An Analysis of the Layered Failure of Coal: New Insights into the Flow Process of Outburst Coal

C oal and gas outbursts are among the most destructive dynamic disasters. It is very important to study the failure process and mechanism of coal and gas outbursts in coal mines. Based on outburst cases and experiments, the outburst process and failure types of coal are discussed, and the flow process of outburst coal is studied in depth. This article concludes that the whole process of outburst generally involves four stages (preparation, trigger, development, and termination), which include both time and spatial dimensions. There are two types of coal failure: shear failure and tensile failure. Under the effect of the gas pressure gradient, the coal exhibits tensile failure, and the coal ultimately fails in the form of multiple similar layers. After the layer is separated, the pressure difference between the two sides of the layer is found to be stable for a period of time, and a pressure difference coefficient, lambda(i), is introduced to characterize the change of this pressure difference before and after the separation of the layer. Furthermore, the flow process of outburst coal is divided into a quasi-static phase, a constant acceleration phase, a freefall phase, and a deceleration phase. Accordingly, a simplified model of the coal flow is established, and the model is calculated using the data from outburst experiments. Based on the layered failure of outburst coal, the flow velocity of outburst coal is obtained. The distribution features of outburst coal in both the experiments and the outburst cases are explained as well.