Study On Broken Mechanism Of Fully Mechanized Top-coal Caving Mining After Extracting The Middle Slice In Extremely Thick Coal Seam

China is a big energy-demanding country,and coal is the main source for power and heat generation.From the perspective of coal mining areas,in East China and North China coal has begun to shrink in recent years and the depth and difficulty of coal seam mining have increased.With the rapid development of the western region and the growing demand for resources,China's coal mining begins to shift to the west.From the reserves,the total coal resources in the west account for 64.8%of the country,and the production capacity in 2018 was 15.13 trillion tons/year,accounting for 43.34%of the country.Therefore,the layout of coal mining in the west is the general trend.There are a large number of extra-thick coal seams which are more than 20m in height in the western China,most of which cannot achieve open-pit mining.There is no mature theory and technology at home and abroad to learn from.It is an unsolved worldwide problem to achieve safe and efficient underground mining of such coal seams.In order to achieve safe and efficient mining of extra-thick coal seams above 20m,it is necessary to carry out basic research on the fully mechanized top-coal caving mining after extracting the middle slice in extremely thick coal seam.The paper aims to figure out the degree and extent of self-caving and breaking of the upper coal and whether top-coal can be draw out safely and efficiently.So,the 5#main mining coal seam of a certain mine in the west is selected as the research object.It belongs to the medium hard coal seam with buried depth 552m,the average dip angle 6°,the average coal thickness 26m,and the Platts coefficient 1.5.In this paper,top-coal fracture blockage,top-coal slumping and migration characteristics,factors affecting top-coal failure of the fully mechanized top-coal caving mining after pressure-relieving in extra-thick coal seam are studied in details.The failure mechanism of top-coal has been preliminary explored in order to provide theoretical support for the future application of this mining technology.The main conclusions are as follows:(1)Breaking size of top-coal of the fully mechanized top-coal caving mining after extracting the middle slice in extremely thick coal seamThe similar simulation technology was used to carry out statistical analysis for the top-coal block during the mining process,and the effect of the collapse and fracture of the top-coal during the pressure-relieving mining stage and the fully mechanized caving mining stage in the extra-thick coal seam after extracting the middle slice was studied,comparing with the effect of layered fully mechanized caving in the same condition and evaluating the caving of the top-coal,the results show that:During the stage of pressure-relieving mining,under the action of supporting pressure and support,as working face advances,the volume of the rear top-coal block is getting smaller and smaller,and the number of blocks is increasing,which reveals the primary crushing effect of the top-coal;Compared with the pressure-relieving mining stage,In the fully mechanized caving mining stage,within the cumulative volume of a certain blocks,the proportion of top-coal blocks has increased,and the number of top-coal blocks has increased significantly,which reveals the secondary crushing effect of the top-coal.Compared with the upper and lower stratified fully mechanized caving mining in extra-thick coal seam,within the cumulative volume range of a certain block,the block size of the top-coal block for pressure-relieving mining is small,and the number of top-coal blocks is large,indicating that the effect of pressure-relieving fully mechanized caving is better than that of stratified fully mechanized caving.(2)Top-coal collapse and migration law in the fully mechanized top-coal caving mining after extracting the middle sliceSimilar simulations and numerical simulations were used to study the slumping characteristics of top-coal and support resistance distribution,migration characteristics,displacement field,vector field,stress field and fracture field of top-coal.The results show:During the stage of pressure-relieving mining,the shape of the fallen space of the roof and the top-coal roof is approximately trapezoidal.The breaking and instability of the stepped cantilever beam structure of top-coal above the bracket have periodic effects.Upper top-coal displacement>median top-coal displacement>lower top-coal displacement,the topping presser accident occurred after the basic top first weighting.In the fully mechanized top-coal caving mining stage,the short cantilever beam structure first broke into smaller block structures and then broke into a bulk structure,and the total displacement of the median top-coal is greater than that of the upper top-coal,which reveals the law and scope of primary and secondary slumping and migration of top-coal in extra-thick coal seams after extracting the middle slice.During the recovery of the pressure-relieving working face,the displacement field of the top-coal in the initial stage stays the same and changes little.In the later stage of the shifting,the displacement of the upper top-coal>the displacement of the middle top-coal>the displacement of the lower top-coal.There is no significant change in the displacement field of the top-coal accumulating in the goaf and the displacement field of the lower top-coal directly distributes above the support.During the recovery of the top-coal caving mining face,the displacement vector field is concentrated in the upper right area of the top-coal caving mining face,which is consistent with the results of speckle