DESIGN flaws helped trigger the deadly collapse of the Samarco tailings dam in Brazil, last year.
An independent investigation carried out by geotechnical engineers on the orders of mine owners BHP Billiton and Vale found that stored mine wastes became oversaturated, liquefied and then behaved like toothpaste being pushed from a tube, leading to the collapse of the dam.
The mudslide buried villages and homes, killing 19 people and polluting local water supplies.
The dam was designed to separate the coarser sand tailings from finer tailings known as slimes that were produced by the mining operation. The sands were also used to build the walls of the dam, increasing their height as it became necessary to store more tailings. The dam included drainage systems to prevent the sands from becoming saturated and losing their stability.
The series of events that led to the eventual collapse began in 2009 when water was seen seeping from the dam.
“These seepages indicated that there were construction flaws in the base drains,” explained lead investigator Norbert Morgenstern in a video address released alongside the findings.
These seepages were sealed and a new drainage system was added but the new design only allowed water to escape from the upper portion of the sands tailings rather than the entire depth of the stored waste.
“The new design allowed saturated conditions within the tailings,” Morgenstern said. “Over time as the dam increased in height the [new] blanket drain was, less and less effective at draining the sands, and thus the degree of saturation of the tailings increased.”
In 2010, a channel was added to the dam that led slimes to enter into the area of the dam used to store sands.
“As the dam was raised more weight was placed on the slimes, which caused them to be pushed forward, outward – in other words to move laterally. This process is similar to squeezing a tube of toothpaste,” Morgenstern said.
Ninety minutes before the failure, three small earthquakes – the largest measuring 2.6 on the Richter scale – accelerated the liquefaction of the tailings and initiated the mudslide.
“Saturated sands can lose their strength and eventually collapse when force is applied on them or stresses are removed from them. This loss of strength takes place rapidly.”
The tailings that had been solid ground just minutes before were transformed into “a roiling river” of mud, the report explains, that swept into the town of Bento Rodriguez.
The investigators have not assigned blame for the dam’s failure to any person or company.
Commenting on the report, Dean Dalla Valle, BHP Billiton’s chief commercial officer, said the company has reviewed its other significant dams and confirmed they are stable.
“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,” he added.
BHP Billiton recorded its biggest ever annual loss last month, including a US$2.2bn impairment charge for the Samarco dam failure. Brazil’s president Dilma Rousseff has compared the event to BP’s Deepwater Horizon oil spill, and Brazilian prosecutors have filed a civil action against the mine’s owners for R$155bn (US$43.5bn) to repair damage caused by the collapse.
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