# Improving Hard Rock Materials Cuttability by Hydraulic Fracturing at Mining Working Face
## 1 Introduction
### Hydraulic Fracturing Test Schemes  
The experimental results in the previous section show that both radial and axial fractures occur during un-directional hydraulic fracturing. When the fracture develops three times longer than the borehole diameter, it will propagate along the plane perpendicularto the minimum principal stress direction. When the borehole axis is not perpendicular to the maximum horizontal principal stress, the HF will re-orientate more quickly. All the radial fractures develop symmetrically and propagate uniformly [44; 45]. Therefore, based on these results, three field test schemes of hydraulic fracturing are implemented. The borehole dip angles are set to be 0\({}^{\circ}\) and 45\({}^{\circ}\).  
In schemes A and B, five boreholes are drilled at the target section where the No. 87-93 hydraulic supports stand below. In scheme C, nine boreholes are arranged at the target section (four boreholes are placed in the upper row and five boreholes are deployed in the lower row). Through the comparisons of the output current of the shearer and the consumption number of picks, the effect of hydraulic fracturing on improving the cuttability of the rock mass and assisting the mechanized rock fragmentation is assessed. Meanwhile, the final hydraulic fracturing scheme is selected. The technological parameters of the three schemes are listed in Table 2. The borehole layouts are shown in Figure 12. The mining height of the working face is 4.6 m. The center distance of the hydraulic supports is 1.5 m. The azimuth angle is the advancing direction of the working face, that is, the direction of the minimum horizontal principal stress. The length of the packer is 0.9 m. An air-leg rock drill with a 3 m drill rod and a 42 mm drill bit is utilized to drill a borehole. The borehole spacing is 3 m. The fracturing sequences in schemes A and B are the same, i.e., Borehole 1\(\rightarrow\)Borehole 2\(\rightarrow\)Borehole 3\(\rightarrow\)Borehole 4\(\rightarrow\)Borehole 5. In scheme C, the fracturing sequence is to fracture the boreholes at the lower row first and then fracture the boreholes at the upper row. The adjacent two boreholes are fractured simultaneously, that is, Borehole 1 and Borehole 3 are fractured at the same time, and then Borehole 5 and Borehole 7 are fractured in turn.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Scheme**} & **Number of** & **Borehole** & **Azimuthal** & **Dip Angle** & **Borehole** & **Row** & **Packer** & **Borehole** \\  & **Boreholes** & **Depth (m)** & **Angle (\({}^{\circ}\))** & **(\({}^{\circ}\))** & **Spacing (m)** & **Spacing (m)** & **Length (m)** & **Diameter (mm)** \\ \hline A & 5 & 3 & 106 & 0 & 3 & - & 0.9 & 42 \\ B & 5 & 3 & 106 & 45 & 3 & & 0.9 & 42 \\ \hline \multirow{4}{*}{C} & \multirow{4}{*}{9} & \multirow{4}{*}{3} & \multicolumn{4}{c}{The dip angle of the borehole at the upper row is 45\({}^{\circ}\); the dip angle of the borehole at the lower row is 0.} & The spacing of the borehole at the upper row is 0.} & The spacing of the borehole at the lower row is 0.} & \multirow{4}{*}{1} & \multirow{4}{*}{0.9} & \multirow{4}{*}{42} \\  & & & & & & & & & \\ \cline{1-1} \cline{4-10}  & & & & & & & & & \\ \cline{1-1} \cline{4-10}  & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Borehole parameters.  
Figure 12: _Cont._