The Response Mechanism Of Groundwater In Unconsolidated Aquifer To Coal Mining In Thick Loess Area

In Loess Plateau,which located in the Midwest of our country with dry climate,due to the limited availability of surface water systems,the groundwater in the Quaternary unconsolidated aquifers is the primary resource for national economic development,residents'livelihoods,and ecoenvironmental maintenance.In some mining area with the thin rock strata,underground mining activities had made impacts on the unconsolidated aquifer overlying the coal measure strata,and eventually lead to the decline of a water table,intensifying conflicts between supply and demand of water resources,as well as deteriorating ecological environment.But in some deep coalfields covered by the thick loess in the southeast of Shanxi province,the unconsolidated aquifer has not been directly affected by mining.The water table depression caused by leakage was often neglected.Therefore,the integrated study on the response mechanism of groundwater in unconsolidated aquifer to coal mining in mining area covered by the thick loess is the theoretical basis for coalmining with water resource conservation,constructing green mine and ensuring the coordinated development of ecological resources and economy.The Changcun colliery in the southeast of Shanxi province served as the research base in this project from the perspective of water protection.This study focused on the indirect effects of underground mining on unconsoliadated aquifer,in which the groundwater leakage through the clay aquitard was the result of the hydraulic head pressure difference.Using the physical modeling test with similar materials,this paper studied the movement and deformation discipline of the rock mass and the aquitard under the unconsoliadated aquifer at various mining conditions,and investigated the development and distribution characteristics of the residual fractures in mining overburden,and analysed the mechanism that affected unconsoliadated aquifer by mining.In the permeability experiment,the hydraulic conductivity of mining-induced rock mass with different fracture rates in caving zone and fractured zone was tested,and the rule of permeability variation and the distribution characteristics of the mining-induced rock mass were analyzed.By using the numerical simulations,this study discussed systematically the response mechanism and the effect degree of groundwater in unconsolidated aquifer to excavation.The main conclusions obtained are as follows:(1)With a constant mining geological background and exploitation method,deeper mining and bigger working thickness results in greater vertical displacement at the same layer.For different layers,the vertical displacement grows in negative correlation to the distance from roof.The more the layer moves towards to the coal seam,the nearer ground subsidence approximates to working thickness.(2)The growth of fractures in overburden rock induced by mining has a positive correlation with the width of the mined-out area,the cumulative working thickness and the height of overlying strata.The fracture ratio of mining overburden is reduced from the bottom up generally,and which of the sides is larger than that of the middle part.(3)After the mining over,the hydraulic conductivity in different areas of mining-induced fracture field varies greatly;the permeability of fractured rock improves with the increase of fissure ratio.The hydraulic conductivity of damaged rocks in caving zone is between 0.081 cm/s and6.847 cm/s.The vertical hydraulic conductivity and the horizontal hydraulic conductivity of the damaged rocks in fractured zone is 0.067cm/s?0.228 cm/s and 0.744 cm/s?2.546 cm/s respectively.The maximum hydraulic conductivity is in the fractured area of regular caving zone,and the minimum value is in the coal wall support area and recompaction area of irregular caving zone.(4)Variation in lowering speed of groundwater in the unconsolidated aquifer is a direct result of the deformation extent of aquitard underlying unconsolidated aquifer,which has a negative correlation with the height of overburden,but a positive correlation with the working thickness.The loss of groundwater from the overlying unconsolidated aquifer does not accelerate until the maximum subsidence of the aquitard exceeds approximately 0.9 m,or the subsidence-bending area of the aquitard exceeds approximately 156 m²,or the ratio of maximum subsidence of the bottom clay to the width of the mined-out area exceeds approximately1:300,which further leads to poor impermeability and structural damage,the stability of unconsolidated aquifer is suffered the extraction effects.(5)Underground coal mining has effects on groundwater in the unconsolidated porous aquifer in one of two ways:directly impact and indirect impact.First,the fractured water-conducting zone grew up to the aquifer or connected with the downward crack zone underneath the aquifer,groundwater flows directly into the mined-out area.Second,the groundwater in the bedrock fissures of the aquifers drained to the working face,and the head difference between the upper and lower aquifers increased continuously.The groundwater in the overlying Quaternary unconsolidated aquifer does not flow directly into the gob but instead leak and recharge the dewatered fissured aquifer through the aquitard.Then,the water level of unconsolidated aquifer is decreasing.(6)The degree to the unconsolidated aquifer which was affected by coal extraction can be separated into three categories in general:the unconsolidated aquifer is located within the direct influence zone that the water levels fall abruptly and the aquifer is unwaterting.With the second type,the aquifer is located within the disturbing influence zone that the aquitard is damaged that the decreasing rate of groundwater could fluctuate and increase.The third type is that the aquifer and its aquitard lie in the slight influence zone that the decreasing rate of groundwater is steady,and the drawdown is only caused by hydraulic head pressure difference.(7)The degree of underground mining influence to unconsolidated aquifer depends on mine working coditions,the property of aquitard floor underneath the unconsolidated aquifer and the hydraulic characteristics of the aquifer.With a constant mining geological background and exploitation method,the maximum drawdown changed slightly when the thickness of nonbreaking bedrock is more than 140 m or the hydraulic conductivity is less than 10^-6 cm/s,or the aquitard thickness is greater than 40 m.