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Lesson 2/4

2.1 Introduction

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2.2 Topic 1
2.3 Topic 2
2.4 Topic 3
2.5 Topic 4

2.2 Critical thinking required

The results shown in Figure 20 are interesting and appear to show some significant trends, but there are some issues that should be understood. Firstly, the number of failures during a decade is not compared with the number of dams of a particular type constructed during that decade. It is likely that more tailings dams than water retention dams were built during the 1960s and 1970s. If so, the number of failures of tailings dams relative to the number of such dams would be lower than the relative number of failures of water retention dams.

Secondly, in an engineering sense failure of a structure or system means an inability to perform its intended function. In the case of a tailings impoundment, the intended function is to safely store tailings for an indefinite period. The failure of a tailings dam may be catastrophic and lead to permanent closure of an operation or it may be temporary. There are no details on what kinds of failure are shown in the data of Figure 1 and thus the data could include a repairable leak, a localized failure of the soil placed to build the impoundment, or an inability to maintain water or land to a particular environmental standard, each of which imply that the impoundment has failed to perform its intended function, but only until the problem causing the failure is fixed.

The design of tailings dams falls into the specialized field of geotechnical engineering—the design of structures using soil and/or rock with foundations on some combination of soil and rock. In general, there is an empirical side to any kind of engineering design in which the behaviour of combinations of materials or systems cannot be predicted with complete certainty. This is very true of geotechnical engineering where the behaviour of soil and rock is uncertain and can exhibit significant variability even within the same geographical region. The best guide to behaviour in this situation is experience. It is unlikely that the increase in tailings dam failures in the 1970s followed by a decrease in the 1980s and 1990s can be attributed to chance; more likely the decrease in failures is the result of geotechnical engineers incorporating the science of soil mechanics and the knowledge gained from previous failures into the design, construction, and operation of tailings dams.

2.3 Omai, Guyana

In January 1993, the Omai gold mine began processing gold ore at a rate of 13,000 tpd using carbon-in-pulp cyanide extraction. At the time it has the largest open pit gold mine in South America. The earth fill tailings dam was in a small valley located on a bank of the Omai River about three kilometres from its confluence with the much larger Essequibo River.

At about midnight of August 19, 1995 a haul truck driver noticed a stream of water coming from one end of the tailings dam. By dawn another stream was discharging from the other end. A large crack was visible along the dam crest suggesting the presence of a consistent defect along the entire length of the dam.

The effluent escaping the impoundment consisted of some tailings but was mostly water which contained a cyanide residue. Diversion ditches and a cofferdam were constructed by August 24 and managed to divert about 1.3 million cubic meters of the effluent to the mine pit. However, from August 19 to 24 another 2.9 million cubic meters of effluent was discharged into the Omai River and ultimately into the Essequibo River.

From a tailings dam engineering standpoint, the interesting aspect of this failure was that the effluent did not flow over the top of the dam, i.e. the dam did not breach. Rather the effluent flowed through the core of the dam which had completely failed along the entire length of the dam due to a process known as internal erosion. Internal erosion (or piping) occurs when fine particles are placed next to coarse particles and, under the influence of gravity or water pressure, flow through the coarse particles resulting in a complete loss of support for any solids overlying the fine particles. Figure 22 illustrates this process and also points out the remedy which is to use soils with particles sizes intermediate between those of the coarse and fine particles to construct a transition zone, a gradual change in particle size between the fine and coarse particles.

[IMAGE]

Figure xx: The process of internal erosion caused by fine particles percolating through coarser particles due to water pressure.

The problem at Omai was that the dam was constructed in such a way that a transition zone effectively did not exist. This happened because of the combination of a design flaw and a lack of construction control (Vick (1996); Repetto (2003)). The mistakes made in the design and construction of the Omai tailings dam have become an illustrative example used in many geotechnical engineering classes.

There were several environmental, financial, and legal consequences of the failure. About 350 fish were killed in the Essequibo River and trade in fish taken from the Essequibo was curtailed for some time due to the possibility of contamination. Tax revenue from the mine constituted about 25% of Guyana's gross domestic product and this revenue was curtailed during the six month shutdown period following the failure. Cambior Inc and Golden Star Resources were joint owners of Omai Gold Mines Ltd, the mine operator, and the share value of each company decreased significantly following the failure. Several lawsuits against Cambior, Golden Star and Omai were launched but were unsuccessful because no evidence of catastrophic or permanent damage could be produced. Fishermen were compensated by Omai Gold Mines, but there were complaints about the adequacy of the compensation. There were also lingering doubts about whether the mine continued to leak cyanide contaminated water into the river system.

Poison in a Lifeline - Part 1 (Optional)

Poison in the Lifeline - Part 2 (Optional)

Poison in the Lifeline - Part 3 (Optional)

References

Repetto, R, 2003. Silence is Golden, Leaden, and Copper, Disclosure of Material Environmental Information in the Hard Rock Mining Industry. Yale School of Forestry & Environmental Studies. Available at www.yale.edu/environment/publications .

Vick, S.G. 1996. Failure of the Omai tailings dam. Geotechnical News, September, BiTech Publishers, Richmond, BC.

2.4 Marcopper, Phillipines

In 1969, Marcopper Mining Corporation started mining by open pit methods on the island of Marinduque in the Philippines. The Mt. Tapian site was mined from 1969 to 1990 when its reserves were depleted and the San Antonio pit was opened. The company was given a permit to dump mine tailings into Calancan Bay at the north end of the island. Fishermen complained that this was affecting fish stocks and residents of Calancan Bay complained of dust and tailings accumulation on the shores. As a result it was decided to dispose of the tailings into the Mt. Tapian pit and a permit to do this was granted to Marcopper in 1990.

