Engineering contracting and con- struction services company Murray & Roberts Cementation has taken an unconventional approach to a project that will see the establishment of two ventilation shafts at platinum miners Anglo American Platinum and Atlatsa Resources’ Bokoni joint venture, in Limpopo, scheduled for completion in August 2014.
The mine is located 80 km south-east of Polokwane. It forms part of the north- eastern part of the Bushveld Complex and operates under a mining right covering a total area of 147 km2. The operating depth for the current workings is between surface and 500 m below surface.
Current mining rates are about 80 000 t/m and the mine is expected to increase production in 2013, with continued operational improvements and the introduction of opencast Merensky reef mining at the Klipfontein farm.
The company has departed from the conventional blind sinking option (drill-and- blast) in favour of bulk earth- moving and excavation methods, says Murray & Roberts Cemen- tation project manager Dirk Visser.
He explains that the company conducted a study during the tender phase, which revealed that taking a bulk earthmoving and excavation route would be more cost effective.
He adds that it is also the safer option and will reduce the manpower requirement substantially, with personnel on site at the projects peak likely to be less than 45 people.
The upcast 88 m ventilation shaft will have a completed diameter of 4.6 m and the downcast 350 m ventilation shaft a completed diameter of no less than 5.9 m. Both shafts are being constructed using the same method, with variations on the dimensions and quantities.
On both shafts, Murray & Roberts Cementation began by excavating and removing the soil overburden from depths of about 24 m to 50 m to reach competent rock. Portions of the initial excavation process were carried out simultane- ously and, after the overburden was excavated, contiguous piles were sunk from the bottom of the open excavation to the refusal depth.
After piling, a curtain grout precementation method will be applied to a depth of 80 m. A concrete reinforced pile cap will be constructed as a foundation for the raisebore machines. Once raiseboring has been completed precast concrete rings, with diameters that correspond with the diameters of the shafts, will be installed from the bottom of the installation, back to natural ground level.
Concurrent with the precast concrete ring installation, the previously excavated soil materials will be backfilled and compacted to natural ground level.
A remote shaft lining machine will be used to inspect the shaft and, based on the client’s recommendation, a 100 mm fibre-lined Drycrete shotcrete will be applied to the shaft walls in layers of 25 mm.