# Effect of Thick-Hard Main Roof Fracturing on the Spatiotemporal Evolution of Overburden Fractures
## 3 Advanced Abtment Pressure Response Characteristics of the Main Roof Fracturing Weakening
### Effect of Fracturing Mode on the Volume of Potential Gas Storage Area in Goaf  
As the working face just advanced by 100 m, during the process of the overlying strata transitioning from movement to stability, the volume of the goaf changed. This can be regarded as one of the causes of the gas disaster in that the gas that accumulated in the goaf was squeezed to the working face within a period of time, resulting in the overrun of the gas on the working face and affecting the safety of production. Through the binary processing of images, the variation law of the volume of the potential gas storage area in the goaf under different fracturing schemes was quantitatively analyzed. Since the model is a two-dimensional model, the thickness of the model was considered to be 1 m for the convenience of description and analysis. The simulation results were analyzed and processed by the binarization method mentioned in Section 4.2 above. When the main roof was not treated and the working face advanced by 100 m, the main roof became broken for the first time. The volume of the blank part of the goaf before and after the break was reduced from 461.2 m\({}^{3}\) to 309 m\({}^{3}\), a decrease of 33.01%. At this time, the reduced volume (152.2 m\({}^{3}\)) could be regarded as the main source of gas overrun in the initial mining stage of the working face. When the main roof was treated with vertical fracturing, horizontal fracturing, and Voronoi fracturing, respectively, the strength of the main roof was weakened, the bearing capacity decreased, the main roof was broken when the working face advanced by 80 m, the overburden fracture developed, and the gas migration channel and gas enrichment area were initially formed. The volume change of the blank part of the goaf when the working face advanced by 80 m and 100 m under different fracturing schemes is shown in Figure 9.  
In Scheme 1, the gas was squeezed into the working face when it advanced by 100 m, and this process occurred within ten minutes. In order to facilitate the analysis of the gas volume extruded to the working face, the following assumptions were made: (1) assuming that the gas is an ideal gas, the gas is incompressible during the extrusion process of rock stratum migration. (2) We assume that the total volume of gas remains unchanged, that is, the total volume of gas is conserved during the period from \"the overburden has just advanced to 100 m, and the overburden has not stabilized\" to \"the overburden migration is balanced, and the volume of the goaf no longer changes drastically\", and no new gas is added to the goaf. (3) We assume that the gas concentration fraction transferred to the overburden fracture is 100%. (4) We assume that all the reduced volume of the goaf after fracturing is transferred to the overburden rock, and the fragmentation and expansion coefficient of the soft rock are not considered. (5) We assume that the gas is evenly distributed in the goaf. When the main roof fracturing treatment was completed, part of the gas was transferred to the overburden fractures, and it can be seen from the gas diffusion law that the gas migrates to the overburden fractures under the drive of density difference. As is observed in Schemes 2 to 4, the actual volume of gas disaster caused by extrusion to the working face was lower than the volume change directly calculated. Taking the goaf gas concentration of 40% as an example, the pure gas volumes that caused the gas overrun in the initial mining stage of the working face were 24.95 m\({}^{3}\), 21.03 m\({}^{3}\), and 20.32 m\({}^{3}\), respectively. Compared with the 60.88 m\({}^{3}\) of Scheme 1, the decrease was apparent, reaching 59.02%, 65.46%, and 66.62%, respectively. 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 so as to solve the problem of overrun gas in the working face.