# Effect of Thick-Hard Main Roof Fracturing on the Spatiotemporal Evolution of Overburden Fractures
## 6 Conclusions  
1. Hydraulic fracturing of thick and hard main roofs significantly alleviates the advanced stress concentration in the working face. The numerical simulation analysis showed that the vertical fracturing scheme had the best effect on weakening the advanced abutment stress of the working face.
2. The main roof hydraulic fracturing achieved remarkable results in improving the fissure ratio of the overburden rock. Based on the binary method, this study calculated the spatiotemporal evolution law of overburden fractures and the dominant channel of gas migration when the working face advanced by 80 m and 100 m under the four schemes calculated. Among them, the fissure ratio of the left fracture zone of the horizontal fracturing scheme increased most significantly, reaching more than 3%; the width of the left fracture zone of the vertical fracturing scheme experienced the most drastic rise of about twice that of the non-fracturing scheme; and the fissure ratio of the gas migration channel increased to over 2.5%.  
Figure 12: Calculation results of fractal dimension of overburden fracture when the working face advanced by 80 m and 100 m under different schemes: (**a**) Scheme 1, unfractured; (**b**) Scheme 2, vertically fractured; (**c**) Scheme 3, horizontally fractured; (**d**) Scheme 4, Voronoi-fractured; (**e**) summary and comparison of the four schemes’ fractal dimensions.  
3. Fracturing weakening effectively reduced the potential storage volume of gas disasters in goaf and achieved remarkable results in controlling gas overrun in the initial mining stage. When the gas concentration in the goaf was less than or equal to 60%, the fracture space generated by the migration of the overburden after the main roof fracturing could meet the requirements of storing gas to reduce the gas concentration in the goaf, thus solving the problem of overrun gas in the working face. 4. The permeability model of \"permeability-fissure ratio-fractal dimension\" was established. When the scale coefficient and the maximum fracture length were constant, the permeability and fissure ratio were proportional to the fractal dimension. The box-counting dimension method was used to obtain the fractal dimension of the overburden fracture network when the working face advanced by 80 m and 100 m under the four schemes, respectively. The results showed that the overburden fracture network of the horizontal fracturing scheme had a larger fractal dimension, indicating that the overburden fractures under the horizontal fracturing scheme were more fully developed, thus more conducive to gas migration. 5. The fracture area of overburden is an important channel for gas uplifting, diffusion, and migration, and it is of great significance to study the morphology and fracture rate of the fracture area for gas extraction and coal safety mining. The research on the development and changes of fractures under the effect of the softening of rock mass generated by hydraulic fracturing water will be further developed in the future. Meanwhile, the flow of liquid and gas in fractures of different scales and the \"solid-gas-liquid\" coupling model will be further explored.  
Conceptualization, J.J. and Z.L.; methodology, J.J. and Z.L.; software, C.Q.; formal analysis, J.J.; data curation, J.J. and C.Q.; writing--original draft preparation, J.J.; writing--review and editing, Z.L.; project administration, Z.L. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by the National Key Research and Development Program of China (grant number: 2021 YFC2902101), the Natural Science Research General Program for Shanxi Provincial Basic Research Program (grant number: 202303021211060), and the Joint Funds of the National Natural Science Foundation of China (grant number: U22A20169).  
Not applicable.  
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.