Aerial footage of destruction from the Brumadinho dam collapse, taken two days after the collapse occurred. When the mining dam broke, it sent a wave of toxic mud into the town underneath the mine, killing at least 160 people. Now, people are looking for answers. (Image courtesy of Mauro Pimentel/AFP.)

A mining dam collapse in Brazil that killed over 160 people has put the world's largest producer of iron ore under the microscope for its safety practices.

On January 25, a tailing dam at the Córrego do Feijão ore mine collapsed, burying the mine's administrative office and the nearby town of Brumadinho in toxic mud. Brazil-based multinational Vale insists that it monitored the dam carefully, and claims that there was no way of knowing it would collapse.

But critics of the company say that the tragedy was entirely preventable; Brumadinho Mayor Avimar Barcelos called the company “incompetent and reckless.” And an investigation into the collapse reveals a controversial dam design, substandard maintenance, and a disregard for repeated warnings.

The Tragedy

Just after noon on January 25, Tailing Dam No.1 suffered a collapse. From videos of the collapse, it appears to have occurred almost spontaneously—a small failure that quickly became catastrophic. “It started as a pinpoint, and maybe in one second it looked like a whole patch down there," William F. Marcuson III, former president of the American Society of Civil Engineers, told the New York Times. “The dam fails and the tailings pour out and the dam goes to hell in a matter of seconds. And the dam just crumbles up.”

When the dam collapsed, it released the tailing pond that it had been supporting. Tailing ponds are filled with finely ground rock waste, water and whatever chemicals were used to extract the metal, creating “toxic mud.”In this case, 11.7 million cubic meters of mud burst out of the dam, flowing downhill. The wave hit the foot of nearby Dam No. 6, and swept down toward the mine's administrative area and cafeteria. While the dam had an alarm system in place to alert people nearby if the dam burst, the mud simply hit too fast for workers to sound it. Many workers were unable to make it out.

From the mine, the slurry traveled downhill to the nearby town of Brumadinho, reaching speeds of up to 120 km/h. It then raced through one of the town's suburbs, destroying buildings and burying their inhabitants. Finally, the flood mingled with the Paraopeba River, killing a significant amount of the nearby wildlife.

An aerial view of the collapsed dam (on the right) and the trail the mud carved through the surrounding countryside. (Image courtesy of Reuters.)

This isn't the first time that Vale has had a dam collapse in the region. In 2015, a dam that the company co-owned with BHP Billiton collapsed and killed 19 people. At the time, the collapse was considered the worst environmental disaster in the country’s history. The death toll from the more recent collapse has reached 166 people, with 147 people still officially missing.

The tragedy has prompted scrutiny of Dam No.1, with people trying to figure out exactly how it could have failed.

The Dam

Like most dams built to contain tailing ponds, Dam No. 1 was a raised embankment dam, where compacted earth is piled into a “mound” to restrain the slushy mud. And it was both the cheapest and the most controversial kind of embankment dam: an upstream structure.

An upstream dam starts with a single “starter dike”; a permeable dam that lets water trickle out while containing the tailings and soil. As the tailing pond fills, the workers will build higher, impermeable dams on top of the now-dry mud. Over time, the dam grows taller and taller, depending on the hardened mud for its stability.This type of dam gets its name from the fact that it’s built continually upstream from the starter dike.

A typical upstream embankment dam. Note how the later layers are builton top of the tailings mud. (Image courtesy of John Engels.)

Upstream dams are attractive because they have the lowest initial cost and the least amount of material required for further additions. But they also have a hidden cost: the potential for liquefaction.

Liquefaction occurs when a solid under stress behaves like a liquid. Soil liquefaction specifically occurs when waterlogged soil loses its stiffness because of an applied stress and flows like water. It's difficult to predict when mud will liquefy but, if the mud is unstable enough, even small changes like an increase in water content or drilling in the ground nearby can make liquefaction more likely to occur.

From video footage of No. 1's collapse, it appears that the dam underwent liquefaction, although it's unclear whether the liquefaction caused or resulted from the dam's collapse, or what factors may have caused the liquefaction.

Because upstream dams require building the upper layers on top of hardened mud, liquefaction presents a serious risk. For this reason, they're controversial, and are certain safeguards are recommended when building them.

