Redevelopment of Fractures and Permeability Changes after Multi-Seam Mining of Shallow Closely Spaced Coal Seams

Mining the lower seams in a sequence of shallow, closely spaced coal seams causes serious air leakage in the upper goaf; this can easily aggravate spontaneous combustion in abandoned coal.Understanding the redevelopment of fractures and the changes in permeability is of great significance for controlling coal spontaneous combustion in the upper goaf.Based on actual conditions at the 22307 working face in the Bulianta coal mine, Particle Flow Code (PFC) and a corresponding physical experiment were used to study the redevelopment of fractures and changes in permeability during lower coal seam mining.The results show that after mining the lower coal seam, the upper and lower goafs become connected and form a new compos ite goaf.The permeability and the number of fractures in each area of the overlying strata show a pattern of "stability-rapid increase-stability" as the lower coal seam is mined and the working face advances.Above the central area of goaf, the permeability has changed slightly, while in the open-cut and stop line areas are significant, which formed the main air leakage passage in the composite goaf.