The construction of the large TSF-4 tailings storage facility at Mimosa platinum mine – the second largest TSF in Zimbabwe, has demonstrated the ingenuity of local firms, working in close collaboration with SRK Consulting.
The Mimosa Mining Company, owned by Sibanye-Stillwater and Implats, had appointed SRK Consulting (South Africa) to conduct the detailed design of TSF4, and this was completed in May 2022. The next part of the contract, lasting about two years up to March 2024, covered the civils works and the electrical infrastructure and instrumentation to operate the facility. According to SRK partner and principal civil engineer Andrew Robertshaw, SRK was also involved in oversight of the project implementation, its quality and its scheduling.
“This was a sizeable project that has resulted in a technologically advanced TSF structure measuring some 1 km by 1,3 km,” said Robertshaw. “The storage capacity of the facility is substantial, based on a production rate of 466,000 tonnes per month over a 20-year deposition period.”
Empowerment
Key to the client’s strategy was the involvement and empowerment of local companies; among the main players in the construction and related duties were Harare-based Forit Contracting and local laboratory services company Kumba Resources Zimbabwe. Paterson & Cooke Consulting Engineers were engaged for the mechanical and electrical design.
Designed and built to comply with the Global Industry Standard on Tailings Management (GISTM), published in 2020, the TSF-4 also had to deal with specific local conditions such as a high level of talc in the tailings. This material stays in suspension for lengthy periods, requiring a longer time to settle.
“There are further challenges when the material does settle, as the fine material can cover and block the drains under the TSF,” he explained. “This could lead to water forcing its way out of the dam through other routes, such as under its foundations or through its walls, creating a serious structural risk.”
Complex drainage
Working with Robertshaw was SRK corporate consultant Robert McNeil, who said that the drainage system design had to be complex and cutting-edge to deal with the challenge of the talc content.
“It was important for us to remove the water in the tailings as quickly as possible, via a robust drainage system through the dam foundations at ground level,” said McNeil. “As the water drains, a series of conduits collect the water and channel it down to one of lowest topographical points.”
An in-line pump station pumps the water around the eastern side of the structure and into a silt-trap dam where remaining suspended solids have more residence time to settle out before the water continues to the return water dam.
“A great deal of work went into understanding the seepage on this facility, and how we decant water from the structure,” he said. “We also wanted to include ‘future-proofing’ elements in the design, which would, at little extra cost, extend the operational life of the facility.”
Drainage components
The TSF has been provided with a complex network of drains, designed to ensure drainage of the TSF outer shell, and a means to monitor the efficiency of the desaturation at many locations.
Internal paddocks have been designed to run alongside the inside walls of the TSF. The purpose of the paddocks is to provide smaller areas for deposition, allowing the settlement of the coarser fraction of tailings and the creation of a well-drained outer shell. The paddocks fill quickly with coarse tailings, while the finer tailings and talc, with a longer settlement time, flow over the inner paddock walls into the main basin – where the finer tailings and talc settle. Forcing the fine material away from the main drain reduces the opportunity for talc to settle over and clog the main drain.
The main drains are strategically placed vertically below the crest of the full level. This allows the operator to build the TSF to the initial 30 m height, or to extend the TSF life to a full development height of 60 m. These main drains aid drawdown of the outer shell’s phreatic surface to reduce the risk of failure.
The paddocks are an important element of McNeil’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 McNeil. “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.”
Robertshaw noted that the interface between TSF-4 and an existing TSF also required a specific solution to potential seepage, comprising a complex array of pipework to monitor and understand where water flows were emanating.
Penstocks
Impala’s philosophy to duplicate critical infrastructure was reflected in the choice to provide 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 60 m.
Local laboratory
The on-site laboratory, established by Kumba Resources Zimbabwe with technical guidance from SRK, was important for talking samples of various materials and delivering quick results to allow the project to proceed expeditiously. This was a key component of quality assurance on the site, he said. Typically, the contractor would run their own laboratory and consultants would need to check if these results were accurate. The Kumba laboratory team numbered almost a dozen, including specialist samplers and testing technicians, who worked 24-7 in three shifts towards the end of the project to complete the necessary work in time.
“One of the joys of this project was the dedication, expertise and attention to detail exhibited by the lab team,” said Robertshaw. “With our regular visits and collaboration, we were able to facilitate considerable upskilling of the team.”
The lab now has a solid track record of providing on-site quality assurance to a large TSF development – testing earthworks materials and concrete, and keeping detailed records over the span of the project.
A similar developmental approach was taken with the role of Forit Contracting in this project, with the mine and SRK working with the contractor to ensure the necessary resources were available and better 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.
SRK’s construction specialists were on hand to advise on various aspects of plant scheduling and availability, to ensure the contract was kept on track while meeting all required specifications. SRK provided the project with a highly experienced scheduler who reviewed and updated progress on a weekly basis. A local safety, health and environment (SHE) practitioner was appointed to oversee project safety.
“The developmental approach led to a real sense of achievement among all involved, showing how the local economic impact of a project can be enhanced as part of a strategic commitment to responsible mining,” said Robertshaw.
