Study On Movement Laws Of Roof Strata And Its Control In Slicing Mining Extra-thick Coal Seam In Dajing Mining Area

During the past decades,Xinjiang has become to be one of the biggest national principal coal producing regions in China.The super thick coal seam makes either the caving method or the overburden subsidence significantly different from existing research being on the conventional thickness of coal seam.Unfortunately,few studies in this area have been conducted,remaining a great challenge not only for research but also for practical application.This thesis therefore presents the first ever investigation on the deformation characteristic of overlying strata and corresponding optimum multilayer caving method for underground coal mine with super thick coal seam.In particular,the research focus on the influence of some critical parameters on the fracture characteristics and structural evolution of overlying strata by employing various methods including theoretical analysis,similar simulation and numerical simulation.The results obtained from this research are believed to provide a valuable and referable guideline for other coal operators in Dajing mining area,Xinjiang.According to the results and discussions in this thesis,the following conclusions have been drawn up:(1)Considering the coal resource distribution and the development plan of underground coal mines with super thickness in Xinjiang,Dajing mining area was finally selected to be the research area in the present research.Subsequently,both the physical and numerical simulation model were set up based on the real geological conditions.The results demonstrates that the both the cumulative caving thickness and stratified caving thickness,as two main factors to be considered,affect the characteristics of fracture and the structural evolution of overlying strata.In detail,the hinge structure moves up with the increase of cumulative mining thickness.However,for a given cumulative caving thickness,the greater thickness of stratified caving thickness,the smaller overburden subsidence.In addition,the relationship between the stratified caving thickness and the movement law of surrounding rock has been clarified while the fragmentation of rock was put into consideration.(2)The innovative concept termed “Three-stage evolution(TSE)theory” was proposed by the author of this thesis based on the gradually broken and discrete feature of overlying strata with the caving of super thickness coal seam.The theoretical model consisting of initial hinge,medium-term cantilever and post-loose is consequently established to determine the static breaking distance of rock beam,for which the thickness of stratified caving is denoted as the controlling parameter.Whereas,the cumulative caving thickness is used to be the calculation criterion for the threshold point transferring from initial stage to the second stage.The dynamic calculation of overburden damage height and rock mass breaking under several caving procedures was then proposed based on the better understanding of the relationship between the roof and shield support.The mentioned calculation method provides the critical criterion for the transaction from the second stage to third stage.Finally,the “Hertz sphere contact theory” was introduced to determine the failure of discrete block in the third stage.Then,the required loading capacity of support under different stages can be successfully obtained.(3)This paper puts forward the mining principle of first thick,then thin,and as few layers as possible,and establishes the division system of the layered mining thickness of the huge thick coal seam,which is constrained by the caving property of top coal,the stability of coal wall and the destroyed depth of bottom coal.On this basis,it is determined that the layered thickness range of the 50 m thick coal seam in Dajing mining area should be controlled between 15.5m and 18 m,and the cutting height should not greater than 4.5 m,and the suitable working face length is 300 m.