An improved model to predict the water-inrush risk from an Ordovician limestone aquifer under coal seams: a case study of the Longgu coal mine in China

The mining of stratigraphically low coal seams in North China-type coalfields is subject to water inrush from the underlying Ordovician limestone aquifer. The water-inrush coefficient method that is currently used for the evaluation of the water-inrush risk has inherent shortcomings, because it takes into account only the aquifer head pressure and the aquiclude thickness. Therefore, an improved water-inrush coefficient (IWIC) model is proposed. Based on the normalized water-inrush parameter, water-resisting parameter and structural parameter, the IWIC model is established using a linear weighting method. The first-order weights of each parameter are determined by the analytic hierarchy process, and the second-order weights are determined by the trapezoidal fuzzy number technique. Contour maps of the water-inrush risk calculated with the IWIC model are then obtained. The water-inrush risk grades are classified by thresholds derived via the Jenks natural breaks technique. The IWIC model is applied to the Longgu coal mine, as a typical coal mine in China, to evaluate the water-inrush risk of the lower four coal seams (L4CS). The evaluation results show that the risk of water inrush in the L4CS can be divided into five grades: safe, slightly safe, slightly dangerous, dangerous, and extremely dangerous. Overall, the L4CS mining in the Longgu coal mine is seriously threatened by the underlying Ordovician limestone aquifer. As the depth increases, the risk of water inrush increases from the No. 15 1 to No. 18 2 coal seams. Among the L4CS, No. 17 and No. 18 2 have the highest grade of water-inrush risk, and it is proposed that these two coal seams should not be mined to prevent water-inrush accidents.