Study on a Mechanical Model of a Long Cantilever Beam Structure Formed by the Fracture of Thick Partings Transversely Penetrating a Coal-Caving Ellipsoid and Its Application

Due to the large number of layers and different thickness of partings, the poor caving performance of top coal (CPTC) in thick coal seam with complex structure has always been an important problem affecting the top-coal recovery rate (TCRR) in the fully mechanized caving face. Based on the theory of coal-caving ellipsoid, a mechanical model of a long cantilever structure formed by the fracture of the thick parting crossing coal-caving ellipsoid was established, and the influence mechanism of the long cantilever beam structure on CPTC was revealed. Besides, a method to distinguish the influence of multi-layer partings on CPTC was put forward, and the influence of parting thickness, position and the number of parting layers on CPTC was obtained. The results show that the critical parting thickness h(g) increases with the increase of axial deflection angle theta, mining thickness h and volume weight of parting gamma(g), and h(g) decreases with the increase of tensile strength of the parting R-t. The main influencing factors of h(g) can be ordered as follows: h > theta > R-t > gamma(g). The position of the thick parting plays a controlling role in CPTC. The fundamental measure to improve CPTC is to reduce the cantilever beam length formed by the thick parting so that top coal above the parting can be caved. Based on this, the technical scheme of hydraulic fracturing for promoting top-coal caving was proposed and applied. During the initial mining period of the working face, TCRR was effectively increased from 45.99 to 77.18%.