A panel of experts hired by Vale has concluded that the Brumadinho tailings dam broke down when the tailings suffered a sudden and rapid loss of strength, a process known as “static liquefaction”
Police investigations have already pointed to liquefaction as the main hypothesis to explain the collapse in January that left 257 dead and 13 missing. According to the expert group, problems in structure construction also contributed to the disruption.
For the experts, the rupture occurred due to deformations of the dam structure. They also pointed to reduced resistance in certain areas of the structure due to heavy rainfall infiltration that had fallen in the region in the days before the tragedy. “The dam was essentially very steep and damp, and the material retained by the dam was fluffy, saturated, very heavy and very fragile in behaviour,” said PhD in Geotechnics Peter Robertson, who led the group hired by Vale, in a video about the report.
The 88-page report by four experts was hired by Vale’s legal department in February to report the technical causes of the breach. Vale pointed out that the expert panel did not evaluate the responsibilities of the company or those involved in the tragedy.
The experts’ conclusions, said the mining company, are exclusive to the panel members. In addition, the Board of Directors has set up a committee to investigate, in parallel, the causes of the accident, whose report will still be released.
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Experts say that the tailings dam construction type worsened the scenario
Also according to experts, the design and construction of the structure contributed to its disruption. “Specifically, the project resulted in a steep dam, lacking sufficient drainage, generating high water levels, which caused high shear stresses within the dam,” the report says.
The dam was built using the upstream construction method from 1976 onwards, having made ten elevations, the last one in 2013. Vale purchased the mine by acquiring Ferteco in 2001. The last tailings from mining activities were made. July 2016. For panel experts, the dam did not contain sufficient internal drainage and therefore high water level in the downstream slope. “This caused a significant portion of the tailings to remain saturated, which is a prerequisite for undrained static liquefaction.”
They pointed out that no larger capacity drainage systems were installed during the construction of the ten subsequent elevations. Additionally, the design configuration changed from the third elevation, making the structure steeper and pushing the tops over the weaker tailings.
Overloading along the dam has generated a phenomenon known as creep, which is when a material under constant load eventually deforms. This favoured instability at the site as the cemented appearance of the tailings made the deposited materials in the dam brittle and poorly resilient, the panel said.
In February, the government of Minas Gerais sanctioned a law providing for the prohibition of damming by the upstream method. Companies that already had these structures in place should submit a plan to the government to replace or disable the dams. In the same month, the National Mining Agency (ANM) lowered a measure with similar content, the construction or elevation of dams with these characteristics.
Vale knew of excess water which contributed to Brumadinho disaster
Geotechnician Peter Robertson said Vale was aware of excess water flow, one of the factors contributing to the accident. “They knew about the high level of water in the structure and were taking steps to address it. The company was also adopting other solutions to increase dam stability.”
Another surprise in the analysis, according to Robertson, was the high iron content in the tailings, which made the material heavier. In addition, studies have indicated the presence of cementation in the tailings, making them the most fragile and less resistant materials.
Another factor considered was the dam construction and extension technique. The initial stretch of the dam, built from 1976, contained technical features that prevented drainage by its foot. As a result, the combination of heavy and at the same time fragile tailings, lack of adequate water drainage and technical characteristics of the project created the conditions of dam instability, leaving the site susceptible to successive small events that could act as a trigger. to collapse. Stress analyzes indicated that significant parts of the dam were under very high loads.
About the fact that the tailings are high in iron and cemented, Robertson exempted Vale from guilt. “Because the tailings were buried, there was no way Vale could know about this feature. Today we know more about the dam than Vale knew at the time,” said Robertson.
Specialist avoids points responsibility of Brazilian company
Asked if the report exempted or showed that Vale had been negligent in dam management, Robertson said the study was not intended to ascertain liability for the accident.
In stating that too much rainfall and the creep effect triggered the dam rupture, the expert dismissed the hypothesis that drilling on the day of the accident would have caused the structure to collapse. Another hypothesis disregarded was the incident at the installation of the fifteenth deep horizontal drain (DHP), one week before the site ruptured.
Robertson said he will have a meeting on Friday, December 13, with Vale to present the findings of the panel report, composed by the professor and three other experts. The report will also be sent to the authorities involved in the accident investigation. The amount paid for the study by the experts was not disclosed.
You can download the full report here
An additional review is being been undertaken by the ICMM and Bruno Oberle.
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