20 January 2020
Brumadinho: the Expert Panl report on the failure of the Feijão tailings dam
In a few days time it will be the first anniversary of the failure of the Feijão tailings dam at Brumadinho in Brazil. Last month the official expert panel released its report on the disaster – there is a website dedicated to the findings, which includes the full report in English and all of the appendices.
As expected, the report finds that the failure occurred as a result of static liquefaction. The investigation has deduced that failure initiated close to the crest of the dam but very rapidly progressed through the entire structure, allowing a comparatively shallow failure to develop. This was then followed by a series of retrogressive failures that released the large volume of mine waste.
There is a particularly interesting aspect of the report that will cause deep concern for those responsible for such structures:-
“The failure is also unique in that it occurred with no apparent signs of distress prior to failure. High quality video from a drone flown over Dam I only seven days prior to the failure also showed no signs of distress. The dam was extensively monitored using a combination of survey monuments along the crest of the dam, inclinometers to measure internal deformations, ground-based radar to monitor surface deformations of the face of the dam, and piezometers to measure changes in internal water levels, among other instruments. None of these methods detected any significant deformations or changes prior to failure.”
That such a catastrophic failure can develop with no signs of distress, and no indication that stability was being compromised, is a great surprise. It implies that the failure was extremely brittle. The Expert Panel looked in detail at all of the monitoring data, and undertook a back analysis of historical InSAR data. They conclude that the dam was settling at rates of up to 30 mm per year (as illustrated schematically in the image below), but this was expected and could not be used to infer that stability was compromised.
In terms of the failure, the Expert Panel blames the upstream construction method deployed by Vale at Brumadinho. In particular, the Expert Panel is critical of the lack of effective drainage installed during the early phases of construction, which allowed very high pore water pressures to develop both during deposition (which meant that the tailings were loose) and then afterwards (which promoted failure). At the same time, the tailings contained a very high content of iron, which allowed them to become bonded, introducing the brittleness highlighted above. The upshot was that the dam was, in the words of the Expert Panel, “composed of mostly loose, saturated, heavy, and brittle tailings that had high shear stresses within the downstream slope, resulting in a marginally stable dam (i.e., close to failure in undrained conditions)”. In other words, Brumadinho was a disaster waiting to happen.
The final failure did not need a specific trigger – it was in essence progressive. Long term heavy rainfall had reduced suction forces whilst creep of the tailings caused strain to localise, which promoted creep rupture.
There are many lessons to learn from this failure, but absolutely critical will be the finding that failure could not have been anticipated through monitoring. This means that to understand the behaviour of the dam the local team needed a much more nuanced view of the properties of the tailings (and in particular their propensity to fail in a brittle manner) and the conditions within the dam. This is a critical lesson for the industry.