MINE TAILINGS DAMS: WHEN THINGS GO WRONG

8/9/2019 MINE TAILINGS DAMS: WHEN THINGS GO WRONG

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MINE TAILINGS DAMS:WHEN THINGS GO WRONG

Michael Davies, Todd Martin and Peter Lighthall AGRA Earth & Environmental Limited, Burnaby, BC

Abstract

Mine tailings impoundment failures continue to occur at unacceptable rates. Theworldwide mining industry has experienced roughly one significant impoundment failureper year over the past 30 years. Many of these failure events have resulted in massivedamage, severe economical impact and, in several cases, loss of life.

A tailings impoundment failure case history database has been developed. Inaddition to an overview of this database, the basic features of a number of specific casehistories are presented that provide valuable lessons to the industry. From the overall

database, failure modes, failure impacts, and failure frequency are identified. The reviewof failure modes shows that most events can be attributed to easily preventable causes - adisappointing conclusion but one that offers a readily identifiable solution. The review of failure impacts indicates the large scale of immediate economic losses and expensivelonger-term harm resulting from tailings dam failures.

The paper shows there are clear trends that arise from objectively reviewing tailingsdam failure case histories. Understanding these trends greatly assists in enhancingdesign, construction, operation and closure stewardship of mine tailings facilities. Asdemonstrated by a review of case histories, an ignorance of past failure events and thelessons offered by these events can be highly contributory to subsequent failures.

The mining industry is at a crossroads with tailings impoundment performance - is

the relatively constant failure frequency trend for the past 30 years going to continue or decrease as we enter this new century?

Introduction and Perspective

Dams have been used for water supply and/or flood control purposes for thousandsof years. More recently, dams have been developed for both hydroelectric power generation and the retention of industrial byproducts such as mine tailings. Mine tailingsdams, which really became recognized as "structures" near the beginning of the 20

th

century, rival or in many cases exceed the scale of conventional water supply, floodcontrol or hydroelectric dams. Despite their size, and despite tailings impoundments

representing some of the largest man-made structures, tailings dams have only gainedrecognition as "dams" in the last few decades.

Conventional dams have had a generally good safety record although catastrophicfailures have occurred. Examples from each of the past three centuries include:


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• The 46-m high Estrocho de Rientes dam in Spain breached in April 1802 following firstfilling of the reservoir. The town of Lorca was inundated and approximately 600 peoplelost their lives.

• On May 31, 1889, the 22-m high South Fork dam in Pennsylvania initially overtoppedand, within three hours, fully breached. The flood damage included 2209 fatalities.

• On October 9, 1963, an overtopping event of the 266 m high Vaiont Dam in Italyoccurred as a result of a reservoir landslide. The resulting landslide induced wavepassed over the dam roughly 250 m above the crest and swept down more than 500 minto the valley below killing about 2500 people in the villages of Longarone, Pirago,Villanova, Rivalta and Fae. The actual dam structure was essentially undamaged bythe overtopping event.

Conventional dams continue to be constructed to greater heights with greater storage volumes. However, the safety record of conventional dams has been steadilyimproving over the past 40 years to the point that the probability of a conventional damfailure in any given year is roughly 1 in 10,000. As will be shown in this paper, this safetytrend is not the case for mine tailings dams which appear to be failing at a rate at least tentimes higher than that for conventional dams. Some make a different argument (e.g.

Bruce et al., 1997), implying that tailings dams are equally "safe" as conventional damsand that both are being built to at least the same "state-of-the-art" practice. This latter interpretation of the statistical database is common and worrisome as it can lead to acomplacent attitude. It also does not appear to account for the fact that tailings dams canundergo environmental failures while maintaining physical integrity - an issue not readilyassociated with conventional dams.

The authors support efforts to show the mining industry in a good light with respectto the tailings dam performance history. Recent trends and initiatives in tailings damstewardship, spearheaded by the mining industry, are extremely positive and encouraging(Martin and Davies, 2000), though these initiatives tend to get ignored by a relativelybiased news media. However, an objective evaluation of the tailings dam failure database

illustrates that many tailings dams are not being designed, constructed and/or operated toadequate standards. Moreover, the safety record of tailings dams cannot be consideredacceptable given the tremendous damage to the overall mining industry that every newfailure provides.

Tailings dams currently have a higher profile in the mining process than at anyprevious period. There has been a dramatic increase over the past ten years in thenumber of regulatory agencies involved in setting prescriptive and/or rigid guidelines. Thenumber of mining companies with internal programs aimed specifically at assessingcurrent and planned tailings dams likely outnumbers those who do not have suchprograms; at least for medium to large sized organizations. An increasing number of undergraduate programs offer at least some form of training in the basics of tailings dam

design and the number of graduate theses published on tailings dams has roughlydoubled over the past decade. Design professionals have an increasing number of technical forums to update their skills and compare design competency with their peers.

