# Improving Hard Rock Materials Cuttability by Hydraulic Fracturing at Mining Working Face
## 1 Introduction
### Experimental Scenarios  
The elastic modulus of gelatin is a basic mechanical property for investigating the propagation behavior of fractures [26]. A reasonable magnitude of the confining pressure applied in laboratory experiments is closely related to the elastic modulus of gelatin. Hence, the elastic modulus of the gelatin samples should be measured before conducting hydraulic fracturing experiments.  
The elastic modulus of the gelatin samples is determined through indentation tests [35]. After comparing the gelatin samples with different weight concentrations [22; 23; 24], well-solidified gelatin with a weight concentration of 8% is employed. A cylindrical steel indenter is installed to apply a vertical stress to the sample at a loading rate of 5 mm/min [36]. The loading time is maintained for more than 30 s [37]. In order to ensure the accuracy of the test results, the indenter diameter should not exceed one-tenth of the diameter or the height of the gelatin sample [38]. The deflection change of the gelatin surface is read directly by the computer monitor. A digital electronic scale is placed below the sample to record the whole process. After the deformation amount of the gelatin is obtained, the  
Figure 3: Stress condition in open borehole (white—tube; red—packer; green—open bore area).  
elastic modulus (\(E\)) of the gelatin sample with a weight concentration of 8% is calculated to be 38.5 KPa according to Equation (1) [39].  
\[E=\frac{M_{\mathrm{L}}g\big{(}1-v^{2}\big{)}}{D_{\mathrm{L}}w} \tag{1}\]  
where \(M_{\mathrm{L}}\) is the mass of the cylindrical steel indenter, kg; \(g\) is the acceleration of gravity, m/s\({}^{2}\); \(v\) is the Poisson's ratio (\(v=0.5\)) [40; 41; 42]; \(D_{\mathrm{L}}\) is the indenter diameter (\(D_{\mathrm{L}}=10\) mm); and \(w\) is the deformation amount of the gelatin, mm.  
During the hydraulic fracturing pretreatment of the hard rock fault at Guojiawan Coal Mine, the azimuthal angle of the borehole is arranged along the advance direction of the working face, and the borehole intersects the minimum principal stress (\(\sigma_{3}\)). The device used in this experiment applies a horizontal confining stress to the gelatin sample. The vertical direction is the minimum principal stress direction without any confining stress (i.e., the confining stress in the vertical direction is zero). Therefore, the prefabricated borehole in the sample is parallel to the minimum principal stress, which is consistent with the onsite borehole layout and the principal stress conditions (Figure 4).  
A total of seven groups of gelatin samples are prepared. Table 1 exhibits the experimental schemes. The control group is not subjected to any confin  
Figure 4: Relation between borehole axis and the direction of \(\sigma_{3}\): (**a**) borehole axis parallel to the direction of \(\sigma_{3}\) in experimental study; (**b**) borehole axis parallel to the direction of \(\sigma_{3}\) in field test study; (**c**) borehole axis intersects the direction of \(\sigma_{3}\) in experimental study; (**d**) borehole axis intersects the direction of \(\sigma_{3}\) in field test study.  
groups are mainly for studying the effects of the borehole dip angle (0\({}^{\circ}\) and 45\({}^{\circ}\)) and the differential stress on the HF initiation and propagation. In the table, the borehole angle refers to the angle between the borehole axis and the horizontal direction. The confining stresses are the stresses applied to the sample in three directions, which are determined by means of the dimensionless analysis based on the ratio of the compressive strength of the fault rock to the elastic modulus of the sample. [PERSON] et al. [30] applied the confining stress ranging from 1400 to 4800 Pa for loading the gelatin sample. Thus, the confining stress in this study is determined in the range of 1400 to 2400 Pa.