A DAM BIG PROBLEM
20 AUG 2020, by Warren Cornwall, Science dot org

The dam, a 40-meter wall of rocks & dirt, gave way without
warning, unleashing a torrent of mud. Within a day, some
21 million cubic meters of gray goo & water—the tailings
waste left behind by 16 years of copper & gold mining at
the Mt Polley mine in western Canada—escaped from a holding
pond behind the dam, buried a creek, & poured into Quesnel
Lake, home to 1/3 of British Columbia's legendary Fraser
River sockeye salmon.

The 2014 Mt Polley disaster shocked mining engineers
around the world. Many considered Canada a leader in
developing rules aimed at preventing the failure of such
tailings dams, & respected the mine owner, Imperial Metals.
"That wasn't supposed to be able to happen," Jim Kuipers,
an engineer & former tailings dam manager who now consults
for environmental groups, recalls a colleague telling him.

Since then, the sense of crisis has deepened. In 2015, a
tailings dam in Brazil collapsed, unleashing a mammoth mud
spill that killed 19 people, contaminated 668 km of river,
& reached the Atlantic Ocean. In 2018, a dam failed at a
major mine in Australia; luckily, a 2nd barrier prevented
disaster. Last year, a dam disintegrated at a decommissioned
Brazilian iron mine, releasing a torrent that killed 270
people.

Engineers fear more catastrophes await, as the world
confronts a swelling volume of muddy mine tailings, contained
by more & larger dams. Some rise to nearly the height of
the Eiffel Tower & hold back enough waste to fill Australia's
Sydney Harbor. "The consequences of a failure are getting
much bigger," says Priscilla Nelson, a geotechnical engineer
at the Colorado School of Mines.

In response, scientists, governments, environmentalists, &
miners are searching for safer ways to handle the tainted
mud. Some are trying to simply inventory the world's tailings
dams—estimates of the number range from 3500 to 21,000—and
identify those most at risk of failure. A few have called
for a ban on one common but failure-prone design. Others
are working on regulatory and management fixes. "The mining
industry," says Joseph Scalia, a geotechnical engineer at
Colorado State University, "is realizing they can't just
spend as little as possible and the problem is going to
go away."

TAILINGS ARE THE TRASH of the mining world. To extract
most metals, from iron to gold, miners often mix pulverized
rock with water, creating a milkshake of silt & gravel.
As higher quality mineral deposits run out, miners are
turning to lower grade sources that generate more waste.
Worldwide, the metal content of copper ore has fallen by
nearly half since the mid–20th century. Extracting a single
kilogram of copper can now produce 200 kg of sludge. The
muck is often contaminated with toxic metals or minerals
that produce sulfuric acid when exposed to air.

Tailings dams, unlike those built to store water or
generate power, don't earn revenue, creating an incentive
for mine owners to minimize costs. Many are built piecemeal
throughout the life of a mine. And the barriers are often
made from a mixture of rock and the tailings themselves,
rather than a more uniform and predictable material such
as concrete. Those factors contribute to a failure rate
that, over the past century, was over 100 times higher
than that of reservoir and power dams, according to one
estimate.

Each disaster has its own constellation of causes, but
some arise from seemingly trivial errors. At Mt Polley,
investigators led by Norbert Morgenstern, a geotechnical
engineer at the U. of Alberta concluded that part of the
dam was built on a weak patch of silt & clay. Exploratory
boreholes drilled prior to construction were too shallow
to find the problem. Builders further weakened the dam by
making its walls steeper than planned, after the company
ran short of rock. One night, the weight of the sludge
became more than the dam could bear.

It could've been much worse. No one died. Workers ultimately
repaired the dam & shoveled up much of the mud that had
buried the creek. (The company says the spill didn't cause
long-term harm to the Quesnel Lake ecosystem, but some
ecologists say it's still too early to tell.)

Morgenstern, who also led the investigations into the
2015 Brazilian incident and the 2018 Australia failure,
has found that faulty engineering, including inadequate
scrutiny of the underlying geology, was at the heart of
all but two of 15 major incidents between 1980 and 2015.

One major problem, he says, is the "normalization of
deviance." The phrase, coined after the 1986 explosion of
the space shuttle Challenger, describes how engineers can
be lulled into accepting a series of seemingly small risks
that snowball into a catastrophe.

There is an unwritten covenant that regulators & mine
owners can count on engineers to design a safe tailings
system, Morgenstern told a gathering of Brazilian geo-
technical engineers in 2018. "That covenant," he said,
"has been broken."

