Evolution Mechanism And Control Of Overburden Structure For Gently Inclined Ultra-Thick Coal Seam Multi-Layer Mining In Xinjiang East Junggar Mining Area

As the fourteenth billion-ton coal bases in China,Xinjiang is an important energy replacement area and strategic energy reserve area.The presence of ultra-thick coal seams is an important feature in East Junggar,Turpan-Harmi and Ili coalfields.Based on the occurrence conditions of the typical thick coal seam in East Junggar coalfield with intensive development in Xinjiang,this paper aims at the mining characteristics of large-scale mining space expansion and multi frequency stress disturbance for ultar-thick coal seam.Systematic research is carried out on the evolution of overburden bearing structure and strata behavior control in multi-layer mining for ultar-thick coal seam,by using research methods such as field investigation,theoretical analysis,experimental testing,physical simulation,and numerical calculation.The main results are as follows.(1)Based on the occurrence conditions of the typical thick coal seam in East Junggar coalfield,the mechanical properties of roof strata are mastered.The form of 70 m ultra-thick coal seam has the remarkable characteristic of coal seams merging and bifurcating.And the occurrence conditions of ultra-thick coal seam are divided into two types: single ultra-thick coal seam(including ultra-close coal seam group)and close coal seam group.(2)The characteristics of overburden fracture hinge movement,under different slicing mining thickness of ultra-thick coal seam,are studied.It characterized by " in the early mining stage,broken rock block beam hinge in lower hard rock — in the middle stage,the lower hinged structure weakened and the beam hinged structure moved up — in the later stage,the continuous increase of the mining space leads to the horizontal O-X fracture of far-stope rock strata,and broken rock blocks are extruded into shells".In the process of multi-layer mining,the overburden bearing structure shows the evolution process of “beam structure — higher beam structure — shell structure”.(3)The stability condition,position determination and stratification fracture characteristics of thick main roof are studied.And the forming conditions,dimension parameters and articulated block stability mechanism of shell structure are defined.The calculation formula of stress arch height,based on slicing thickness and advancing speed,is obtained.Based on the key parameters of slicing thickness and the crushing coefficient of caving strata,a method for the identification of roof bearing structure of "beam structure-high beam structure-stress arch structure" was proposed.(4)The mechanical model of the relationship between the support and surrounding rock is established,under the condition of large-scale mining space expansion and multi frequency stress disturbance.The change characteristics of roof bearing structure and support controlling roof under different mining stopes are revealed.And the calculating method of support load is put forward.The load on support shows the change characteristic of “increases slowly under near-stope roof bearing structure and tend to stabilize under the far-stope roof bearing structure”.The reasonable working resistance and initial support force of hydraulic support are determined.(5)The strata behavior control principle of "avoiding the cantilever beam structure and ensuring the stability of roof bearing structure near the stope" is made clear,in East Junggar coalfield 70 m ultra-thick coal seam mining process.And the control measures of "pre-splitting main roof and pre-crack the roof at the cut side in first layer mining","reducing the workface length and rapid advancing in later mining stope" and "grouting in goaf and separation area" are put forward.There are 184 figures,24 tables and 164 references.