| It is a common phenomenon that there is a gangue layer in the extra thick coal seam.In the fully mechanized top coal caving mining of the extra thick coal seam in the domestic mining area,there are many layers of gangue.The gangue layer with high strength not only has poor crushing effect and many large blocks,but also is easy to cause the surrounding top coal to be broken into large blocks,difficult to release,and easy to block the coal caving opening.Therefore,it is of great significance to study the failure mechanism of top coal block in extra thick coal seam with complex structure containing gangue and analyze the influencing factors of composite top coal failure in extra thick coal seam,so as to improve the output and recovery rate of fully mechanized top coal caving face in extra thick coal seam.In order to master the caving characteristics of coal gangue interbedded composite top coal body in fully mechanized top coal caving mining of extra thick coal seam,and ensure the rationality of caving process and high recovery rate of top coal,this paper takes the fully mechanized top coal caving mining of 20 m extra thick coal seam as the on-site actual conditions,takes the block caving mechanism of top coal body as the main research content,and uses the methods of laboratory test,numerical simulation and on-site observation to reveal the full cycle motion loading and bearing mechanism of roof composite structure,The mechanical properties of coal gangue interbedded composite top coal are obtained,the structural instability criterion of composite top coal is determined,and the block crack and caving mechanism of composite top coal under variable roof load is obtained,which provides a theoretical basis for improving the top coal recovery rate of coal gangue interbedded composite top coal in extra thick coal seam.The main research results are as follows:(1)Based on the structural theory of "combined short cantilever beam hinged rock beam",the corresponding relationship between the full cycle movement of the combined roof structure and the stress change law of the top coal body is studied.It is analyzed and determined that the direct roof thickness of the working face is 15.74 m.According to different top coal stress states,the roof structure of extra thick coal seam is divided into three combination forms: "combined short cantilever beam" + "hinged rock beam" hybrid bearing structure,The "combined short cantilever" single bearing structure and the "hinged rock beam" single bearing structure determine the non deformation pressure rock stratum of three roof structures.The top coal deformation is "combined short cantilever" single bearing structure > "combined short cantilever" + "hinged rock beam" mixed bearing structure > "hinged rock beam" bearing structure.(2)The mechanical model of the interaction between the roof combined structure and the top coal body in different positions is established,and the stress of the extra thick top coal body is analyzed.The distribution characteristics of the horizontal stress in front of the coal wall and the horizontal stress near the goaf above the support are obtained: the horizontal stress increases from small along the advancing direction of the working face,and the horizontal stress in the top coal body decreases first and then increases from bottom to top;The horizontal stress near the goaf side above the support increases gradually from bottom to top,and the difference of vertical stress in the top coal body is small.(3)The mechanical properties of the top coal of the coal gangue interbedding are quite different.The residual strength of the coal gangue interbedding specimen is large,and the volumetric strain is larger than that of the pure coal specimen.The brittleness of the pure coal specimen is more obvious than that of the coal gangue interbedding specimen.The ratio of gangue has a significant impact on the strength of coal gangue interbedding structure.With the increase of gangue ratio,the volume strain decreases,the deformation required before failure decreases,the critical volume stress of coal gangue interbedding structure decreases,the stability of coal gangue interbedding structure increases,and the unit failure energy required during failure increases.In the process of fully mechanized top coal caving mining,the higher the proportion of gangue in the top coal body,the greater the increase of overall strength,and the more difficult it is to break,thus affecting the caving ability of top coal.(4)The proportion of gangue in the coal gangue interbedded composite roof is negatively correlated with the bulk modulus,and there is a polynomial corresponding relationship;During the unloading process,the top coal of coal rock interbedding will be damaged and damaged in varying degrees and asynchronously.The block fracture scale is concentrated in 12-15 cm,accounting for more than 60%.In the vertical direction,it will show various forms of failure characteristics according to the change of horizontal stress gradient;The progressive failure process of coal rock interbedding under loading and unloading can be divided into three stages: elastic stage I,stable crack propagation stage II and unstable crack propagation stage III;With the increase of gangue ratio,the overall failure degree of composite top coal decreases,the unstable crack propagation stress increases gradually,and the incomplete X-mode conjugate shear failure characteristics become more and more obvious;With the increase of loading axial pressure,the distribution range and density of fractures increase,the rock horizon presents tensile splitting failure,and the coal seam presents shear failure of dense fractures.(5)With the increase of the proportion of gangue,the arch forming probability of top coal increases,and the scale of balance arch increases.The upper top coal block and gangue released by 20 m composite top coal caving are very easy to form a spherical arch in the coal caving space by extrusion.According to the position of the arch foot of the spherical arch,the spherical arch can be divided into three categories: type I arch,the arch foot is located in the goaf and tail beam;Class II arch,the arch foot is located in the goaf and shield beam;Class III arch,the arch foot is located in the goaf and roof beam.(6)The volume and weight of the released blocks of composite coal and rock mass conform to the normal distribution.The proportion of coal gangue block with length less than 40 cm is 66%,but its weight is less than 15%.The weight of coal gangue block is mainly concentrated in the range of length 40 ～ 100 cm.The main way to improve the recovery rate of top coal in extra thick coal seam is to improve the release efficiency of block with length of 40 ～ 100 cm.(7)Through the actual measurement and analysis of the weakening effect of the top coal and roof,in the fracturing affected area,the end cycle resistance decreases by25.4% during non pressure period and 15.3% during pressure period.The frequency of microseismic events is significantly reduced,the scope of microseismic activity intensive area is reduced,the fracturing expansion range is more than 280 m and the height is less than 30 m.The top coal is covered.After surface fracturing,the layered damage degree of overburden is high,and the hard thick rock stratum is thinned,The recovery rate of top coal has been improved,which is 6.1% higher than that when the top coal is weakened without surface fracturing. |
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