Abstract:
Shallow buried extra-thick coal seams are widely distributed in the western mining areas of China, due to their shallow burial depth, thin bedrock layers and thick loose layers, and other storage characteristics, in the process of mining both overburden rock fracture and cracking and surface subsidence perturbation of the dual disaster-causing characteristics. The FDEM method based on continuous-discontinuous coupling can not only realise the transition of deformation, fracture and breakage from continuum to discontinuity, but also take into account the superposition effect of stress perturbation caused by continuous excavation, which is a feasible path to realise the accurate prediction of surface subsidence of shallow buried extra-thick coal seam mining. The FDEM simulation analyses the overburden breakage process and the cycle pressure law during the mining process, traces the stage settlement caused by the breakage, and finally realizes the reliable prediction of subsidence based on the on-site monitoring and calibration. The study shows that: The initial breakage of the overburden key layer in the back mining of shallow buried extra-thick coal seam leads to the upward development of overburden fissures, and the shear slip instability of the cut-off body is accompanied by the increasing height of the fallout zone and the fissure zone, whose peaks are 5.6 times and 6.1 times of the mined thickness, respectively, the fracture type of overburden is transformed from tension-tension-shear dominated to shear damage; The FDEM model accurately captures the mineral pressure characteristics induced by overburden fracture: the initial fracture step is consistent with the actual measurement, and the simulation error of the peak mineral pressure and the cycle pressure step is <5%; Under the condition of shallow buried extra-thick coal seam, the fracture of the main key strata provides subsidence space for the instability of the high main control strata, and the fracture of the high main control strata further enlarges the settlement amplitude and controls the scope of the subsidence area. The FDEM model shows that the change trend of surface subsidence at different mining distances is consistent with the field measured data; The FDEM simulation results accurately reproduced the breakage law of overburden rock in shallow buried extra thick coal seam mining and accurately predicted the surface subsidence, and the error between the simulation and monitoring results mainly came from the mismatch of the time scale and the complex engineering conditions in the field.