Principal stress rotation in longwall top-coal caving face adjacent to the gob

Mining induced fracture development is closely related to the stress redistribution in longwall top coal caving(LTCC) face. In order to provide the stress field for investigating the fracture propagation mechanisms in top coal,the distribution of the principal stresses in the LTCC face adjacent to the gob is analyzed by using numerical simulation,theoretical analysis and field investigation. The results indicate the progressive failure process of top coal is greatly influenced by the magnitude and orientation of the principal stresses in the LTCC face. The increase of the major principal stress and the decrease of the minor principal stress cause the failure of top coal. The efficiency of the latter in breaking top coal is 3.0 to 4.5 times of that from the former. The coal seam belongs to complex geological materials with many pre-existing fractures,which means the coal presents anisotropic mechanical behavior. Thus,the mechanical behaviors of top coal evolve with the rotation of the principal direction. Moreover,the failure initiation and propagation orientation of mining-induced fractures are influenced by such stress rotation. In the LTCC uninfluenced by ad-jacent gob,the mining induced stress presents a symmetrical distribution. There is one group of rotation trajectory on the stereonet,which has small rotation degrees. Under the influence of the adjacent gob,the principal stress presents an unsymmetrical distribution. The concentration degree at the gob side is significantly larger than that at the unmined coal seam side. Besides,the rotation trajectory of the principal direction is categorized into two groups on the stereonet,which have large rotation degrees. Under the influence of coal seam extraction,the major principal stress rotates to a vertical direction while the minor principal stress rotates to a horizontal direction. According to the spatial relation between the principal stress and vertical plane parallel to face advance direction,the rotation trajectory is divided into four stages,including external rotation stage,external reverse rotation stage,internal reverse rotation stage and inconsistent rotation stage. At last,the unsymmetrical deformation phenomenon existing between the gates on two sides of the LTCC face adjacent to the gob is reasonably explained by mining induced rotation in the principal stress.