Investigation of the formation mechanism of coal spallation through the cross-coupling relations of multiple physical processes

Coal spallation is a visible product of outburst, and it represents a unique failure type for coal. A theoretical analysis of coal spallation was constructed to study its formation mechanism. Then, physical experiments and numerical simulations were performed to determine the physical features of coal spallation and evaluate the theoretical analysis. In addition, the effect of gas, stress and coal strength on coal spallation was discussed. The results show that coal spallation is a comprehensive product of multiple physical factors, including stress, gas and the coal strength. The formation process is related to stress transfer and mechanical behavior changes, gas migration and the evolution of the coal strength. Coal is eventually subjected to tensile failure, and the mechanical condition for coal spallation requires that the gas pressure difference (pi−pa) near the exposed surface is enough to overcome the tensile strength (σt). The gases in coal provide the power necessary for coal spallation; thus, coal gas promotes plastic failure and is the leading factor for tensile failure. Stress is the leading factor for plastic failure and has a great influence on the extent of coal spallation. Meanwhile, with an increase in the coal strength, which represents the resistance to coal spallation, the acreage of the plastic area decreases, and thus, a greater pressure difference is required for tensile failure.