So why do failures of tailings dams continue to occur? The failures are not just of older facilities constructed without formal designs, but include facilities designed and


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commissioned in the past 5 to 20 years - supposedly the "modern age" of tailings damengineering.

The first step in evaluating the reasons for continued tailings dam failures comesfrom recognizing the uniqueness of mine tailings dams. The unique attributes include:• Tailings impoundments are among the largest manmade structures with several

approaching 1 x 109 tonnes of stored slurried tailings;

• Tailings dams are built on a continuous basis by mine operators; and• Tailings dams are a cost to the mining process - they do not generate a revenue

stream akin to a hydroelectric dam.Mining companies typically do not have in-house geotechnical expertise, instead there isreliance on periodic design and perhaps construction monitoring from consultingengineers. Most large-scale water supply and/or hydroelectric agencies more often thannot have very capable dam designers and surveillance engineers/technicians in-house.Owners of large conventional dams also typically retain an independent board of eminentconsultants to provide expert third-party review. This is not a typical practice in mining atthis time.

Are the unique features of tailings dams the reason for the failure trends? The

authors suggest that a combination of factors including a lack of input from appropriateexternal consultants and/or the reliance on third-party consultants without adequate reviewof their work are highly contributory to the failure trends. As noted by Davies and Martin(2000), there are basic requirements for a designer working in tailings dam engineeringand these requirements need to be followed.

This paper examines the phenomenon of tailings dam failure, or, “when things gowrong." The paper is not geared at assigning blame for dam failures but takes theapproach that most, if not all, of the failures that have occurred fit into a very consistentset of trends. This consistency is emphasized with the clear premise that if one becomesfamiliar enough with these trends, future failure events will not arise from an ignorance of the lessons offered by the failure database. If the mining industry collectively embraces

the lessons from these trends, the current profile surrounding tailings dams can perhapswane considerably as the safety record for tailings dams improves to the standarddemanded by those who are so quick to criticize the industry.

Definition of Failure

When tailings dams go wrong, it is to say that they have failed. Websters'dictionary offers the following for defining failure: falling short, weakening, breakdown inoperation, neglect, not succeeding, becoming bankrupt. All of these have someappropriateness with tailings dam incidents. Leonards (1982) in his Terzaghi lecturedefines failure as "…an unacceptable difference between expected and observed

performance".The authors suggest the terminology offered by Leonards (1982) captures what

failure means in the context of tailings dams. Failures need not be catastrophic flowfailures for those who wish to learn the most from the errors of others. In fact, there aredramatically more "mundane" failures to learn from (e.g. compare the USEPA "failure"


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case histories, USEPA, 1997, with the USCOLD, 1994, "failure incident" summarydocument). While the more catastrophic failures gather the most attention and certainlydominate the typical failure databases that get developed, the same trends and lessonsare available from the lesser failures (also called "upsets" by many in the industry). As thelesser failures tend to get very little publicity, and almost never any technical publication,practitioners of tailings dam design should keep their own database developed from

observations obtained from reviews, audits and the like.

Tailings Dam Failure Database

There is a very poor database of the world’s tailings dam inventory. From anextensive literature review and discussions with regulatory officials worldwide, it isestimated that there are somewhat more than 3500 tailings dams worldwide. This total ismade up of contributions that include the following where relatively good inventory listsexist: 350 in Western Australia, 65 in Quebec, 130 in British Columbia, 400 in South Africaand 500 in Zimbabwe.

As far as performance of these dams, there are a number of publications that

summarize portions of the worldwide tailings dam failure incident database. These includethe four most often referenced:1. 1994 USCOLD database of tailings dam failure incidents2. 1996 UNEP database on mine waste incidents3. 1997 USEPA summary of relatively recent tailings dam incidents largely focusing on

non-compliant events and limited to certain jurisdictions of the United States.4. WISE Internet site.

The authors, through reviews and similar assignments, have been made aware of asignificant number of failure case histories not captured by any of the above documents,but which occurred within the timeframes and jurisdictions reviewed in each case. Thisdoes not condemn any of the above efforts - these summary documents are of

tremendous value. The point illustrated is that these publications do not offer the entiresuite of information available on tailings dam failures. A great many failures (and thevaluable lessons associated with them) go unpublished due to sensitivity and legalimplications.

