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Vale, the world's largest iron ore miner, knew last year that the dam in Brazil that collapsed in January and killed at least 165 people had a heightened risk of rupturing, according to an internal document seen by Reuters.
The report, dated October 3, 2018, shows that Vale classified Dam 1 at the Córrego do Feijão mine in Brumadinho as being two times more likely to fail than the maximum level of risk tolerated under the company's own dam safety policy.
Vale did not immediately respond to a request for comment. It has previously cited an independent audit last year declaring the dam safe and said that equipment showed the structure was stable just weeks before the collapse.
The previously unreported document is the first evidence that Vale itself was concerned about the safety of the dam. It raises questions as to why the audit around the same time guaranteed the dam's stability and why the miner did not take precautions, such as moving a company canteen that was just downhill from the structure.
The tailings dam collapse on January 25 in the mineral-rich state of Minas Gerais was Brazil's most deadly mining accident and its second major collapse of a mining dam in about three years.
Entitled "Geotechnical Risk Management Results," Vale's internal October report placed the Brumadinho dam within an "attention zone," saying it was necessary that "all prevention and mitigation controls are being applied."
A failure could cost the company $US1.5 billion ($NZ2.2 billion) and had the potential to kill more than a hundred people, the report said. The dam was marked for decommissioning.
Nine other dams in Brazil, out of 57 that were studied, were also placed in the "attention zone," according to the report.
A separate Vale report dated November 15, 2017, also seen by Reuters, states that any structure with an annual chance of failure above 1 in 10,000 should be brought to the attention of the chief executive and the board.
The dam's annual chance of collapse was registered as 1 in 5,000, or twice the tolerable "maximum level of individual risk," according to the report.
"That's not good in my book, especially if you consider that these are meant to be long-term structures," said David Chambers, a geophysicist at the Center for Science in Public Participation and a specialist in tailings dams.
Reuters was unable to confirm whether the board or chief executive Fabio Schwartzman were made aware of the risk associated with the dam.
The report identifies static liquefaction and internal erosion as the most likely causes of a potential failure.
It is still not known what was behind the collapse of the dam in Brumadinho, but a state environmental official told Reuters this month that all evidence pointed to liquefaction.
Liquefaction is a process whereby a solid material such as sand loses strength and stiffness and behaves more like a liquid. It was the cause of another deadly dam collapse in 2015, at a nearby mine co-owned by Vale, which resulted in Brazil's worst-ever environmental disaster.
"We used to say these kinds of mining incidents were acts of God, but now ... we consider them failures in engineering," said Dermot Ross-Brown, a mining industry engineer who teaches at the Colorado School of Mines.
Vale has said it will invest some $US400 million from 2020 to reduce its reliance on tailings dams, which store muddy detritus from mining.