Posted December 13, 2019
By Samantha Pearson and Luciana Magalhaes, WSJ
Team hired by Brazilian miner says original design prevented correct drainage at the base, allowing water to build up over time
The original design of the dam and the way it was built up over decades allowed dangerous amounts of water to accumulate inside the structure, meaning it was only a matter of time before it gave way, according to the report released by Vale on Thursday.
The failure of the mine-waste dam, one of the world’s deadliest in decades, is the subject of criminal investigations in both Brazil and Germany, home to TÜV SÜD, the dam’s Munich-based safety inspector.
The report was commissioned as a technical study, designed to spell out the specific engineering shortfalls and causes of the collapse. It wasn’t designed as a report to affix blame or responsibility, the report’s drafters said.
When the dam was first built in 1976 by Ferteco Mineração, a Brazilian miner that Vale acquired in 2001, its design prevented correct drainage at its base, according to the panel of four international engineering experts hired by Vale’s lawyers. The dam was also initially constructed with a particularly steep slope, leaving the structure more unstable as subsequent layers of the dam were added over the next 37 years, they said.
The dam was built via the so-called upstream method, the cheapest design, whereby dried out mining waste is gradually piled up over time to form the wall of the structure. Vale built the last layer in 2013, taking the dam to a height of 280 feet, and stopped depositing mining waste there in 2016.
The experts said the mechanisms introduced at later stages of its construction were also insufficient to solve the structure’s drainage problem, which was exacerbated by intense rainfall during Brazil’s wet season at the end of last year.
Adding to the dam’s problems was the stiff and heavy nature of its contents, the so-called tailings. The tailings contained a particularly high level of iron ore, making the structure more brittle, according to the experts from universities in Canada, Australia and the U.S.
When the dam finally ruptured at 12:28 p.m. on Friday, Jan. 25—as many workers were having lunch in the mine’s canteen lower down the valley—the heavy weight of the tailings meant they also burst out with great force. The slope of the dam collapsed in less than 10 seconds. About three quarters of the dam’s contents rushed out over the next five minutes, obliterating the canteen, as well as nearby homes and a small guesthouse.
This occurred via a process called liquefaction, a trick of physics that makes solids behave like a liquid, the report said. Liquefaction was also responsible for the collapse in 2015 of another mine-waste dam jointly owned by Vale in the nearby town of Mariana.
The experts ruled out seismic activity or detonations in neighboring mines as the trigger, saying that the dam was under so much internal stress that it likely underwent a gradual deformation and collapsed.
In September, Brazilian federal police accused 13 mostly lower-level employees from Vale and TÜV SÜD of covering up structural dangers at the dam.
A spokeswoman for Vale said the company wasn’t aware of any imminent or critical risks at the structure before it collapsed. She said the company was also carrying out a detailed technical investigation into all its upstream dams, and would share the findings of Thursday’s report with the authorities.
TÜV SÜD, who police said were aware of the dam’s drainage problems, has declined to comment on the police inquiry, saying the company was cooperating with the authorities.