Mechanical behaviors of anthracite coal subject to low-cycle compressive differential cyclic loading (DCL) after wetting-drying (WD) treatment: an experimental study

Stability of coal pillar (walls) is of great importance to security of underground mining. Periodic excavation and water table variation subject coal pillars (walls) to cyclic load with varying stressing rates and wetting-drying (WD) circumstances. Therefore, investigation on mechanical responses of coal exposed to effects of WD and compressive differential cyclic loading (DCL) was conducted to experimentally reveal coal's mechanical behaviors. 22 cylindrical coal samples, 7 WD schemes (natural, soaking, 0, 10, 20, 30, 40 WD cycles) and 2 DCL loading modes (loading rate is 4 times of unloading rate and vice versa) were applied. The measurements covered mass loss, P-wave velocity, energy dissipation, crack characterization and hysteretic phase shift. P-wave velocity shows an exponential attenuation versus WD cycle. Mode 1 possessing loading rate 4 times of unloading rate corresponds to a higher rate of premature failure, incurring 4 out of 7 samples failed in cyclic stage, whereas all 7 samples survived in cyclic stage in mode 2 with opposite stressing rates. Stressing rate induced impact on deformation and energy dissipation is qualitatively revealed, with 6 out of 7 groups exhibiting more dissipated energy (approx. 10-50% larger than mode 2) in mode 1. More dissipated energy is attributed to a pronounced phase lag incurred by a higher loading rate and elastic-after effect. WD action induced meso-cracks were characterized and the width of crack is proportional to applied WD cycles. The testing results can provide practical guidance for in-situ application, such as evaluating the stability of coal mine based ground water reservoirs.

Automated summary

The mechanical responses of coal exposed to wetting-drying conditions and compressive cyclic loading were investigated to reveal its behaviors. Experimental results showed that wetting-drying cycles had an exponential attenuation effect on the P-wave velocity of coal, while different loading rates led to different failure modes and energy dissipation. In addition, wetting-drying action induced macro-cracks, and the width of the cracks was proportional to the number of cycles. These testing results provide practical guidance for evaluating the stability of coal mines in in-situ applications.