Contributed by SRK Consulting
Extending the tailings storage facility (TSF) at Zimbabwe’s Mimosa platinum mine has involved the construction of a new dam, based on a design which addressed a range of interesting challenges.
View of TSF4 from Northwest showing the northern return water pumpstation, the internal paddock arrangement, the Starter Wall with Toe drain, Main Drain One and Main Drain Two. All images supplied by SRK Consulting
The new facility – TSF4 – was designed by SRK Consulting (South Africa), who also managed the two-year construction phase which was completed in March 2024. Leading the SRK team was partner and principal civil engineer Andrew Robertshaw, who pointed out several unusual aspects of the project that required an innovative approach.
“The existing TSF at Mimosa had experienced challenges related to a fine talc in the discharged tailings, which would settle and impede the crucial function of the TSF drainage system,” said Robertshaw. “This would lead to standing pools of water above the drains, as the fines would clog the outlets and prevent.
SRK corporate consultant Robert McNeill emphasised that the drainage system had to be specially designed to deal with the challenge of the talc content. The nature of the talc was that it would ultimately settle as soon as the tailings slurry stopped moving. In response, SRK’s design provided for a comprehensive drainage network while also significantly increasing the drains’ cross-sectional area allowing for deposition of talc to take place quickly towards the centre of the TSF – so that it would not settle on the drains.
Focus on deposition
“The drains themselves were similar to the many other TSF projects we have conducted, but what made this unique was the design that allowed the tailings to be deposited in a particular way,” said McNeill.
Of the two sets of drains designed into the TSF, the one that will serve the facility at its full height of 60 metres would not normally be covered by coarse tailings early on – but needs to remain unblocked for an extended period.
“For the other drain – which is 80 metres from the edge of the TSF – we created a series of secondary paddocks to force the flow of tailings at the required discharge volumes,” Robertshaw explained. “The overspill from the paddocks keeps up the energy of the flow, so that the talc remains in suspension in the tailings and is taken away from the outside edge. This outside edge is the critical area of the TSF – allowing for stability and drainage.”
Clay and HDPE lining
The paddocks were an important element of McNeill’s commissioning plan, which allows early deposition of tailings while keeping the talc material away from the critical main drains. He pointed out that, as part of the application of best practice, the TSF was not only clay-lined but there was also extensive use of high- density polyethylene lining in the drains.
“Within the footprint of the dam itself, all the drains are lined with HDPE as well,” said McNeill. “Using both the clay lining – including a four-metre layer of clay on the walls – and the HDPE represented quite a unique approach. This was the optimal response to both the geochemistry and the very flat gradients at the site.”
Green wall
Another innovation was the application of the ‘rising green wall’ concept – in which the outside wall of the TSF is progressively vegetated as the facility grows in height. As each bench is raised to accommodate the continued discharge of tailings, a layer of topsoil is applied and vegetation is cultivated.
“This is a positive advance from the traditional practice, when operators would wait until the TSF was at its full height and at the end of its operating life before starting to rehabilitate the outside of the walls,” said Robertshaw. . “An advantage of this strategy is that dust is reduced, and the cost of vegetating the TSF is spread over the life of the facility – rather than demanding all this expenditure at once at the end of its active life.”
By facilitating the rising green wall, SRK’s design also aligned with best practice in TSF closure – which favours progressive and ongoing
rehabilitation during the life-of-mine, so that the practical and financial demands are not concentrated very late in the mining life-cycle.
“To allow vegetation the best chance to take root on a TSF, the design should avoid long slopes that are susceptible to erosion from rainfall run-off,” he said.
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- SRK partner and principal civil engineer Andy Robertshaw.
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- SRK Consulting (SA) corporate consultant Rob McNeil
Prioritising empowerment
At the heart of this project was a close working relationship with the client that prioritised the engagement and empowerment of Zimbabwean firms, highlighted Robertshaw. With its focus on quality standards across the project, SRK Consulting (SA) provided technical guidance for local company Kumba Resources Zimbabwe to set up an on-site laboratory.
“This testing facility was important for taking samples of various materials and delivering quick results – which streamlined the pace of the project,” he said. “Testing earthworks materials and concrete, and keeping detailed records over the length of the project, was a key component of quality assurance on the site.”
The Kumba laboratory team numbered almost a dozen, including specialist samplers and testing technicians. Towards the end of the project, they worked around the clock in three shifts to complete the necessary work in time. Robertshaw commended the dedication and expertise of the laboratory team, which has now developed a strong track record of providing on-site quality assurance to a large TSF development. Kumba has established a new laboratory in Harare following the project, building on their foundation of extended mentoring on the Mimosa site.
Supporting local contractor
A similar developmental approach was taken with the role of Forit Contracting – the main earthworks contractor. SRK Consulting (SA) worked with the mine and the contractor to ensure the necessary resources were available and fully utilised. The early site clearing was undertaken manually, but as the project gathered momentum, Forit was able to increase its earthmoving plant fleet on site to around 150 items at the peak.
Tasked by the client to provide a safety officer on site, SRK Consulting (SA) appointed a local safety officer and raised his skill level through training to achieve an international safety qualification. Among the critical safety factors was the capability of working at heights on the 40 m penstock towers, which required the relevant expertise and experience. Training in working at heights was therefore conducted for a number of the workers on the project.
View from TSF3 north along western flank of TSF4. Visible are the outer paddocks, the Starter Wall, clay lining under Toe Drain and excavation for Main Drain One and Main Drain two.
Designed for the long run
Future-proofing has also been an important element of the TSF design, he pointed out, as the first brief was for a TSF height of only 30 metres.
“The final design allowed us the opportunity to provide drainage infrastructure to take the dam to 60 metres,” he explained. “The penstock towers and the internal main drain were included to allow a second phase of elevation; these elements would have been very difficult to retrofit at the point in the TSF life when it had reached 30 metres in height.”
The main drains are strategically placed vertically below the crest of the full level, aiding the drawdown of the outer shell’s phreatic surface to reduce the risk of failure . The storage capacity of the facility is substantial, as the mine is expected to generate production of 466 ,000 tonnes per month over a 20-year deposition period.
The client also wanted to duplicate critical infrastructure, so the design provided two penstock towers and pipelines discharging separately into the two-compartment silt trap. This duplication both allows one component to be taken out of commission to enable cleaning of the associated silt trap compartment, and to provide redundancy should one system become inoperable. The towers have been designed with three outlet heights which, when combined with the ringing up method, allow decanting all the way to 60m.
Robertshaw highlighted the value of taking a developmental approach on the Mimosa TSF project, which led to a heightened sense of achievement among everyone who contributed. While meeting the highest standards of TSF design and construction, the project had also leveraged its local economic impact through a responsible mining approach.