| Since long time age,there have existed many theoretical and technological blind zones in the short distance coal seam mining,which brings a large number of difficult problems for the stability control of the workface and backstopping roadway surrounding rocks.Based on the deep research on the current situations of stope overburden strata structure theory,their movement research,mining pressure appearance in short distance coal seams,proper layout of the lower coal seam mining roadways,this subject has discovered that the current short distance coal seam researches concentrate on strata between seams,and are short of systematical studies on overburden strata structure and its movement law under the influence of multiple mining.More intensive researches are required on such aspects as the stress distribution quantitative judge in workface and roadway top during lower coal seam mining,systematical analysis about factors affecting stress distribution,the influence the roadway layout causes on roadway surrounding rock stability under the effect from repeated backstopping dynamic pressure in lower coal seams,etc.Aiming at some main problems in the existing researches,this subject focuses on overburden strata and their movement.Based on the study on the control and support for the stability of backstopping roadway overburden strata and the workface top under coal pillars,with the techniques,such as theoretical analysis,laboratory experiments,numerical modeling and industrial tests on site,this project does research on overburden strata movement laws and surrounding rock deformation mechanism in the short distance coal seam mining of Shaping Mine,builds up systematic mechanical models of stope overburden strata structure and stress distribution,reveals the basic characteristics and rules of mining pressure appearance in short distance coal seam mining workface,provides solid theoretical basement for the researches on overburden strata structure and movement laws in short distance coal seam mining,stope stress distribution and strata control,backstopping roadway layout and its stability control in lower coal seams.(1)Based on the analysis about the affecting factors for the mining pressure appearance in upward mining,downward mining and the mining between,it is found out that the height of caving zone and the key coal seam stability in both upper and lower coal seams are the most critical influence parameters for the workface mining pressure appearance and backstopping roadway stability in short distance coal seam mining.(2)Based on the caving zone height statistics measured in the national mining sites,with the data generalization about the workface support stress and the stability of the key seam between upper and lower coal seams in 5 coal mines,this project analyzes their relationship with the calculation of caving zone height in upper and lower coal seams,thus dividing the short distance coal seam mining into two types: lower coal seam mining doing no harm to the stability in the key seam of upper coal seam(type ?)and lower coal seam mining leading to the instability in the key seam of upper coal seam(type ?).These two types are also divided into double cantilever structure(type ?A)and masonry beam above with cantilever below structure(type ?B),double cantilever structure(type ?A)and masonry beam above with cantilever below structure(type ?B).(3)With theoretical analysis,this project studies the four types of overburden strata structures in short distance coal seam mining and the damaging effect on coal seam bottom caused by supporting pressure,sets up the mechanical models of the four types of overburden strata structure in the lower coal seam mining workface.It turns out that when the key upper coal seam doesn't lose stability,the separation distance between upper and lower coal seams is relatively large and the key upper coal seam has limited possibility of rotation and deformation.When it reaches stability again,it still has a certain bearing capability and will not generate a large area of pressure on the lower mining workface.In the situation that the key upper coal seam loses its stability,when the middle stratum thickness is relatively small,with the moving forward of the workface,there will form the upper cantilever and lower cantilever structure between the key upper coal seam and the key lower coal seam.The fracture line position and length of the rocks in the key upper coal seams will all perform various influences on mining pressure appearance in the lower coal seam workface and either big or small periodic weighing phenomenon will come into being in the workface.When the interlayer rock is relatively thick and in good completeness,with the moving forward of the workface,the interlayer rock will form the masonry beam structure and there will be a double structures of upper cantilever and lower masonry beam between the key upper coal beam and lower coal beam top rock.Under the influence of some factors,such as fracture line position in the key upper coal seam and the length of the rocks,the mining pressure appearance in workface varies.When the upper coal seam cantilever beam bulk is longer than the key bulk of masonry beam and the fracture line position in upper cantilever rock bulk is aligned with the one in the key bulk of the masonry beam in the lower coal seam,the workface will show the phenomenon that the weighing intervals are consistent and show obviously different large or small periods.During the large period weighing,mining pressure appears intense.Therefore,in the study of overburden strata structure,double cantilever beam structure and double masonry beam structure cause obviously different influences on lower stope stress distribution and mining pressure appearance,which is the emphasis of our research.(4)The damaging depth the upper coal bearing pressure causes on coal seam bottom not only relates to the upper coal periodic weighing law and upper overburden strata structure and its movement rules,and also shows regular change with the changing of the bottom special position.Through the evaluation of the bottom damaging level,the bottom damaging is divided into relatively stable zone and dangerous zone.The workface support choice and roadway layout before the backstopping in the lower coal seam should consider comprehensively the zone control to ensure the safe and efficient production in lower coal seam workface.(5)Through analogous simulation experiment,constructive analysis is done to the upper overburden strata movement law both with 3m interlayer spacing and 18 m interlayer spacing in the short distance coal seam mining.The results of the analogous simulation experiment show the following conclusions.