Failure of hanging roofs in sublevel caving by shock collision and stress superposition

Hanging roofs or high hang-ups, a common problem in sublevel caving mining, usually result in a large ore loss and undermine mining safety. This paper analyzed the formation of a hanging roof and showed that increased confining pressure and reduced free surface were its main characteristics. In order to break down a hanging roof, a new method based on shock wave collision and stress superposition was developed. In this method, two blastholes containing multi-primer at different positions are simultaneously initiated at first. By doing this, a new free surface and a swell room can be created. After these holes are fired, a long delay time is given to the next blasthole so that the fragments from the first two-hole blasting have enough time to fall down. This new method was applied to three hanging roofs in one production area, and all of them were successfully broken down. Field inspection indicated that almost no damage was caused in the nearby drifts/tunnels due to the new method. In addition, the far field vibrations were found to be smaller than the maximum vibrations induced by some other blasts.