# Double-Borehole Superimposed Effect of a New Non-Explosive Directional Rock-Breaking Method
## 1 Introduction
To realize interconnected fractures in double-boreholeslit charge blasting, it is necessary to ensure overlap of damage zones between boreholes, which requires regulating the superimposed stress fields between energy-focused boreholes by adjusting borehole spacing [21]. In the case of cracks between double holes, unequal lengths of cracks, distance from the free surface, and height difference between cracks can lead to the formation of hook cracks, which affects the penetration between cracks [22]. However, explosive blasting is prohibited in certain scenarios due to its significant disturbance and safety risks [23; 24]. Therefore, non-explosive methods are needed for the destruction of rocks. Hydraulic fracturing is often used as a non-explosive rock-breaking method instead of explosives to accomplish fracturing rock. The crack propagation patterns in multi-borehole hydraulic fracturing are predominantly influenced by in situ stress and rock mechanical properties. Increased in situ stress induces fracture zone contraction and width enlargement, while a higher elastic modulus promotes elongated and narrow fracture geometries [25], and crack propagation directions are significantly affected by the direction of maximum principal stress as well as principal stress interpolation [26; 27]. In multi-borehole hydraulic fracturing conditions, the initiation pressure of inner boreholes is consistently higher than that of outer boreholes. Additionally, the initiation pressures of all three fracturing boreholes exhibit a significant increase with larger fracture borehole spacing. When the spacing between fracturing boreholes is small, both outer boreholes generate hydraulic fractures perpendicular to the orientation of the minimum horizontal principal stress, while the central borehole produces hydraulic fractures parallel to this orientation. These fractures extend and interconnect, forming complex fracture networks. In contrast, larger borehole spacing results in relatively independent fractures [28]. The competition between the boundary stress and the internal stress of the holes controls the fracture growth patterns, and the reduction of the boundary stress difference exacerbates the stress concentration between the holes, causing planar fractures formed by a single borehole to evolve into spiral fractures interconnecting two boreholes [29]. Fractures created between multiple holes can expand directionally along the borehole connecting lines, but fractures formed outside the multiple holes fail to expand in that direction [30]. Hydraulic fracturing is difficult to generate directional fractures as expected, so directional hydraulic fracturing is often required to control the direction of the fractures [31]. Directional hydraulic fracturing of a multi-borehole is significantly affected by the principal stress. The rock damage process is dominated by tensile damage, and the deformation damage is most intense at the center line of the holes, which forms a \"strip-like\" fracture surface along the direction of the borehole layout [32]. However, the directional hydraulic fracturing is significantly influenced by in situ stress, resulting in suboptimal directional efficacy [33; 34]. Although directional hydraulic fracturing can control the direction of fracture propagation within a certain range by grooving the hole wall, the direction of fracture propagation will re-orientate as the differential stress states and the distance increases [35]. Moreover, the fracture surface generated by the directional hydraulic fracturing is usually extending radially along the radial direction of the borehole at the groove [36], which is perpendicular to the fracture surface that extends along the axis of the borehole in the shaped charge direction by shaped charge blasting. Therefore, it is difficult to replace shaped charge blasting with directional hydraulic fracturing. Due to the above factors, the development of novel non-explosive directional rock-breaking methodologies is urgently required.