The developed database includes a compilation of available case historiespublished as single events or in compilations such as those noted above. The databasehas been further augmented with largely unpublished information gathered by the authorsover time. From the overall developed database, it can be concluded that for the past 30years, there have been approximately 2 to 5 "major" tailings dam failure incidents per year.During no year were there less than two events (1970-1999, inclusive). If one assumes aworldwide inventory of 3500 tailings dams (a tenuous extrapolation at best), then 2 to 5

failures per year equates to an annual probability of between 1 in 700 to 1 in 1750. Thisrate of failure does not offer a favorable comparison with the 1 in 10,000 figure thatappears representative for conventional dams. The comparison is even more unfavorableif less "spectacular" tailings dam failures are considered.


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Public Perception and Tailings Dam Failures

The public has high expectations for the mining industry in stewarding mine tailings.There are "fringe" groups who appear opposed to mining of any sort that have either notthought out their position with any real effort or advocate a return to a Paleolithic lifestyle.Given that society, at least implicitly by consumption patterns, places a high value onmined products, public perception of the industry should be commensurate with the valueof the industry. Tailings dams, particularly the well-publicized failure events, are lighteningrods for public scrutiny of the industry. However, as summarized below, this is not as newa public sentiment as many would believe:

“The strongest argument of the detractors of mining is that the fields aredevastated by mining operations…further, when the ores are washed, the water used poisons the brooks and streams, and either destroys the fish or drives themaway…thus it is said, it is clear to all that there is greater detriment from mining than the values of the metals which the mining produces”

Agricola - 1556The public now has instantaneous access to tailings dam events (see discussion on

the recent Baia Mare event later in this paper). Given the relatively constant frequency of tailings dam failures over the past thirty years, the public perception is that such eventsare on the rise due to the increase in publicity each successive event receives. Theinfluence public sentiment can have on the viability of a proposed or existing miningproject has never been higher. Public, and some regulatory, perception considerationsare now largely driving project design decisions, as opposed to appropriate experienceand technical logic.

Failures of tailings dams tend to get viewed as events caused by the collectivemining industry. It is naïve to assume that an individual corporation or regulatory

jurisdiction is not affected by the dam failures of others. Whether the industry deservesthe situation, each failure incident "raises the bar" with both the public and regulatory

bodies for the "next" project.

Tailings Dams Failure Impacts

Tailings dam failures can have any or all of the following impacts:• Extended production interruption• Loss of life• Environmental damage• Damage to company and industry image• Economic consequences company, and even industry, wide

• Legal responsibility for company officersFor the mining company, the most tangible impact after ensuring public safety is the

immediate and longer-term financial impact. Table 1 presents some approximate costs of recent tailings impoundment failures (note that Marcopper was not a dam incident).


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• (Esmeralda Exploration WebSite) - Director resigns to pursue other opportunities,February 11, 2000.

The authors are not fully aware of all of the facts of this case history that occurredliterally during final edits to this paper. Experience would tend to indicate that theenvironmental impacts have been overstated and cyanide degradation will be rapid withlittle, if any, long-term impact to receiving waters. However, the nature of the news reports

indicates the climate in which the mining industry finds itself. Any failure, anywhere in theworld, can cause immediate and devastating damage to the mining company and itsshareholders. The failure also serves to graphically illustrate the need to maintainadequate flood storage volume in tailings impoundments - storage that is based uponappropriate design criteria.

Sull ivan Mine, Canada - 1948 and 1991

Davies et al. (1998) describe the static liquefaction event that occurred to the ActiveIron Pond tailings impoundment at the Sullivan Mine in August of 1991. The eventresulted in a flowslide but, fortunately, another tailings dyke contained the flow and no off-site impact was experienced. The dam had been built on a foundation of older tailings

that were placed as beach below water (BBW) material. The failure occurred to theupstream constructed facility by the initiation of shear stresses in the foundation tailings inexcess of their shear strength. As the material strained, the pore pressures rose anddrainage was impeded leading to liquefaction event. The downstream slope of the dykewas roughly 3H:1V, imposing stresses in excess of the collapse surface for the foundationtailings in an extensive stress path and near to the collapse surface in compressive shear.The Sullivan tailings facility had been under the design and monitoring stewardship of arecognized consulting organization. This event served to demonstrate that "a wellintentioned corporation employing apparently well-qualified consultants is not adequateinsurance against serious incidents" (Morgenstern, 1998).

Ironically, the 1991 event was similar in nature to a dyke failure that occurred in

1948. The passage of more than forty years should not have been enough to induce thedesigners into TDA (Tailings Dam Amnesia). As defined by Martin and Davies (2000),TDA refers to a state of tailings dam design or stewardship where lessons available at thatvery site are ignored in spite of ample available information on-site, visual evidence of previous event occurrence and/or published accounts of incidents on a given project.