There was a 2.5 km long drainage tunnel from the bottom of the Tapian pit to the Boac River. This was used to drain the pit when it was mined as heavy rainfall made pumping too expensive. Before tailings could be placed into the pit, the tunnel had to be closed. This was done by means of a concrete plug placed at the mouth of the tunnel as shown in Figure 24.

In 1992 deposition of tailings into the Tapian pit began. By early 1995 leaks from the rock above and around the tunnel portal were observed and were plugged. However, on March 24, 1996, the rock around the concrete plug failed and 1.6 million LaTeX: m^3 of tailings discharged into the 26 kilometer long Boac River. The effects on the river and the people whose lives depended on it were disastrous. Flash floods isolated villages and one village was buried under two meters of floodwater. Much of the valley floor was buried under mine tailings. Agricultural fields were inundated and drinking water was contaminated. Fish, shrimp and other food sources were immediately killed. The government declared the Boac River dead.

Five officials of the company and some government officials were arrested and faced various criminal charges. Nothing has come of these cases. Marcopper closed all mines on Marinduque. Placer Dome, who had bought a 40% share of Marcopper in 1969, assumed responsibility and spent about $80 million on various clean-up operations. In 2006 Placer Dome was acquired by Barrick Gold who must now deal with a lawsuit by the province of Marinduque, seeking more than $100 million for alleged environmental damage. The lawsuit and the required clean-up remain unresolved issues.

The key bit of physics here is that water transmits pressure in any direction and water pressure at a point depends only on the height of water. How much water pressure was on the rock around the plug? That is easily computed:

$LaTeX: \text{P = density of the rock} \times \text{acceleration due to gravity} \times \text{depth of rock}$

$LaTeX: \text{= (2000 to 2500)} kg/m^{3} \times 9.8 m/sec^{2} \times 30 m$

That's about 426 pounds per square inch (psi). But what was the pressure in the ground where the rupture occurred? That is also easily computed. The depth of rock over the rupture was about 30 meters. The density of most rock is between 2000 and 2500 LaTeX: kg/m^3 . Let's estimate a range of rock pressure:

That's a range of 85 psi to 107 psi. So there is 426 psi of water pressure up against 85 to 107 psi of rock pressure. What do you think will win? (Hint: The concrete plug would likely not crack—way too strong.)

Remembering the Marcopper mine disaster

References

University of Michigan. Environmental Justice Case Study: Marcropper in the Phillippines. Undated. Source: www.umich.edu/~snre492/Jones/marcopper.htm (accessed May 2013).

Hoffman, Andy. "Big messes for miners." The Globe and Mail. October 11, 2009. Source: www.theglobeandmail.com/report-on-business/industry-news/energy-and-resources/big-messes-for-miners/article4289199/#dashboard/follows/ (accessed May 2013).

2.5 Los Frailes, Spain

Los Frailes is a zinc, copper, lead and silver mine discovered in 1988 located about 35 km west of Seville in southern Spain. Boliden Apirsa started mining the deposit in 1997 after the reserves at the nearby Aznalcollar pit were depleted. The concentrator used for the ore from the Aznalcollar pit was modified to take ore from Los Frailes. The existing tailings impoundment contained about 15 million m3 of tailings and it was decided to use this to store tailings produced by the Los Frailes operation since the impoundment had a capacity of 33 million m³.

Just before 1:00 am on April 25, 1998, it was noticed that a power line near the toe of the dam had failed. Then at 3:00 am, an electrician inspecting the line noticed cracks in the dam crest and very little water in the impoundment. About 5.5 million m³ of acidic water and 1.3 million m³ tailings water had been released into the Rio Agrio and other rivers downstream (Figure 26). There was no sign of instability before April 25 and the sequence of events between 1 and 3 am suggested that most of the tailings outflow occurred between 1:00 and 3:00 am, i.e. the failure was quite sudden.

Several independent investigations concluded that the failure was the result of high water pressures induced by the extra load of tailings from the Los Frailes operation in a layer of marl (a calcium carbonate rich mudstone containing clays) at a depth of about 14 meters below the dam. The layer failed in undrained conditions and displaced causing part of the tailings dam to break and move a distance of about 60 meters.

There were several environmental consequences. The tailings affected the banks of the Rio Agrio and other rivers downstream for a distance of 40 km almost entering the Doñana national park. About 2600 hectares of mostly agricultural land were covered in tailings and irrigation wells were contaminated. Most of the tailings were removed by December 1 1998, but treatment of wells and further clean-up of areas having high metal concentrations continued through 1999. The tailings impoundment was closed in 2000.

Following the accident there was an immediate drop in the share price of Boliden. The share price continued to drop up to the end of 1998 despite an increase in general market indices. The mine was never fully re-started and ceased operations completely in 2001.

As might be expected a complex sequence of litigation occurred after the accident involving suits and counter-suits between the regional government, Boliden, consultants, and contractors. The details can be found at the WISE Uranium Project website (accessed May 2013). As of 2012, a EUR 89.9 million claim against Boliden by the regional government of Andalusia for the cost of repair of damages has been reinstated by the Spanish Supreme court.

There were several arguments made that this accident was the result of a foreseeable risk, that the clays in the marl layer would swell due to water seepage and this should have been accounted for. This is a bit of 20/20 hindsight. There was no indication, prior knowledge or experience that suggested the marl layer could not withstand the additional load of tailings. However, as a result of this accident, the foundations of existing and proposed tailings dams are under much more scrutiny than before.

20 años del desastre de Aznalcóllar | España (Optional - In Spanish)

La catástrofe ambiental de Aznalcóllar en Canal Sur Televisión (1996-98) - Optional, in Spanish