Fractal and volume characteristics of 3D mining-induced fractures under typical mining layouts

Mining-induced fractures, which can release natural gas and enhance gas migration, are indispensable in the co-extraction of coal and methane (CECM). Investigations into mining-induced fracture systems ahead of the coalface are beneficial in elucidating the complex differences between mining processes and their related static and dynamic strata stress fields. Triaxial tests were conducted on coal samples to simulate the mining-induced mechanical behaviors of the coal seam under three widely used mining processes, i.e., top-coal caving mining (TCCM), non-pillar mining (NPM) and protected coal seam mining (PCSM). Mining-induced fractures were created at the laboratory scale. An industrial computed tomography scanning system was employed to scan the ruptured coal samples and measure the geometric characteristics of the fractures associated with each simulated process. Coronal and sagittal views as well as a precise 3D solid geometrical model were reconstructed for each process. It is shown that the mining layout influences the spatial morphology of mining-induced fractures. Using the box-counting method, a quantitative fractal characterization of the fracture system was estimated for each mining method. The estimated average fractal dimensions of the fracture systems generated in the simulated TCCM, NPM, and PCSM processes were 2.0557, 2.0362 and 2.0129, respectively. Additionally, a scale-independent fracture intensity, FI, was defined to further characterize the volume features of the mining-induced fracture systems. The fracture intensity was 0.6 for the NPM process, 0.5 for TCCM and 0.21 for PCSM.