The Surrounding Rock Structure And Control Mechanism Of Mining Roadway At The Lower Part Of Pillar Boundary Of Close Coal Seams

There are many close coal seams in the west of China,in which the problem of surrounding rock controlling gradually appears with the close coal seam mining.Due to the close distance between the coal seams,the lower coal seam roof is damaged seriously,and surrounding rock shows the non-uniform stress state near the upper boundary pillar and goaf,so reasonable arrangement and support of gateway in the lower close coal seam become very prominent problem in production.Aiming at the technical problems of crushed surrounding rock control and gateway layout in lower close coal seam(two kinds of typical close coal seam: 2m and 20m),through investigation monitoring,laboratory test,physical similarity,theoretical analysis,field test,and other means,this paper discussed the deformmation failure characteristics and instability reason of gateway in close coal seam,found out basic mechanical characteristics and failure pattern recognition model of the grouting body specimen,studied the strata movement rule and the stress distribution characteristics during close distance coal seam mining,analyzed butterfly leaf shape of pillar stoping in the lower gateway,and proposed the collaboration bearing technology of the internal grouting reinforcement and external structural-steel bearing.The main conclusions are as follows:(1)Affected by mining,the interlayer surrounding rock produced plastic failure between two close coal seams,the roadway roof in the lower coal seam expanded,failed laminarly,and showed large subsidence.Under the influence of the stress concentration of the upper coal pillar,the surrounding rock of the lower gateway was broken in a large area,the range of the gateway loosening circle was large,the gateway deformed seriously.(2)The instability modes of the lower gateway in close coal seam included two types: the roof caving plastic collapse stability due to broken roof-rock,and block falling catastrophe instability of roof rock subjected to crushing,breaking,and collapse.Main factors of controlling surrounding rock stability in lower close-seam gateway included reasonable gateway layout and grouting reinforcement bearing of surrounding rock.(3)With the increase of particle size grading,the strength and axial strain of grouting body were "convex" shape,while the transverse strain was opposite.With the increase of water-cement mass ratio,the strength and axial strain of grouting body were in the form of exponential decay,and the transverse strain was in the form of "convex".With the increase of grouting pressure,the strength of grouting body,the axial and transverse strain showed a slight increase trend.(4)The ratio of grouting pressure and water-cement mass promoted the chemical change of mineral composition at the interface of rock and fluid,and enhanced the interface cementing strength and overall bearing strength.The change of mineral diffraction determined the failure area of the specimens,and the grouting specimens of the three groups showed five failure modes and three instability modes.(5)After the mining of the upper coal seam,the upper boundary of coal pillar at both ends formed an plastic disaster area with 55～68° inverted "octave",that was to say,the area of upper large butterfly leaf was formed due to plastic butterfly extension,and the bearing load of coal pillar increased.After the mining of the lower coal seam,the lower boundary of coal pillar at both ends formed an plastic disaster area with50～54° "octave",that was to say,the area of lower small butterfly leaf was formed due to plastic butterfly extension.(6)Under the action of mine pressure,the plastic zone of the lower butterfly eventually developed to an upper open and lower closed oblique triangular "V-shaped" plastic failure zone.The bottom of the "V-shaped" plastic failure zone was the bottom of the stress reduction zone,and the stress of the upper oblique "V-shaped" zone increased successively.The optimal location of the lower mining gateway was in the relative stability zone of surrounding rock on both sides of the "V-shaped" plastic failure zone,and the inner(near coal pillar)stability zone was larger than the outer(far coal pillar)stability zone.(7)After crushing gangue being filled,the supporting force of overlying strata was proportional to the span and height of the basic roof.The interaction between the basic top layers and its own weight had little influence on the load of filling body.The reasonable roadway layout was on both sides of the "V-shaped" plastic zone.(8)When the distance between the coal seams was relatively far,and the bolting net plus routing support scheme in the normal section and the reinforcement scheme in the special section(grouting anchor cable reinforcement in the more serious section and U-shaped steel shed reinforcement in the extremely serious section)were designed.When the distance between the coal seam was relatively close,the anchor net+grouting support scheme for the stable section of surrounding rock and the U-shaped steel+anchor(cable)support scheme for the special section were designed.In this study,two kinds of field industrial tests were carried out under the condition of close coal seam.The deformation of surrounding rock was controlled and the supporting effect was remarkable.