A report into how and why a tailings dam at a mine in Brazil failed has highlighted a series of flaws during construction and use that led to the tragedy which killed 19 people.
The Fundão tailings dam at the iron ore mine of Samarco Mineração failed on 5 November last year. The disaster destroyed the village of Bento Rodrigues and led to water pollution.
Samarco’s joint owners BHP Billiton and Vale commissioned the report, which was led by University of Alberta professor of civil engineering Norbert Morgenstern.
The report concluded that the tailings dam failed when the supporting sand structure that was containing fines – the smallest waste particles after minerals are taken out of the mined material – changed from a solid structure to behaving as a liquid, known as liquefaction.
During the mining process, after the material being mined has been removed for processing, the remaining waste – the tailings – is pumped into the tailings pond from the crest of the dam – the tailings dam. The tailings separate – the larger particles of sand settling out first to form a “beach”, while the fines or sludge flow into the pond. The coarser sand from the beach is used to form the dam.
The most important factor that caused the liquefaction was the presence of fines underneath the sand. This occurred at the same time as the dam reached its full quota of reserve resistance.
The report said: “The saturated mass of tailings sands was growing, and by August, 2014 the replacement blanket drain intended to control this saturation reached its maximum capacity.”
Subsequently, the weight of the sand caused the fines underneath to extrude. The report likened what happened to toothpaste being squeezed out of a tube. It said: “As the softer slimes were loaded, they compressed. At the same time, they also deformed laterally, squeezing out like toothpaste from a tube in a process known as lateral extrusion. The sands immediately above, forced to conform to this movement, experienced a reduction in the horizontal stress that confined them. This allowed the sands to, in effect, be pulled apart and in the process become looser.”
The report cited the ways the construction process had allowed the fines to build up under the sand. This started in a design change with the original dam construction, which allowed more extensive saturation than the initial plans when a drainage blanket was placed at a higher level within the tailings. In addition, the criterion that a 200m width of “beach” of sand should be maintained at the upstream toe of the tailings dam often was not met. At times some parts of the beach were only 60m wide, meaning that fines were deposited in places they were not meant to be. Then in 2012, further construction work on the dam meant moving part of the sand embankment, which was placed directly over some previously-deposited fines.
A further factor in the failure was three small earthquakes on the day of the disaster.
“In November 2015, BHP Billiton committed to making the findings of this investigation public, and we are determined to learn from this tragedy,” said BHP Billiton’s chief commercial officer Dean Dalla Valle.
“This important technical study will improve our understanding of what happened at Samarco. We have shared these findings so that the sector can learn from the dam failure and develop and implement further standards that can help prevent a similar event like this happening again.”
Dalla Valle added, separate to the external investigation, BHP Billiton had also conducted its own reviews of other dams at its operations and of its non-operated minerals joint venture arrangements.
“In the wake of this event, we have separately undertaken a comprehensive review of our significant dams, which has confirmed that those dams are stable. We will take a number of actions to further enhance risk management at these facilities,” he said.
“We have looked comprehensively at tailings dam management and benchmarked to global leading practice. We have assessed our portfolio of dams against these global standards and are implementing actions to enhance the management of our dams.
“We also conducted a review into our non-operated minerals joint venture arrangements and have identified a number of opportunities for improvement. This is consistent with our commitment to continuously improve risk management and our determination to learn from, and share the lessons from, this tragedy.”