28 May 2019
Barão de Cocais: another failing tailings dam in Brazil
In Brazil another tailings dam, at the Gongo Soco mine in Barão de Cocais to the east of Belo Horizonte, is at risk of failure. Multiple local news agencies are covering the story, and contingency plans are in place. Fortunately, unlike at Brumadinho and Samarco, the potential failure of this dam has been detected prior to a major release of tailings (I noted that there was a risk at this site in March), and a suitably cautious approach is being adopted.
The best oversight of the issue at Gongo Soco can be found in a detailed article on em.com.br (in Portuguese – see the Google Translate version), with a less detailed (but still useful) English analysis available via France 24. The failure scenario in this case is quite complex. This is the Google Earth view of the site, on which I have marked the major features. The location is -19.973, -43.584:-
At Gongo Soco, a large section of the pit wall in the abandoned Gongo Soco mine is displacing. Movement rates of 10 cm per year were routine in this section of wall, but in recent months this has rapidly accelerated, and in the last few days movement rates of 20 cm per day or more have been recorded. The volume of rock that is showing displacement is reported to be up to 10 million m³. The risk is that the collapse and runout of the slope will generate vibrations that trigger the failure of the Barragem Sul Superior (Sul Superior tailings dam) shown at the bottom of the image, allowing a major release of mine waste.
To deal with the threat, Vale and the emergency services have evacuated 400 people, and a further 6,000 people are on notice to move should a release occur. High intensity monitoring of the slope is continuing. A Youtube video that describes the preparatory work being undertaken at the site, which includes modelling of the inundation zone for a major failure:-
On Monday night (27th May), the movement rate reportedly reached 21.8 cm per day. However, predicting the time of failure is notoriously difficult, so it is a waiting game. Ironically, a difficult scenario will arise if the slope does not fail during this movement period, which will leave a potentially dangerous slope intact. Knowing how to manage it in that case will be very challenging.