Evolutionary mechanisms and models of strain, porosity, and permeability of compacted broken coals

Residual coal areas in caving zones of goaves in coal mines serve as primary sites for low-temperature reactions between coal and oxygen. Hence, exploring the evolutionary models of the strain, porosity, and permeability of coals in these areas holds great significance for gaining a deep understanding of the spontaneous combustion process and regularity of coals. Based on the self-developed experimental device for gas seepage in compacted broken coals, this study explored the evolutionary patterns of the strain, porosity, and permeability of compacted coals with single/mixed particle sizes during gas seepage. Furthermore, this study analyzed the variations in the strain, porosity, and permeability with stress under different particle sizes. The analysis results show that the variation process can be divided into two stages: the linear variation stage in the case of the axial stress ≤ 6 MPa and the exponential variation stage under the axial stress >6 MPa. The variation process preliminarily indicates that the deformation, porosity, and permeability of compacted broken coals exhibited consistent, particle size-independent variation mechanisms. The axial stress of 6 MPa was proved to be a critical node connecting two different variation mechanisms of strain, porosity, and permeability. Under axial stress ≤ 6 MPa and >6 MPa, the varia-tions in the strain, porosity, and permeability primarily resulted from the compression and slip of coal par-ticles, respectively. In the case of axial stress of 6 MPa, the strain, porosity, and permeability of compacted broken coal with different particle sizes exhibited one-to-one mapping with their variation paths. Accordingly, this study established the stress-strain, stress-porosity, and stress-permeability models. As revealed by the comparison and verification results, the models, despite being derived from experiments of broken coals with various particle sizes, were not influenced by the coal particle sizes and agreed well with the experimental results, yielding satisfactory effects. The results of this study can provide a scientific basis for the early prevention and control of the spontaneous combustion of coals in goaves of coal mines.