The Bushveld platinum group metal deposits are two distinct, shallow-dipping stratiform tabular ore bodies which strike for many hundreds of kilometres. Mining is extensive, with depths ranging from close-to-surface to 2300 m. The mining method is a variation of planar open stoping. Pillars are widely employed to support the open stopes. In the deeper levels, in-stope pillars are required to fail in a stable manner soon after being cut, and the residual pillar strength is used to stabilize the hangingwall. These pillars are commonly known as crush pillars. Little work has been done in the past to determine pillar peak and residual strengths, and pillars have been designed using experience and formulae developed for other hard-rock mines. This has led to over and undersize pillars with consequential loss of ore, pillar bursts and potential collapses. This paper describes a crush pillar design methodology, and provides design charts. Three mining environments were incorporated in the investigations, which included underground and laboratory measurements, analytical solutions, numerical models and back analyses. The results of the study are suitable for the areas where the research was carried out, and may also be applied with caution in other similar environments. Read the full paper here.
B.P. Watsona, J.S. Kuijpersb and T.R. Staceyc
a. Goldfields Ltd and University of the Witwatersrand,Johannesburg, South Africa
b. CSIR, Johannesburg, South Africa
c. University of the Witwatersrand, Johannesburg, South Africa
The Bushveld Complex is a large layered igneous intrusion which spans about 350 km from east to west. It is situated in the northern part of South Africa (Fig. 1). The platinum group metals are concentrated in two planar ore bodies known as the Merensky Reef, a mineralised pegmatoidal pyroxenite 0.7 m to 1.4 m thick, and, underlying this, the UG2 Reef comprising one or more chromitite seams of similar thickness.
Fig. 1 The extent of the Bushveld platinum exposure.
The strata generally dip at 8o to 15o toward the centre of the complex. The horizontal to vertical stress ratio (k ratio) varies from about 0.5 to over 2.5. The depth of mining ranges from outcrop to 2300 m.
If a sufficiently large mining span is achieved, or the stope abuts a geological feature, a large volume of hangingwall rock can become unstable, resulting in a stope collapse, or colloquially, a "backbreak"1. In order to prevent these backbreaks a high resistance support system is required. This is universally achieved by the use of small in-stope chain pillars oriented either on strike for breast mining (Fig. 2) or on dip for up- or down-dip mining.
Fig. 2 Plan view of a typical stope on one of the platinum ore bodies
In the past, pillars on the Merensky and UG2 reefs have been designed using experience and peak strength formulae derived for other hard-rock mines. The consequence of this uncertain methodology is to cut oversize pillars, which lowers the extraction ratio. In addition, pillars cut in the deeper levels are required to fail in a stable manner soon after being cut. These pillars are known as crush pillars and their residual strengths provide the required support resistance to prevent backbreaks and keep the stope hanging wall stable. A recent series of pillar bursts, with serious consequences, has raised questions about the design of these pillars.
The main objective in 'crush' pillar design is to ensure that the residual strength of the 'crush' pillars is sufficient to arrest a backbreak. Pillar size should therefore be designed with the residual strength in mind. However, the pillar bursts show that the peak strength and loading environment also need to be considered in the design.
In the Bushveld platinum mines, the residual strength criterion is 1 MPa across the stope (Roberts et al2). This is achieved if the residual strengths of pillars are between 8 MPa and 13 MPa and the pillar lines are spaced 30 m apart.
The aim of the research described in this paper is to provide a crush pillar design methodology, based on solid Rock Mechanics and backed by underground and laboratory measurements. The investigations were conducted on the Merensky Reef at the Impala Platinum Mine, and in the Thabazimbi and Kruidfontein areas. Therefore the results described in the paper are applicable to the Merensky Reef, but the concepts may be used in any crush pillar design. It is recommended that separate measurements of loading-stiffness and pillar strength be made for other environments.