Numerical studies toward a determination of longwall mining impacts on groundwater resources

A methodology is presented to determine the local and regional effects of mining subsidence on ground water resources. The methodology requires that subsidence induced strains are determined from forward modeling of the deformation processes using a nonlinear finite element model. From the known strain field, the post-mining hydraulic conductivity field is evaluated from knowledge of initial conductivities and a modulus reduction ratio that incorporates the scale effect. This model is relatively robust, requiring only parameters that are generally obtainable in practice. From knowledge of the revised conductivity field, the hydraulic responses of the overburden to longwall mining are evaluated in terms of the distribution of post-mining overburden desaturation and depressurization zones under both completely drained (long-term) and undrained (short-term). The impacts of longwall mining on ground water resources are then determined based on these hydraulic responses.;The nonlinear FE model is successfully verified against two case studies with a variety of general conclusions being drawn. Hydraulic response of the overburden to longwall mining are primarily determined by both post-mining hydraulic conductivity field and drainage into the mine for a particular aquifer. The hydraulic response of aquifers which remain hydraulically isolated from the mine post-milling is determined only by changes in hydraulic conductivity regardless of drainage into the mine. However, the hydraulic response of aquifers which are hydraulically connected with the mine post-milling is determined both by changes in hydraulic conductivity and by drainage into the mine. Piezometric levels in different aquifers fluctuate as mining advances. The fluctuation of the piezometric levels in different wells, and the post-mining availability of the ground water resources, depend on the relative location of these wells to the mine layout, the proximity of mining, longitudinal drainage, the thickness and the location of less permeable layers, and the magnitude of their pre-mining hydraulic conductivity. The influence of these factors is thoroughly evaluated.