Research On The Roof Structure And Support Resistance Of The Working Face Under Close Coal Seam Goaf In Shallow Buried

Yu-shen-fu coal mining area mainly mines the shallow and close seam group,which has entered the stage of mining the lower seam group.The production practice shows that there is a significant difference between the law of ground pressure and the top coal seam mining when mining under the closely spaced goaf,and there is no scientific basis for the selection of support and roof control in the stope.It is of great theoretical and practical significance to research the roof structure of the working face under the shallow buried closely spaced goaf,reveal the mechanism of the weighting from the working face,and determine the reasonable support resistance.Based on the actual measurement and analysis of ground pressure in the working face under the shallow buried closely spaced goaf of Halagou and Ningtiaota Coal Mines,combined with the features of ground pressure in 14 working faces under similar conditions,it is concluded that,compared with the top single coal seam,the weighting strength and dynamic load coefficient of the lower mining area of the close seam are increased,and the periodic weighting step is reduced;the mining height of the lower coal seam and the thickness of the interval strata are the main factors that affect the weighting appearance of the lower coal seam.The physical simulation experiment obtained the breaking characteristics of the key layer of the interval strata:the first weighting step of 1-2 coal seam working face in Halagou coal mine is 28m,and the periodic weighting step is 9m,the key layer of interval strata breaks periodically to form a "voussoir beam" structure;the first weighting step of 2-2 coal seam working face in Ningtiaota mine is 59 m,the average periodic weighting step is 14 m,and the key layer of interval strata breaks periodically to form a "step voussoir beam"structure;and the two working faces both shows obvious weighting of large and small period.The numerical calculation results show that the first and periodic break step of the key layer and the step subsidence of the key block are positively related to the mining height of the lower coal seam.Based on the actual measurement of the ground pressure,through physical simulation and numerical calculations,the roof structure characteristics of the working face when the working height of the lower coal seam is 1.75m and 5m and the distance between the upper and lower coal seams is 11m and 33m are studied.The experiment results show that the key layer of interval strata and the key layers of the collapsed roof of the upper coal seam can form two kinds of combined structure:"voussoir beam-voussoir beam" and "step voussoir beam-voussoir beam".The establishment of the mechanical model of the roof structure reveals the mechanism of the synchronous and non-synchronous breaking of the upper and lower key layers resulting in large and small periodic weighting on the working face.The analysis shows that the structural instability of the key layer of the interval strata has a major effect on the force of the support,and the key layer that has been disturbed by the upper coal seam has a relatively minor effect.Based on the "voussoir beam" and "step voussoir beam”mechanical models,combined with the "load transmitting" analysis of the key layers of the disturbed roof,the calculation formulas for the working resistance of the support under different roof structures were proposed,and mining examples were used verification,it can provide a theoretical basis for the determination of reasonable support resistance at the working face.