Why Did the Dam in Brazil Collapse? Here’s a Brief Look.

Why Did the Dam in Brazil Collapse? Here’s a Brief Look.

On Jan. 25, a mining dam that sat above Brumadinho, a large town in southeastern Brazil, collapsed and unleashed a tidal wave of waste and mud that engulfed homes, businesses and residents in its path. It killed at least 157 people; 182 are still missing.

It was one of the deadliest mining accidents in Brazilian history — a tragedy, but not a surprise, experts told The Times in an investigation into the dam’s collapse. All the elements of a potential catastrophe had been present, and warning signs overlooked, for years.

The structure, owned by the giant Brazilian mining company Vale S.A., strained the very definition of “dam” — it was an enormous, bare-bones reservoir of mining waste held back by little more than walls of sand and silt. It had no separate concrete or metal to hold back its contents. Instead, the dam relied on the lake of mud to remain solid enough to contain itself.

There are 88 mining dams in Brazil built like the one that failed. And all but four of them have been rated by the government as equally vulnerable, or worse. Even more alarming, at least 28 similarly built dams with poor safety ratings sit directly uphill from cities or towns, with more than 100,000 people living in especially risky areas if they failed, an estimate by The New York Times found.

This type of construction — known as an upstream tailings dam — is one of the oddest structures known to engineering. And unless it is designed, constructed and monitored with great attention to detail, it can also be one of the most terrifying.

Dams like the one that failed are, in essence, lakes of thick, semi-hardened mud, consisting of water and the solid byproducts of ore mining, which are known as tailings.

Their unique construction makes them vulnerable to a potentially devastating process called liquefaction, when seemingly solid material can abruptly become a murky liquid, flowing downhill.

Even a subtle change like an increase in water content — because of especially heavy rains or poor management — can liquefy the mud.

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