# Improving Hard Rock Materials Cuttability by Hydraulic Fracturing at Mining Working Face
## 1 Introduction
### Experimental Setup  
Gelatin is a kind of light-yellow solid powder with a density of 1.2-1.4 g/cm\({}^{3}\) and has similar mechanical properties to rock materials (elastic and brittle) [26]. Using gelatin as the experimental raw material, [PERSON] and [PERSON] [15] investigated whether the HF propagation direction was perpendicular to the minimum in situ stress direction. [PERSON] [22] analyzed the propagation mechanism of HFs under different types of fracturing fluids. [PERSON] et al. [23] demonstrated that gelatin can be used as a physical model material to solve hydraulic fracturing problems and also concluded that both the gelatin strength and the initiation pressure have a great influence on fracture propagation. In this experiment, gelatin material is applied to observe the initiation and propagation trajectory of un-directional HFs. Hydraulic fracturing experiments can be divided into directional [27]  
Figure 1: In situ stress condition in this study (borehole parallel to the direction of \(\sigma_{3}\)). and un-directional ones. According to the types of prefabricated boreholes, they can also be classified into open borehole [28] and cased borehole experiments [27]. This paper aims to study the initiation and propagation mechanism of HFs under un-directional open borehole conditions, which is consistent with the field condition at a coal working face. A self-developed experimental system for un-directional hydraulic fracturing is composed of the sample preparation system, the pressure loading system, and the fracturing system. Figure 2 shows the schematic of the experimental system. The sample preparation system includes a glass stick (with dimensions of 1 cm (in diameter) \(\times\) 55 cm (in height) for pre-fabricating a borehole), a cold closet and gelatin powder. The pressure loading system contains an air compressor, a digital pressure gauge, a polyurethane (PU) tube, a T-pipe, an acrylic container (30 cm (in diameter) \(\times\) 35 cm (in height)) and a polyolefin (POF) shrink film. The fracturing system consists of a fracturing tube (1 cm (in diameter) \(\times\) 30 cm (in height)), which is wrapped by a set of O-ring seals to simulate the hydraulic fracturing packer and has several small fluid outlets at the lower end [29], a PU liquid injection tube, a digital flowmeter and a syringe pump. Gelatin powder (2 kg) and hot water at 80 \({}^{\circ}\)C (23 kg) are poured into a mixing container and stirred until the gelatin powder is completely melted [30]. After the gelatin solution in the mixing container cools down to room temperature, it is poured into the acrylic container. Meanwhile, a glass stick is placed in the gelatin solution at a specific angle. Then, the acrylic container filled with the gelatin liquid is placed in a cold closet for 24 h at 4 \({}^{\circ}\)C [31]. Hereafter, the glass stick is slowly removed from the solid gelatin sample to form a prefabricated borehole. After the sample is prepared, the pressure loading device is first connected according to Figure 2, and then the fracturing device is slowly placed in the prefabricated borehole. The distance of the fracturing tube to the bottom of the borehole is 5 cm. Afterward, the air compressor is started to pressurize the acrylic container, making the pressure constant at a pre-set value. [PERSON] et al. [30] set the confining stress on the gelatin sample in the range of 1400 to 4800 Pa; hence, the range of 1400 to 2400 Pa is adopted in the present study. After all parts of the experimental device are installed, the syringe pump is applied to inject plaster-of-Paris slurry into the packer device [15]. High-definition cameras are set up on both sides and above the acrylic container to simultaneously record the whole experimental process. Different from previous laboratory experiments, the experimental device and procedure described in Figure 3 can simulate the real situation of hydraulic fracturing at a coal working face. [PERSON] et al.