Directional fracture mechanism and surrounding rock damage characteristics of slotted cartridge blasting

According to the elastic wave theory combined with the propagation law of shock waves in rock or rock-like media, a slotted pipe wall is assumed to be elastic, irrespective of the attenuation process of detonation and shock waves in the coupling medium. The reflection of detonation and shock waves at the slotted pipe and blasthole walls is also assumed to be positive reflections, and the flow rate and internal energy constantly change with time. The proportional relationship between the peak stress of the hole wall, the range of the crushing area, and the fracture area in the cutting and non-cutting directions was established in this study. Moreover, AUTODYN software was used to establish a slit charge blasting model. Five measuring points were set at equal intervals in slit and non-slit directions. The peak stress, peak blasting vibration velocity, and peak arrival time at the measuring points were analyzed. Thereafter, based on the tunnel blasting test of the Gubei Coal Mine in the Huainan mining area, blasting tests of ordinary charge packs, slit charge packs, and varying peripheral hole spacing were performed. The crack state of the surrounding rock before and after blasting was tested through drilling peeping. Then, a two-dimensional graph of the crack development of the surrounding rock before and after blasting was imported into the customized MATLAB box dimension calculation program for calculation. The results before and after blasting were obtained based on the linear fitting curve and the box-counting dimension of the pixel information matrix of the surrounding image of rock burst cracks after blasting. The influence of the size of the slit charge and the surrounding hole spacing on the damage degree of the surrounding rock was also investigated. The results indicate that when the slit charge is used for blasting, the detonation product jet and stress concentration are produced in the slit direction. Further, the stress peak value and blasting vibration speed in the non-slit direction decline, the energy propagation speed in the non-slit direction decreases, and the energy propagation size in the non-slit direction decreases to achieve the goal of directional fracture. The results of the field test indicate that the degree of damage of the surrounding rock is reduced by >30% after using slit charge blasting compared with ordinary charge blasting. When the slit charge is used for blasting, the degree of damage to the surrounding rock reduces with an increase in the distance between the surrounding holes.