In-situ high-speed 3D-DIC experiment on blast-induced second free surface characteristics at initial stage of cut blasting in a tunnel

To obtain the law that blasting damage the rock mass and the evolution features of the second free surface, this study designed an in-situ tunnel blasting experiment and then got high-speed images using a three-dimensional digital image correlation (3D-DIC) method. The study clarified the movement law of rock mass in the inclined cut zone by analyzing the change of displacement fields under delayed blasting. After analyzing the off-plane displacement data collected from the cross-section, the study found the critical displacement criterion for rock failure, then analyzed the dynamic process and characteristics of the second free surface formation. The study shows that the cut blasting leads to the rapid extension of rock mass deformation in the cut zone and higher off-plane displacement. The focus of the study coincides with the cut-hole charging section, which is less affected by the smoke. The off-plane displacement value reached 204 mm at 26 ms. It is sufficient to analyze the complete rock breaking induced by cut blasting. Statistical analysis in the area not affected by smoke reveals that the rock mass will be damaged and thrown out if it is beyond a critical displacement value. This value is 100 mm here. The study inferred that under double-hole delayed blasting, the second free surface formed at 310 mm from the first initiated hole 11 ms after detonation. According to the extension velocity, it has three stages. The smoke diffusion rate is much lower than the free surface extension rate. The research content is crucial to reveal the quantitative relationship between cut parameters and rock-breaking effectiveness.