4 February 2019

The speed of the Brumadinho tailings dam landslide

Posted by Dave Petley

The speed of the Brumadinho tailings dam landslide

The two important videos showing the Brumadinho tailings dam landslide allow some estimates to be made of the speed of the movement.  Both videos are time-stamped, allowing some parameters to be deduced.

The video showing the collapse itself is available here:-

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The failure process is first visible at 12:28:25:-

Brumadinho tailings dam landslide

The initiation of the failure that generated the Brumadinho tailings dam landslide. Still from a Youtube video.

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Incidentally, I have seen some suggestion that the failure initiated at the toe.  I see no evidence for this in the video, as the image above shows.  The dark line at the crest of the dam is the first sign of the rear scarp opening as the main portion of the dam subsides.  There is no obvious failure event in the toe at this point (though it was undoubtedly moving):-

The still below shows the front of the Brumadinho tailings dam landslide / debris avalanche reaching a fairly distinctive structure below the dam:-

Brumadinho tailings dam landslide

The passage of the Brumadinho tailings dam landslide. Still from a Youtube video.

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This structure can be clearly seen in the Google Earth image below:-

Brumadinho tailings dam landslide

Google Earth image of the location of the Brumadinho tailings dam landslide

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The distance from the toe of the dam to this point is about 300 metres, the imagery suggests that the landslide covered this distance in about 9 seconds, giving a velocity at this point of about 33 m/sec, or about 120 km/h.  This is a very rapid acceleration, suggesting extremely rapid weakening of the debris mass.  This seems to point to a liquefaction mechanism.

The next video shows the front of the flow arriving in the processing area below the dam.  This is the video:-

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Brumadinho tailings dam landslide

The arrival of the Brumadinho tailings dam landslide at the processing plant. Still from a Youtube video.

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This site can also be clearly seen in the Google Earth imagery:-

Brumadinho tailings dam landslide

Google Earth image of the location of the processing plant shown in the Brumadinho tailings dam landslide video.

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The direct line distance from the structure below the dam considered above is about 460 m (total distance from the dam toe is about 760 m), and the time stamp is 12:29:09.  Thus, in this second phase the landslide traveled 460 m in the time between 12:28:44 and 12:29:09, i.e. 25 seconds.  This indicates an average velocity of about 18.4 m/sec, or 66 km/h.  Thus, this was still a very rapid landslide, although as the video shows it had slowed appreciably by this point.  The structure of the landslide mass at this point is quite interesting, as the still below shows:-

Brumadinho tailings dam landslide

The structure of the Brumadinho tailings dam landslide at the processing plant. Still from a Youtube video.

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The red mass is undoubtedly the tailings, whilst the grey is probably the remains of the starter embankment (and perhaps other embankments) at the toe.  If so the deposit has remained well segregated, which may tell us something about the failure process.

There has been a lot of speculation in the media about the activities of the people who were on the dam at the time of the failure.  Vale has released a statement saying that they were undertaking routine work:

“In the specific case of the footage broadcast by the media on Friday (01/02), which showed the exact moment when the Dam I breached at the Córrego do Feijão mine in Brumadinho, the professionals who appear in the footage in the dam area were performing routine tasks. One of the activities performed included the collection of data for compliance with legal requirements, as determined by the Brazilian Mining Agency. It is important to highlight that no construction works were being carried out at Dam I.”

The footage suggests that this is true.  There does appear to be some coring activity though, with the rig being visible on the footage:-

Brumadinho tailings dam landslide

Possible coring activity prior to the initiation of the Brumadinho tailings dam landslide. Still from a Youtube video.

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And there is an article online that describes the experiences of the operator of this rig (who, amazingly, survived the landslide, though his colleagues did not).  This includes this image of the equipment on the dam:

Brumadinho tailings dam failure

A coring rig on the tailings dam at Brumadinho. Photo: Arquivo personal / Lieuzo Luiz dos Santos, via Globo.

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Apparently, this was being used as follows:-

“The drilling is responsible for drilling and monitoring the movement of dams. On the day of the tragedy, Lieuzo had just drilled to install a gauge that would gauge the pressure of the dam.”

I am reminded of the Rissa quick clay landslide in Norway in April 1978.  Famously, this landslide of about 5 million cubic metres was triggered by a farmer undertaking a small excavation of a pit as part of the construction of a new barn.  Is it possible that this landslide was so close to failure that minor disturbance from the drilling triggered a runaway liquefaction event, and thus the collapse?  If so this of course would not the fault of the individuals operating the equipment, it would simply be the straw the broke the camel’s back.