# Double-Borehole Superimposed Effect of a New Non-Explosive Directional Rock-Breaking Method
## 3 Results
### In Situ Test Results of IESF  
After the in situ roof-cutting test, the borehole inspection camera was used to observe the fractures generated in the borehole wall, and the results of its observations are shown in Figure 24. The same rock-breaking method of the two boreholes showed similar fracture development characteristics. Through the observation of fracture generation, it can be found that the fractures in CB boreholes were generated randomly, and cause greater damage to the rock mass. Due to the action of the slit-oriented tube, the SCB generated two symmetrical main fractures, and there were certain secondary fractures around the main fractures. In contrast, IESF generated only two directional fractures along the slotted orientation, symmetrically distributed on both axial sides of the borehole with an interfacial angle of approximately 180\({}^{\circ}\). Borehole inspection results demonstrated the following conclusion. After CB, the surrounding rock was subjected to stress much greater than the compressive strength of the rock, as it was initially subjected to detonation waves generated by the explosives. Consequently, crushed areas developed along the borehole wall, as illustrated in Figure 24a. Subsequently, blast-induced gases exerted pressure again on the borehole wall and penetrated existing fractures as \"gas wedges\", promoting further crack propagation and ultimately generating multiple fractures. After blasting, the surrounding rock of the boreholes showed asymmetric fragmentation, with more severe fragmentation occurring on the inter-borehole rock mass side. This intensified fragmentation was the result of the superimposed effect of double-borehole loading. After SCB, the slit-oriented tube suppressed detonation waves in non-slotted directions, prioritizing energy release along the slotted orientation. This reduced shock impact on the rock mass around the borehole in non-slotted directions, ultimately forming a fracture along the slotted orientation, as illustrated in Figure 24b. However, the detonation wave infrequently generated compressive stress on the rock mass, leading to the formation of fine secondary fractures in non-slotted directions. Overall directional roof cutting was possible by SCB. Since IESF did not generate detonation waves during its reaction process, the tensile stress concentration generated under the action of the slit-oriented tube caused directional tensile failure in the rock mass along the slotted orientation, as illustrated in Figure 24c. The borehole walls exhibited no blasting-induced fragmentation or secondary fracture, demonstrating that the directional roof-cutting efficacy of IESF surpassed both CB and SCB. Figure 23: The strain of the concentric circle with a 0.135 m radius in IESF. In order to quantitatively characterize the effect of directional rock fragmentation, crack rate \(\phi\) was introduced as a criterion [47]. \[\phi=\frac{l}{2L} \tag{26}\]  
where \(l\) is the total length of the cracks generated. \(L\) is the length of charge section on both side of slotted orientation. The slit rate can be used to analyze the ability to generate directional cracks. A higher crack rate indicated a greater ability to generate directional cracks in slotted orientation, while a lower crack rate indicated a weaker ability. The crack rates of SCB and IESF in the ten boreholes were counted separately and the results are shown in Tables 1 and 2. By calculating the crack rates separately, the average crack rate of SCB was found to be 85.0%, while that of IESF was 95.5%. The results showed that the average crack rates of IESF were 10.5% higher than those of SCB. Combined with the results of the borehole inspection, it can be found that the length of directional cracks generated by the SCB was lower than that of the IESF because part of the energy of the SCB was used to generate secondary  
Figure 24: Roof-cutting results for three fracturing methods: (**a**) CB, (**b**) SCB, and (**c**) IESF. cracks.