Numerical study of changes in coal permeability in a mining workface.

At the front of a working coalface, the stress state can differ significantly across different zones. This seriously affects the permeability of the coal seam. In this study, we adopt a discrete element method to study the coal permeability under different stress states. The results indicate that an increase in stress (when sx = sy) causes the permeability to decrease significantly according to an exponential attenuation model. As the stress ratio (sx/sy) increases, the change in x-direction permeability is initially small. However, when the stress ratio reaches a certain value, the permeability starts to increase significantly. This is because of cracks that open under shear dilation. The change in coal permeability is examined in terms of the stress state at the front of the workface in three typical mining layouts. Our results show that the x direction permeability to the left of the peak stress increases slightly, but the y-direction permeability increases significantly. The unloading of x- and y-direction stress to the right of the peak stress causes cleats to open, resulting in a significant increase in both x and y-direction permeability. [ABSTRACT FROM AUTHOR]