analysis and reveals the displacement field and vector field evolution of top-coal.During the pressure-relieving mining stage,due to the role of the support,the lower top-coal was significantly damaged,resulting in two vertical fissures about 10 cm in length.In the lateral direction,as the pressure-relieving working face advances,the coal body cracks gradually develop and expand,the fractal dimension value increases,and the coal body is more broken.In the longitudinal direction,the number of coal body fissures is significantly increases,and the top-coal fractal dimension of the fully mechanized caving face is greater than the pressure-relieving working face,which determines the relationship between the evolution of the top-coal fracture and the fractal.In the pressure-relieving zone,the vertical stress of the upper top-coal>the vertical stress of the median top-coal>the vertical stress of the lower top-coal,in the goaf,the stress concentration phenomenon occurs in the top-coal near both sides of the goaf,It reveals the distribution characteristics of stress in the top-coal.The displacement of the top-coal does not change much during the initial mining stage.as the pressure-relieving face advances in the middle stage,the displacement of the top-coal above the goaf shows multiple undulating peaks.As the pressure-relieving face continues to advance,the number of peaks and undulations continues to increase.In general,the displacement of the lower top-coal>the displacement of the median top-coal>the displacement of the upper top-coal.During the advancement of the fully mechanized top-coal caving mining face,the displacement is redistributed.The displacement law of the top-coal in different positions is that basically the displacement of the lower top-coal>the displacement of the median top-coal>the displacement of the upper top-coal,which reveals the displacement distribution characteristics of the top-coal.(3)Factors affecting top-coal damageNumerical simulation and small-scale similar simulation were used to study the factors affecting the top-coal breakage such as the depth of the coal seam,the thickness of the pressure-relieving layer and the location of the pressure-relieving layer.The results show that:As the depth of the top-coal increases,the displacement of the top-coal is greater,and the damage effect is better,which reveals the influence of buried depth on the degree of top-coal fracture in pressure-relieving mining.The displacement of the upper top-coal increase with the thickness of the pressure-relieving layer increase.The deformation law of the lower top-coal and the medium top-coal is similar to that of the upper top-coal.Due to the staggered accumulation during pressure-relieving,a large number of macroscopic open cracks are formed in the middle top-coal and the lower top-coal,resulting in a zigzag distribution of displacement,which reveals the variation law of the top-coal crushing effect with the thickness of the pressure-relieving layer.When the pressure-relieving layer is at the position I,the overall failure effect of the top-coal is better,that is to say that,the flushing property is better.When the pressure-relieving layer is at position II,the overall failure effect of the top-coal is general,and the overall failure effect of the top-coal is poor when the pressure-relieving layer is at the position III,which reveals the influence of the position of the pressure-relieving layer on the crushing effect of the top-coal.As the number of loading and unloading increases,the number and length of cracks in the top-coal gradually increase.The top-coal fissure can be divided into vertical fissure zone and layered lateral fissure zone,and the fractal dimension of top-coal fissure increases gradually with the number of top-coal loading and unloading increases.(4)Failure mechanism of fully mechanized top-coal caving mining after extracting the middle sliceThe theoretical analysis is used to investigate the fracture mechanism of top-coal of the fully mechanized top-coal caving mining after extracting the middle slice in extremely thick coal seam.The results show that:In the stage of pressure-relieving mining,since the top-coal is not supported by the coal and rock block body in the goaf,so it is periodically broken and acts directly on the working face support in the form of inverted stepped cantilever beam structure.The structural model of the stepped cantilever beam and the instability of the top-coal are obtained.In the fully mechanized top-coal caving mining stage,the block structure model of the secondary failure of the top-coal and the mechanical conditions of the top-coal failure were established.The coal body in front of the pressure-relieving face experienced a complete process from the original rock stress to the axial stress increase and the confining pressure gradually decreased.and the horizontal stress is gradually reduced by the three-way isobaric hydrostatic pressure state,and reduced to zero at the coal wall,which is a simple pressure-relieving process.In the fully mechanized top-coal caving mining,the coal body in front of the working face also experienced a complete process as mentioned above,which reveals the stress environment of top-coal in primary crushing and secondary crushing.The continuous progressive damage process of the top-coal from the original state after stress re-balancing to the fall behind the support is artificially divided into horizontal third zones and vertical second zones.From the left to the right,the third district is the fracture zone,the block zone and the bulk zone.The vertical zone is a block zone and a bulk zone from top to bottom,and the range of the bulk zone is smaller than the block zone.