Mechanism of dilatation catastrophe of coal and gas outburst

Coal and gas outburst mechanism, as a century old global problem, has entered a bottleneck period in its prevention and control at this stage. Faced with the continuous occurrence of outburst accidents, it is still difficult to answer numerous scientific questions behind them, such as the coupling effect among in situ stress, gas pressure, and coal mechanical strength in the process of outburst incubation, and the characteristics of the main control stages are not yet clear. Therefore, based on the mechanical background triggered by typical outburst accidents, starting from the dynamic response law of mechanical damage in mining coal masses, in this work the dilatation characteristics in the process of coals damage are revealed, clarifying the law of mutual feedback response between mining stress and gas pressure. And the main control mechanism of initial failure of coals in the process of an outburst incubation. Then the dilatation catastrophe mechanism of coal and gas outburst is proposed. The results show that the dilatation characteristics in the damage and instability process of coal containing gas can be divided into four stages: slow dilatation, rapid dilatation, sudden dilatation, and surged dilatation. And the gas pressure undergoes the enhancement of strengthening action ability (slow dilatation), the decrease of strengthening action ability (rapid dilatation and sudden dilatation), and and the weakening gas pressure action ability (after rapid dilatation, or sudden dilatation, or surged dilatation). The original strength of coals cannot determine the actual degree of coal failure, or even the difficulty of failure. The key lies in the stress loading and unloading path. The essence of an outburst is a sudden change dynamic process with typical time attributes (known as the “dilatation catastrophe mechanism” of outburst) that occurs because of dilatation catastrophe phenomenon of coal containing gas during the phased transformation process of main controlling action between in situ stress and gas pressure, causing structural instability of coals and being ejected out. The incubation process of an outburst can be divided into three stages: firstly, the coal masses undergo initial damage (slow dilatation and rapid dilatation) under the dominant action of in situ stress with the assistance of gas pressure, which is called the first stage of outburst incubation; Then, the coal masses undergo regional spallation under the dominant action of gas pressure with the synergistic effect of in situ stress, and further fragmentation occurs (sudden dilatation and surged dilatation), which is called the second stage of outburst incubation; Finally, under the action of gas pressure, the fractured coals undergo fragmentation and pulverization (i.e. dilatation catastrophe), which is called the third stage of outburst incubation. In turn, the fractured coals are thrown out under the action of gas pressure, triggering an outburst. The research results can provide a new perspective for deepening the understanding of coal and gas outburst mechanism, with laying a theoretical foundation for the innovation of outburst prevention and control technologies.