On 25 January 2019, a tailings dam at the Córrego do Feijão iron ore mine in Brumadinho, Brazil, failed suddenly without warning. Two hundred and seventy people were killed as 9.7Mm3 of stored tailings were released, with mudflow running through the Paraopeba River, demolishing infrastructure, and seriously affecting the regional ecosystem. This disaster occurred just three years after the failure of the Fundão dam in the same state, and both dams were owned by mining company Vale.
As the team from ETH Zurich explains in their research published in Communications Earth and Environment, with the rapid growth of mining activities and increased incidents of such tailings dam incidents, it’s important to understand the dominant triggering factors behind failed structures so as to effectively avoid recurrence in the same region.
Of particular concern, the team notes, is the fact that the failure at Brumadinho occurred without any notification, the dam hadn’t receiving any tailings since 2016, was equipped with state-of-the-art monitoring instrumentation, while geotechnical reports prior to the failure indicated the dam was safe.
Brumadinho
Built in 1976, the tailings pond at Brumadinho was impounded by Feijão dam and, as is customary in ore mining, was raised by a few metres at a time over the years to create additional space for the storage of processing residues. When the structure failed in January 2019, three years after it had last been in use, the dam was 86m high.
An expert panel appointed by Vale after the disaster suggested the failure was associated with internal creep and the loss of suction induced by heavy rainfall at the end of 2018, but in contrast, the report of the Federal Police of Brazil concluded the disaster was triggered by vertical perforations in a weak point of the dam structure.
Despite considerable investigations, the cause of the failure remains debated, mainly due to its delayed nature. And such uncertainty is worrying, warns Professor Alexander Puzrin who led the study team from ETH Zurich, as it could impede avoiding similar tragedies in the future.
Numerical and analytical modelling
In their research, Puzrin and his colleagues Fangyuan Zhu and Wangcheng Zhang, used numerical and analytical modelling of all stages of the evolution of the dam, in order to investigate the causes of the failure and identify a physical mechanism that may have triggered the mining accident.
As the authors explain, their research shows that failure was initiated by a delayed slip surface growth along weak layers of fine tailings within the dam body. They show how this growth was preconditioned by dam construction and tailings discharge history and further driven by creep deformation during the post-closing stage, until the slip surfaces reached their critical length, resulting in unstable propagation and the rapid collapse of the entire dam. While other triggers such as rainfall and borehole drilling could have accelerated the slip surface growth, the authors demonstrate that creep alone would be sufficient for the slip surface to reach the critical length, leading to inevitable dam failure. They add that this is a worrying outcome for existing dams where tailings exhibit brittle and rate dependent behaviour, as the absence of unfavourable environmental and human factors does not guarantee long-term dam safety.
As the authors go on to explain, a closed dam is supposed to be on the safe side compared with one receiving tailings. This is quite simply because tailings become compacted with long-term consolidation, and the formation of an iron ore bonding structure gives strength and hence reduces risk of slope failure. External factors like earthquakes or exceptionally strong precipitation combined with inefficient drainage conditions leading to piping can have an impact, but there was no significant seismic shaking or evidence of piping reported prior to the Feijão dam collapse. Instead, the researchers believe that this internal failure mechanism was already in motion, turning the dam into a ticking time bomb.
As the ETH study shows, danger may no longer be averted by ceasing loading of a pond with new tailings. However their model can be used to carry out a risk analysis for existing dams and predict the likelihood of a dam failure, and if identified it can be reduced by pumping water out from the boreholes in the tailings ponds, or dismantling the tailings dam. In urgent cases, endangered villages can be temporarily evacuated to protect inhabitants until the danger has been averted.
The authors say the study’s findings are relevant to all tailings dams that process residues from ore mining, and can also provide an indication of how to further improve the safety of earthern dams impounding water in the event of an earthquake, which can generate an initial slip surface.
“In this respect, our work helps to make dams safer in general,” Puzrin said.
Fifth anniversary
The fifth anniversary of the Feijão dam collapse “serves as a powerful reminder to all of us in the mining industry about the tragic consequences when something goes wrong, and the need to remain focused on improving how tailings facilities are managed across the entire industry”, the International Council on Mining and Metals said in January 2024.
As an immediate response to the disaster, ICMM co-convened a multi-stakeholder review, with the UN Environment Programme and Principles for Responsible Investment, that led to the development of the Global Industry Standard on Tailings Management. This was described as “setting a high bar for the industry” and had 77 requirements integrating social, environmental, local economic and technical considerations towards achieving the goal of zero harm to people and the environment. The Global Tailings Management Institute is also being established to oversee conformance with the industry standard and promote its continued uptake.
When it was published in August 2020, ICMM members committed to conform with the standard for tailings facilities classified as ‘extreme’ or ‘very high’ consequences by August 2023, and all other facilities by August 2025. ICMM says this commitment galvanised immediate and sustained action by its members who represent one third of the global mining industry.
The mining council goes on to add that implementation of the standard has already resulted in ‘transformative improvements’ in tailings engineering, management, governance and monitoring. It has elevated tailings management to the highest levels of company oversight and accountability, while promoting greater transparency, collaboration, and meaningful engagement with stakeholders. And although there has been a sustained effort to bring the highest consequence facilities into conformance with the standard in just three years, ICMM says that some companies have still not been able to achieve this.
“ICMM members have an unequivocal commitment to implementing the Global Industry Standard on Tailings Management. I am encouraged by the considerable progress that companies have made so far, and acknowledge that meeting our three-year milestone has proven to be ambitious and challenging because the standard sets a very high bar. We still have a way to go, and members are working diligently to bring their tailings facilities into full conformance,” said Rohitesh Dhawan, CEO and President of the ICMM. “We recognise that trust stems from transparency, which will be foremost in how members disclose the progress they have made, and how they intend to meet the full requirements of the standard.”
References
The slip surface mechanism of delayed failure of the Brumadinho tailings dam in 2019 by Fangyuan Zhu, Wangcheng Zhang & Alexander M. Puzrin. Communications Earth & Environment (2024) 5:33. https://doi.org/10.1038/s43247-023-01086-9
www.icmm.com/en-gb/news/2024/brumadinho
www.icmm.com/progress-implementing-gistm
