Separation and fracturing in overlying strata disturbed by longwall mining in a mineral deposit seam

Mining-induced fractures influence the stability of overburden and may provide pathways for transferring heat and mass in underground environment. A novel approach was proposed to quantify the separation and fracture evolution in the undermined overburden, consisting of (1) theoretical distribution models of the void ratios of fractures (VRFs), based on analytical evaluation of key strata subsidence, and (2) numerical modeling using the universal distinct element code and a sequence of image post-processing procedures. For a longwall mining panel, both theoretical calculations and numerical simulations indicated that VRF first increased rapidly, then gradually decreased, and finally stabilized to a minimum from the surrounding edges of disturbed strata to their centers, and gradually decreased from deep to shallow strata. The fractures exhibited a "fracture-rich arch" distribution type along a vertical section. Numerous fractures occurred around the arch feet near the perimeters of mined-out area and/or arch crown near the center of disturbed strata. The distributions of VRFs show the heights, shapes and damage intensities of fractured zones, and can also be used as porosity parameters to determine the permeabilities of mining-disturbed overburdens. Therefore, the VRF models can be used as a quantitative parameter to assess the extent of possible risk zones, e.g. for water inflow into the mine or escape of hazardous fluids to the surface.