A Numerical Investigation of the Stress Relief Zones Around a Longwall Face in the Lower Seam for Gas Drainage Considerations

Extraction of a protective coal seam (PVCS)-below or above a coal seam to be mined with the potential of coal and gas outburst risk-plays an important role not only in decreasing the stress field in the surrounding rock mass but also in increasing the gas desorption capacity and gas flow permeability in the protected coal seam (PTCS). The PVCS is mined to guarantee the safe mining of the PTCS. This study has numerically evaluated the stress redistribution effects using FLAC3D model for a longwall face in Shanxi Province. The effects of mining depth, mining height and inter-burden rock mass properties were evaluated using the stress relief angle and stress relief coefficient. Vertical stress distribution, stress relief angle and stress relief coefficient in the PTCS were analyzed as the face advanced in the PVCS. The results showed that the stress relief achieved in different locations of the PTCS varied as the face advanced. Sensitivity analyses on the pertinent variables indicate that the stress relief in the PTCS is affected most by the mining depth followed by the inter-burden lithology and the mining height. Furthermore, the elastic moduli of different layers within the inter-burden rock mass are more important than their uniaxial compressive strength (UCS) and Poisson’s ratio. These observations can guide gas drainage borehole design to minimize the accidents of coal and gas outbursts.