Study On Evolution Of Overlying Cracks And Gas Migration In Layered Mining Of Extra-thick Coal Seam

Extra-thick coal seam layered mining is a process of repeated roof disturbance.Especially when mining under layering,the overhanging distance of the upper part of the goaf gradually increases,making the overburden have more space for migration,resulting in the overburden The structure movement has changed greatly.In this paper,similar material simulation test and UDEC numerical simulation software are used to analyze the development law of overburden cracks and breakage characteristics of overburden rock,and further use FLUENT fluid software to analyze the distribution law of gas migration in mined-out area.The research results show that:(1)Using the method of similar material simulation to study the fracture characteristics and crack development rules of layered mining in extra-thick coal seam.When the layer is mined in 2621 working face,the direct roof collapses and the overlying thick rock layer is separated from the layer-broken-The phenomenon of delamination,the development of overburden cracks is mainly based on the fracture lines on both sides and the central fracture line.When mining the lower layer of the 2621 working face,due to the mining action of the upper layer,the long-structured rock blocks are further broken into small rock blocks,which reduces the stability and bearing capacity of the overburden hinge structure,and the overburden cracks and separation Through,the fissure development is more obvious.After mining the adjacent 2721 working face,there was a free space distribution between the 2621 working face and the 2721 working face.The stress was concentrated in the inverted trapezoidal area and there were tiny through cracks.The coal pillars were crushed and ganging occurred.(2)The UDEC5.0 numerical simulation method was used to study the movement law of overburden rock after coal mining.After mining three layers in 2621 working face and one layer in 2721 working face,the rock blocks during the collapse of the overburden rock They squeeze each other to form a hinged cantilever beam structure,and the overburden cracks develop up to the delamination area.The delaminations alternately close and separate.Post-processing using UDEC5.0 numerical simulation software shows the distribution and coverage of the overburden crack field.Rock subsidence displacement law.(3)The modeling software ICEM CFD and fluid software FLUENT in the ANSYS module are used to simulate the different layers in the mining of the 2621 working face and the 2721 working face,and the gas movement law in the mined-out area is analyzed.By simulating the single layering of the working face of 2621,it is concluded that the gas migration distribution cloud map in the goaf is fan-shaped as a whole.Due to the influence of the air leakage factors in the goaf,the volume fraction of gas in the air inlet side is small,and the gas in the return air side Concentration is more obvious.The free diffusion of gas in the upper part of the goaf results in a gas volume fraction reaching about 0.6.The gas volume fraction distribution curve grows slowly within 100m,and the gas volume fraction increases rapidly between 100m and 200m,showing a"concave" growth.trend.In the second layer of mining 2621 working face,the gas volume fraction reached about 0.75,and the gas volume fraction distribution curve showed a "Z" type growth distribution.In the third layer of mining 2621 working face,the gas volume fraction between 120m and 200m reached 0.7,which eventually formed a trend of high gas concentration on the side of the return air lane,and the overall distribution was arc-shaped.After mining the adjacent 2721 working face,there are 5m small coal pillars between the return air lanes of the two working faces,and the gas in the return air lanes is concentrated on both sides of the coal pillars,so that the gas volume fraction on both sides of the coal pillars is high,and the inlet air lanes are on both sides.The gas has a small volume fraction and is distributed in a "ridge" shape.(4)By arranging tiered boreholes in the 2721 working face of Baijigou Coal Mine and stratifying boreholes in the three-layer return air lane of the 2621 working face,large-aperture boreholes and tiered goafs in the 2721 working face are arranged along the remaining lanes Implemented the arrangement of buried pipe extraction to solve the gas overrun in the process of the 2721 working face.At the same time,the gas left in the goaf of the 2721 working face was extracted to make the coal mine safe and stable.