A New Model for Calculation of the Plastic Compression Index and Porosity and Permeability of Gob Materials in Longwall Mining

In coal longwall mining the accuracy of gob methane drainage system design, control of methane leakage into the longwall stope, and control of spontaneous combustion potential all depend on the porosity and permeability of the gob material. However, longwall mining conditions do not allow for any direct measurements of the gob material characteristics. Therefore, the simulation of gob material as a granular medium contributes to the ability to predict gob porosity. Many features of a granular medium, like gob material, are directly and indirectly affected by its particle size distribution. Fractal models are commonly used for porosity prediction which allows for a better understanding of how porosity can change with stress states based on the fragmentation size distribution of gob material. The plastic compression index (PCI), which describes the degree of compression of the broken material in gob, can be calculated using the physical properties of the broken material. However, using the existing fractal models, it is difficult to determine this parameter because of the multiplicity of parameters. In this research, a new fractal model was developed to facilitate the calculation of the PCI parameter and to increase the accuracy and validity of the results. Using data from the Tabas coal mine, the porosity-stress and permeability-stress curves were investigated for the gob material. The porosity results from this study agreed with those of Fan's gob compression model in a comparison. Although the obtained permeability results were within the range of previous studies, Berg's permeability model provided a better tool for estimation of the gob permeability.