The orientations of discontinuity sets and major geological structures greatly influence the stability of an excavation. Excavations should therefore, where possible, be orientated such that major discontinuity sets and structures are considered, allowing the likelihood of rockfalls in a longhole stope to be kept to a minimum. The longhole stopes are non-entry excavations, which are remote cleaned to avoid exposure of personnel. However, excessive rockfalls can significantly influence productivity and cause damage to mining machinery. This paper presents a risk-based approach to design, where the failure potential of mining stopes for the Platreef mine was determined considering the influence of major structures. The probabilistic keyblock analysis software JBlock was used in this capacity. In this study, the failure potential of keyblocks was determined in JBlock for the longhole stopes of various proposed orientations, considering structural domains and a major geological fault which was identified across the project area. Results of the analysis indicated that stopes orientated sub-parallel to Platreef's major fault (Tshukudu Fault) resulted in the largest rockfalls. Overall, JBlock was used successfully as a design tool to assess the stability of the longhole stopes. Outcomes of the study have provided Platreef mine with valuable information that will assist in the decision making process when finalising stope orientations. This study therefore attempts to serve as a reference point for solutions to similar situations, with the objective that risk-based designs are called upon more frequently in the future.
Ivanplats (Pty) Ltd, a subsidiary of Ivanhoe Mines Ltd, has undertaken an investigation to assess the feasibility of developing a new 4 Mt/a underground platinum mine (the Platreef mine) accessed via a vertical shaft system. Platreef mine is located on the northern limb of the Bushveld Complex in South Africa, near the town of Mokopane, approximately 280 km northeast of Johannesburg.
It is planned to extract the wide, shallow dipping orebody using longhole stopes (15 m wide, 25 m high, and between 40 m and 200 m long), which are post-filled. The stopes will be mined in a primary-secondary sequence using mining machinery and will be non-entry once blasted. In areas which are flat dipping (<10°), the stopes can be orientated in any direction, but where the dip is greater than 10°, they can either be oriented longitudinally (parallel to strike) or transversely.