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Engineers say Brazilian disaster shows world-wide danger from Hoover Dam-size
earthen structures holding ‘tailings’ waste
From Chile to Australia to the U.S., the quest for economies of scale has prompted mining companies to
dig larger and deeper pits, creating record volumes of waste. To house all that detritus they have
constructed some of the most colossal man-made structures on the planet. Known as tailings dams, these
earthen embankments hold back sprawling reservoirs of mud, finely ground rock and water—what is left
after a mill separates metals from ore. In theory, tailings dams are intended to last forever. In
practice, they fail often enough that industry engineers themselves are sounding alarms. Fifteen months
before the Brazilian disaster, Canada suffered its biggest tailings-dam failure at a copper mine that was in
full compliance with local regulations. Experts estimate that between one and four breaches occur each
year at tailings dams worldwide, roughly 10 times the failure rate of water dams. According to Andrew
Robertson, a Vancouver-based consultant who has designed a number of very large tailings dams for
mining companies, Canadian dumps are among the highest-risk structures on Earth. He notes that the
biggest mines increase their waste output by 10 times every third of a century. Accidents in countries
governed by autocratic regimes often go unreported, particularly in China, experts say.
Unlike dams that store water or generate hydroelectric power, tailings dams are not designed and built all
at once. They are gradually raised by mining companies as they exploit mineral deposits.
While many water dams can be drained and often removed at the end of their working lives,
mining companies design their dams with a different goal in mind: to leave them behind when the
ore runs out. The most common, “upstream,” design involves letting the tailings closest to the dam dry
out. These dry tailings are then used as the foundation for new levels, raised by plowing earth or tailings
into successive embankments. As it requires the least amount of bulldozing, the upstream method is the
least expensive way of building a tailings dam and was employed by Samarco. Upstream dams are said to
be the most likely to fail. Earthquake-prone Chile bans the design.
A 2002 paper reported that the failure rate for tailings dams was approximately 10 times that of waterretention dams. Back then, it was estimated that there were more than 3,500 tailings dams world-wide.
Current figures are not known, since the International Commission on Large Dams (ICOLD) does not
include the structures in its 58,000-entry World Register of Dams due to internal concern that their high
failure rates would tarnish the reputation of all dams. Dangerous tailings dams are not just a developingworld problem. Canada, home to one of the world’s most advanced mining sectors, saw its largestever tailings accident in 2014 when the Mount Polley dam collapsed. Some 8 million cubic meters of
gold and copper byproduct poured into a pair of glacial lakes in British Columbia, temporarily cutting off
local drinking-water supplies.
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Researchers argue that the typical culprit for tailings accidents is too much water, which can cause
earthen dams to liquefy. A review panel after the Mount Polley accident recommended that miners adopt
technology to remove water from their waste before storing it. Another way to make a dam safer is to
expand it by building outward, rather than upstream atop dried tailings. Called the downstream design,
this results in a bulkier structure that more closely resembles a water dam. But either technique is more
expensive than upstream dams.
Regulation and enforcement vary wildly among different jurisdictions, often leaving mining companies to
police themselves. Fundão dam, which belonged to Samarco Mineração SA, is a joint venture between
two of the world’s biggest mining companies: Australia’s BHP Billiton Ltd. and Brazil’s ValeSA. Vale
and BHP Billiton have said the dam was Samarco’s responsibility, not theirs. In the case of Brazil,
officials acknowledge that regulatory oversight is sparse. Prosecutors say the National Department of
Mineral Production, or DNPM, had only two dam-safety specialists responsible for monitoring more than
300 tailing facilities. The agency was also hit by funding cuts amid fiscal crisis. The exact cause of the
burst dam remains unknown but it appears that state mining regulations are inadequate and mine owners
BHP and Vale, and operator Samarco, have failed to comply with their due diligence responsibilities, as
outlined in The Organisation for Economic Co-operation and Development (OECD) guidelines for
multinational enterprises in Minas Gerais state before the accident.
Fonte: Texto adaptado de:Engineers say Brazilian disaster shows world-wide danger from Hoover Dam-size earthen structures holding
‘tailings’ waste. Disponível em: http://www.wsj.com/articles/brazils-samarco-disaster-mining-dams-grow-to-colossal-heights-and-so-do-therisks-1459782411. Último acesso: 6 abril 2016.
