Research On Evolution Characteristics And Instability Mechanism Of Composite Pressure Arch Of Overlying Strata During Shallow Coal Mining

The reserve of coal resource is huge in western China and the developing advantage is obvious,shallow buried depth,thin bedrock and thick aeolian?alluvial?sand cover are typical characteristics of the coal seam,but the fragile ecological environment and mining-induced disaster also restrict the development of western mining area seriously.The bedrock of shallow coal seam is easily broken in total thickness for mining influence,the strong and weak periodicity exits in roof weighting,the disaster for roof cutting and the support crushing are prominent problems,sliding instability of the caved zone of overlying strata is serious threat to the safety of the mining area.Taking the typical shallow coal seam mining with thin bedrock and thick bedrock in Shendong mining area respectively,based on the key strata theory of strata controlling and the pressure arch theory,according to the breaking and migration law of the roof in each mining stage,the mechanical models of symmetrical pressure-arch,step pressure-arch,rotative pressure-arch in key blocks of near filed overlying strata and global pressure-arch in far filed overlying strata are established,and the mechanical criterion of stability of the pressure arch structure and the characteristic index of load bearing capacity are deduced.The evolution characteristics of composite pressure-arch in near and far field overlying strata are verified and analyzed by using similar simulation test and numerical simulation,the migration and roof weighting mechanism of overlying strata of shallow coal seam mining area is revealed.Combining the stepwise loading and unloading test and triaxial compression test with different confining pressure on water bearing sandstone samples,the coupled responsing mechanism of the pressure-arch evolution and energy filed transferring and dissipation in overlying strata is established.The main research results are as follows:?1?Taking the typical shallow coal seam mining of Daliuta mine and Shangwan mine in Shendong mining area as engineering background,the mechanical models of symmetrical pressure-arch,step pressure-arch and rotative pressure-arch in key blocks of near field overlying strata are established,the mechanical criterion of stability of the pressure-arch structure in key blocks is deduced,and the influence of boundary horizontal stress on stability of the pressure-arch structure is analyzed.Using UDEC software to conduct the numerical simulation computing of shallow coal seam mining with different bedrock thickness and different initial horizontal stress,the evolution characteristics of the composite pressure-arch in near field and far field overlying strata and the distribution law of principle stress are analyzed.?2?Based on stability theory of elastic system and catastrophe theory,the buckling and breaking equation of fractured roof strata is deduced by establishing the Euler structure mechanical model of the hanging strata of mined out area,proposing different instability modes of the pressure-arch structure of near filed fractured strata and the influence of deflection,thickness/span ratio and initial angle of rotation.Applying numerical simulation computing in analyzing sliding instability and rotating instability of the pressure-arch structure in basic roof key blocks of thin bedrock and corresponding roof weighting,according to the alternated instability of multi-layer overlying strata of thick bedrock,the mechanism of roof step cutting and long/weak periodic roof weighting is revealed.?3?Considering two typical shallow coal seam mining conditions of thin bedrock with thick loose layer and thick bedrock with thin loose layer,using similar material simulation test to analyze the evolution characteristics of overlying strata structure and stress distribution in surrounding rock during shallow coal seam mining,the structure type and combined relation of each layer overlying strata in different mining stage is obtained.By analyzing the difference of load bearing capacity between the thin bedrock mining area and thick bedrock mining area,the compound load bearing mechanism of the pressure arch in near field overlying strata and the pressure arch in far field overlying strata is elaborated.?4?According to the load transferring mechanism of the hanging strata of mined out area and the arching property of pressure in the strata,the pressure arching index k of far field overlying strata,pressure-arch core judging index kc and core ratio index Rc are proposed,the above indexes are used to make reasonable division of each region of the mining moved strata.Using FLAC^3D software to conduct numerical simulation analysis of the evolution characteristics of pressure-arch in far field overlying strata during shallow coal seam mining with thick bedrock,the pressure-arch range of far field overlying strata and load bearing condition in different mining stage is analyzed with fish language programming,and analyzing the influence of different working factors of working face length,mining height,quality grade of surrounding rock and combination of rock property on the pressure-arch range of overlying strata and load bearing condition.?5?According to the loading features of overlying strata in mined out area under the influence of pressure-arch,designing the stepwise loading and unloading and triaxial compression test on water bearing sandstone specimens,the energy evolution characteristics of the rock specimen under different loading way is revealed,and the effect of the confining pressure and the water content on energy evolution characteristics of the rock specimen is expounded.Based on the fitting equation of elastic energy of the rock in the test,the catastrophe model for energy accumulation and release of rock specimen is established.Applying the built-in strain energy calculation function of FLAC^3D software and fish language programming in the simulation analysis of characteristics of energy accumulation,release and dissipation of overlying strata during shallow coal seam mining,the evolution characteristics of the energy of strata in various regions of pressure-arch is revealed in different mining stage.