Differences in fragment size distribution between air-coupling blasting and water-coupling blasting and their formation mechanism

Through using integrated theoretical analysis and numerical simulation, this study addresses the differences in fragment size distribution (FSD) between air-coupling blasting (ACB) and water-coupling blasting (WCB) and reveals their formation mechanism. Firstly, the borehole wall blast load, the stress field of the rock near the borehole, and the extent of the rock failure zones were theoretically calculated. FSD differences between the blasting methods were qualitatively analyzed. Then, a three-hole blasting model was used to simulate blast-induced rock damage under both blasting methods. Based on this, image recognition was used to quantify FSD data, revealing quantitative differences in FSD between the blasting methods. By combining the rock-breaking process with the rock fracture characteristics, a novel failure zone division method was developed to investigate the formation mechanism of the differences. The results show that the fragment size in the two blasting methods follows the Rosin-Rammler distribution. Under identical decoupling coefficients, WCB exhibits a smaller characteristic size parameter x_c yet a higher uniformity index n compared to ACB. With increasing decoupling coefficient, x_c increases linearly while n decreases linearly in both ACB and WCB. Corresponding formulas for x_c and n were established, respectively. In addition, with increasing decoupling coefficient, the difference in overall fragment size between the two methods increases, while the difference in the inhomogeneity of the rock fragments remains stable. The above differences are mainly caused by the different blast loads, which result in different rock fracture patterns and degrees of rock fragmentation in the fracture zone around the borehole, the reflected tensile zone around the free surface, and the superposition zone of compression waves and tension waves. ACB generates fine fragments near boreholes and the free surface in front of the center position of adjacent boreholes, whereas WCB primarily generates them near free surfaces. The formation of boulders in both blasting methods is primarily caused by insufficient rock fragmentation within the fracture zone around the borehole and the superposition zone of compression waves and tension waves.