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DECEMBER • 2016
Construction
WORLD
52
Professional Services
C
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
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.
ACID MINE DRAINAGE PROJECT:
EASTERN BASIN TREATMENT PLANT
98% of the gold that has been mined in South
Africa has come from the Witwatersrand
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.
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
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.
Highly Commended