Impact of the closure of a coal district on the environmental issue of long-term surface movements

The environmental impact of deep underground coal mines using the longwall mining method is diverse, e.g., short- and long-term subsidence, damage to surface infrastructure, disturbance of the hydrogeological conditions, and the quality of groundwater and surface water. The study presented focusses on the long-term surface movements after the closure of an entire coal district. Due to the flooding of the underground infrastructure and rock mass, an upward surface movement or uplift is observed. For a specific site in the Campine coal district, Belgium results are presented of satellite data (radar-interferometry). However, the main aim of the study is to better understand the process of uplift and to determine the various mechanisms that are involved. For this purpose, an analytical framework was developed recently, and it was applied successfully in a relatively easy case. The case study of the paper is more challenging, but the usefulness of the analytical framework is clearly confirmed. The most important conclusions are that (i) the uplift is induced by an increase in water pressure after the closure, i.e., re-establishing the original hydraulic gradient, (ii) the expansion of both the goaf volumes and the volumes of the non-collapsed rock mass must be considered, and (iii) the assumption of a linear decrease of water pressure variation from the top to the bottom of the mined area at the end of the mining phase provides the most realistic results. However, the next step in the analysis should focus on a more advanced hydrogeological model of the complex underground environment.