Ore passes at Premier Mine are located in the norite country rock fairly close to the Contact with the kimberlite pipe. They are usually excavated by initially raise boring a 2.1 m diameter hole, and then enlarging this to the required size by blasting. The problems which have been experienced with the ore passes include both blockage of the initial raise bore, and blockage of the enlarged pass. In some cases failure has occurred before the passes have been used. This has required excavation of replacement passes.
To remedy the problems, consideration was given to support of the passes. Initially conventional forms of support such as lining were considered. However, it was noted that the norite in which the passes are located is a competent, tough rock with good wearing properties, and the necessity for lining was questioned. An alternative approach, which is described in the paper, was therefore adopted. This involved the measurement of the rock structure, and, from these data, the assessment of the probability of Occurrence of potentially unstable blocks of rock in the walls of passes. This allowed the determination of the required spacing of support elements to reduce the probability of failure of the rock to acceptable block sizes and volumes. The results were expressed as a design curve correlating support spacing and likely failure volume.
Ore and waste passes are extremely important components of the mining system. They are often critical elements, and their blockage or failure may cause major losses of production. In high stress mining environments, with hard, abrasive Ore and waste, problems of excessive scaling and blockage have been experienced (Gibbon 1977). In similar operations, geological structure has played a significant role in the failure of the passes (Schoombee and van Wyk 1983, Stringer and Lategan 1983). Repair of passes is costly, time consuming and sometimes not practical.
Premier Diamond Mine has had experience with the raiseboring of holes for various purposes in felsite, gabbro and norite rock types. Major problems have occurred with some of the holes in the norite, which is a strong and hard rock material, but which is blocky with well-defined Jointing. The problems which have occurred have been a function of the diameter of the hole and the use to which it was put. Numerous 1.8 m diameter ventilation shafts have been bored, and no problems have been experienced with these holes. However, in two separate 6 m diameter raise-bored ventilation shafts major collapses occurred in the norite sections of the shafts.
Twenty-four raisebore holes have been drilled on the north side of the Mine as part of the pass system for development and production ore and waste from the underground block cave mining operation. All of these holes were drilled to a diameter of 2.1 m and vary in length from 90 m to 120 m. One of the holes collapsed shortly after it had been drilled, and it was subsequently found that it intersected a 1 m wide shear zone.
