Study On The Changing Law Of Effect Of Upper Protective Layer Mining With Changing Distance Within Coal Seam Group And Its Engineering Application

Protective coal seam mining is the most effective approach for regional coal and gas outburst treatment and the determination of its effect is the key of related study.Due to large amount of factors and their complexity of way to influence the effect of protective coal seam mining,most related studies mainly focus on one particular minig condition.To investigate the effect of coal seam distance on the protective mining effect systematically,numerical simulations were taken according to experiment design of Respond Surface Method(RSM),which invovled factors of burial depth,tectonic stress,thickness of protective seam,distance between seams and dip angle of seams.The sensitivities of these factors were obtained from RSM finally.Comparison of each sensitivities revealed the great influence of coal seam distance and further study by similar simulation method would be taken both for this reason and for verification of entailed numerical calculation.By variation of the distance between seams,changing law of stress relief and protected range of the protected layer was obtained.For the control of the gas emission rate of tunneling in the protected zone,the relation between the residual gas pressure of protected zone and gas emission rate of tunneling in the protected zone was studied.The relation between residual gas pressure of protected layer after protective seam mining and gas emission when tunneling in the protected layer,based on the view of control of gas emission when tunneling by protective seam mining.Finally,based on upper protective seam mining engineering of Dongling coal mining in Nantong mining district,different methods to determinate the protected range were compared and analyzed.This paper comes to main conclusions as follows:(1)To solve the problem caused by large number of mining condition parameters and indexes of protection effect,response surface model for effect of upper protective seam mining based on quadratic polynomial was built.By which,influence of various mining conditions were quantified and compared.It was found the coal seam distance be the most influential factor for the effect of protective seam mining.The length of protected rang and upper and lower stress relief angle are most sensitive to the coal seam distance.(2)By analysis of similar simulation test of different layer distance,the change law of stress relief and protected range of protected seam were obtained.The result show that the protected range accelarate decrease with coal seam distance and is smaller than given in state law of the same coal seam distance.The stress relief curve of theprotected seam holds convex shape,whose center axis is biased to the lower direction.Both the length of stress relief range and the intensive stress relief range denoted by top part of curve show decreasing tendency.As for the inclination condition,both cantorial axes move to lower direction with increasing seam distance,while the latter one moves faster.(3)In the context of protection seam mining,calculation method of gas emission flux combining solid gas coupling FEA model of cross section and time integration method was built,which serves the prediction and control of gas emission flux.The relation of gas emission flux of continuous tunneling with constant speed to distance and time obtained by numerical calculation is well fitted by exponent decay polynomials.Discontinuous extraction holds zigzag shape but similar overall gas flux trend and the flux difference tends to be constant when tunneling time is large enough.(4)Through study of influence of residual gas pressure of protected layer on gas emission flux of tunneling in the protected zone,the critical surface representative of relation of residual gas pressure,tunneling speed and tunneling time was obtained as well as the critical residual gas pressure.The critical residual gas pressure is not only accounted for prevention of coal and gas outburst but also for gas emission control of tunneling in the protected seam.The level of gas concentration in roadway can be determined by analysis of the distance of point representative of certain tunneling condition to the critical surface.By the critical surface of certain tunneling condition,the method of determinate the critical residual gas pressure was presented,which is smaller than 0.74 MPa for prevention of gas outburst and ensures the gas concentration when tunneling within safety level(1%).This approach provides a new view for the protective seam mining application and guidance for treatement of gas problem in the coal ming.