Longwall Top Coal Caving Mechanisms in the Fractured Thick Coal Seam

In this study, the finite difference method (FDM) coupled with a discrete fracture network (DFN) was utilized to analyze longwall top coal caving (LTCC) behaviors. The integrated FDM-DFN model enabled the influence of the preexisting fracture, stress redistribution, stress rotation, caving material compaction, and periodic rupture of roof strata to be superimposed on the failure process of top coal. It was revealed the LTCC influenced both magnitude and orientation of the principal stress within top coal. The minor and major principal stresses experienced successive peak points as top coal approached the LTCC face, and the corresponding principal axes rotated toward horizontal and vertical directions, respectively. The concentration of the major principal stress and the release of the minor principal stress resulted in the shear failure of top coal ahead of the LTCC face. The failure mode transferred from shear to tension at the rear of the face line. The principal stress rotation led to continuous variation in internal cohesion of top coal and brittle fracturing of the main roof resulted in dynamic load at the LTCC face. The stress rotation and roof rupture greatly promoted the failure process of top coal. This type of promotion was also provided by the preexisting fractures and adjacent goaf.