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.
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 effective drainage".
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.
