (3) Different dip angles expose the macroscopic cracks to different tensile and shear forces, leading to diverse failure modes. Cracking along the interface between the soft and hard rock layers occurs in the tunnel surrounding rock at various dip angles. This indicates that reinforcement support should be considered at the interface.

Key words: impact loads, composite strata, tunnel, dynamic response, surrounding rock fracture, crack propagation