Fractal evolution and connectivity characteristics of mining-induced crack networks in coal masses at different depths

In situ capture of crack network evolution is extremely difficult but necessary in fundamental research on mining mechanics and coalbed methane extraction. Systematic field monitoring of the mining-induced crack networks near three mining faces with vertical depths of 700 m, 850 m, and 1100 m in the Pingdingshan coal mine in China was conducted. By utilizing fractal geometry and an algorithm that predicts the connectivity rate, the changes in the fractal dimension and connectivity of the mining-induced crack network in these coal-rock masses with mining face advance were determined. The results indicate that the evolution of the crack network can be divided into three stages: a slow linear growth stage, an exponential growth stage, and a stable growth stage. As depth increases, the crack connectivity in a coal mass increases. The ranges of the mining influence zones at mining faces with vertical depths of 700 m, 850 m and 1100 m are approximately 50 m, 60 m and 75 m, respectively. The distances between the maximum mining pressures and the three corresponding coal mining faces are 15 m, 18 m and 25 m, respectively. At a depth of 1100 m, the crack connectivity of the coal-rock mass is 1.36 times that at a depth of 850 m and 1.58 times that at a depth of 700 m. A high crack connectivity can easily arise in thousand-meter-deep mines, which can lead to increased gas emissions and pose risks to production safety at the mining operation face.