(1)In the short distance coal mining with 3m interlayer spacing,since a large number of cracks have been formed in the upper overburden strata during the upper coal seam mining,the lower coal seam mining will cause these cracks to open again,develop and widen,causing the distance between the working face and the compacting area in the goaf behind the lower coal workface to shorten greatly and bringing severe threaten to the workface top stability control and safe production.(2)With the interlayer spacing distance increasing,the distance that the overburden strata movement stability fall behind the workface gradually increases.(3)With the relatively small interlayer spacing distance,masonry beam structure formed in the upper coal seam mining turns into the upper cantilever beam structure.The caving gangue in the upper coal seam transfers the load to the lower coal workface top strata,upper coal cantilever beam structure and lower coal cantilever beam structure,leading to the phenomenon that the periodic weighing interval in the lower coal seam workface consists basically with that of the upper coal seam.(4)Since the interlayer rock thickness increases,the interlayer rock meets the requirement of the main roof formation.So it is the main roof between the interlayer rocks that influences the workface in the lower coal seam mining.Since the caving gangue in the lower coal seam workface goaf can form support toward the workface top,when the workface experiences the normal backstopping period,the double masonry beam structure model can be formed.Part of the overburden strata load transfers to the lower workface top strata through the upper coal beam caving gangue.Therefore,when the space between coal seams is large,the synchronous coordination of weighing pressure law in both upper and lower coal seam workface will constantly weaken.(6)In the study on stress distribution and roadway layout in the short distance coal seam stope,through MATLAB and discrete element software 3DEC,research are being done on overburden strata movement law with different interlayer rock spaces in the short distance coal seam mining,mining pressure appearance law in workface,characteristics of stress distribution layout in stope and the determination of the proper position for backstopping roadway in the lower coal seam.The law about the influence interlayer space causes on stope stress distribution layout is revealed.With the increasing of the interlayer space,the strongest stress zone in the stope falls behind the workface and the distance is gradually increasing.And with the filling of the caving gangue in the goaf,the mining pressure appearance in the lower coal seam workface shows the trend of weakening.Based on the numerical simulation analysis about the reasonable roadway layout with 3m interlayer distance in the short distance coal seam mining,a important conclusion is made,which says it is reasonable to remove the lower coal seam backstopping roadway outward by 5m.(7)Based on the analysis about overburden strata structure and movement theory in the short distance coal seam with both small and large spacing,we build fracture and rotation mechanic model of the key bulk beside the coal pillar during the upper coal seam backstopping.During the upper coal seam backstopping,the lower backstopping roadway overall bending moment is large in the middle and small in both ends and the bending moment beside the coal pillar is larger than that beside the integrated coal,which indicates that the shallow immediate roof in this position is most likely to be broken and damaged and it is the important zone for roadway control.Experiments in site show that with the constant decreasing of the interlayer spacing between upper and lower coal seams,the deformation of the lower coal seam rock increases obviously and when roadway surrounding rock deformation speed increases,advance workface distance increases constantly.When the interlayer space is relatively small,the upper support zone carries out remarkable influence on the lower coal seam roadway rock deformation,leading to a more intensive surrounding rock deformation in the dynamic pressure zone of the lower coal seam roadway.When the surrounding rock deformation becomes stable,the lag upper coal seam workface distance is smaller.Through theoretical analysis,the interactive relationship between “support and surrounding rock” in various interlayer spacing situations during the lower coal seam mining is analyzed in detail.Short distance coal seam mining conditions in Shaping Mine considered,the reasonable working resistance of workface support with double cantilever structure and double masonry beam structure is calculated.The result shows that when the interlayer spacing between coal seams is relatively small,and the fracture positions of both upper and lower cantilevers remain on the same plumb line,the load on the workface support is the largest,7692 KN,which is obviously smaller than the load on the double masonry beam structure.It explains that the working 8000 KN resistance of workface support is suitable.(8)According to the research results of this project,the industrial experiments are being carried out in the 1808 workface of Shaping Mine,which shows that with the constant decreasing of the distance between the roadway and the lower coal work face,cracks on the roadway top rock redevelops,extends and cuts through downward.When the coal seam spacing becomes relatively small,the upper support pressure has already caused a large number of cracks in the interlayer strata.During the backstopping in the lower coal seam workface,the cracks will develop,widen and seriously affects the roadway roof stability.The lower coal seam advance support pressure obviously influences the roadway surrounding rock deformation.The increasing speed of the top and bottom roof moving and both ends moving in the roadway with small interlayer spacing is obviously faster than period ? with lager interlayer spacing.Therefore,during the backstopping in 1808 workface,the advance support design with plastic transport gateway should vary according to the different interlayer spacing and the interlayer stratum mechanical characteristics.The experiment result in site shows that when the coal seam spacing is small,support pressure is greatly influenced by the key upper coal seam structure and the synchronous coordination of weighing pressure interval between upper and lower coal seams will increase.During the backstopping in lower coal seam workface,the key upper coal seam structural fracture and rotary instability will cause the support in the upper coal seam workface to bear more widely overburden strata load.The site experiment results are satisfying.These results consist with the research outcomes and can offer some theoretical basement and application value for the short distance coal seam mining in the similar conditions. |
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