First of all, the potential for the mud to become waterlogged means that they're best in arid climates, where sudden rain won't change the water content of the mud significantly. Second, it's best to avoid building these structures uphill from towns or cities: “When you have this sort of structure upstream of a population center, that sends up all sorts of red flags,” Marcuson said.

Finally, the mine operators need to carefully monitor the state of the dam, and pay close attention to any red flags.

The Warnings

According to Vale, the dam’s collapse couldn’t possibly have been predicted. The dam featured 94 piezometers (an instrument that measures water pressure) and 41 water level indicators, and went through “biweekly field inspections.” Vale’s official statement concludes, “All these inspections did not detect any change in the state of the structure.”

But public record suggests otherwise.

The most obvious warnings came from a report by Tüv Süd, an inspection company that examined the dam back in September 2018 (an English language translation of the report is available here). The firm concluded that the dam was currently stable, but noted the potential for future problems.

The surface report noted blocked drainage pipes and cracks in the dam, water visibly seeping from parts of the dam’s façade, and a lack of internal drainage in either the starter dike or the first dike above the starter dike (which can cause the mud to become too wet). The report also noted that these problems had been cited during earlier surveys and had not been fixed, with the exception of a new blockage in the dam’s overflow pipe.

The report on the monitoring equipment found inconsistencies in the data, missing records, a lack of instruments to measure stability above the water table, and incorrectly installed monitoring equipment. Finally, the company’s analysis of the dam’s critical slip surface (a measurement of soil stability) showed that the entire dam would fail simultaneously in the event of liquefaction.

Tüv Süd recommended installing monitors and a better overflow system, conducting a study of the foundation, and taking steps to prevent the water level from rising. It’s unclear how many of the recommendations that Vale heeded, except that the workers were fitting new monitors on the dam the day of the flood—more than four months after the report was issued.

Surviving workers also report that there were visible problems with the dam before the collapse. Mine worker Fernando Coelho told the Guardian that he helped procure sand and gravel to repair a leak, and that his father (who also worked at the mine) was called in to deal with it directly. “The supervisor found the mud leaking,” he said. “They called for my father, because he had worked his whole life with dams and mines.” While the repair was made, Coelho Senior told his son, "That’s going to burst at any time."

José de Gouveia said that he saw coworkers repairing a leak at the base of the dam around the same time. And Washington Alves remembers hearing from his electrician brother that there had been a serious leak at the dam. “He said he heard someone say it was leaking water through a crack,” Alves said. “He was scared—so much that he was only going to stay another two or three months.”

Captured footage of the dam collapse shows the river of mud sweeping over the mine’s administrative area (the advance wave of the flood is visible on the right. (Image courtesy of TV BANDEIRANTES.)

And a recent set of reports suggests that at least some of Vale’s management may have had direct information about the dam's defects. Portuguese-language Globo TV reported on an email chain between Vale execs, TÜV SÜD and another inspection company on January 23—correspondence that touched on data discrepancies and monitoring breakdowns. And news organization Reuters reported that it had seen an internal report from Vale, dating back to October 2018, that classified the dam as two times more likely to fail than the maximum tolerable risk.

The Fallout

After the collapse, anxious Brazilians have looked at the country’s other dams to see if another tragedy could happen somewhere else. Unfortunately, the answer is “probably yes.”

A 2017 report from the National Water Agency classified over 700 dams nationwide as having a “high risk of collapse.” Currently, there are 88 other upstream embankment dams, and 84 of them have been rated as vulnerable as Dam No.1—or worse. And 28 of these dams sit directly uphill from towns or cities.

Traditionally, the Brazilian mining industry has gone largely unregulated. A 2016 government audit found the state had only 20 percent of necessary staff at the agency charged with regulating mines. And new laws have reduced the say that environmental groups have in the mine licensing process. In fact, last December, the state council on mining regulations fast-tracked Vale's request to expand the operation at Córrego de Feijão and another mine.

Now, the tragedy means that politicians are once again calling for stricter regulations on mining companies, and have discussed banning upstream dams entirely. As of publication time, the government has arrested eight Vale employees as part of its investigation, two of whom are executives. And there is a mounting sense of public anger: the sense that this tragedy could have easily been avoided.

“We are not dealing with an accident, but with a crime against people and nature,” said Greenpeace Brazil director Nilo D'Ávila, in a statement after the collapse. “How many lives do we still have to lose (until) the Brazilian state and mining companies learn from their mistakes?”