A report into the disastrous tailings dam burst at BHP and Vale’s Samarco mine has found drainage and design flaws to be the culprit.
A panel consisting of four geotechnical specialists was commissioned by the companies involved in the disaster to investigate the incident which killed 19 people and flooded rivers and villages in both the Minas Gerias and Espirito Santo states of Brazil.
The report indicated that the dam’s design change between 2011 and 2012 created less efficient drainage, saturating the sand in the dam, Reuters reports.
This saturation caused liquefaction – a process where vibrations or water pressure causes sand to behave more like a liquid, a common cause for tailings dam collapses as the walls are mainly built with dried tailings made from a combination of sand and mud.
The report said “there was a fundamental change in the design concept whereby more widespread saturation was allowed and accepted”, adding that the increased saturation created the potential for this sand liquefaction.
As the dam’s height increased – reaching 100m when it collapsed – the added weight on the tailings led to liquefaction. The weight pushed the mud in the dam outward, similar to toothpaste squeezed from a tube, loosening the sand and making it flow like a liquid, leading to the dam wall’s collapse.
The findings of the investigation were made public, which BHP COO Dean Dalla Valle said was “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.”
The report also highlighted a small earthquake on the day of the burst as a factor that “accelerated” the dam failure.
It did not, however, blame any regulatory or corporate malpractice.
A separate police investigation accused Samarco of wilful misconduct, saying they ignored indications that the dam risked collapse.
The company has denied the claims.
A prosecutor in the case against Samarco, Carlos Eduardo Pinto, said a majority of the findings in the report were already identified in the police investigation.
“Samarco has long claimed it performs good practice but the report did not demonstrate this,” he said.
“Quite the opposite, it showed the company altered the project in a way that would not have been permitted.”
On the back of the main investigation, BHP also launched an internal investigation into the disaster, with Dalla Valle taking futher actions to enhance risk management at their current dams.
“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.”
The Samarco joint venture has reached a settlement with the Brazilian government for a minimum of $US1.7 billion as compensation over six years for the incident, in addition to another reported civil proceeding that seeks an additional $58 billion in compensation.