The damage to the surrounding rock under the adjacent explosive load generally manifests as fracture growth. In order to in-depth understand the fracture growth law and mechanism, fracture growth process in the surrounding rock with a nearby adjacent tunnel under the explosive load is studied using the dynamic caustics method. Test results indicate that the original fracture growth process can be divided into two stages, demarcated when the main fracture penetrates the original fracture. The original fracture grows in the vertical direction due to the free surface of the tunnel in the first stage. The main fracture from the explosive source plays a dominant role in the growth direction of the original fracture in the second stage, and the original fracture deflects and grows parallel to the main fracture. Based on kinematic and energy analysis, the penetration process is companied by energy transfer and superposition. The neglected back-facing side also contains fracture on the lower side. The fracture growth direction changes after the original fracture penetrates the free surface of tunnel under the combined effect of the original fracture and unloading wave.

The surrounding rock damage of a tunnel under adjacent explosive load is often manifested as the growth of the original crack. In order to thoroughly understand the crack growth mechanism, in this study, the growth process of the original crack was investigated in detail by the dynamic caustics experiment. The experimental study shows that the growth of original crack is the result of the combined action of explosion stress wave and explosive gas. The quasi-static stress generated by the explosive gas was superimposed on the weakened stress field, resulting in the formation of the peak of the main stress difference in the surrounding rock, moving towards the adjacent tunnel in the form of an arc wave. With the arc wave moving towards the original crack, the growth rate of the original crack increases rapidly. During the period of 200 μs to 250 μs, the crack growth rate oscillates at the peak, and its size is approximately equal to the moving speed of the arc wave. Based on the experimental results and microscopic damage mechanics, the high stress concentration at the original crack tip under the stress wave first causes damage localization and the weakest chain is formed by the penetration of the damage localization zones by microcracks by special distribution law. Subsequently, the original crack starts to initiate and grow along the direction of the weakest chain.