# Improving Hard Rock Materials Cuttability by Hydraulic Fracturing at Mining Working Face
## 1 Introduction  
Currently, the application of hydraulic fracturing in coal mining is mainly targeted at improving the cuttability of hard roofs to enhance the coal seam permeability [1, 2]. The essence of the art of hydraulic fracturing is to inject high-pressurized fluid into coal and rock masses so as to increase artificial fractures in the coal and rock masses.  
In northern Shaanxi Province, China, hard geological structures (such as faults [3] and collapse columns [4]) are often revealed and impede the normal cutting of the shearer during the coal excavation. Borehole blasting, as a traditional method for hard rock pretreatment [5, 6], has some disadvantages, such as limited access to explosive materials, high construction cost, high labor intensity and harmful gas production during the blasting process [7, 8, 9]. Therefore, hydraulic fracturing, as a safe, green and efficient pretreating measure for coal and rock masses, has the potential to be a substitute for traditional blasting in promoting the cuttability of the rock mass and facilitating mechanized rock fragmentation. Specifically, in this paper, the objective is to solve two crucial issues: (1) revealing how HFs initiate and propagate at the hard geological structure region of a coal working face under a complex stress environment; (2) understanding how to determinereasonable on-site fracturing parameters to assist the shearer in cutting hard geological structures smoothly.  
Figure 1 describes the stress conditions at the hard geological structure region of a coal working face. As can be seen in Figure 1, regarding the stress condition involved in this paper, the borehole is parallel or nearly parallel to the minimum principal stress direction, while in most hydraulic fracturing numerical simulations [10; 11; 12], the borehole is perpendicular to the minimum principal stress direction. Moreover, previous laboratory experiments on hydraulic fracturing mainly focus on the intersection of fractures [13; 14], whether the fracture is perpendicular to the minimum principal stress or not [15] and the fracture re-orientation under the influence of artificial directional fracturing [16]. The propagation trajectory of HFs after un-directional fracturing under the stress and the borehole conditions in Figure 1 has not yet been studied.  
In this work, the stress and the borehole layout conditions shown in Figure 1 are constructed. Hydraulic fracturing experiments are performed in transparent gelatin samples to obtain the initiation and propagation mechanism of HFs. This is to investigate the pretreatment effect of single-hole hydraulic fracturing at the hard geological structure region of a coal working face. Moreover, field fracturing parameters are optimized. The feasibility of hydraulic fracturing in terms of improving the cuttability of the rock masses and hence assisting the shearer in rock fragmentation is also verified.  
Experimental Study on Initiation and Propagation of Un-Directional Hydraulic Fracturing in Transparent Samples  
At present, most experiments on the initiation and propagation of un-directional HFs are carried out by the use of rock or concrete samples [17; 18; 19; 20; 21]. However, the initiation and propagation process of HFs inside the rock or the concrete is invisible. A hydraulic fracturing experiment in gelatin is able to visibly simulate the initiation and expansion of HFs in rock masses [22; 23; 24; 25]. Therefore, gelatin is used as the raw material to make samples for un-directional hydraulic fracturing experiments.