## 2 Experimental Configuration
### Self-Developed Experimental Device  
Figure 1 shows the schematic of the self-developed experimental device for studying DHF. The experimental device mainly consists of three parts: a container for placing gelatin samples, a pressurizing device, and a fracturing device. Each gelatin sample is cylindrical (with dimensions of 30 cm (in diameter) \(\times\) 30 cm (in height)). The experimental container for the gelatin sample mainly consists of a polyvinyl chloride (PVC) pipe (with a diameter of 32 cm) and a cylinder-shaped polyolefin (POF) shrink film (with a diameter of 30 cm). Glass cement is squeezed into the gap between the PVC pipe and the POF shrink film (at the top and the bottom) to form a confined space. In each test, an air compressor is used to produce air pressure in this confined space in order to apply the confining stress to the gelatin sample [27; 28]. The magnitude of the confining stress is monitored by a digital high-resolution (1 Pa) pressure gauge over the whole test. a horizontal radial confining stress is applied to the sample to induce the required differential stress for HF re-orientation (the confining stress in the vertical direction is zero) [25]. The gelatin type chosen for this experiment is 220 power, and the manufacturer is Fuyuan Gelatin Company. Weighing the specific weight concentrations of gelatin powder (i.e., 6%, 8%, and 10%) into about 80 \({}^{\circ}\)C hot water to fully melt into gelatin fluid. After gelatin fluid is poured into the experimental container (inside of the POF shrink film), a deviated plastic stick is put into the fluid and fixed by a wood plate. Then the experimental container is placed in a refrigerator for 24 h at 4 \({}^{\circ}\)C to solidify the gelatin fluid [31]. Afterwards, the plastic stick is removed from the solid gelatin sample to leave an open space in the sample as the hydraulic fracturing borehole (with a diameter of 10 mm). A steel cutting tool is used to create a radial initial notch around the borehole wall as an artificial weakness for DHF. The notch has a depth of 10 mm, which means the notch diameter is twice the borehole diameter [32]. Different from the metal molds used in previous studies (to simulate initial notches) [14], the steel cutting tool in this study creates a realistic radial notch in the gelatin sample, and the fluid pressure can directly act on the notch surfaces to effectively simulate the stress concentration induced by the initial notch in DHF. Then an injection tool is inserted into the open borehole, which includes a set of O-ring seals to simulate the hydraulic fracturing packer and several small holes as outlets for fracturing fluid [33]. The injection tool connects a syringe (with a maximum capacity of 500 mL) through a plastic tube. Plaster-of-paris slurry is taken as fracturing fluid since this material is allowed to set after hydraulic fracturing, thus providing a permanent record of HF geometries [25]. At this stage, the experimental device in this study basically has two limitations. First, the true triaxial confining stress condition is unrealized, and only a radial confining stress is applied to the gelatin sample in the horizontal direction. [PERSON] et al. [12; 34; 35] demonstrated that HF re-orientation is dictated by differential stresses, and two different in situ stress conditions result in an identical HF re-orientation trajectory if they have the same differential stress magnitudes. Therefore, the gelatin sample in this study is radially confined in the horizontal direction to create the differential stress for DHF in a reverse faulting in situ stress state, which is common at block cave mines in Australia and coal mines in the northwest of China [36; 37]. In addition, [PERSON] and [PERSON] [25] also hydraulically fractured a cylindrical gelatin sample sustaining radial confining pressure to examine the HF propagation plane relative to the minimum in situ stress direction.