Planning mechanised cave with coarse fragmentation In kimberlite

Abstract

Premier Diamond mine had to plan, develop and operate a low cost, mass mining method to recover extensive ore reserves below a 75 metre thick, dipping gabbro sill. The mining method had to preclude the extraction of as much of the 52 million tons of barren waste contained in the gabbro sill as possible and ensure the safety of personnel and operations. Consequent on the failure of the open stope mining method first attempted, a geotechnical investigation showed that ore recovery by caving methods was possible. The fragmentation that reported to the drawpoints would be coarse and the production horizon would be situated in relatively weak rock. The increased depth or mining, experience on the mine in using LHD's, changes in mining technology and the high production rate required, determined that Premier Diamond Mine could implement a mechanised cave using LHD's for ore extraction. Parameters that need to be defined to successfully exploit any orebody considering cave mining methods include (Cummings et al., 1984 ): * The area that must be undercut to induce continuous caving. * The fragmentation that will result as the orebody caves and the fragmentation size distribution that \viii report to drawpoints. The size distribution will determine drawpoint spacing, secondary blasting procedures and equipment. ore pass diameters, as well as tunnel and LHD sizes. * The rock mass response to the mining operations must be understood and used to optimise the mining sequence. Once the rock on the production level has been damaged by high abutment stresses, maintaining the stability of excavations can be expensive and time consuming. * Support systems and time or installation must be carefully planned and controlled. The function and potential method of failure of the support clements must be understood * Draw control and analysis or draw control data is important to ensure that premature waste ingress is minimised and that stress related problems that can result in cave-sitdowns" do not occur. Research was undertaken by Premier Mine personnel into aspects of cave mining prior to the implementation of a panel retreat cave in the BA5 mining block. This included visits to cave mines using LHD's for extraction in other areas of the world. Problems after initial implementation of the BA5 cave forced further investigations by the Geotechnical Department. Premier Diamond Mine had experienced problems in predicting the area that would need to be undercut to induce continuous caving in caves above the gabbro siII. D.H. Laubscher's correlation of Mining Rock Mass Rating with hydraulic radius was found to be the most accurate method of predicting the area that needs to be undercut to induce continuous caving. An expert system to predict the fragmentation that will result as ore caves and moves through the draw coIumn to drawpoints below was developed and successfully calibrated at Premier Diamond Mine. Prediction of the fragmentation size distribution and hangup frequency have been used to plan several aspects of cave mining.