Deformation, failure and permeability of coal-bearing strata during longwall mining

To mitigate water shortage, a few groundwater reservoirs are being built at the Daliuta coal mine, Shaanxi Province China. It is essential to investigate the deformation, failure and permeability of coal-bearing strata during longwall mining and their influences on the construction of groundwater reservoirs. In this paper, the local geology was introduced briefly and the complete stress-strain and permeability for different rock samples were investigated using laboratory experiments, and then a three-dimensional coupling model between stress and permeability was proposed; after that a series of numerical simulation was performed to investigate the dynamics of stress, permeability coefficient using FLAC3D. It turns out that, due to the relatively low mechanical strength of the coal-bearing strata at the Daliuta coal mine, there are only two zones above the goaf, that is, caving zone and fracture zone, and the fracture zone extends directly to the surface; the rock mass permeability is closely related to the stress-strain and the permeability decreases with the rise of stress in the stage of elastic deformation while it increases significantly in the stages of plastic deformation and failure; the maximum value of permeability coefficient appears in the stage of strain softening and the value varies greatly with the rock lithology; during the advancement of longwall face, the redistribution range is much wider for permeability than for rock failure and the permeability is the largest on the periphery of goaf; rock mass with increased permeability is horizontally oval-shaped and vertically saddle-shaped; the permeability of coal seam roof and floor experiences three stages: decrease, increase and gradual recovery during coal mining. The results are in good agreement with those of physical modeling and field observation. The outcomes may contribute to the site selection and construction of the groundwater reservoir at the Daliuta coal mine.