The report, commissioned by BHP Billiton and Vale, the 50-50 partners in the Samarco iron ore joint venture, was put together by a four-person expert panel chaired by Canadian geotechnical expert Norbert Morgenstern. Other members were Steven Vick, Cassio Viotti and Bryan Watts.
Part of the report includes an animation explaining how the incident occurred.
BHP Billiton chief commercial officer Dean Dalla Valle stressed the report was only about finding the causes of the disaster and sharing it with the mining industry. It was not, he said, about apportioning blame, which he felt would be inappropriate given the myriad legal actions surrounding the incident.
Dalla Valle said the company would be adopting the Canadian Dam Association guidelines – considered the most rigorous in the industry – as a minimum standard for the management of both its own dams and those on its non-operated minerals joint ventures.
However, he is uncertain how much that will cost BHP in Australia and globally to bring its dams up to that standard.
“The fact that we have assessed our dams and they are sound shows we are managing them well,” Dalla Valle said.
The incident has also led BHP to review how its tailings dams are monitored and how its non-operated joint ventures are managed.
“We are looking at how we manage these joint ventures,” Dalla Valle said.
“We’re in discussion with Vale on how we will manage Samarco going forward.”
BHP has decided to create a centralised dam management function. That will bring additional specialist expertise in-house at the company. It will also be looking at technology options to improve dam management across its portfolio.
Dalla Valle also hit back at claims the report was simply a public relations exercise, instead insisting it was designed to improve mining industry understanding of tailings dam management.
The company has also contributed to a review initiated by the International Council on Mining and Metals into tailings management to help provide industry with better guidance on how to manage the risks.
The Fundao dam was designed to take the two types of tailings formed by Samarco’s iron ore beneficiation process: sand tailings and slimes.
It was originally planned to deposit sands behind a compacted earth fill starter dam and then raise it by the upstream method to progressively increase its capacity.
The sands would, in turn, retain the slimes deposited behind them in a way that the two would not intermingle.
To maintain the free-draining characteristics of the sands a 200m beach width was required to prevent water-borne slimes from being deposited near the dam crest where it would impede drainage.
A high capacity drainage system at the base of the starter dam would allow water to drain from the sands to reduce saturation.
About 2pm on November 5 2015 the Germano plant complex at Samarco experienced a tremor lasting several seconds. It shook the windows and caused objects to fall from tables but did not appear to do any serious damage.
At 3.45pm shouts started coming over the radio that the dam was collapsing. The tailings that had been solid just minutes before transformed into a roiling river. That river overtopped but did not breach the downstream Santarem Dam before entering the town of Bento Rodriguez shortly thereafter enroute to the sea.
The panel’s investigation discovered that things at the Fundao tailings dam had started to go wrong in 2009, shortly after the starter dam was completed. Due to construction defects in the base drain the dam was so badly damaged the original concept had to be abandoned. Instead, a revised design substituted a drainage blanket at a higher elevation.
Together with the revised design there was a fundamental change in the design concept where widespread saturation was allowed and accepted.
According to the panel this increase in saturation introduced the potential for sand liquefaction.
The second incident associated with slimes and water management occurred over an extended period of time in 2011 and 2012 while the revised design was being built.
During operation the 200m beach width was often not met. Instead water was encroaching to as little as 60m from the crest, which allowed slimes to settle out in areas where they were not meant to be.
Another incident occurred in late 2012 when a large concrete conduit beneath the dam’s left abutment, the Secondary Gallery, was found to be structurally deficient and unable to support further loading.
This meant the dam could not be raised over it until it had been abandoned and filled with concrete.
To keep operations going in the meantime the dam was realigned with the left abutment being set back from its previous position. However, this placed the embankment directly over the previously-deposited slimes, putting all the necessary conditions for liquefaction triggering in place.
As dam raising continued, surface seepage began to appear on the left abutment setback at various elevations and times during 2013.
The saturated mass of tailings sands was growing and by August 2014 the replacement blanket drain intended to control this saturation reached maximum capacity.
Meanwhile, slimes beneath the embankment were responding to the increasing load being placed on them by the rising embankment. The way those slimes responded and what that response did to the sands is what ultimately caused the sands to liquefy.
As the softer slimes were loaded they compressed and also deformed laterally, like toothpaste being squeezed from a tube, in a process known as lateral extrusion.
This caused the sands immediately above the slimes to be pulled apart and essentially become looser.
A related aspect of the failure was the series of three small seismic shocks that occurred about 90 minutes earlier.
By then the left abutment of the dam had reached a precarious state of stability.
These events all led to the liquefaction flowsliding and the failure of the dam.