Study On Seepage Failure Of Loose Aquifer Mining With Sand Proof Coal (Rock) Pillar

It is an important theoretical basis for safe coal mining under retaining sand proof coal(rock)pillar to accurately grasp the seepage failure characteristics of the overlying unconsolidated aquifer.However,there are few research results on hydraulic gradient calculation of aquifer seepage failure,which makes it difficult to determine water inrush and sand burst.In this paper,aiming at the problem of water inrush and sand burst caused by coal mining under unconsolidated confined aquifer,firstly,the aquifer water inrush and sand burst is regarded as one-dimensional channel flow,and the differential equation of pressure to non pressure channel flow is established and the analytical formula is given;at the same time,the permeability volume strain coupling equation is established in the numerical simulation to study the evolution process of permeability coefficient,and finally,based on the original exploration data of Wobei mine Through collecting the relevant geological and hydrogeological data of Wobei coal mine,this paper analyzes the properties of the aquifer in Wobei coal mine,and uses the groundwater dynamics method and numerical simulation method to solve the hydraulic gradient of the aquifer,and systematically evaluates the risk of water inrush and sand burst in the working face when the sand proof coal(rock)pillar is reserved,so as to provide the basis for the safe mining of coal seam.The achievements are as follows:(1)If the length of the communication channel is much larger than its width,the aquifer groundwater flow is regarded as one-dimensional ditch flow.In order to obtain the hydraulic gradient of aquifer,the partial differential equation of confined to non pressure unsteady flow at constant drawdown is established.The analytic solution of hydraulic gradient is obtained by using the method of separating variables,Laplace transform and Fourier transform,and verified with the numerical solution.On this basis,the influence of hydrogeological parameters of aquifer on hydraulic gradient is discussed.Parameter analysis shows that the maximum hydraulic gradient at the gully wall has a negative correlation with the water storage coefficient μ*,time t and permeability coefficient K,but a positive correlation with the aquifer thickness M.the hydraulic gradient decreases faster when it is closer to the gully wall,and is more gentle when it is far away from the gully wall.(2)The triaxial permeability test of coal seam roof rock is carried out,the permeability variation characteristics of rock in the process of deformation and failure are analyzed,and the permeability volume strain coupling equation is established,which is embedded into FLAC3D to analyze the evolution law of rock sample fracture permeability.It is concluded that the post peak plasticity of weathered rock and mudstone is strong,while the post peak brittleness of sandstone is strong.After the peak stress,the permeability will jump to the maximum value,and the slope of permeability curve is very large.In the post peak strain softening stage,the new cracks begin to close under the action of confining pressure.The research results can provide a theoretical basis for FLAC3D numerical solution of hydraulic gradient.It can effectively solve the problem that the groundwater dynamic solution can not combine the complex evolution of mining stress and rock permeability,and can not realistically reflect the whole process of water inrush and sand burst,which brings inconvenience to the accurate calculation of aquifer hydraulic gradient.(3)Based on the example of mining under loose aquifer in 8201 working face of Guobei coal mine,the paper analyzes the sedimentary structure characteristics and water resisting property of aquifer and water resisting property at the bottom of loose aquifer,determines the mining grade of water body under loose aquifer,finds out the water resisting property of bedrock weathering zone,and defines the shallow overburden structure type of 8 coal seam and the height of sand control coal pillar.The actual hydraulic gradient of unconsolidated aquifer is calculated by using the analytic solution of constant drawdown from confined to pressureless gully flow and FLAC3D numerical simulation method,and compared with the critical hydraulic gradient,the numerical solution of hydraulic gradient is basically consistent with that of gully flow,which is larger than the critical hydraulic gradient of aquifer.It is suggested to take drainage and depressurization measures,and the head reduction value is given to ensure the safety of coal mine production.Figure[47]Table[2 2]Reference[90]...