## 4 Conclusions  
Severe mining-induced stress was observed in the I030902 working face return roadway of Liming Coal Mine, and hydraulic fracturing was used to relieve pressure on the narrow coal pillar roof. The following conclusions were drawn from the hydraulic fracturing experiments using gelatin material and on-site industrial hydraulic fracturing tests in the return roadway at the I03-0902 working face:  
Under the same gelatin concentration but different confining pressure conditions (in different key strata), the propagation law of hydraulic fracturing cracks exhibits significant differences. Under lower confining pressure conditions, the propagation of cracks is more symmetrical, with smaller deflection angles and deflection distances, and less shear failure. In contrast, under higher confining pressure conditions, the propagation of cracks shows more deflection, spiral propagation, and shear failure.  
Under the same confining pressure and gelatin concentration conditions, the arrangement angles of the pre-drilling holes have a significant impact on fracture propagation. When the holes are arranged at an inclined angle, the deflection distances and deflection  
Figure 16: The monitor data curve of the surface displacement of the surrounding rock and the stress condition of the bolts: (**a**) roof to floor convergence; (**b**) the amount of roadway between two sides.  
angles are smaller than those with a vertical arrangement, where the propagation of cracks exhibits more deflection and spiral propagation, resulting in a more complex trajectory. The asymmetric stress field intensifies branching and spiral propagation, and the fractures can transition from radial to axial propagation more quickly, forming radial cracks and promoting changes in the propagation mode.  
Based on the analysis of the on-site test results, it was found that after hydraulic fracturing, the roof pressure decreased by 17.8%, the average pressure step distance reduced by 18.0%, the average roof to floor convergence rate in the tunnel decreased by 63.86%, and the average roof to floor convergence rate dropped by 72.4%. The results verify the feasibility of hydraulic fracturing used in addressing the strong dynamic pressure caused by mining stress in the return roadway and controlling the deformation of the tunnel by forming a weak structural structure in the hard roof.  
In the future, we will focus on the in-depth study of stress shadow effects in transparent gelatin materials.  
Conceptualization, T.S. and Z.L.; data curation, Z.L. and B.L.; formal analysis, T.S. and B.L.; funding acquisition, Q.H. and D.M.; investigation, Z.L., D.M. and X.G.; methodology, T.S. and Q.H.; project administration, Q.H.; supervision, Q.H.; validation, T.S., Z.L., B.L. and X.G.; visualization, D.M., B.L. and X.G.; writing--original draft, T.S. and Z.L.; writing--review and editing. D.M., B.L. and X.G. 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) and the National Natural Science Foundation of China (U23B2091).  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.  
Author [PERSON] was employed by the Shaanxi Bureau of State Mine Safety Supervision Bureau. Author [PERSON] was employed by the Inner Mongolia Limin Coal Char Co., CHN Energy Wuhai Energy Company. The remaining authors declare that this research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.