A35-million-litre reser- voir has been built in the Moses Katane municipal area, north of Rustenburg, in the North West province. This reservoir will provide water for investment company Boynton Investments’ opencast platinum- group metals (PGMs) mine, situated on the western limb of the igneous Bushveld Complex in the region, says engineering company Thuthuka MD Bill Pullen.
Thuthuka designed and managed the construction of the reservoir, which supplies bulk water to the Boynton platinum mine in the Pilanesberg area, in June. The first phase of the project, a 30-km 762-mm-diameter steel pipeline, was completed in February 2009, says Thuthuka senior manager for civil construction Vivian Pullen.
The target area of the Boynton-owned platinum mine, in the Bushveld Complex, is the Merensky reef – a layer of igneous rock, which, together with an underlying layer, is called the upper-group two reef. This reef contains most of the world’s known reserves of PGMs. “The Merensky reef varies in thickness from 30 cm to more than 1 m and is a pyroxenite (an intrusive igneous rock that is mined for platinum and palladium) unit, often with coarse-grained, thin chromitite stringers,” Bill Pullen says.
The mining operation will recover all six platinum-group elements (platinum, palladium, rhodium, ruthenium, iridium and osmium), as well as gold, nickel and copper, that are present in the ores, he adds.
The Magalies Water Board (MWB) will administer the supply of the water to the platinum mine, as well as to the local communities served by the Moses Katane munici- pality, says MWB’s Roelf le Roux. He adds that the MWB contributed about 50% of the funds needed to build the reservoir. The water is drawn from the Vaalkop dam.
The reservoir is a post- tensioned reinforced concrete structure and, with a 64-m diameter, is the largest domed reservoir in South Africa, says water reservoir specialist company Level Construction MD Onner Jager.
The reservoir used 200 t of reinforcement, including 30 t of prestressing reinforcement and about 2 500 cubes of concrete. There were 30 to 60 people that worked on the project.
“The domed reservoir is a patent of Level Construction. The project took ten months to complete because the team involved is highly skilled and specialised in water reservoir construction,” he says.
The precast concrete shutters used for the roof of the structure are made in a steel mould on site. The shutters are then hoisted into place on the walls and on top of staging supports. The shutters are 45 mm apart, do not touch before they are cemented in place, and are formed into six rings, with more shutters used in the longer bottom rungs than the top rungs.
The shutters have thickened ridges at certain places, called ribs, which carry the load of the concrete down the length of the ribs and into the rest of the structure, Jager explains.
The top of the reservoir is closed off with a concrete cap that must be cast after the walls and rings have been completed as it is too heavy to hoist into place like the shutters. Ventilators pierce the dome and enable air to flow into and out of the reservoir to prevent pressure damage when the water level changes, he adds.
The walls are post-tensioned with a number of cables running around the walls inside aluminium sleeves. The sleeves and steel cables are placed at certain heights in the walls before a new layer of concrete is poured. The tips of the cables emerge from the sleeves at a raised buttress on the southern side of the structure. The cables are tensioned after construction is completed by using a hydraulic tensioning machine, and the ends are then cut before the buttress sides are sealed with concrete. “The cables provide reinforcement and tension points that add strength to the structure when water is inside the reservoir,” Jager explains.
Vivian Pullen says that a 762-mm-diameter pipe leads into three 300-mm pipes. The three control valves are auto- matic and use hydraulic pressure to regulate inflow. The diaphragm is the only moving part in each valve, making the system robust and low maintenance. The reservoir is fitted with telemetry boxes that send signals of the water level and the actions of the valves to a control room at the MWB, where water engineers can monitor and adjust certain functions.
The sourcing of materials and the volatility of supply were challenges experienced during the construction of the project. Level Construction and Thuthuka report that the local community was involved and was supportive of the project.
“The involvement of the local community was key to the success of this project. The Bakgatla community provided unskilled labour for the con- struction and provided positive input. The pipeline passed through local settlements and created local employment opportunities, including 180 jobs for the Bakgatla tribe,” says Pullen.