Failure and Permeability Characteristics of Coal Pillar in Closely Coal Seams Gob Under Multiple Mining

Coal pillars are loaded and unloaded repeatedly when mining, which lead to fractures in the coal close, open, generate and expand. As a result, the permeability of coal is changed. The high permeability fractures in coal and rock between the upper gobs and the lower working faces are the main channels for fresh air entering the upper gob, which could induce spontaneous combustion of coal in gob. To identifying the air leakage channels, multiple mining of closely coal seams was numerically conducted with three working face layouts. The failure and permeability characteristic of coal pillar in closely coal seams gob under multiple mining were obtained and analyzed. When the working faces are mined, the vertical stress and horizontal stress of the upper coal pillar in gob load and unload synchronously in all three working face layouts. The laterally directed horizontal stress could unload to zero due to no confine on the lateral side of coal pillar. The stress in the middle of upper coal pillar loads continuously until the lower working face is mined. When the lower coal seam working face is mined, the coal and rock between the upper and lower coal seams damage in shear and tension. When the lower coal seam working face is staggered from the upper coal seam working face, the permeability of the coal and rock pillar increases more than 22000 times due to tension damage of the coal and rock pillar. As a result, the coal and rock pillar is the main channel for fresh air flowing into the upper gob. The high permeability coal pillar provides favorable conditions for spontaneous combustion of coal in gob.