Evaluation of the weakening behavior of gas on the coal strength and its quantitative influence on the coal deformation

The coal strength and deformation properties are key factors affecting safe coal mining and high-efficiency coalbed methane (CBM) development. In this paper, reconstituted coal samples are chosen to investigate the weakening behavior of gas on coal strength, meanwhile, its effects on coal deformation are quantitatively evaluated. The results indicate that the weakening degree of gas on coal strength is closely related to the confining stress and gas pressure. Compared with non-gas-saturated coals, the maximum weakening ratios of adsorbed gas to coal strength are 10.58%, 18.12%, 8.55% and 14.65% under the conditions of confining stress CS = 3 MPa and gas pressure GP = 1 MPa, CS = 3 MPa and GP = 2 MPa, CS = 4 MPa and GP = 1 MPa, and CS = 4 MPa and GP = 2 MPa, respectively. Furthermore, the maximum weakening ratios of free gas to coal strength are 18.27%, 36.54%, 14.79% and 29.58%, respectively, under above four conditions. The maximum coal bulk strain decreases as particle sizes of coal powders increase, and it has a maximum value of 0.0227 and a minimum value of 0.0191 in particle size ranges of 0.01–0.041 and 0.5–1 mm. Under the same conditions, the coal bulk strain increases with increasing gas pressure, revealing that coal deformation properties can be enhanced by gas.