A new experimental apparatus for sudden unloading of gas-bearing coal

To study the characteristics of the deformation and failure of coal during a coal and gas outburst, we developed an experimental apparatus that subjected coal samples to sudden unloading under various gas conditions. The apparatus consisted of a coal sample cell, a fast pressure unloading system, a monitoring and control system, and a gas filling system. The gas filling system was equipped with safe, explosion-proof solenoid valves for gas pressure unloading and an accurate strain chip for measuring sample strain. The apparatus was designed to probe the influence of the gas on coal and gas outbursts. The study focused on the mechanism by which high-pressure gas influences coal and gas outbursts, leading to the deformation characteristics of the sudden unloading process. The experimental results showed that the plastic deformation of coal samples in the presence of methane gas can be divided into three stages: compression induced by increased pressure, expansion induced by adsorption, and deformation or failure induced by sudden unloading. The results of tests performed in the presence of helium revealed that the deformation could be divided into just two stages: a compression stage induced by increased pressure and a recovery stage induced by sudden pressure unloading. Coal samples were found to deform much more rapidly and to a greater degree in methane than in helium, and the expansive deformation and failure of the coal sample become increasingly apparent with increasing initial gas pressure. Overall, our results demonstrate that the experimental apparatus developed here provides a feasible means to test the characteristics of the coal damage and gas migration that occur during a coal and gas outburst.