Construction World December 2016

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ACID MINE DRAINAGE PROJECT: EASTERN BASIN TREATMENT PLANT

techniques, used to optimise the detail design, add value to the engineering design and ensure cost savings for the client. Some of the technology used on the project was tried and tested systems which perhaps were lacking originality, but simply due to the size of the project these are now the largest examples in the world. An improvement over the typical HDS process was the enhanced sulphate removal system by providing additional retention time in a mixed Gypsum Crystallisation tank. On commencing with construction, an underwater camera was lowered into the flooded mine shaft to ensure that there were no blockages. A modified sonar was also lowered into the shaft to give a wider field of view to reduce the risk of damage to the abstraction pumps when they were lowered. The mine shaft used as the AMD abstraction point had been in disuse for some years and had suffered from lack of maintenance and vandalism by illegal miners. Use was made of professional mining rescue teams with the necessary equipment and experience to remove debris and clear obstacles from the mine shafts above the water line. Constrained by a railway on one side and a wetland on the other, the site was challenging. The plant was designed so that the large thickener recycle pump station excavations (8 m deep from natural ground level and requiring lateral support) were moved as far out of the wetland as possible. The excavations for the thickeners were still a cause of considerable construction difficulties. The material being excavated was a silty clay that was prone to shear failures. Extreme care had to be taken during excavation. A number of solutions were considered by the team including drilling with casings and other forms of lateral support. The decision was taken to use self-drilling anchors, which flush the area with grout while drilling occurs, to enable the hole to stay open while voids are grouted and stabilised in the same operation. This solution was successfully executed and the lateral support was successfully installed in the large thickener excavations. Concern about the environment steered the project as a whole. If the project did not stick rigidly to the programme dates, the AMD in the mining void would rise to a point that it would pollute natural aquifers and potentially decant at surface. Construction on the Eastern Basin AMD Site occurred within the bounds of the Blesbokspruit, which had previously been registered as a RAMSAR area of natural importance. Detailed measures and processes were put into place to ensure that construction activities did not negatively impact the wetland.

While operational, large dewatering programmes must be implemented to allow the miners to tunnel, blast and to ultimately extract gold. With the closure of the mines and the turning off of the dewatering pumps, water started accumulating in the old mine workings, leaving deep shafts that provide direct access for water and oxygen to underground rock surfaces. Through natural oxidation of pyrite bearing rock strata, the water that filters into these empty shafts via recharge and runoff water is characterised by acidity, high metal content and high salinity, and is known as Acid Mine Drainage (AMD). Decant of AMD started in the West Rand in 2002. Similar decant was anticipated in Central and Eastern Basins. It was imperative that something be done to stop the AMD contaminating the groundwater to a point where it is unsuitable for domestic or other uses. The Treatment Plant designed by AECOM for the Eastern Basins is a High Density Sludge (HDS) Plant which is treating a maximum of 110 Ml/day of AMD. The scheme’s design followed the typical industry practice as used for water/wastewater treatment works. Some of the aspects of the detailed design were verified using state of the art goldfields. For more than a century, gold was the most important export commodity, but over the past 25-30 years gold production has suffered a drastic decline. In 1970 South Africa was still producing 1 000 tonnes of gold each year, two thirds of world production; but by 2010 this had dropped to just 192 tonnes, a mere 10% of world production. 98% of the gold that has been mined in South Africa has come from the Witwatersrand

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Project information • Company entering: AECOM • Client: Trans Caledon Tunnel Authority

• Start date: June 2014 • End date: August 2016 • Main contractor: CMC PG Mavundla JV • Project manager: Trans Caledon Tunnel Authority

• Quantity surveyor: AECOM • Consulting engineer: AECOM • Subcontractor: Tecroveer

Construction WORLD

DECEMBER • 2016