Dynamic law and mechanism of groundwater induced by medium-deep coal mining

The influence on underground water aquifers especially weathered bedrock aquifer and phreatic aquifer caused by middle-deep coal mining has become the focus of protection of water resources.In order to systematically reveal the dynamic law and mechanism of groundwater induced by the mining of middle-deep buried and thick coal seam in Yushen mining area,the first coalface of a coal mine was taken as the study area,the methods of drilling,groundwater level observation,field exploration and comprehensive analysis were adopted to study the dynamic law and coupling relationship with overlying rock damage and surface movement.The results show that with the advance of coal mining,the water level of the weathered bedrock aquifer located in the curved subsidence zone shows a dynamic process that is falling first and then rising to the initial water level.The dynamic process is divided into four stages,namely “start-up stage”,“rapid decline stage”,“recovery stage” and “stable stage” with an average duration of 102 days.In the stage of rapid decline of water lever,the maximum depth and average falling rate of water level are in accordance with the exponential function with the distance from the center of the working face,while in the stage of water level rising-recovery,the duration and average rising rate are negatively correlated with the distance from the center of the working face.When the distribution of the phreatic aquifer is limited and the lateral recharge is lacking,the water level of the phreatic aquifer located in the curved subsidence zone has two dynamic changes:“falling first and then rising” and “continuous descending” to stable,and both of them have not recovered to the initial water level.The dynamic process of water level of the weathered bedrock which located in the working face is closely related to the bending and subsidence of the bedrock.The increasing and decreasing process of subsidence velocity of weathered bedrock corresponds to the falling and rising process of water level,respectively.The decline of water level is related to its subsidence,temporary fissures and the development of detachment,while the rise of water level is related to the slowing down of subsidence,the closure of temporary fissures and separations and the lateral recharge of aquifers.The dynamic process of water level of the weathered bedrock outside working face,is controlled lateral replenishment.The dynamic process of phreatic water level in working face is closely related to the surface subsidence,and the falling period of the water level is consistent with the violent subsidence in the early stage of surface active period.The diving head difference caused by surface subsidence and the seepage exposure of phreatic water are the main reasons for the drop of water level,while the rise of the water level is caused by the slow subsidence of surface,lateral recharge of high level of phreatic water and the replenishment of atmospheric precipitation.The main reason of the continuous decline of phreatic water level outside the working face is the lateral seepage and insufficient recharge.Under the mining condition of medium and deep buried coal seams,the water diversion fracture zone generally does not lead to the weathered bedrock,the water of the weathered bedrock aquifer and the phreatic aquifer will not be lost due to the mining water diversion fracture,and the water level of the weathered bedrock can be restored.The water level of phreatic aquifer will change due to mining surface movement.