Numerical simulation on mechanisms of dense drilling for weakening roofs and its application in roof control

This study proposed a method of dense drilling that could induce the formation of a discontinuous surface to weaken the roof. According to the geological conditions of the Chahasu coal mine, a PFC 2D numerical model was established to explore the stress response mechanism and crack expansion law around dense drilling. The study found that, after the dense drilling excavation, progressive damage occurred around the boreholes, and then an elliptical pressure relief zone was created concentric with the direction of the minimum horizontal principal stress as the long-axis and the direction of the maximum horizontal principal stress as the short-axis. Meanwhile, the cumulative area ratio of pixels ( λ ) increased from 0 to 6.42% and the pressure relief zone width ratio ( μ ) increased from 11.55% to 54.6% when the drilling diameter increased from 30 mm to 133 mm. When the spacing of drilling was increased from 300 mm and 700 mm, λ decreased from 30.86% to 9.74%; μ decreased from 63.9% to 33.2%, which means that larger diameters and smaller spacing are beneficial for pressure relief. Field tests found that the discontinuous weak surface induced by dense drilling effectively improves the roadway stress environment. This study can provide reference and experience for hard roof control in coal mines.