The Mechanism Of Ground Pressure Induced By The Breakage Of Far-Field Hard Strata And The Control Technology Of Ground Fracturing

Super-thick coal seams buried under hard roof,if mined out,will cause overlying strata to move and break in the wide range,making the strata in different depth break in different ways and show various pressure characteristics,which inevitability leads to complex stress distribution in the large space and ground behaviors.When there is a hard and thick layer(HTL)lying over super-thick coal seams,the breakage and instability of HTL would easily cause a strong ground pressure in working face,so it is urgent to study ground pressure mechanism and control technology of HTL.Taking the 14~20 m thick coal seam of Carboniferous under hard roofs in Datong mining area as the engineering background,the methods of field measurements,physical simulation,numerical simulation,theoretical analysis and laboratory test are used to investigate the structure feature,abutment stress and ground pressure mechanism.The novel technique of ground fracturing is adopted to control strong ground pressure,and the fracturing performing parameters,both in time and space,were studied and selected in accordance with fracturing effect.The innovation points of this thesis are mainly embodied in the following.(1)Fracturing height and breaking span in overlying hard roofs are revealed from in-situ measurement.Accordingly the concept of ‘far filed’ and ‘near field’ are proposed,along with the characteristic and difference of ground pressure induced by the breakage of hard roofs in near and far field.(2)The mechanical model of hard roof breakage in far field is established,with which the structure characteristics and pressure effects of the hard roofs in far field are analyzed to reveal the mechanism of strong ground pressure.The range of key stratum in far field that results in the strongest pressure is clearly defined.(3)The crack propagation rule during hydraulic fracturing was obtained from experiments.Moreover,the effects of ground fracturing on the vertical and horizontal fracture plane were given to explain the mechanism of the strong ground pressure controlling.The theoretical calculation model for the rational location of ground fracturing is constructed,providing the engineers with efficient tool in ground fracturing.(4)The system of ground fracturing hard roofs in far field is established,which realizes a high efficiency control of fracturing level,timing and crack propagation of hard roof.The successful application of ground fracturing opens up a new way of hard rock strata control and successfully solves the problem of strong ground pressure in the working face.The mechanism of strong ground pressure induced by the breakage and instability of far field hard strata is revealed in the dissertation.The successful application of the ground fracturing technology provides a theoretical support and technical reference for effectively control of hard rock in the far field,and opens up a new way of hard roofs control in coal mine domain.The method of ground fracturing has a great significance in solving the ground pressure disaster caused by the instability of far field strata and structure.