Engineering consultancy Aecom received a ‘Highly Commended’ citation at Construction World’s 2016 Best Projects Awards for its work on the Eastern basin acid mine drainage (AMD) treatment plant commissioned by State-owned water infrastructure developer the Trans-Caledon Tunnel Authority (TCTA). The awards dinner took place on October 11.

The AMD plant services a surface area of about 768 km2, comprising a catchment area that stretches roughly from Modderfontein in the north to Springs in the east, Nigel in the south and Germiston in the west. The plant itself is located in Springs. It is one of the largest high- density sludge plants in the world, with a maximum treatment capacity of 110-million litres a day.

Aecom notes that AMD poses a major environmental threat on the Witwatersrand. The depletion of gold reserves in the area has meant a cessation of mining activities and related dewatering operations, which has resulted in the flooding of mining voids. AMD is generated when ore and other sulphide-containing mining waste are exposed to oxygen and water. The water in the mining voids, thus, becomes acidic and contaminated with heavy metals.

The TCTA was mandated in 2011 by the Department of Water Affairs to implement the necessary AMD water management and establish the treatment infrastructure. The project, conceived in 2013, encompassed treatment plants in the Randfontein Estates area (Western basin), the dormant East Rand Proprietary Mines South-West Vertical Shaft area (Central basin) and the Grootvlei mine Shaft 3 area, in Springs (Eastern basin). The plant at the Eastern basin was completed last.

The Eastern basin plant, designed by Aecom in conjunction with civil engineering firm Golder Associates, followed typical industry practice for water/wastewater treatment works, says Aecom senior civil engineer for Africa Claire Hurrell. “Some aspects of the detailed design were verified and optimised using state-of-the-art techniques, which not only added value to the engineering but also ensured cost savings for the client.”

Hurrell explains that, owing to the size of the project, Aecom decided to standardise the plant design by using tried and tested technology, which has been proven to work best globally. “However, we were able to improve on the standard abstraction method. The three, 20-m-long, 1-m-diameter super duplex stainless steel deep-level abstraction pumps selected were installed from ground level down into the 370-m-deep shaft, allowing for the abstraction of the AMD without having to re-establish underground workings.”

Construction began in June 2014 and was completed in August, costing just under R1-billion. The main contractor was the CMC/PG Mavundla Eastern Basin Joint Venture, with Austrian plant engineering group Andritz supplying the deep-level abstraction pumps.

Prior to construction, an underwater camera was lowered into the flooded mine shaft at the Eastern basin to establish if there were any blockages or significant damage to the shaft. A modified underwater sonar system was also deployed to provide a wider field of view to reduce the potential risk of damage to the abstraction pumps.

Hurrell explains that the preliminary investigation was essential, as the mine shaft had been in disuse for several years and maintenance work had ceased; the shaft was also vandalised by illegal miners. Properly equipped and experienced professional mining rescue teams removed debris and cleared any obstacles from the mine shaft.

Additional constraints were posed by a railway line on one side of the construction site and a wetland on the other. Therefore, the plant design allowed for deep excavation to enable the thickener recycling pumpstation to be moved as far as possible out of the wetland area for ease of construction.

However, these excavations still posed a considerable construction challenge, as the excavated material was silty clay prone to shear failures. Hence, extreme care had to be taken during excavation.

“A decision was taken to use self-drilling anchors, which flush the area with grout while drilling takes place. This allows the hole to remain open while the voids are grouted and stabilised in the same operation,” Hurrell highlights, adding that the solution was implemented successfully, allowing for the necessary lateral support to be installed in the large thickener excavations.

Environmental considerations were also paramount for this project. It was essential to stick to the programme dates to avoid AMD rising in the mining void and reaching a point where it threatened to pollute natural aquifers and decant at surface.

The plant, which has been successfully operating since August, is reducing the AMD level in the mine shaft. The process is significantly reducing the iron and manganese levels of the AMD and increasing the pH to a level acceptable for discharge into the environment.

Hurrell says the project affirms Aecom’s commitment to ensure not only South Africa’s but also the world’s need for a safe, reliable water supply while also protecting the critical natural resource.

“Combining our knowledge of local African markets with a global network of creative engineering and technical expertise, our sustainable, multidisciplinary approach to water engineering produces high-quality, cost-effective, energy- efficient, functional and visionary solutions.”

Aecom works with clients across the entire project life cycle – from initial planning studies to final construction and operations and maintenance services – on both traditional and alternative project-delivery options, including design build, public–private partnerships and financing.

While Aecom exhibited at the 2016 Investing in African Mining Indaba, in Cape Town, it will not do so this year, but will still send a representative to attend what it considers a premier event. The 2017 Mining Indaba will be held at the Cape Town International Convention Centre from February 6 to 9.