Propagating mechanism of top-coal fracture in longwall top-coal caving mining

In order to understand the fracturing mechanisms of top coal,lab testing,numerical modeling,theoretical analysis and field measurement are used to study the driving forces of top-coal fractures. Top-coal cavability is posi- tively proportional to the development of mining-induced fractures. Stress conditions for different propagation modes (I,II and I-II) of top-coal fractures are determined where dominant propagation angle is considered. Propagation and associated pattern are dependent on the magnitude and direction of principal stresses. There are peak points of major and minor principal stresses occurring ahead of the longwall face. The major principal stress remains in loading state while minor principal stress undergoes loading,unloading and reverse loading processes. Major and minor principal di- rections rotate to the gob in a vertical plane (α) parallel to the advancing direction in front of the longwall face. Rotation angle of two principal stresses keeps equal in plane α. Reverse rotation occurs subsequently at the rear of the long- wall face. The major principal direction deviates from plane α and becomes parallel to dip direction. In horizontal plane,the rotation angle of minor principal stress is equal to the angle between advancing direction and initial minor principal axis and that of major principal stress is zero. Due to the increase of principal stress magnitude and rotation of principal direction,top-coal fracture propagates in mode I first. Then,the mode turns into mode II because of high mi- nor principal stress. In the proximity of longwall face,the fractures propagate in the mix mode I-II. Because of principal stress rotation,mining-induced fractures in top-coal incline to the gob consistently. Based on the propagation modes, top coal is divided into five zones,including initial stress zone,microcrack-increasing zone,shear failure zone,tensile- shear failure zone and caving zone.