In the wake of the November 5, 2015 dam collapse that killed 17 people and left hundreds homeless, Samarco joint owners BHP and Vale appointed Cleary Gottlieb Steen & Hamilton to investigate the cause.
A panel of four geotechnical experts, led by Canadian civil engineer Dr Norbert Morgenstern, handed down their findings overnight.
The 88-page report said the way the sands and slimes in the dam interacted was key to understanding the failure, as was how their deposition was influenced by a series of unplanned occurrences during the dam’s construction and operation.
The panel found that damage to the original Starter dam resulted in increased saturation, which contributed to the liquefaction of the sands, while deposition of slimes in areas where that was not intended was another factor.
There were also structural problems with a concrete conduit that caused the dam to be raised over the slimes.
According to the report, there were warning signs in as early 2009, when construction defects in the base drain of the Starter dam were so badly damaged that the original concept could to deposit sands behind a compacted earthfill and then raise it by the upstream method could no longer be implemented.
The revised design allowed more widespread saturation of sands, increasing the potential for liquefaction.
The panel said the second incident associated with slimes and water management occurred over an extended period of time in 2011 and 2012 while the new design was being constructed.
“During operation, the 200m beach width criterion was often not met, with water encroaching to as little as 60m from the crest,” the report said.
“This allowed slimes to settle out in areas where they were not intended to exist.”
Another incident occurred in late 2012 when a large concrete conduit beneath the dam’s left abutment was found to be structurally deficient and could not support further loading, meaning the dam could not be raised over until it had been abandoned and filled with concrete.
“In order to maintain operations in the interim, the alignment of the dam at the left abutment was set back from its former position,” the report said.
“This placed the embankment directly over the previously-deposited slimes. With this, all of the necessary conditions for liquefaction triggering were in place.”
The panel found that surface seepage began to appear on the left abutment setback during 2013, and by August 2014, the blanket drain intended to control the saturated mass of tailings reached capacity.
The slimes responded to the increasing load, compressing and deforming laterally, with the sands above forced to conform to the movement, causing them to be pulled apart and become looser.
The panel acknowledged that there were three small seismic shocks that occurred around 90 minutes before the dam failure, which wasn’t the cause, but accelerated the accident.
“By then the left abutment of the dam had reached a precarious state of stability,” the report said.
“Hence the failure of the Fundão tailings dam by liquefaction flowsliding was the consequence of a chain of events and conditions.”
BHP chief commercial officer Dean Dalla Valle said company would learn from the report, with the results made public so the sector could develop and implement further standards to prevent another incident like Samarco.
BHP completed a separate review into its own dams, confirming their stability.
“We have looked comprehensively at tailings dam management and benchmarked to global leading practice,” Dalla Vale said.
“We have assessed our portfolio of dams against these global standards and are implementing actions to enhance the management of our dams.”
The company also conducted a review into its non-operated joint ventures in the Minerals Americas group and 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,” Dalla Vale said.
BHP shares were trading A17c higher at $21.10.