Geotechnical Input Into Shaft Sinking on the Platreef Project

Two shafts are planned to service the massive mining operations on the Platreef Project. The ventilation shaft, Shaft #1, has a lined diameter of 7.25 m with a planned depth of 980 m. The main shaft, Shaft #2 will have a lined diameter of 10.00 m and depth of 1104 m. In this paper we describe the geotechnical work that has been conducted and the future geotechnical work to be conducted on shaft #1, in a sequential approach. The planned positioning of Shaft #1 was determined through development of robust geological, structural and PGE grade models. Geotechnical data obtained from the drilling of a vertical borehole (GT008) to a depth of 1 225 m was used to provide an indication of rock mass conditions within the shaft barrel and empirical shaft support design recommendations. Forty seven percussion boreholes surrounding the Shaft #1 position were drilled and grouted to a depth of 60 m to form a ‘grout curtain’, intended to minimize the ingress of groundwater into the shaft. To construct a stable shaft collar within the soil horizon, boxcut was excavated and blasted to a depth of 12 m with the side walls supported with mesh and soil nails. Following completion of the collar construction and backfilling of the boxcut, sinking operations commenced with a pre-sink until approximately 53 m depth and a slow / main sink thereafter. Prior to shaft sinking commencing, a number of geotechnical systems focused on the mitigation of rock mass failure in shaft sinking were drafted for implementation. The side walls of the shaft have been supported with mesh, pinned by 1.8 m length split sets, followed by a 0.3 m thick concrete shaft lining. Current geotechnical activities include support-installation audits and discontinuity mapping. The Rock Mass Rating (RMR) of Shaft #1 is determined from the geotechnical inspections and continuously compared with the original shaft geotechnical borehole (GT008) study together with the updating of the geological model and structural information obtained from ATV downhole geophysics. Wedge analysis is also conducted using Unwedge and geological sections to ensure adequate support length. Breakaway station developments for Shaft #1 are planned at 450 m, 750 m, 850 m and 950 m levels.