Influence of cover stress re-establishment on the permeability evolution of the mining-induced fractures

This paper presented an investigation of the influence of cover stress re-establishment on the permeability of mining-induced fractured zone and its application to the calculation of the height of water flow fractured zone for slices mining and re-mining of abandoned coal seams. An experiment on the hydraulic conductivity of the fractures under axial loading with lateral restraint was conducted to simulate influence of cover stress re-establishment on the permeability of fractured zone. Based on the in-situ measured logarithmic distribution of the bulking factor with the distance from the gob in the mining-induced fractured zone, the distribution of permeability in the water flow fractured zone has been proposed, and the evolutions of permeability and aperture of the mining-induced fractures under cover stress re-establishment were analyzed by the fractured rock mass mechanics method. The experimental results show that the aperture and the hydraulic conductivity of the fracture decrease more significantly in the earlier stage of the axial loading than that in the later stage, even though the stress is smaller; the later stage of the axial loading plays a less role on the influence of the fracture's permeability, and the aperture is hardly compressed to be closed under an axial loading of 20 MPa. The theoretical calculation results show that the permeability of the fractures with larger aperture in the lower part of the water flow fractured zone has been decreased but still has a good permeable pathway for water flow; and the permeability of the upper part of the water flow fractured zone has been decreased obviously. According to the code for engineering geological investigation of water resources and hydropower, the hydraulic conductivity of 0.01 m/d is determined as the standard for impermeable layer, the maximum height of the water flow fractured zone after the cover stress re-establishment is decreased by 26% compared to the original one. The formula for calculating height of water flow fractured zone is modified based on the research results, which would provide a more suitable approach to determine the size of safety pillars for mining the lower slices of coal seam and re-mining the abandoned seams.