LAUSR

The permeability of the caved zone in a longwall operation impact many issues related to ventilation and methane control, as well as to interaction of gob gas ventholes with the mining environment. Insofar as the gob is typically inaccessible for performing direct measurements of the stresses and permeability, the latter values of the caved zone have to be assessed indirectly, which requires the application of the most reliable prediction techniques. To study the permeability evolution of the broken coal and its influencing factors during the coal seam group repeating mining, the particle deformation of the broken coal sample (BCS) is assessed in this study based on the Hertz contact deformation principle. Using the experimental results of the BCS cyclic loading and unloading seepage tests, the effect of BCS parameters on the stress sensitivity for permeability is analyzed. The laboratory test results imply that the re-crushing, re-arrangement, and compressional deformation of particles in the loading process lead to a drastic drop in the caved zone porosity causing the permeability reduction. During the unloading process, only the permeability loss caused by the particle deformation can be recovered. The secant modulus of BCS during unloading is stable and can be assessed by fitting the permeability stress curves. The stress sensitivity of the BCS permeability during unloading process drops with an increase in the secant modulus, while the re-crushing capacity and re-arrangement ability of BCS particles gradually deteriorate due to an increase in the secant modulus with the number of loading cycles. The effect of Poisson’s ratio on the permeability stress sensitivity at the later loading/unloading stages is found to be quite feeble, while the stress sensitivity is indirectly related to the particle size via the secant modulus: the greater the particle size, the higher the unloading stress sensitivity.