Mining through water-bearing fissures at Northam platinum mine (Zondereinde)

Abstract

Mining through water geological fissures can result in a fall of ground and loss of revenue if proper mining and support strategies are not employed. Sound mining practices and good design are recommended to ensure safe and stable excavation. The host rock at Northam Platinum Mine is Norite, with a uniaxial compressive strength of 130 MPa. The host rock is supported with conventional tendon support in the form of 1.8m length, 20mm diameter, full column resin grouted bolts. Unfortunately, prominent and complex geological water-bearing fissures exist on the eastern and western sides of the shaft, striking North-South (N-S). The structures are associated with closely spaced joints, dykes, shear zones, and water fissures, making it challenging to install conventional tendon support due to friable ground conditions. The study investigated a geotechnical strategy to develop excavations through these complex structures with arch sets and void fill. Three development sites at Northam Platinum Mine approaching major complex fissures are discussed in detail in this report. The strategy used for water sealing when approaching these structures and increasing tunnel development dimensions to accommodate arch sets and void fill are discussed. Geotechnical analyses using empirical methods were used to designate the characterisation and mechanical properties of the rock mass. Geological core logging and physical mapping of joints, faults, and dykes were conducted on boreholes drilled ahead of mining faces for the three development tunnels. Numerical modelling using Vantage elastic software was used to assess rock wall condition factor (RCF) on tunnels and assist in selecting suitable support units. The support used in the form of steel arches to support these structures and consolidation to enhance the strength of the rock mass is analysed and discussed. Smooth wall blasting limits hanging wall and sidewall damage, whose application requires evaluating and optimising blasting designs and was a focal point of this study. Moreover, other related case studies involving similar conditions, like tunneling under the Himalayas, and running dykes, amongst others, are also dealt with.