Overburden movement and failure law of coalface in end slope and the slope stability control method

Due to the limitation of technology and safety factors,the end side slope angle of open pit coal mine in China is relatively low,which is only 20°-35° under normal circumstances,therefore,billions of tons of coal resources are trapped under the end side slopes,and after the completion of internal drainage and reclamation,most of these trapped resources will not be recovered.With the decrease of coal resources and the progress of mining technology,coal mining is gradually transferring from shallow coal resources to deep coal resources and complex and difficult coal resources,and more attention has been paid to the coal under end side slope and residual coal recovery in open pit coal mine.Engineering practice shows that the mining technology of end side shearer is a fast,flexible,safe,efficient,economical and environmentally friendly end side coal mining process in open pit coal mine.The essence of this technique is to construct a series of roadways along the coal seam at the exposed position of the coal seam at the lower part of the slope,and the roadways are perpendicular to the slope.There is no support in the whole mining process,and the weight of the overburden is supported by the supporting coal pillars retained between each roadway.As a result,the mining effects of “surface mining” and “roadway mining” are superimposed and influenced each other,and finally formed a dynamic complex system with diversified spatial forms.Once the supporting coal pillar is damaged,it is likely to induce the chain instability of the pillar group,which will lead to the large area landslide at the end side coal mining area.Therefore,it is of great importance to ensure the safety of the mining area,and the stability of supporting coal pillar.Slope is the key to ensure the safety of the mining area.To overcome the problems above,taking the end slope coal extraction of an open pit mine in Erdos area in China as the background,the influence of the superposition of “surface mining” and “roadway group mining”,which effects on slope stability,is investigated comprehensively.The 3DEC numerical analysis model for the deformation and failure of rock and soil mass in the coalface of end slope under near horizontal condition is constructed,the deformation movement and stress distribution characteristics of overlying rock in the whole process of coal mining are simulated.The research results show that according to the moving deformation characteristics of rock and soil mass,the deformation and failure process of mining slope can be divided into four stages:supergene transformation stage,structure modification stage,time dependent deformation stage and final instability (stability) stage.the structure modification stage is the critical stage of slope deformation and failure,and the upper part is the key area of deformation and instability in the process of end side coal mining,and the possible instability modes are the circular sliding inside the bulk material or the bedding sliding along the soft rock layer.Through the research,the formation process and evolution law of “distensible pressure arch” in the overlying strata of roadway groups are revealed,a method for the stability control of overburden rock of the coalface in end slope based on reasonable supporting coal pillars reservation is presented,and a method for determining the size parameters of permanent isolation pillar is established,finally,the applicability of the theoretical formula is verified by numerical analysis and engineering practice.