# Double-Borehole Superimposed Effect of a New Non-Explosive Directional Rock-Breaking Method
## 1 Introduction  
To achieve desired rock fragmentation outcomes in practical applications, typically it is necessary that the synergistic interaction of multi-borehole acts collectively on the rock mass. Therefore, it is imperative to investigate the superimposed effect of double-borehole fracturing to elucidate the varying patterns of stress-strain fields during practical rock fragmentation and their impact on crack propagation. Explosive blasting is a commonly used method of rock destruction at present, and has been widely used in engineering practice because of its characteristics of high power, low cost, and simple operation [1,2]. In order to control the effect of explosive blasting, the energy release law can be changed by using a slit-oriented tube [3,4], so as to achieve the effect of directional blasting [5, 6, 7]. The stress-strain fields of double-borehole blasting are different from that of single hole blasting. Different from the single-borehole loading of conventional blasting of explosives, the double-borehole loading will cause the superposition of stress waves, making the radial cracks formed between the two holes easy to completely break [8]. Under the condition of hydrostatic in situ stress, the detonation cracks propagating perpendicular to the direction of the hole line are more likely to be inhibited by in situ stress. When the in situ stress is low, although the low uncoupling coefficient can promote the crack propagation significantly, it also increases the bifurcation density and expanding range of the crack, increases the damage range of the surrounding rock mass, and is not conducive to the pre-crack forming. With the increase of in situ stress, the superimposed effect of stress waves decreases, and the crack propagation length between boreholes decreases as the stress level increases, and the cracks mainly expand along the direction of the maximum principal stress, which further affects the slit cutting effect between double holes [9,10]. The time delay between double-borehole loadings also affects inter-borehole fracture development. When the time delay is less than the arrival time of detonation energy at the second borehole, the number of blast-induced cracks between the boreholes increases with longer time delays [11]. At the same time, delayed blasting between two holes is limited by the hole spacing, and delayed blasting has a greater effect on the superposition of tensile stresses and requires a higher degree of accuracy in the delay [12,13]. The influence of different joint angles between the double holes on the propagation of inter-hole cracks is significantly different. When the joints are perpendicular to the inter-hole line, the blocking effect on cracks is the smallest, with the angle decreasing, the blocking effect gradually increases. The blocking effect is the strongest at 30\({}^{\circ}\), and the direction of crack extension is guided when the angle continues to decrease [14]. In addition, the presence of empty holes at suitable spacing also acts as a guide for crack propagation between the two holes [15]. The stress field at the fracture tip during shaped charge blasting is altered by the presence of neighboring boreholes, which enhances the process of directional extension of the fracture [16]. For multi-borehole shaped charge blasting in the same section of the detonation conditions, the distance between the boreholes is the main factor affecting the fractures between the boreholes; the appropriate distance between the boreholes through the superposition of the stress can effectively promote fracture penetration, improving the effect of directional rock fragmentation [17,18]. Crack propagation between two holes of shaped charge blasting is still dominated by tensile stresses, and the explosive gas is the main driving force [19]. Slit charge blasting relies on the slit-oriented tube to control the direction of energy transmission to realize the directional rock fragmentation [20].