The causes of the rupture were still being investigated, Vale said. It has repeatedly said the collapsed dam was declared sound by an independent auditor in September.
However a report, dated Oct. 3, 2018, classified the dam at Brumadinho in the state of Minas Gerais as being two times more likely to fail than the maximum level of risk tolerated under internal guidelines.
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.
It raises questions as to why the independent October audit 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.
Vale said that the report, called “Geotechnical Risk Management Results,” comprised the views of specialist engineers, who are obliged to work within strict procedures when they identify any risks.
The audit by Germany-based TÜV SÜD, which was seen by Reuters, said the dam adhered to the minimum legal requirements for stability but it raised a number of concerns, particularly about the dam’s drainage and monitoring systems.
The auditor made 17 recommendations to improve the Brumadinho dam’s safety, primarily suggesting the company address various areas eroded away by static 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 the 2015 dam collapse nearby, which resulted in Brazil’s worst-ever environmental disaster.
The dam was marked for decommissioning and placed in the “attention zone,” requiring “all prevention and mitigation controls to be applied or be penalized US$1.5 billion and possible kill over 100 bystanders.
Weeks later, as predicted, on January 25, Brazil experienced its most deadly mining accident, leaving 165 dead.
“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.