Evolution of Acoustic Emission and Microcrack in Jointed Coal under Loading and Their Correlations

Understanding of continuous damage and cracking process of compressed jointed coal is critical to predict its stability. Herein, for jointed coal under uniaxial compression with bedding planes perpendicular (sample A) and parallel (sample B) to the loading direction, an acoustic emission (AE) monitoring device and a computed tomography (CT) scanner were employed to study the damage and cracking process. Furthermore, the correlations between the AE signals and internal fracture parameters were analyzed. Results show that (1) shear and tension cracks are respectively main damage forms in samples A and B; (2) there is a drop valley of variations of AE count fractal dimension (D-1) over time before the stress peak, and compared with sample B, the drop valley for sample A are much wider; (3) many fractures between joint planes formed at yielding and postpeak stages, fracture fractal dimensions (D-2) first increase slightly with loading and then increase significantly at yielding and postpeak stages; especially D-2 of sample A is larger than that of sample B; (4) the accumulative value of D-1 (AD(1)) increases with D-2 monotonously, and an expression of porosity or connectivity of the compressed jointed coal was developed, by which the porosity and connectivity of jointed coal could be calculated. The study outcomes could contribute to predict coal stability and the variations of water and gas flowing channels in seam in underground coal mines.