Drainage Features Of Karst Aquifer Under Coal Mining Conditions

The influence of coal mining on groundwater is continuous and dynamic. Generally, coal mining would destroy aquifer and aquitard in original strata, break the natural balance of groundwater recharge, runoff and discharge, and cause groundwater recession and aquifer unwatering, which make groundwater system increasingly complex. In recent years, with the expansion of mining scale and increase of exploitation depth, the probability of water inrush of the limestone aquifer within coal seam roof and floor is climbing. Therefore, it is important for the safty of coal mining to research on groundwater seepage field rule of karst aquifer under the conditions of coal seam mining and predict the mine discharge. Based on the detailed review of related researches in China and abroad, we conducted the mine discharge numerical simulation on a coal mine in Shanxi Province.On the basis of previous research findings about hydrogeological exploration, pumping tests and water chemical characteristics, we synthetically analyzed the geological and hydrogeological conditions in the research area. Combined with the analysis of water filling sources and filling channels, the research area was generalized as a heterogeneous anisotropic three dimensional seepage system. Then we established the mathematical model of karst groundwater, where Carboniferous Taiyuan formation and Ordovician Fengfeng formation limestone were treated as the water-bearing media. Through the space and time discretization and the determination of definite condition, source and sink terms and hydrogeology partition parameters, the parameters in the model were identified and verified by applying the finite difference numerical method. Finally, groundwater numerical simulation model was established to predict mine discharge in this area.Additionally, we employed the water inrush coefficient method to determine the water inrush risk zone of limestone aquifer. The area of the danger zone of Taiyuan formation limestone under2#coal seam floor and Fengfeng formation limestone under11coal seam floor takes up3.74%and11.26%of the total area of the research zone, respectively. Then we must conduct drainage and depressurization to the aquifer for safety production. We used the big-well method and numerical simulation model to predict the required time and mine discharge when the karst water reached safety water-head. The research has shown that, when2coal seam was exploited, the predicted mine discharge calculated by the big-well and numerical method was99.36m3/h and140.4m3/h, respectively. And the limestone aquifer level of Taiyuan formation fell below the critical water level after discharged70days. When11coal seam was exploited, the predicted mine discharge calculated by the big-well and numerical method was400.41m3/h and396m3/h, respectively. And the limestone aquifer level of Fengfeng formation fell below the critical water level after discharged90days. The forecasting results can be used as a reliable reference for effective water prevention measure and safety production.