Continuous Compaction and Permeability Evolution in Longwall Gob Materials

Understanding the evolution of gob compaction and related gas transport behavior is necessary for the planning and optimization of gas ventilation and control in longwall coal mines. In particular, the detachment of the undermined roof into the gob leaves a loosely compacted perimeter that skirts the longwall panel. This permeable gob perimeter in plan view forms as a result of shear separation from support provided by the solid ribs. This detachment and the resulting rotated and reduced stresses limit compaction, elevate permeability and exert significant control on gas flow during active longwall mining operations. We report gob compaction experiments on in-mine-collected fragmented rock and conduct mechanical compaction on stacked samples that are either uniformly coarsening upwards (case A) or are coarsening upwards, but capped by a segregated upper layer of coarse rock (case B). Observed compaction is linked to a capillary model representing porosity reduction and permeability evolution. As applied uniaxial stress increases from 0 to up to similar to 2000 kPa, the porosity decreases from 0.64 to 0.41(similar to 36%) for the uniform stacked material (A) and but only from 0.66 to 0.51 (similar to 23%) where the gob is topped with a layer of coarse "roof" rock simulants (B). Particle-particle self-adjustment dominates the compactive behavior at initial low stress and results in significant strain-followed by a linearly elastic region through the remainder of loading. The elastic regime is used to predict the permeability of the loosely compacted gob, considering the redistribution of stresses induced by shear collapse at the rib. Permeability evolution is scaled through the evolving compactive strains and particle size distribution of the fragmented rock, enabling results to be up-scaled to mine scale. These results provide a first rational method for analyzing the interactions between caved gob and the ventilation system towards mitigating gas concentrations and minimizing the hazard.