Authors: Andrews, PG; Barsanti, BJ
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A reliable stability assessment for stoping is critical at the design stage of mining and the Conqueror mine at Gold Fields’ St Ives operations is no exception. The mining method at Conqueror was longhole open stoping with diminishing rib to island pillars. The Mathews Stability Graph Method (SG) was initially used to assess the Conqueror primary stopes but proved unwieldily with the onset of the extraction of the secondary stopes from rib to island pillars. The St Ives Geotechnical Department therefore had to search for an alternative stability assessment method. A Canadian stability method, Radius Factor (Milne, 1996), was investigated and subsequently trialled. Whilst Hydraulic Radius as considered in the Stability Graph method is based on boundary geometry (length x width), the Radius Factor method is based on surface geometry. Two new terms were introduced to site personnel, Radius Factor (RF) is related to the overall stability and maximum deformation of a surface and Effective Radius Factor (ERF) is related to the local stability and deformation of a point on the stope surface. The method was trialled firstly as a tool to back analyse old stoping areas. Back analysis showed the method to be effective for determining maximum stable spans. The method was then used as a design tool for determining maximum spans and pillar placement in the mine design. The Radius Factor method was continually calibrated to actual mine results over the life of the mine. This calibration enabled stope geometry and sequencing changes to incorporate best geotechnical practice into the mine extraction sequence. As a result the St Ives geotechnical and mining teams were able to safely extend two stope lengths and extract one central pillar resulting in an extra 404 kg.


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