The purpose of this study was to evaluate the potential of a novel technique for the drilling of production blastholes. An improved technique is attractive if it can reduce the costs of blasthole drilling at mining operations or allow for greater flexibility in the operational aspects of blasthole drilling.
In the investigation, a waterjet was used to assist the standard rock cutting rotary action of a tricone roller bit. The waterjet uses a pressure up to 90MPa with a flow rate of up to 230 litres per minute.
The baseline performance of drill bits using conventional drilling was determined within the test rock type, which was a hard (Uniaxial Compressive Strength: 220 MPa) coarse-grained granite (Enogerra granite). Modification to the drill bits and the experimental drill rig itself were made, to produce a cavitating jet directed at the base of the hole.
The best performing waterjet was determined from a number of pure slotting tests where a waterjet was rotated in front of a rock sample. The most efficient waterjet at creating a slot, based upon its specific energy, was found to be a 2.0mm Leach and Walker (L&W) design. In addition, pre-slotted rock samples were used to determine the most effective position of the slot. While this was partially successful in demonstrating the effect of a slot placed at the outside diameter of the hole, it was difficult to quantify between slots at different positions.
The first waterjet trial tested the initial waterjet assisted design of the drill bit and revealed that further improvements to the design could be made. It also demonstrated that correct bailing was very important.
The second waterjet trial revealed that the most effective use of waterjets in terms of the improvement in drill performance was not with the larger diameter nozzles. Rather, the performance of the 2.0mm diameter nozzles outstripped or was equal to the performance of the larger diameter nozzles.
The study demonstrated that the waterjet assisted technique could increase the rate of penetration over conventional drilling. For the best-case experimental result, the relative improvement was 41.3%.
Finally, an economic model was developed to understand the specific drilling costs of waterjet assisted drilling. This model demonstrated the possibility of reducing overall drilling costs through the reduction in the drill fleet size. More importantly, it is a starting point for examining alternative uses of the additional drill productivity obtained from waterjet assisted drilling, such as increasing the density of the drill pattern.