Techniques of inverse drop raise blasting and slot drilling

Mining with blast holes drilled upwards may be done to recover sill pillars, or for economic reasons when the distance between the levels is relatively large. If the rock is sufficiently competent to accommodate large stope size, the distance between sublevels could be increased to as much as drilling accuracy allows. The same drill drift can be used for the drilling of both down holes and up holes. In the case of down holes, the initial opening of the stope (drop raise) is not as much a problem as for the inverse drop raise for mining with up holes. This is because, with a drop raise, a lot of space is available at the toe for the blasted rock to expand. In addition, the raise can be blasted in many shots thus allowing smaller blasts each time. Inverse drop raises are generally much more difficult to blast because of the limited space for muck flow and must, therefore, be taken down in a single blast, requiring special considerations in order to ensure a success. A case study of a 29 m (95 ft) inverse drop raise successfully blasted in a single shot at Heath Steele Mines is described. The paper covers technical considerations such as drilling accuracy, hole deviation survey delay extension and protection, initiation methods and initiation sequence. Finally a novel method for creating the initial opening called 'slot drilling' is presented. This method, inspired from LKAB in Sweden, consists of drilling a row of interconnected holes using a special guide mounted on a regular production drill hammer With a case study of blasting an inverse drop raise of 29 m (95 ft) in one blast this paper further discusses some technical issues that are critical for the development of inverse drop raises. A novel method for creating the initial opening with a slot of interconnected holes is also presented with the experience acquired at different mine sites across the Noranda group.