Stress-strain-acoustic responses in failure process of coal rock with different height to diameter ratios under uniaxial compression

Residual coal pillars play an important role in mining the adjacent coal seam safely, managing the gobs and maintaining the stability of abandoned coal mines. The height to diameter ratio (H/D) affects the stability of residual coal pillars. In this study, uniaxial compressive tests of coal specimens with five H/D (2.0, 1.5, 1.0, 0.8 and 0.6) were performed, and the stress, strain and acoustic emission (AE) were monitored. Results show that the uniaxial compressive strength (UCS) and peak strain increase with H/D decreasing. An empirical equation is proposed to calculate the UCS based on the H/D. The AE activities during coal failure process can be separated into four periods. The span of quiet period and rapid decline period shorten with H/D decreasing. The smaller the H/D is, the more complicated the failure characteristics of coal will be. The failure form of coal with H/D of 2.0, 1.5, and 1.0 is primarily shear failure, while splitting failure along the axial direction is the mainly mode when H/D is 0.8 or 0.6. The initiation, expansion, aggregation and connection of micro-cracks can be reflected by the real-time spatial evolution of AE event points.