# Improving Hard Rock Materials Cuttability by Hydraulic Fracturing at Mining Working Face
## 4 Conclusions  
This paper proposes that hydraulic fracturing can be used to improve the cuttability of the hard rock mass and assist the shearer in cutting the hard fault at a coal mining working face. Hydraulic fracturing experiments in transparent gelatin samples are carried out to study the initiation and propagation mechanism of un-directional HFs under different conditions. The feasibility of the hydraulic fracturing pretreatment to the hard rock mass for rock fragmentation is verified through field tests at the 51207 working face of Guojiawan Coal Mine. The following conclusions are obtained.  
* When the borehole axis is parallel to or intersecting with the minimum principal stress, axial and radial HFs initiate simultaneously in the process of un-directional hydraulic fracturing. Under a low differential stress, the HF tends to propagate radially. With the increase in the differential stress, the HF gradually re-orientates axially. Due to the differential stress, both axial and radial fractures eventually propagate along the plane perpendicular to the minimum principal stress direction. When the borehole intersects the minimum principal stress (e.g., 45\({}^{\circ}\)), both axial and radial HFs re-orientate more quickly towards the plane perpendicular to the direction of the minimum principal stress, and the re-orientation distance is within three times the borehole diameter.
* Hydraulic fracturing can enhance the cuttability of the rock mass and assist the shearer in breaking rocks. After hydraulic fracturing pretreatment, the cutting current ratio of the shearer is reduced by 51.70% and the cutting efficiency is increased by 89.93%. The average time of fracturing a single borehole is 6.2 min.  
* Compared with a single layout of boreholes (only horizontal holes or only inclined holes), the layout of inclined boreholes at the upper row and horizontal boreholes at the lower row can be used for multi-dimensional pretreatment of the hard rock mass. The average current ratio of the shearer, the consumption number of picks and the cutting time required by the shearer passing through the fault area decrease significantly.  
Conceptualization, Q.H., D.M. and G.L.; Methodology, D.M.; Validation, J.Y.; Investigation, J.C.; Resources, Q.H.; Writing--original draft, H.L.; Writing--review & editing, B.L.; Visualization, H.Y.; Supervision, Y.G.; Project administration, Q.H.; Funding acquisition, D.M. All authors have read and agreed to the published version of the manuscript.  
The work is financially supported by the National Key R&D Program of China (2022 YFC2905600), the National Science Fund for Excellent Young Researchers of China (52122404) and the National Natural Science Foundation of China (U23B2091).  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
Author [PERSON] was employed by the company Jiangxi Copper Company Limited. Author [PERSON] was employed by the company China Railway 19 th Bureau Group Corporation Limited. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.