# Improving Hard Rock Materials Cuttability by Hydraulic Fracturing at Mining Working Face
## 1 Introduction
### Experimental Results  
Figure 5 shows the results of Test 1-1, where no confining stress is applied. The HF initiates along the borehole axial direction. With the injection of the fracturing fluid, the axial fracture gradually re-orientates towards the borehole radial direction until it reaches the sample boundary. The re-orientation of the axial fracture is mainly due to the fact that the HF initiation position is close to the bottom of the borehole, where the stress concentration changes the local stress state and causes the HF to deviate from its initial path. As can be seen from Figure 5b, when no confining pressure is applied to the sample, the axial HF probably re-orientates towards the radial direction, finally forming a complete macroscopic failure plane in which the radial HF and the axial HF intersect. From Figure 5c,d, it is observed that a short radial HF initiates at the bottom of the borehole, indicating an obvious stress concentration in this area. The distribution of the HFs at the left and right sides is asymmetrical. The development rate of the fracture at the right side (right fracture) is obviously higher than that of the fracture at the left side (left fracture). The gelatin concentration (8%) and the confining stress in Test 1-1 in Figure 5 are consistent with those in the literature [43] but the experimental results are different, that is, the fracture distribution is asymmetrical. Figure 6 presents the experimental results of Test 1-2 under a horizontal confining pressure of 1400 Pa. The right fracture firstly initiates in the middle part of the borehole along the axial direction, then gradually re-orientates to the horizontal direction, and finally, reaches the sample boundary. From Figure 6a, it can be noted that a smooth fracture surface is produced in the axial direction of the borehole, and several folds are generated at the edge of the right fracture, suggesting that the HF undergoes a shear failure during its development. Figure 6b shows that an axial fracture occurs at the left side of the sample. It is clearly seen that the HFs are asymmetrically distributed during un-directional hydraulic fracturing. From Figure 6c,d, it can be found that the axial fracture has a slight re-orientation. The development rate of the axial fracture is higher than that of the radial fracture. The re-orientation angle (the angle between the fracture and the borehole axis) is 13\({}^{\circ}\). Figure 7 provides the experimental results of Test 1-3 under a horizontal confining pressure of 1900 Pa. The right fracture firstly develops in the middle area of the borehole along the axial direction. By continuously injecting the fracturing fluid, secondary fractures are produced above the primary HFs at both sides. The primary and secondary fractures gradually re-orientate towards the horizontal direction. It can be noticed from Figure 7a that a smooth HF surface is created in the axial direction of the borehole until the primary HF extends to the bottom of the borehole. In Figure 7b, it is clear that the fractures at both  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Test Case** & \begin{tabular}{c} **Gelatin** \\ **Concentration (\%)** \\ \end{tabular}  & **Elastic Modulus (KPa)** & **Borehole Angle (\({}^{\circ}\))** & \(\sigma_{H}\) **(Pa)** & \(\sigma_{h}\) **(Pa)** & \(\sigma_{v}\) **(Pa)** \\ \hline
1-1 & 8 & 38.5 & 90 & 0 & 0 & 0 \\
1-2 & 8 & 38.5 & 90 & 1400 & 1400 & 0 \\
1-3 & 8 & 38.5 & 90 & 1900 & 1900 & 0 \\
1-4 & 8 & 38.5 & 90 & 2400 & 2400 & 0 \\
1-5 & 8 &sides are roughly symmetrical. Figure 7c,d shows the schematic of the primary HF at the left side. The left fracture re-orientates obviously. The re-orientation angle is 30\({}^{\circ}\). Figure 8 displays the experimental results of Test 1-4 under a horizontal confining pressure of 2400 Pa. Only one axial HF initiates. The HF firstly initiates in the middle region of the borehole along the axial direction.