Development of overburden fractures for shallow double thick seams mining

Three models including three-dimensional physical model,PFC2D numerical model,and theoretical model were established for the shallow double-thick coal seams in Shendong mining area in China. The development rule of o- verburden fractures after double coal seams excavation was studied. The results show that the height of fractured zone is about 55. 5-60. 3 m after upper seam excavation. The key stratum between two seams is first fractured resulting in the connectivity of upper and lower gobs when advance distance of working face was 60 m. After that,the main key stratum is fractured,and the fractures extends to surface. The ultima height of fractured zone is about 146. 5-153 m. The ends of gob with higher porosity is taken as a dominant pathway of air flow,as the compaction degree of gangue at there is more than that at the middle of gob with lower porosity. Mining-void field presents a double-arc shape,com- bined with the theory of key stratum,overburden strata can be divided into irregular void zone,periodic void zone, dominant void zone between arcs,and micro void zone under arc. Engineering values of these void zones are also illus- trated in detail. A method is proposed for calculating the equivalent mining height of close double coal seam mining. And it is used in predicting the height of fracture zone combined with modified predicting formula of top coal caving.In conclusion,the study reveals the air leakage mechanism of surface-upper mining section-lower mining section in shallow double-thick coal seams mining. The engineering measures for air leakage and spontaneous combustion are also put forward according to the characteristics of void field. It could provide a reference for other similar coal mines in the actual operations of fire prevention and extinguishment.