Research On Overlying Strara Structure And Stability Of Fully Mechanized Coal Face In Close Distance Shallow Coal Seams

Shendong mining area,the typical occurrence region of close distance shallow seams,has turned into the mining of the second coal seam step by step with the first coal seam gradually depleted due to high strength mining.Ground pressure behavior appears strong while the second coal seam mining,which was highlighted in the phenomenon of long and short periodic pressure phenomenon in the working face,and support crushing accidences happened while the working face out of the upper coal pillar or during the gully terrain.It is generally known that the abnormal pressure of working face is usually caused by the movement and failure of mining overlying strata structure.Under certain conditions,the law of rock pressure is closely related to the movement characteristics of overlying strata structure.Based on the means of field measurement,similar simulation experiment,numerical simulation experiment and theoretical analysis,the evolution characteristics of overlying strata structure and its stability in close distance shallow seams are deeply researched.Based on mechanical effect of coal seam excavation and pressure arch phenomenon of surrounding rock stress transfer,the three basic structure forms of the near horizontal coal seam mining field with the rock support structure,the rock pressure movement structure and the protective structure of working face are put forward,which respectively referred to as the "big structure","small structure" and "micro structure",and also the relationship between the three basic structure and mine pressure appearance of stope is discussed in this paper.The transverse and longitudinal spatial structure of close distance shallow coal seams are analyzed,and put forward the new method of "three zones" division and the principle of "short side control and long side".The coal pillar which is composed the arch foot of large structure is pointed out,which is the carrier of load transfer.The key strata of rock strata of the upper coal seam and lower coal seam(hereinafter referred to as the middle rock strata)are classified.Through the analysis of measured data of mine pressure of a plurality of fully mechanized coal face,the characteristics of mining pressure and the abnormal pressure location during the working face of lower coal seam mining under the key strata of the middle rock strata has been basically grasped.The characteristics of mining pressurewhen the working face of lower coal seam mining under the solid coal,coal pillar along strike and the goaf area has been mastered.The evolution law of the large structure is studied when the working face of the lower coal seam mining under the upper coal seam goaf area and the coal pillar.The stress state of the roof of the working face is divided into 4 zones,which is the tensile stress zone,the supporting pressure concentrated area,the unloading area and the compression zone.According to Promojiyfakonov theory,the trace equation of structure arch are derived before the middle rock strata completely broken.Mechanical properties of goaf pressure arch is analyzed by three hinged arch theory of structural mechanics.The discriminant of large structural instability and the stability analysis method of working face pressure arch.is established.The mechanism of dynamic mine pressure disasters is revealed while mining out of the upper coal pillar and during uphill of gully terrain in close distance shallow seams,and the preventive measures is put forward.On the basis of the thickness of immediate roof and fragmentation degree of key stratum,the small structure of overlying strata in close distance shallow seams is divided into four types: masonry beam structure,cantilever beam-masonry beam structure,combined cantilever beam-masonry beam structure and step voussoir beam.The formulas for calculating the working resistance of various small structures are given.The breaking characteristics of the key stratum in the middle rock strata are analyzed,and the reasons of the phenomenon of the big and small periodic weighting under the thicker key strata of the middle rock strata.The principle of "three zones" division of overlying strata in close distance shallow seams is put forward.Taking penal22307 of Bulianta coalmine as an example,the zone characteristics of overburden failure in close distance shallow coal seams and the influence of layer spacing and mining height on the structure of the overlying strata were analyzed.