Overburden failure laws in working face of short distance thick coal seams group

In this paper,to deeply study the overburden failure rules in sub-level caving mining of close distance thick coal seams group,and provide basis for the reasonable setting of safety coal and rock pillars in the outcrop area to control the water disasters(e.g.water inrush,sand inrush) occurred in shallow part coal face,taking one working face in close distance thick coal seams group(No.7 and No.8 Coal Seams) of Datun Mining Group as example,on the basis of early observation of overlying strata failure rules in the upper coal seam(No.7 Coal Seam),the underground two zones visualizer was recently used to observe the two zones height development characteristics of the lower coal seam(No.8 Coal Seam),thus taking the leading and comprehensive measurement of overburden failure laws in the working face of close distance thick coal seams group in China.The results show that when there exits coincidence area between the floor damage range of the upper coal seam and the caving zone development range of the lower coal seam,the caving zone height of the lower coal seam will increase.Meanwhile,the ratio of the fractured zone height to the mining height in sub-level caving mining of close distance thick coal seams group is similar to that in the one single coal seam according to the calculation of comprehensive mining thickness,while the ratio is higher than that in one single seam about 52% according to the calculation of lower coal seam thickness.Therefore,the calculation formula of two zones height in Mine regulations in three special needs further improvement under the conditions of sub-level caving mining in the close distance thick coal seams group.