Mining-induced stress rotation and its application in longwall face with large length in kilometer deep coal mine

Due to the increase in bury depth and face length,the mining induced stress environment becomes increas- ingly complex. The damage degree and controlling difficulty of surrounding rock grow rapidly in longwall mining. Such changes drastically threaten the mine safety. In order to improve surrounding rock control in such longwall face,the mining induced stress rotation and its influence on surrounding rock stability is investigated with theoretical analysis, numerical simulation and field measurement. The results show that pre-existing fracture shows an increasing trend in rock masses surrounding longwall face with large face length in kilometer deep coal mine. Surrounding rock stability is thus influenced by both the magnitude and orientation of mining induced stress. The pre-existing fracture causes the appearance of optimal propagation angle of the failure fracture. Mining induced stress rotation leads to the decrease in the load-bearing capacity of surrounding rock,whose stability is negatively related to the rotation angle of mining in- duced stress. After mining the 121304 longwall face,stress rotation phenomenon occurs,and rotation trace is closely re- lated to the influence intensity of the mining and face advance direction. With the decrease in the distance from the edge of gob,the rotation angle and rotation speed of mining induced stress increase quickly. The major principal stress of coal seam and lower overburden strata rotates toward horizontal direction in the vertical planes parallel with and per- pendicular to the face advance direction. The minor principal stress first rotates into the referred vertical planes and then rotates consistently with the major principal stress in the vertical planes. At the same time,the minor principal stress rotates toward vertical direction gradually. Rotation angle experienced by mining induced stress increases with the growth in vertical distance from coal seam. In higher overburden strata,the rotation trace of mining induced stress is influenced by the gob remaining after the extraction of the 121303 longwall face,which deviates to the gob. Accord- ing to the relation between face advance direction and the rotation trace of mining induced stress,the determination method of face advance direction is proposed for the surrounding rock with one set and many sets of fractures with sta- ble orientation as well as that with random-distributed fractures. Besides,the influence of mining induced stress rotation on the voussoir beam structure of overburden strata is discussed.