Geomorphic designs inform tailings dam sustainability20 October 2022
Civil engineer Justin Walls of SRK Consulting considers how geomorphic designs are being applied to tailings dams, emulating natural landforms to render these structures more stable, economic and sustainable.
The field of tailings dam design and management has recently seen a ratcheting of standards following a particularly severe and well-publicised dam failure in Brazil in 2019. The event led to the formulation of the Global Industry Standards on Tailings Management (GISTM), to which all members of the International Council on Mining and Metals (ICMM) subscribe.
The GISTM’s raising of the bar has meant that many of the traditional maintenance techniques for tailings dams are likely to become even more costly and perhaps unaffordable for mining companies. For instance, the GISTM has lengthened the timelines for responsible post-closure dam management as it is necessary for tailings dams to be considered ‘permanent landforms’. The cost implications of this shift are unsurprisingly significant, opening the door to exploring design philosophies that can optimise cost efficiency while remaining uncompromising on safety over the long term.
One of the opportunities being considered – and indeed actively proposed – by engineers and scientists is the value of geomorphic designs that emulate natural landforms. While these concepts are not new, it has taken some time for them to start gaining currency in the mining sector. Traditionally, tailings dams have been designed to limit the geographic footprint – making use of design features such as steep side slopes and angular benches to achieve considerable vertical height. This contrasts, of course, with nature’s more undulating forms which are intrinsically more stable. The Tailings Management Good Practice Guide, in support of the GISTM, even calls for tailings dams to ‘mimic natural landforms’ to allow them to have ‘much lower long-term maintenance and surveillance requirements’.
Another important driver behind innovative thinking in tailings dam design is the impact of climate change. With changing weather patterns – particularly the intensity and volume of rainfall – it is becoming more difficult to forecast the water-related parameters necessary for safe designs. As water volumes are a critical element in tailings dam safety and stability, uncertainty about rainfall tends to raise operational and long-term risk.
Climate change resilience
Again, the GISTM mentions this specifically, requiring tailings dam operators to “enhance resilience to climate change”. It also requires that mines “use climate change knowledge throughout the tailings facility lifecycle in accordance with the principles of Adaptive Management.”
Our design approach can learn a great deal from the equilibria created by nature over millennia, where natural topography tends to follow rounded shapes that are stable and not quickly eroded away. Even the way that natural watercourses are armoured by rock and vegetation can help inform this approach. Applying these lessons in the design of tailings dams could include changing their outer geometry to mimic natural analogues, and armouring areas of concern such as water concentrations and dust sources.
It is hoped that these designs could contribute to more sustainable long-term solutions for safe tailings dam management – reducing the mine’s liability both in terms of risk and cost. The GISTM has emphasised that designing and operating tailings dams for mine closure requires a long-term view. These standards assume that tailings facilities will be permanent landforms – and should be planned, constructed, and closed accordingly. This requirement applies as much to safety and stability of the structure as it does to its environmental, health and visual impact.
The design approach also aligns well with the GISTM’s highlighting of concurrent rehabilitation or reclamation. This seeks to leverage the cashflow of operating mines in planning for closure – and to avoid the risk that mines’ profits will diminish as they approach closure, becoming unable to afford the necessary rehabilitation at the end of mine-life. It could be argued that the most practical way of implementing concurrent reclamation is by taking account of how nature achieves equilibria.
The industry is certainly not unanimous in supporting this direction, and there is considerable hesitation in departing from past practices. Among the advantages of this approach which are likely to gain it more acceptance is that changes can be made without necessarily involving any greater upfront cost. The aim would be to apply a design which could endure for the full life-of-mine, without changes being required in decades to come. For instance, whereas benches on tailings dam side slopes would usually need to be ‘smoothed out’ at closure, these elements could be designed out from the start – as part of an alternative geometry that the tailings dam could maintain into the future.
A geomorphic approach is currently being applied in mine closure design with regard to water management, but its application to early-stage tailings dam design is still in its infancy. It is becoming clearer, though, that many of the ‘hard engineering’ solutions we have used in the past – especially those which demand intensive and ongoing maintenance – are unlikely to be optimal in dealing with future uncertainties.
Among the immediate challenges that the mining industry faces in developing and applying innovative tailings dam design is a pronounced shortage of specialised engineers in this field. This scarcity is certainly being felt in South Africa, which has been a leader in developing tailings related technologies and knowledge, but the problem is a global one. The evolution of this science will require ongoing dedication to skills development and applied research, as industry encourages more engineers and scientists to explore the options for a more sustainable future.