# Improving Hard Rock Materials Cuttability by Hydraulic Fracturing at Mining Working Face
## 1 Introduction
### Analysis of Test Schemes  
During the field application of hydraulic fracturing, the section at which the Nos. 80-86 hydraulic supports deploy and the section at which the Nos. 94-100 hydraulic supports stand are considered as the comparison section where no hydraulic fracturing pretreatment is performed. The section at which the Nos. 87-93 hydraulic supports stand is the pretreatment section by hydraulic fracturing (Figure 14). The fracturing effect of the target section is evaluated by the cutting efficiency of the shearer. The fracturing effect can be characterized by the peak value recorded by the pressure gauge. Figure 15a,b shows the details of the fracturing duration and the peak pressure for each borehole in the three test schemes. During the implementation of schemes A and B, it is obvious that the pressure peak value is smaller when the hydraulic fracturing is performed in the borehole closer to the center of the target section. The region near the center of the target section is affected by the fracture penetration into the adjacent boreholes, resulting in a decrease in the peak pressure during the fracturing of the middle borehole. In the implementation of scheme C, the peak pressures of the boreholes (boreholes 1, 2, 8 and 9) near the untreated hard rock fault are larger. The peak pressure of the borehole in the central part is lower due to the presence of other boreholes. After a certain distance of fracture propagation, the radical HFs penetrate into the adjacent boreholes. As a result, the peak pressure of the borehole located in the middle of the target section in scheme C is lower. The average time of fracturing a borehole is 6.2 min. Figure 14: Schematic diagram of the positions of the target section and the comparison section. Figure 13: Schematic of system equipment: 1—borehole; 2—packer; 3—starter valve; 4—pressure gauge; 5—pressure relief valve; 6—t-pipe; 7—hydraulic support; 8—emulsion pump. The MG400/940-WD drum-type AC haulage shearer is used at the 51207 working face. The maximum haulage speed is 7.1 m/min. The maximum tractive power is 55 kw. The current variation during the rock mass cutting can be observed and recorded through a constant haulage speed. The record of the cutting current is not the \"indication number\" of the current, but the percentage of the instantaneous cutting current to the rated current. When the current on the display screen exceeds 100, it indicates that the electric engine is overclocked and the shearer is overloaded. The variation of the cutting current recorded in the three schemes is shown in Figure 16. Through data analysis, it is found that when the shearer cuts the rocks at the comparison section at a constant speed, the cutting current increases significantly, resulting in overclocking the electric engine and making the shearer severely shake. When the shearer cuts through the target section, the cutting work goes smoothly. Also, the cutting current of the shearer is obviously decreased. Rock blocks constantly appear in the hauling process (Figure 17a). This directly proves that hydraulic fracturing can create a series of artificial fractures in the rock mass. Figure 16: Variation of the shearer cutting current. Figure 15: Test records: (**a**) peak pressure of boreholes; (**b**) fracturing duration of boreholes. Before carrying out the hydraulic fracturing application, 56 picks are consumed each day for cutting the full sandstone section (where all the Nos. 80-100 hydraulic supports stand). After the implementation of scheme A, only 24 picks are consumed by the shearer. After the implementation of scheme B, the shearer consumes 4 picks for cutting through the target section and 18 picks for cutting through the comparison section. After the implementation of scheme C, the shearer consumes 3 picks and 18 picks for cutting through the target section and the comparison section, respectively.