RESPONDA ÀS QUESTÕES CONFORME INFORMAÇÕES CONTIDAS NO TEXTO.
QUESTÃO 1
Analise as seguintes afirmações sobre o texto:
I. Waste output has increased exponentially as large scale mining ventures become more common.
II. The accidents reported in Canada demonstrate that developing countries house the most
dangerous mining projects.
III. Accountability concerning mining-related incidents is unusual in autocratic countries.
IV. Water-dams related incidents are substantially less common than tailing dam failures.
São VERDADEIRAS:
(A) apenas II
(B) apenas II e III
(C) I, III e IV
(D) apenas I e II
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QUESTÃO 2
O que diferencia a barragem que armazena água da barragem de rejeitos?
A barragem que armazena água é projetada e construída ao mesmo tempo, enquanto a de rejeitos vai sendo
aumentada à medida que as mineradoras continuam com a exploração dos depósitos de minério. As
barragens que armazenam água podem ser drenadas e frequentemente são removidas ao fim da sua vida
útil; quando o minério se esgota, por outro lado, as mineradoras simplesmente abandonam as barragens de
rejeitos, sem movê-las.
QUESTÃO 3
Por que barragens como a da Samarco e de outras usinas são construídas a montante e qual é o
risco apontado?
Elas são as barragens de menor custo, já que demandam menos despesas de demolição do que qualquer
outro tipo. No, entanto, elas têm maior probabilidade de se romperem, e por isso são proibidas em países
como o Chile.
QUESTÃO 4
“Unlike”, em negrito no 2º parágrafo, indica
(A) probabilidade
(B) improbabilidade
(C) contraste
(D) semelhança
,
QUESTÃO 5
Na afirmativa em negrito no 2º parágrafo, os pronomes sublinhados estão relacionados a
(A) water dams, mining companies and dams
(B) water dams, water dams and working lives
(C) mining companies, water dams, working lives
(D) dams, water dams and mining companies
QUESTÃO 6
A que se refere cada um dos seguintes números, grifados no 3º parágrafo: 2002; 10 times; 3,500; 8
million e 2014?
2002 foi o ano em que se reportou que as barragens a montante possuem um risco 10 vezes maior de
rompimento do que as barragens de retenção de água. 10 times refere-se à probabilidade do risco de
rompimento. 3,500 era o número de barragens de rejeitos existentes no ano de 2002. 8 million refere-se ao
número aproximado de metros cúbicos de subprodutos de ouro e cobre depositados em dois lagos glaciais
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na British Columbia, em consequência do desastre de Mount Palley. 2014 foi o ano em que se registrou o
maior acidente envolvendo barragens de rejeitos, quando a barragem de Mount Polley desmoronou.
QUESTÃO 7
O marcador “due to”, em negrito no 3º parágrafo, poderia ser substituído pelos marcadores abaixo,
mantendo-se o mesmo sentido, EXCETO no caso de
(A) owing to
(B) provided that
(C) because of
(D) on account of
QUESTÃO 8
Quais são as sugestões oferecidas pelos pesquisadores para aumentar a segurança das operações?
Utilizar uma tecnologia que permita a remoção da água dos resíduos antes de armazená-los; construir as
barragens a jusante, o que resultaria em uma estrutura mais semelhante a uma barragem com capacidade
de reter água.
QUESTÃO 9
Analise as seguintes informações sobre o texto:
I. Monitoring of the operations is often conducted by external auditing processes.
II. The text argues that DNPM is currently underfunded and understaffed.
III. Vale and BHP Billiton laid the blame on Samarco, the operational arm of the companies.
IV. The companies have not met OECD guidelines for multinational enterprises.
São VERDADEIRAS:
(A) apenas II
(B) apenas II e III
(C) II, III e IV
(D) apenas I e II
QUESTÃO 10
A palavra “oversight”, em negrito no 5º parágrafo, poderia ser substituída por todas as palavras
a seguir, EXCETO
(A) supervision
(B) security
(C) administration
(D) management
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