Instability Mechanism Of Pillars In Multi-section Mining Of Longwall Working Face In Steeply Dipping Seam

Steeply dipping seam is a coal seam with dip angle of 35°-55° which accounts for about 20%of the proven reserves of coal in China,and more than half of them are high quality coking coal and anthracite.However,when mining in steeply dipping seam face,the support-surrounding rock system is complex,which is prone to catastrophe and affects the safe and efficient production of the face.Research what large-scale ground control betweensections for improving the stability of "R-S-F" system and the stability of coal pillars in long-wall working face with steeply dipping seam is of great significance for controlling the surrounding rock activities of Long-wall multi-section mining and ensuring safe and efficient mining of working face.Based on the engineering background of 5#coal seam in 2130 Coal Mine of Xinjiang Coking Coal Group,this paper designs the typ ical working conditions of multi-section mining in steeply dipping seam.By using the methods of physical simulation,numerical simulation and theoretical analysis,the overburden movement law and the instability mechanism of coal pillar in multi-section mining in long-wall working face with large dip angle are studied.The main results are as follows:(1)In the longwall mining of this seam,roof breakage migration is time-series and unbalanced,and the deformation and failure migration of overburden rock in stope is asymmetric.Along the inclination of working face,inclined stacking masonry structure is formed,which has the characteristics of regional filling.There are"space shell"structure and macro-stress arch shell in surrounding rock,and"space shell"structure and inclined stacking masonry structure lead to the complexity of large dip stope structure and rock pressure appearance,and affect the shape of macro-stress arch shell.Stress arch shell is the main bearing capacity system of stope,which can transfer the load distribution of overlying strata to coal and rock around the stope.(2)In the multi-section mining of long inclined working face with steeply dipping seam,the surrounding rock failure in the section coal pillar area is dominated by shear failure,and the upper part of the coal pillar and the floor rock layer are destroyed before the coal pillar.When the size of the pillar is small,the pillar will be destroyed and the surrounding rock will collapse.The surrounding rock activities of the section stop can be divided into "Coal pillar stabilization stage"and"Surrounding rock migration stage"according to the state of the coal pillar.(3)In the stable stage of the isolated coal pillar,the upper and lower sections are successively harvested and fallen,and the migration range of the overlying strata in the two sections is different(the upper section has a higher migration height than the lower section).However,due to the isolation of the coal pillars,the surrounding rock of the section did not have a large-scale overall movement,and the overburden loads of each section were distributed from the respective stress arches to the stope boundary.Therefore,the bearing pressure at the coal pillar increases sharply,and the growth rate increases as the coal pillar size decreases.During the whole migration stage of the surrounding rock,the coal pillar is unstable,and the surrounding rock of the section is completely collapsed and transported.The rock stratum above the coal pillar and the upper strata of the section stope are moved and squeezed under the section stope and lower section.The inclined structure of the section is more compacted by the internal compression joint of the pressure.The bearing pressure at the coal pillar is reduced,and the boundary bearing pressure on both sides is increased(in which the peak value of the coal wall supporting pressure on the return air passage side of the upper section is larger than the lower lane transport lane side).The area of the"empty area"above the working face of the lower section is reduced,and the supporting pressure in the upper middle part of the goaf is increased,the full filling area is extended,and the broken rock mass and the overlying rock layer are bound to the structure of the R(the top plate above the support).It is strengthened to facilitate the integrity and stability of the work surface "RSF"system.(4)After the large dip angle multi-section stope coal pillar is unstable,the surrounding rock will move in the"extrusion-bending"mode or the"compression-dumping"mode,and the surrounding rock movement range of the stope will be expanded.At this time,the rock layer above the coal pillar is subjected to spatial unbalanced motion,and the overlying strata of the mining section in the lower section and the inclined masonry structure of the fallen rock mass in the goaf are strengthened,and the "R-S-F" system on the lower section of the working section is restrained.Conducive to the stability of the "R-S-F" system.Through comparative analysis,the three-dimensional space form of surrounding rock failure and migration under the"collapse-dumping"mode is more conducive to improving the stability of the "R-S-F" system in the upper region of the working face.Therefore,we should take this as the theoretical basis to carry out Large dip angle multi-section large-scale rock formation control.(5)In the mode of "collapse-overturning"combined transportation in multi-section mining of steeply dipping seam,the overburden structure will undergo two structural states of"beam-arch composite beam"and "double-arch continuous beam".When the overburden structure exists in the form of"beam-arch composite beam",the strength of the coal pillar in the section determines the mode of combined transportation in the goaf.When the form of"double-arch continuous beam"exists,the size of the coal pillar in the section will be determined.Inch determines the rock pressure characteristics of the"collapse-dumping"transport mode and the three-dimensional spatial form of the goaf after deformation and destruction.It is concluded that section coal pillars must satisfy both σZHmax<σZH conditions and σSKmax>σSK,and the width of section coal pillars must be larger than kZH and smaller than ksK.This paper studies the law of surrounding rock activity in multi-section mining of large dip working face,the large-scale ground control between sections technology and the calculation method of reasonable width of coal pillars,which has important theoretical value and practical significance.