Drilled-hole number effects on energy and acoustic emission characteristics of brittle coal

Abstract

Energy accumulation and release are the essence of rock deformation and damage. Thus, investigating the damage mode and energy evolution law of coal specimens containing holes to elucidate the mechanism of borehole pressure relief is important. To examine the effect of the number of drilled holes on the energy and acoustic emission (AE) evolution characteristics of brittle coal, uniaxial compression AE tests were performed using four types of drilled coal specimen. The results revealed that the peak strengths and elastic moduli of the specimens exhibited nonlinear and linear decreasing trends, respectively, with the increase in hole number; however, the peak strain initially decreased and subsequently increased. The failure mode changed from shear splitting to penetration crack damage between rock bridges, accompanied by a hole collapse phenomenon. As the hole number increased, the total and elastic energy exhibited a decreasing trend, whereas the ratios of the elastic and dissipative energy to total energy exhibited a decreasing and an increasing trend, respectively. The AEs of different types of drilled coal specimens had similar evolutionary processes, but their activity exhibited an overall increasing trend with an increase in hole number. The AE fractal characteristics of a drilled coal specimen were calculated based on the G-P algorithm. The AE signal of the specimen exhibited fractal characteristics, and the sudden drop in fractal dimension was considered the precursor of specimen failure. The findings provide a reference for understanding the energy evolution mechanism and advance warning for coal pressure relief.