THE SEARCH IS ON for fixes. Some mining watchdogs are
calling for replacing one common type of dam, called an
upstream dam, & banning future use of the design. Upstream
dams are built in stairlike stages, heading upstream over
the accumulating tailings (see graphic, above). Part of
the weight of each added step is borne by the tailings
below. This approach is often the cheapest, because the
tailings serve as construction material.

Over 40% of major tailings dams are the upstream design,
according to a global inventory of over 1700 dams recently
launched by pension funds of Sweden & the Church of England,
which have pressed the mining industry to strengthen environ-
mental & safety measures. A study of 8000 tailings dams in
China found that 95% were upstream dams. And such dams are
involved in 3/4 of tailings dam failures, according to
one estimate.

The problem is that tailings aren't a predictable building
material, & they are often waterlogged. The water can act
like a lubricant, reducing the friction that binds an
earthen dam together. Engineering flaws such as poor
drainage can exacerbate the problem. In extreme cases—such
as the 2019 disaster at the Brazilian iron mine—dam
sections simply liquefy.

In Chile, where earthquakes make upstream dams even riskier,
the govt has forbidden the design since 1970. Brazil banned
them in the wake of the 2019 accident, & has ordered the
mothballing of all upstream dams by 2027. Worldwide, such
a policy could mean the demise of thousands of mines and
tailings dams (which could be replaced by dams with
different designs). Although such a change might be
expensive for companies, right now communities near dams
are bearing the costs of cheaper construction, says Payal
Sampat of Earthworks, a nonprofit group that promotes
mining reforms. "That is unacceptable."

Some experts caution against a one-size-fits-all approach.
Upstream dams can perform safely, particularly in places
with dry climates & few earthquakes, says David Williams,
a geotechnical engineer at the U. of Queensland, St. Lucia.
"You can construct [an upstream dam] to be perfectly safe.
You can also build it in a not so good way."

One knowledge gap is an understanding of the forces that
can suddenly turn an earthen dam into a liquid river of mud.
At Georgia Tech, geotechnical engineer Jorge Macedo is
stress testing tailings in his lab to document the
conditions that trigger liquefaction, particularly in silt,
a little-studied material that is common in tailings used
to build upstream dams.

Other researchers are looking at better ways to spot dams
on the verge of failure. Moe Momayez, an engineer & geo-
physicist at the U. of Arizona, is testing sensors on an
Arizona dam that track temp & moisture levels. Some dams
are already equipped with radar or lasers that watch for
worrying bulges. Momayez's goal is to integrate streams of
data in a computer system that can spot problems that
might escape periodic inspections. "We have a pretty good
idea how these tailing dams fail," Momayez says. "The
question is, can we predict that, can we get ahead of
the curve?"

Some engineers would like to simply eliminate the need
for massive dams. "The best tailings dam is no dam at all,"
Nelson says. She is studying whether mine waste can be
melted into glasslike fibers that could be used for
textiles or reinforcing concrete. In June, mining giant
BHP said it would spend $10 million to study such reuse
of copper tailings.

A more mature approach is to wring the water from tailings,
creating waste the consistency of damp earth, which can be
sculpted into mountains. The leftovers can still be toxic,
but there's less danger of a devastating flood, says Jan
Morrill of Earthworks. "Filtered tailings should be
considered the industry standard," Morrill says.

Although the approach has been around for decades, it's
rarely used, representing just 4% of tailings systems in
the pension funds' inventory. Filtered tailings systems
can cost 5-10 times more than a conventional dam, says
Harvey McLeod, a geological engineer who designs tailings
dams for Klohn Crippen Berger, a private firm. It's also
an enormous challenge to process tailings at big mines
churning out 100,000 tons of waste per day, particularly
in wet climates. "It's easier said than done," McLeod says.

MANY GROUPS are also pushing for regulatory & management
reforms. After the 2019 Brazilian disaster, investment
funds worth over $10 trillion helped bring together
officials from industry, govt, & the investor group
Principles for Responsible Investment to create a set of
global guidelines for tailings dam construction. Earlier
this month, the coalition issued its plan, calling for
stiffer engineering standards for new dams. It also urges
top mining executives, rather than lower level staff, to
be responsible for tailings safety, and for independent
experts to review companies' waste plans. But it doesn't
push for a ban on upstream dams.

Morgenstern notes that similar reforms he & others
suggested in the late 90s, after an earlier string of dam
disasters, were never fully embraced. He expects it won't
become clear until the end of the year whether the new
proposals will fare better. Still, he's heartened that,
after the recent tragedies, muddy mine waste is again in
the spotlight. "The tree," he says, "has been shaken."

https://www.science.org/news/2020/08/catastrophic-failures-raise-alarm-about-dams-containing-muddy-mine-wastes

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