# Effect of Thick-Hard Main Roof Fracturing on the Spatiotemporal Evolution of Overburden Fractures
## 1 Introduction  
Coal is an essential component of China's primary energy consumption, and ensuring safe and efficient coal mining is of great significance to energy supply, economic development, and social stability. With the increase in underground coal mining depth in China year by year, the phenomenon of high gas pressure and gas reserves in coal seams has become remarkably significant. After coal mining, a certain concentration of gas is stored in the goaf before the collapse of the hard roof. When the hard roof breaks for the first time, the gas accumulated in the goaf is instantly squeezed to the working face, resulting in the overrun of gas in the working face and even triggering major safety accidents such as gas explosions. During the roof falling process, a vast shockwave of \"air-gas\" mixture is formed from the goaf to the working face, and the instantaneous air volume generated can be called a mine hurricane. To ensure the safe production of coal mines, it is highly necessary to conduct in-depth research on the prevention and control methods of gas disasters caused by mine hurricanes. Preventing and controlling gas disasters is a crucial research object in coal mine safety production. For a long time, many experts and scholars explored the occurrence mechanism, evolution mechanism, and monitoring methods of gas overrun in the initial mining stage from different perspectives. The \"key strata (KS) theory\" [1; 2; 3] clarified the internal mechanisms of rock strata migration, the periodic pressure of working faces, and the development height of fracture zones in the process of coal mining, thus being widelyused in the field of geology and mining engineering. The initial collapse of the large hard roof and the coordinated migration of the overlying soft rock are the root causes of mine hurricanes. The prevention and control of mine hurricanes requires in-depth research on the migration of rock formations. Regarding the influence of load distribution, strength, block length, and other factors on the rotation velocity of voussoir beam structures formed after the breakage of KSs, [PERSON] et al. [4] established the equation of rotation motion of the block based on elastoplastic mechanics by a theoretical analysis and similar material simulations. It is believed that the angular velocity of the voussoir beam structure blocks increases with the rise of the overlying load and the reduction of block length. [PERSON] et al. [5] used a hollow inclusion strain gauge to measure the stress of the deep protoolith stress field. They concluded that the rock only absorbs a small part of the elastic strain energy, and most of the accumulated elastic strain energy is released suddenly, resulting in the fracture of the rock formation. The hydraulic fracturing process can effectively weaken the strength of rock formation and alleviate the concentration of advanced stress in the working face. The basic principle of hydraulic fracturing is to use high-pressure liquids to generate cracks and fissures in the rock mass which in turn weaken the rock mass's overall strength. In this case, the overburden is more prone to deformation and fragmentation as the working face advances. [PERSON] et al. [6] studied the weakening mechanism and effect of staged fracturing in vertical wells, staged fracturing in horizontal wells, and blasting pre-cracking on hard rock formations through theoretical analyses, numerical simulations, and a large number of industrial experiments. Natural fractured rock mass has high heterogeneity. Based on the fully coupled multi-scale numerical analysis method, [PERSON] et al. [7] studied the cases of fracture initiation and curvature near boreholes; the interaction between hydraulic fractures and natural fractures, multi-stage hydraulic fracturing, and discrete fracture network (DFN); hydraulic fracturing in production wells, and mine earthquakes induced by mining-induced stress changes, and evaluated the degree of various uncertainties.