Study On The Structural Characteristics Of Overlying Strata And Supporting Resistance In Fully Mechanized Mining Face Under Goaf Of Close Distance Coal Seam

The underground pressure law of fully mechanized mining face under goaf is affected by many complex factors,and it shows obvious particularity and difference.In this paper,21305 working face of Daliuta coal mine is the engineering background.On the basis of the study of the law of the actual measurement of 21305 working face,a comparative analysis is made on the possible combination of the direct top and the basic roof strata.The thickness of the direct roof and the basic roof is 5.03m and 12.83m respectively.From the perspective of the obvious influence on the underground pressure of the lower coal seam,the overburden structure in fully mechanized mining face under the goaf is divided into the primary rock state and the secondary rock state.Based on elastic-plastic theory,the failure depth of:floor under longwall goaf is derived,and the height of roof caving zone is calculated by the empirical formula.lt is the starting point that the failure depth of the floor mining in the coal seam is connected with the roof caving belt of the adjacent coal seam,and whether the bottom failure depth of the upper seam mining will cause damage to the basic roof of the lower coal seam,and the sufficient condition for determining the secondary rock state is determined.The overlying strata of 21305 working face are determined by the above standard,and the overlying strata of the 21305 working face have direct top?base top+small isolated rock columns.Through the construction of the stress model in the periodic motion of the rock beam,the criterion for the periodic breaking of the interlayer basic roof structure to the "transferring rock beam,or "cantilever beam",is determined,that is,the limit turning volume of the broken rock block can form a stable hinged structure.Based on this,it is determined that the basic roof of 21305 working face was broken through the form of "transfer rock beam".The basic roof rock beam experiences the periodic motion evolution process of "stability-instability-stability,,which produces the periodic pressure phenomenon of the working face.However,there is a significant difference between the measured pressure value of the roof and the calculated value by tthe”transfer rock beam"during the normal propulsion of the 21305 working face.This is mainly because the gangue in the upper goaf will move in a timely manner in the vicinity of the working face along with the support moving forward,and the additional static load will be generated on the working face.With numerical simulation software,it is proved that the advanced abutment pressure of close seam mining may be larger than that of single seam mining.Finally,combined with the fact that there is additional static load on the roof pressure of 21305 working face,considering the qualitative relationship between the working resistance of the support and the subsidence of the roof,the method combining the equation of state with the measured statistics is applied.Based on the double standard of top control effect and the overlimit ratio of working resistance of the support,the reasonable working resistance of the bracket in 21305 working face is 12326 kN.