Critical failure of overlying rock strata and its criteria induced by high-intensity longwall mining

The overburden failure height caused by longwall mining is of great significance to under water body mining,water conservation mining and gas control in the mining of liberated strata.The overburden failure height,overburden critical failure and its criteria induced by high intensity longwall mining were investigated by means of theoretical analysis and numerical simulation.Based on the definition of overburden critical failure,its characteristics and influencing factors were analyzed,including the mining height,mining size (the strike length and dip length of a panel),mining depth and strata lithology.A theoretical assessment method of overburden critical failure was proposed based on trapezoidal area and quadrilateral prism volume.Overburden failure and surface subsidence due to longwall mining were simplified as four kinds of mining volumes,i.e.,the cuboid volume of gob,the quadrangle volume of pre damaged overburden,the volume of damaged overburden and the volume of surface subsidence.The theoretical equation of overburden failure height when overburden critical failure is achieved and the judgement equation of “two zones” overburden failure mode were obtained.Based on a high intensity mining panel,the 3D numerical models that consider the maximum horizontal principal stress and minimum horizontal stress and their orientations with the panel advance direction were established.The influence of different factors (mining height,mining size and mining depth) on overburden critical failure was analyzed by the numerical simulation.The results show that the advanced distance when the overburden critical failure is reached is inversely proportional to the dip length and depth to thickness ratio of the panel.The criteria of overburden critical failure in high intensity mining and its applicable conditions is given and applied to some panels.It can provide an important theoretical basis for the measurement of the maximum overburden failure height and the safe mining under the water body.