From (David P.)@21:1/5 to All on Sun Sep 12 15:04:23 2021
    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

    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

    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."


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