Enhancing gas drainage efficiency in soft coal seams: Mechanically over-excavated cavities along borehole in floor tunnels

Abstract

In addressing the challenge of gas drainage in soft coal seams, this study introduces an innovative technique involving mechanically over-excavated cavities along boreholes in floor tunnels. Through a combination of laboratory experiments, numerical simulations, and field tests, we investigate the efficacy of this method in weakening the coal structure and enhancing gas extraction. The influence of cavity parameters (inclined angle, cavity length and diameter, cavity number) on the mechanical properties and AE behavior of coal specimens were investigated by biaxial compression tests. Besides, the stress distribution and permeability evolution around the cavity were studied using FLAC3D combined with COMSOL software. Finally, the field tests were conducted in LiCun Coal Mine. The results indicate that a significant reduction in the compressive strength of coal specimens with increasing cavity dimensions. Notably, the peak stress in coal specimens decreased significantly as the cavity length, diameter, and number increased, by up to 42.09%, 39.38%, and 57.83%, respectively. In contrast, the peak stress increased with the cavity inclination angle, and it was about 1.14 times higher for specimens with a 90° angle than for those with a 15° angle. It is also found that the cavity inclination reduces the damage and acoustic emission activity of the specimen, while the cavity volume increases them and weakens the rock-bearing capacity. The failure modes of all specimens are shear failure combined with surface spalling. Numerical simulations employing FLAC3D and COMSOL software corroborated these results, demonstrating enhanced pressure relief and increased permeability around the cavities. Field tests conducted in the LiCun Coal Mine provided practical evidence of the method's efficacy, with a more than 3-fold increase in gas drainage concentration and volume observed compared to traditional drilling methods. By adopting mechanically over-excavated cavities, not only is the efficiency of gas drainage significantly improved, but also the safety and productivity of mining operations are enhanced. This study offers a promising and impactful solution for addressing gas drainage challenges in soft coal seams.