Curtain And Mining Protection Technology For Coal Mining Under Water In Qiuji Coal Mine

As the mining of the eastern part of China gradually develops deeper,the risks of water inrush are increasing under the action of the pressure-bearing karst fissure water of the limestone.The most representative one is the coal seam of the lower coal group in the North China coalfield,which is seriously threatened by the water inrush under the clamping actions of the water diversion structures of the limestone karst water and strong runoff.Currently,the depth of the floor transformed by the comprehensive grouting has reached the limit equilibrium value.As a result,the traditional method of grouting and water blocking is hard to work.For achieving a safe and efficient mining of the coal seam,it is urgent to strengthen the research of related technologies.What's more,in recent years,the effectiveness of curtain grouting technology in governing water disasters under some conditions such as high pressure and strong water containing has been proved and tested by plenty of lessons,experiences and engineering practices.However,the design of the scheme of curtain grouting is still to be improved and the problem that the rigid composite roof transformed by the grouting is difficult to cave during the mining process is neglected.And if the roof is not caved in time or is suddenly caved,the excessive deformation of the rock strata can be easily caused.Moreover,the roof cracks will develop and form a new water-diversion channel,threatening the safety of the mining area.Therefore,in combination with the on-site monitoring,theoretical analysis and numerical simulation,this paper takes the first mining area of the 11 coal seams in Qiuji Coal Mine as the research background to study the curtain and mining protection technology of coal mining under water body.(1)Through the pumping test and the tracer test of aquifer connectivity,it is concluded that the first mining area is supplied by the aquifer of four-five limestone on the roof from the horizontal and vertical-downward directions.Besides,it is also supplied by the aquifer of Xu limestone on the floor from the vertical-upward direction.Affected by the vertical water-diversion channel,a concentrated effluent zone will appear in the bottom of the mining area.And the ordovician limestone aquifer is considered as one of the important supply sources of groundwater of the overlying strata of Xu limestone and four-five limestone.(2)By means of RFPA2D-FLOW,a numerical calculation model is formed.Under different grouting pressures,the grout diffusion laws of the bedding section and vertical section of water-diversion channel of the limestone are analyzed,The results are as follows.?The greater the grouting pressure is,the larger the diffusion radius is.However,when the grouting pressure reaches 12 MPa,the diffusion radius increase slightly.And the effective and farthest diffusion radii of grout are 30m and 35m,respectively.Therefore,the safe thickness of grouting curtain body in the numerical simulation is initially selected from 60 to 70m.?When the vertical channel section is strong permeable,the slurry can diffuse 200-250m under low pressure,and the diffusion range can fully cover the vertical guide channel section.(3)By means of FLAC3D,a three-dimensional numerical model is established,Under different thicknesses of grouting curtain of 60m,65m and 70m,the influence of thickness of three different grouting curtains on 60m,65m and 70m on the pore water pressure and stress distribution of the four-five ash and the first mining area was analyzed,The results are as follows.?As the thickness of the curtain body increases,the pore water pressures of the four-five limestone and the first mining area of 11 coal seams in the grouting area decrease continuously.Their pressures of pore water in the first mining area of the four-five ash and seams at 65m decrease by 21%and 50%respectively in comparison with those in the thickness of 60m,the pore water pressures of 65 and 70m thickness are not much different,indicating that the grouting curtain body effectively prevents the water from penetrating into the interior.?As the thickness of the curtain body increases,the stresses of the four-five limestone and the first mining area of in the grouting area decrease while the effective stresses of them increase,the stresses of 65 and 70m thickness are not much different,the stress concentration area formed by the vertical section of the channel disappears before grouting,which shows that the grouting curtain body improves the stress of the surrounding rock mass and the stable bearing capacity.It can be determined that the safe thickness of the grouting curtain body is 65 m.(4)Geological structure and geostress exert effects on the selection of borehole orientation in the bedding section.Therefore,when arranging the directional hole in the bedding section on the ground,it should be drilled horizontally along the target stratum of the grouting as far as possible under the condition of connecting multiple structures at a wide angle in the lateral direction.Accordingly,more structures and bedding structural planes can be connected.And a better diffusion radius of grouting can be achieved when grouting borehole is drilled in the horizontal section along the vertical direction of the maximum principal stress.(5)By calculating the water inflows and water bursting coefficients of four-five limestone on the roof and Xu limestone on the floor before and after grouting,and combining with the on-site monitoring data,the effect of reinforcement and water blocking after curtain grouting is analyzed.The results can be seen as follows.?After the top floor is grouted,the water inflow from the working faces of 1101,1102 and 1103 is less than 60m3/h,of the single hole is less than 10m3/h.Both of them are smaller than the specified critical value in the mining area.And the water inflows of the four-five limestone and Xu limestone after grouting decreased by 83%and 88%respectively in comparison with those before grouting.?After grouting,the aquifer of Xu limestone becomes the acquiclude and the water bursting coefficient is smaller than the specified critical value in the mining area.The inspection index of the grouting effect meets the requirements of the mining area,which states that the effect of reinforcement and water blocking after curtain grouting is sound and verifies the effectiveness of curtain grouting technology.(6)By simplifying the roof strata into beam model,the stress states of the strata structures of roof cutting and uncutting are analyzed.Based on the criterions of the fracture and caving of rock stratum,this paper calculates the safe distance from the mining location to the boundary of the curtain.Besides,a three-dimensional numerical model is established by FLAC3D to analyze the distribution of stress in the x-direction and y-direction of the grouting curtain after excavation in the first mining area 1102 and 1103.The results show that after excavation,the maximum stress in the x direction of the grouting curtain on both sides of the east and west sides increased by 5%compared with that before excavation;after excavation of the working face of the first mining area 1102,the maximum stress in the y direction of the grouting curtain on both sides of the north and south increased by 6%compared with that before excavation;after excavation of the 1103 working face in the first mining area,the maximum stress in the y direction of the grouting curtain on both sides of the north and south increased by 7%compared with that before excavation.It shows that the excavation only has a slight impact on the grouting curtain body,and the safe distance from the mining boundary of the first mining area is effective.