An ultrasonic-based method for longwall top-coal cavability assessment

Longwall top-coal caving (LTCC) is widely employed in the underground mining of thick coal seams. The technique divides a thick coal seam into a lower and an upper section. The bottom coal is sheared by a cutting head and transported by a conveyor in front of the support while the top coal fails and caves under mining pressure, transported by a conveyor at the rear of the support. In an LTCC panel, accurate assessment of top-coal cavability is critical to the design of mining parameters required to achieve continuous caving of top coal. In the present paper, an integrated detection system in which fracture development and ultrasonic wave velocity are used to characterize top-coal cavability is introduced. The cavability is predicted conveniently due to ease of access of the ultrasonic wave velocity in the top coal. Accordingly, an ultrasonic model is established and coupled to the mechanical model previously proposed by the authors to simulate the evolution of the wave velocity in stressed coal. The mechanical-ultrasonic coupling model is validated by comparison against experimental data and further extended to evaluate top-coal cavability in Dongzhouyao mine with a finite difference type code. The predictions, including the excavation-damaged zone, roof weighting interval, stress distribution, stress rotation and ultrasonic wave velocity evolution, are consistent with the field measurements. With the proposed model, the influence provided by hydraulic fracturing on top-coal cavability is investigated by conducing the finite difference type modeling.