‘An upstream dam uses far less wall material and occupies less land than a downstream dam, but can be less stable particularly if seismic activity is a feature of the region.’ So wrote mining engineer Chandra Durve and then head of technology at Cornwall technical college, Dr Edward Ferrett, in October 1985 in an article for Geographical about tailings dams – the structures used to contain the waste product of mining operations.
The dams, they noted, require constant monitoring using small instruments that measure water pressure: ‘It is this type of continuous monitoring that could provide early warnings of potential failures.’ As recent events in Brazil and around the world indicate, it seems these warnings, already well understood in the 1980s, have not been heeded by some large mining companies.
The most recent upstream tailings dam collapse took place on 25 January in the state of Minas Gerais in Brazil when the 280-foot-high structure owned by Vale SA, which towered over the small town of Brumadinho, collapsed, sending 11.7 million cubic metres of toxic mud downstream. The surge crashed into the Vale administration rooms where workers were eating lunch, overwhelmed trains and vehicles and submerged nearby homes. At the time of writing, an estimated 300 people have been killed and the waste is still on the move, contaminating rivers and water supplies in the area.
The disaster follows a similar event in 2015, when another tailings dam in Minas Gerais owned by Samarco – a joint venture between Vale and BHP Billiton – collapsed, killing 19 people. Shortly before that, the Mount Polley tailings dam in Canada failed, sending eight million cubic metres of gold and copper waste into a pair of glacial lakes. According to the non-profit organisation World Mine Tailings Failures (WMTF), 42 tailings dam failures occurred between 2008 and 2017, resulting in 435 deaths.
Speaking more than 30 years after the publication of his original article, Chandra Durve laments the fact that the lessons of the past have consistently been ignored. ‘These dams have been failing for many years,’ he says. ‘There isn’t a lack of knowledge, though unfortunately there aren’t too many experts around in this field and people who profess to be experts unfortunately may not have a full understanding of the intricacies of tailings.’
Upstream tailings dams have the potential to be less secure than other types of dam, largely due to the way water is passed through the dam wall. Unlike a water dam that usually features an impervious wall of concrete or similar material, the wall of a tailings dam is constructed from the coarse portion of the tailings themselves – the waste pulp that results from the mining process in which ore (containing copper, iron, gold and the like) is crushed and ground with huge volumes of water. The finer particles of the tailings are allowed to gravitate away from the wall. Water is allowed to seep through the tailings and the wall in order to consolidate the mud – the idea being that this will compact the tailings and reduce their volume.
‘The key to the stability of these structures lies in the proper control of the seepage and in particular the “phreatic surface”, which is the upper limit of water flowing through the dam,’ explains Durve. ‘The phreatic surface must be intersected well before it reaches the outer part of the tailings dam wall. If it hits the outer part then the tailings dam will collapse. Equally, if it intersects the wall in an uncontrolled way, the whole wall could slide, not only releasing liquefied tailings but adding to the liquefaction of other material.’ Liquefaction is the process in which solid mud is reduced to liquid and though Durve says it is too early to definitively state why the Brumadinho dam collapsed, most experts agree that the dam failure was one of liquefaction. This process can also occur due to seismic activity or a heavy deluge of rainfall.
Many commentators argue that, as the cheapest and least stable form of tailings dam, upstream technology is inherently problematic and should be scrapped altogether. Some engineers favour the downstream dam, in which successive loads of tailings are deposited downstream of the initial wall rather than upstream. Dr Barry Wills, senior partner of the journal Minerals Engineering International writes that: ‘Downstream dam building is the only method that permits design and construction of tailings dams to acceptable engineering standards.’
However, Durve disagrees and says that both types of structure can be stable or unstable depending on how they are constructed. ‘Instability will arise in either case if sufficient consideration is not paid to seepage and the position of the phreatic surface,’ he says. What’s essential, as he noted in 1985, is constant monitoring of the water pressure in the wall – something he claims is cheap and easy to do using water pressure monitors.
In the aftermath of the disaster, Vale stated that there was no evidence of an imminent risk of collapse at Brumadinho and that it consistently monitored water pressure. But a string of media reports (along with the arrest of eight Vale employees) indicated that the company and one of its contractors knew of significant issues. On 11 February, Reuters stated that it had seen an internal Vale report dated October 2018 which classified the dam at Brumadinho as being two times more likely to fail than the maximum level of risk tolerated under internal guidelines. According to the Guardian, a September 2018 report by the German certifications contractor TÜV SÜD concluded that the Brumadinho dam was stable but highlighted drainage problems and recommended installing new water pressure monitors. Vale stated that ‘workers were fitting the new monitors on the day the dam burst’.
Further reporting by the Wall Street Journal in February, based on police reports, court documents and arrest warrants, revealed that inspectors at TÜV SÜD had known of dangerous conditions for months but certified that the dam was safe anyway, ‘expressing worry about losing contracts with Vale’.
Brazil has now ruled that all upstream dams (there are 88 according to the National Mine Association) must be decommissioned or removed by August 2021. In doing so it joins Chile and Peru as the only countries to ban this type of structure. It’s a move unlikely to provide much comfort to Andréa Zhouri, a professor in the Department of Anthropology and Archaeology at the Universidade Federal de Minas Gerais. Speaking at the British Academy on 23 February, Zhouri emphasised the importance of putting the Brumadinho disaster in the wider context of increasingly worrying environmental deregulation in Brazil. ‘The social and environmental governance package that was established between the 1980s and 1990s has undergone a gradual process of erosion,’ she said. ‘Shortly after the collapse of the Samarco dam in 2015, MPs from the state of Minas Gerais did not hesitate to approve a decree that reduces requirements for environmental assessment in order to speed up mining permits.’
All the evidence suggests that this unravelling will now accelerate under Brazilian president Jair Bolsonaro. For Zhouri this is linked to a wider problem in Brazil known-as ‘revolving doors’, which she says refers to a tight link between state actors and commercial mining companies. She points to a recent report by Brazilian newspaper O Tempo which claims that of the 130 federal and state deputies elected in Minas Gerais in 2014 (the last year private donations to political campaigns were allowed), 102 were funded by mining-related companies.
When it comes to the specifics of upstream tailings dams, Zhouri is firmly on the side of those who view them as dangerous, referring to them as ‘cheap, old technology’. While there are no official figures for the total number of upstream dams worldwide, many large mining companies still utilise them (Anglo American, one of the world’s largest miners, has 32 upstream dams in its portfolio, while the RioTinto Group, the world’s second largest, has 21). What’s more, as Durve makes clear, it is not only upstream technology that can fail, especially if monitoring is weak. There are an estimated 3,500 tailings dams worldwide and WMTF predicts that without major changes to law and regulation, as well as new technology, 19 ‘very serious failures’ of tailings dam will occur between 2018 and 2027.
While the engineering specifics of these structures may be complex, there is one simple thing most experts agree on – without sufficient monitoring and expertise, many more tailings dams are likely to collapse.
This was published in the April 2019 edition of Geographical magazine
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