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CONSTRUCTION WORLD
MARCH
2017
30
PROJECTS & CONTRACTS
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 is exposed to oxygen
and water. The water in the mining voids thus becomes acidic and
contaminated with heavy metals.
TCTA was mandated by the Department of Water Affairs to
implement the necessary AMD water management and treatment
infrastructure. The project encompassed treatment plants in the
Randfontein Estates area (Western Basin), the ERPM South-West
Vertical Shaft area (Central Basin) and the Grootvlei Mine Shaft
No. 3 area in Springs (Eastern Basin).
The Eastern Basin plant designed by AECOM (in association
with Golder Associates) followed typical industry practice for
water/wastewater treatment works, says Claire Hurrell, senior civil
engineer, Africa for AECOM. “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, due to the size of the project, AECOM
decided to standardise on tried-and-tested technology, as this
has been proven to work best globally. “However, we were able to
improve on the standard abstraction method.”
The three 20-m-long by 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
2016. The total construction cost was just under R1-billion. The
main contractor was the CMC/PG Mavundla Eastern Basin Joint
Venture, with 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 check for any blockages
or significant damage to the shaft. A modified underwater sonar
system was also deployed to provide a wider field of view in order to
reduce the potential risk of damage to the abstraction pumps.
Such preliminary investigation was essential as the mine
shaft had been in disuse for several years, and had seen a lack
of maintenance, as well as vandalism 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 on one side
and a wetland on the other. Therefore the plant design allowed
for the deep excavations for the thickener recycle pump station
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 being grouted and stabilised in the
same operation,” Hurrell highlights. “The solution was implemented
successfully, allowing for the necessary lateral support to be
installed in the large thickener excavations.”
Environmental considerations were also paramount on this
project. It was essential to adhere 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 operating successfully since August
2016, is reducing the AMD level in the mine shaft by significantly
reducing the iron and manganese levels and increasing the pH to an
acceptable level for discharge into the environment.
Hurrell indicates this project signifies AECOM’s commitment
to balancing not only South Africa’s, but the world’s, need for
safe, reliable water, while protecting this critical natural resource.
“Combining our knowledge of local African markets with a global
network of creative engineering and technical expertise, our
sustainable, multi-disciplinary approach to water engineering
produces high-quality, cost-effective, energy-efficient, functional
and visionary solutions.”
In fact, AECOM has water professionals who work in and across
the major markets of drinking water, wastewater and conveyance
to deliver comprehensive solutions to safeguard human health
and the environment. Worldwide, including Africa, AECOM designs,
builds, finances, operates, and manages projects and programmes
that unlock opportunities, protects the environment and improves
people’s lives.
The company works with clients across the entire project
lifecycle – from initial planning studies through final construction
and operations and maintenance services – on both traditional
and alternative project-delivery options (design-build, public-private
partnerships, and financing).
COMMENDATION FOR WORK
on AMD treatment plant
AECOM received a ‘Highly Commended’ award at
Construction World’s
Best Projects Awards 2016 for
work on the Eastern Basin Acid Mine Drainage (AMD)
treatment plant on behalf of the Trans-Caledon
Tunnel Authority (TCTA). This is one of the largest
high-density sludge (HDS) plants in the world, with a
maximum treatment capacity of 110 M
ℓ
/d.
“A decision was taken to use self-drilling anchors, which
flush the area with grout while drilling takes place.”