Brumadinho Dam © 2019 by Eric Marmor / Wikimedia Commons 0.000804
On January 25, 2019, the dam of a pond containing residual iron mine sediments burst in the town of Brumadinho, near Belo Horizonte, in the Brazilian state of Minas Gerais, resulting in the death of 270 people and a disaster. Major environmental impacts in the region. A team of researchers from the Polytechnic University (ETH) of Zurich has now discovered the physical mechanism that could have triggered the catastrophe.
The disaster occurred when the dam at the Córrego de Feijã mine, owned by Companhia Vale do Rio Doce, collapsed, spilling millions of cubic meters of water and toxic sludge across the area. It was the second environmental disaster caused by the company in less than four years, after the Bento Rodrigues dam disaster, also in Minas Gerais.
The Brumadinho landslide occurred without warning and destroyed the mine as well as neighboring settlements and a railway bridge.
In short
Until now, it was not clear why the Brumadinho dam collapsed three years after it was last loaded with material. Researchers at ETH Zurich have used models to identify a physical mechanism and influencing factors that explain the delay in the dam failure. The approach can be used in the future to assess the risk of settling ponds that are already closed.
The disaster near the small town of Brumadinho in southeastern Brazil occurred shortly after noon: on January 25, 2019, a pond made of sterile sediments from an iron mine was dammed, in which muddy and fine-grained residues from ore processing (“waste”) were stored.
A massive avalanche of around ten million cubic meters of mud flooded the mine site, destroyed neighboring settlements and washed away a railway bridge. At least 270 people died. The Paraopeba River ecosystem below the mine was destroyed. Although the dam was monitored by a control system, no one had foreseen the disaster.
The collapse of the Brumadinho dam led to several lawsuits against the mining company Companhia Vale do Rio Doce and the technical testing laboratory TÜV Süd in Germany. Shortly before the accident, he had certified that the dam was sufficiently stable. Vale was ordered to pay the equivalent of around six billion euros in damages.
An investigative committee suspected that the cause of the accident was the slow microscopic movements (so-called creep) of the deposited layers of waste, but could not prove this with a credible physical mechanism. In particular, it remained unclear why the dam broke exactly in 2019 – just three years after the tailings pond was last loaded with new waste – and why no significant shifts were detected before the dam burst.
The physical mechanism has been clarified
A study by professor and engineer Alexander Puzrin, director of the Institute of Geotechnics at ETH Zurich and an expert in landslide modeling, has shed light on the Brumadinho disaster. The work appears in the journal external pageCommunications of Earth and Environmentcall_made. The scientists involved analyzed the causes of the dam failure using a model and identified a physical mechanism that explains the mining accident.
The settling pond was built in 1976. As is common practice in mineral extraction, the pond's earth dam was raised by several meters several times in the following years to create additional space for the storage of treatment waste. The steps of the dam were placed one on top of the other like the steps of a ladder (upstream principle). In the end, the dam consisted of ten steps and was 86 meters high. When the structure collapsed in January 2019, the collapse initially occurred at the level of the second stage of the dam. As a result, the ten steps of the earthen dam collapsed and, together with the bare embankments, rolled into the valley in the form of a landslide.
Creep deformations after dismantling
The work of Alexander Puzrin's team now shows how this could have happened. According to the new findings, some slip areas appeared in the barren dams during the construction of the dam, at the height of its second stage. For decades these had a limited extent and remained unnoticed.
But after the tailings pond was dismantled in 2016 – as the ETH team's modeling suggests – the sliding surfaces expanded horizontally and eventually reached critical dimensions. This caused the layers of waste to move, causing the dam to collapse under their weight and triggering the deadly landslide.
According to the model, the reason for the growth of the sliding surface is so-called creep deformation. These are small, slow movements of the earth in the fine-grained and brittle residues, which are caused by the uneven pressure distribution in the overlying deposits.
Professor Alexander Puzrin explains:
Other triggers such as precipitation and drilling can accelerate slip surface growth, but our model shows that creep deformations alone are sufficient for the slip surface to reach the critical extent that causes a dam to fail.
The finding is worrying in two respects: The slip surface that caused the disaster apparently occurred at a time when the settling basin was no longer contaminated with new waste, i.e. without any additional external pollution. And: The growth of the sliding surface did not lead to any clear external deformation of the dam that the monitoring system could have detected and recognized.
You can use the ETH Zurich model to analyze the risks
Tailings ponds for the treatment of waste from the extraction of iron ore and other mineral rocks are used in large numbers worldwide. Since 2000, between five and six cases of damage or failure of dams for various reasons have been recorded each year. After the Brumadinho disaster and other similar accidents, Brazil closed waste ponds with dams based on the upstream principle. However, the study by the Polytechnic University of Zurich now shows that the danger is by no means averted if a settling pond is no longer polluted with new waste.
Dam failures like those in the municipality of Brumadinho cannot be predicted with conventional monitoring systems. The ETH Zurich study creates new opportunities in this area:
According to Professor Puzrin:
Our model can perform a risk analysis of existing dams and predict the probability of their failure.
If a high risk is identified, various measures are possible: The risk can be reduced by pumping water from boreholes into settling ponds. Or the settling tank can be dismantled. In urgent cases, villages at risk can be temporarily evacuated to protect the population until the danger has been averted.
Contribution to the safety of earth dams
The conclusions of the ETH study are relevant for all tailings ponds that are used to treat waste from mineral extraction. If the waste consists of fine-grained and brittle material, in unfavorable cases sliding surfaces can form, which leads to the deposited material slipping and damaging the dam.
The situation is not directly comparable with reservoirs where an earth dam holds back water. But here too, the new findings can contribute to safety, as professor and engineer Alexander Puzrin emphasizes:
Our findings provide insight into how the safety of earth dams can be further improved in the event of an earthquake. In this sense, our work contributes to the overall safety of dams.
Bibliographical note
Zhu F, Zhang W and Puzrin AM. (2023) The slip surface mechanism of the delayed failure of the Brumadinho tailings dam in 2019, Communications of Earth and Environment. doi: external page 10.1038/s43247-023-01086-9call_made