Bord stability evaluation and support design at Thorncliffe Chrome Mine in the Bushveld Complex: a case study

Abstract

Thorncliffe Chrome Mine is one of Glencore’s Eastern Chrome Mines (ECM) bord and pillar mining operations. It is in the process of revising its mine design criteria with a focus on the stability of mining spans between pillars. The process includes the development of appropriate standard support requirements. A methodology developed by the Mine Health and Safety Council (MHSC) for designing stable bord spans in shallow Bushveld Complex operations was applied in the case study. Important factors considered included the rock mass quality, degree of safety required in different production drives, engineering requirements, local rock engineering environment, and various rock mass failure mechanisms. The rock mass classification analysis found that the ground conditions ranged from very poor to very good. Empirical design results indicated that maximum unsupported spans for short-term production drives varied between 3 m to 15 m. Due to higher safety requirements, maximum unsupported spans in medium-term production drives varied between 2 m to 8 m. However, engineering requirements dictated the mine maintains a minimum of 6 m in all the production drives, travelling roadways, and strike conveyor belt drives, to accommodate the manoeuvrability of the Trackless Mobile Machinery (TMM) used in production and belt conveyor equipment. Sufficient space is also required for workers to travel to workplaces. As an additional safety factor in the design of bord spans, the largest (diagonal) spans across intersections were used to design maximum stable bord widths. Poor ground conditions and spans larger than 17 m (formed by 12 m spans of orthogonal bords) showed costly standard support requirements. An optimised tendon support design was proposed. An opportunity to reduce the current tendon length from 1.5 m to 1.2 m was identified. The newly proposed primary tendon support system will achieve the desired stable conditions whilst reducing support costs for the mine.