# Investigation into Influences of Hydraulic Fracturing for Hard Rock Weakening in Underground Mines
## 1 Introduction  
Currently, over 40% of fully mechanized mining faces in underground coal mines in China experience significant problems with hard roof pressure, especially those with extremely thin direct roofs [1; 2]. How to effectively control the hard roof over the working face has become the focus of current research. If the hard roof cannot naturally collapse from above the working face, large areas of suspension on the working face occur, resulting in excessive roof pressure on hydraulic support systems [3; 4]. In addition, if large areas of the suspended roof collapse due to roof cutting, severe impact loads would be generated, seriously threatening the safety of coal mining operations [5; 6; 7]. Hydraulic fracturing is a safe and efficient method for cutting hard rock layers, and its application range is constantly expanding [8; 9; 10; 11]. By injecting high-pressure water into the hard roof, cracks appear in the hard roof and expand, thus changing the structure of the rock layers [12]. After fracturing treatment, multi-level water pressure cracks and secondary cracks appear inside the hard roof, which damage the structural integrity of the rock, weaken its overall strength, and ultimately achieve the goal of reducing pressure [13; 14]. The core research content of hydraulic fracturing technology is the fracturing mechanism, which mainly includes crack initiation and crack extension. [PERSON] [15] found that regardless of the permeability of the fracturing fluid, the direction of fracture propagation on the fracturing surface always remained perpendicular to the direction of the minimum principal stress. This conclusion provides an important basis for further research on hydraulic fracturing technology. In areas affected by geologicalstructural stress, especially near the faults, the minimum principal stress is usually in the horizontal direction. In the geological structure compression area, the minimum principal stress is in the vertical direction, and the hydraulic fracturing surface extends horizontally. [PERSON] [16] described that when the release rate of strain energy increases to a certain extent, cracks experience unstable failure and propagation. When the elastic strain of cracks exceeds the tensile strength of the material, it continuously expands until it reaches a critical value. According to the theory proposed by [PERSON] [17], cracks usually develop in a direction that generates higher strain energy release rates. Once the predetermined energy level is reached, cracks rapidly expand. [PERSON]. et al. [18] proposed a calculation model based on the tensile strength criterion, which can effectively predict the development of hydraulic fractures. By analyzing the level of geostress and the orientation of the wellhead, this model can accurately predict the hydraulic fracturing pressure and the formation and expansion degree of fractures. Extensive research has been conducted in the area of hydraulic fracturing mechanisms [19; 20; 21; 22; 23]. [PERSON] et al. [24] studied the entire hydraulic failure process, including hydraulic fracture initiation and crack propagation through true triaxial testing and a finite element numerical simulation. The effects of the groove angle, length, and injection flow rate of fracturing fluid on directional hydraulic cracks were studied. By analyzing uneven forces, the formation and development of cracks were predicted. [PERSON] et al. [25; 26] studied and explored the propagation of hydraulic cracks in elastic and ductile media, with a focus on the geometric shape of hydraulic cracks under different fracturing times. The real-time monitoring of hydraulic crack propagation behavior in two-dimensional confined gelatin samples was conducted by changing the stiffness of the gelatin, the viscosity of the fracturing fluid, and flow rates. [PERSON] et al. [27] studied the interaction between natural cracks and horizontal radial hydraulic cracks through theoretical analysis and numerical simulation methods.