Study On Migration Law Of Strata In Stope Under Overlying Concentrated Coal Pillar

Shendong mining area is an important coal base in China.When mining the second main coal seam at present,the safety of the stope working face is bound to be affected by the concentrated coal pillars left in the goaf of the first main coal seam.The overlying concentrated coal pillar has a significant influence on the mining safety of the lower coal seam.The migration law of overlying strata in the stope is different from the conventional one,and the strata behavior is assertive,and the phenomenon of frame pressing in the working face occurs from time to time.However,it is still inconclusive whether all the overlying concentrated coal pillars must be pretreated.To solve this problem,this paper takes Huoluowan coal mine in Shendong mining area as the engineering research background and studies the migration law of overlying strata in stope under overlying concentrated coal pillar.In this paper,the influencing factors of overlying strata migration in stope under overlying concentrated coal pillar are analyzed qualitatively.Secondly,a discrete element numerical model is established to analyze the migration of overlying strata in stope under overlying concentrated coal pillars with different widths.The migration law studied from the aspects of vertical displacement change and stress distribution of overlying strata.Finally,a similar model is established to further analyze the migration law of overlying strata in stope through simulation mining.The main results are as follows:(1)The influencing factors of overlying strata migration in stope under overlying concentrated coal pillar mainly include buried depth,mining height,interlayer spacing,size of concentrated coal pillar,stability of residual room pillar coal pillar,advancing speed of working face,etc.Thus,the influencing factors are many and mutually interlaced,which is more complex.(2)Using 3DEC numerical simulation software to establish discrete element numerical model,combined with the specific engineering background of Huoluowan coal mine,the simulation operations of the overlying concentrated coal pillar width of 50 m,45m,40 m,35m were carried out respectively.The results show that,in a certain range,the wider the concentrated pillar in the upper coal seam,the smaller its influence on overlying strata migration;When the upper and lower goaf penetrate with the advancement of the lower coal seam,the deformation and stress of the concentrated coal pillar increase rapidly;With the reduction of the width of the overlying concentrated coal pillar,the pressure in the lower coal seam working face increases when which pushed out the pillar,especially when the concentrated coal pillar is 35 m,the pressure increases the most,50% increase and the dynamic pressure or even frame pressure may occur.Therefore,when the width of the overlying coal pillar is 40 m,it is safer to leave the concentrated coal pillar in the lower stope.(3)Similar simulation research shows that the left and right sides of the concentrated coal pillar of 2-2 coal are trapezoidal caving after mining.The concentrated coal pillar is stable and has a slight bearing capacity time.When 3-1 coal is mined,the mined-out areas of the upper and lower coal seams are connected,and the caving range of overlying strata is expanded.At this time,the overlying strata above the concentrated coal pillar are inverted trapezoid,and its bearing capacity increased.Influenced by the vertical stress of overlying strata of concentrated coal pillar,the rock strata between coal seams change from transverse tension failure to longitudinal shear failure.That results in several vertical fissures,but the rock strata remain stable continuously.When 3-1 working face comes out of the overlying concentrated coal pillar,the overlying strata are cantilever beam structure,which can protect the working face.Those show that the working face can pass safely when it is covered with a 40 m concentrated coal pillar.The research results can guide for safe mining of working faces in similar mines.