Merriespru it, Sou th A fr ica - 1994

TDA struck again but in a slightly different form with the Harmony mine adjacent toMerriespruit, South Africa. The Bafokeng tailings dam in South Africa, also a paddockupstream facility, failed in almost the same manner (static liquefaction involved) with asimilar result; e.g. downstream fatalities. At Bafokeng in 1974, seepage/piping introduced

the retrogressive liquefaction flowslide whereas at Merriespruit, overtopping due toinadequate freeboard was ample trigger for liquefaction once enough toe material waseroded away.

The Merriespruit failure occurred on February 22, 1994 in the evening. A massivefailure of the north wall occurred following a heavy rainstorm. Over 600,000 m

3 of tailings


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and 90,000 m3

of water were released. The slurry traveled about 2 km covering nearly500,000 m

2. Given the downstream population, it is fortunate that not more than 17

people lost their lives in this tragedy.

Stava, Italy - 1985

Perhaps the most tragic tailings dam failure to date occurred on July 19, 1985. A

flourite mine, located near Stava in Northern Italy, had both of its tailings dams failsuddenly and release approximately 240,000 m

3 of liquefied tailings. The liquefied mass

moved up to speeds of 60 km/h obliterating everything in its path for a stretch of some 4-km. The flowslide destroyed the village of Stava and also caused considerable damage atTesero, at the junction of Stava Creek and the Avisio River at the 4 km point from themine.

The tailings dams were both nearly 25 m high with one directly upstream of theother. The failure mechanism began with failure of the upper dam that in turn overtoppedand failed the lower dam as well. The dams were upstream constructed with outer slopesfrom 1.2 to 1.5 horizontal to 1 vertical. Based upon the likely state of the in-situ tailings,the soil mechanics curiosity with this failure is that the dams could attain such a height

prior to failure. There is no question that the design of these dams was not consistent witheven the most elementary of engineering principals available at the time. There are anumber of "rules" for upstream tailings dam engineering (Davies and Martin, 2000) thatwere understood for many years prior to the Stava failure. The Stava dams both broke far more of these rules than they followed.

Lo s Frai les, Spain - 1997

Possibly the most publicized tailings dam failure in history was the 1997 Los Frailesevent in Spain. A shallow foundation failure led to the release more than 3 x 10

6 m

3 of

process water and tailings from one of two adjacent ponds within an overall impoundment.For this failure, a lack of understanding of the prevailing foundation conditions was directly

attributable to a design that was contraindicated by site conditions.The Los Frailes incident, besides demonstrating the immense power of the media

to bring tailings dam failure events to a worldwide audience in a matter of hours, allows acandid assessment of how such incidents can have immediate, and dramatic, impact on amining company's finances. While other events were certainly at play in 1998, the failuretriggered an immediate negative market response. The event occurred at only one of anumber of mines for a relatively major mining company. The dramatic share devaluationin 1998 demonstrated the collective impact a single tailings failure event can have on atleast medium-term investment confidence in a given corporation.

Omai, Guy ana - 1994

Another highly publicized event, the internal erosion failure of the Omai mine'stailings dam, involved a dam breach and the release of cyanide-laden water to the OmaiRiver and then to the much larger Essequibo River. This event caused debatableenvironmental damage with reports of downstream devastation far outstripping the abilityof the dilute contamination to ever accomplish.


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The failure was likely the first incident with worldwide outrage. However, thetechnical debate that was part of the aftermath of this failure was as unique as the degreeof public outcry in comparison with the actual damage to the environment. Followingextensive post-failure investigations, representatives of the original design consultant andthe post-incident Dam Review Team strongly disagreed on relatively basic engineeringissues involved in both the original design and the ultimate failure mechanism(s) (Haile,

1997 and Vick, 1997, respectively).

Trends from the Failure Database - Lessons to Learn

By combining published accounts of dam failures and those available throughreviews, industry contacts and similar sources, several trends from the tailings dam failuredatabase are evident:• active dams are more susceptible to failure - this trend may diminish over time if the

current trend advocated by some to flood all tailings impoundments upon closure gainsmomentum

• upstream constructed dams = more incidents - not quite fair as there are more

upstream dams, however upstream dams are more susceptible to liquefaction flowevents and are solely responsible for all major static liquefaction events

• slope instability/earthquakes for 2/3 of all upstream dam incidents• seepage related phenomena (e.g. piping due to poor filter design such as was evident

in the Omai dam failure) is the main failure mode for non-upstream tailings dams• earthquakes are of little consequence for most non-upstream dams• for inactive dams, overtopping is cited as the primary failure mode in nearly 1/2 of the

incidentsThe list of trends from the database can be continued and has been presented in

the past by many others. However reviewers of the case histories seldom make the mostimportant conclusion; that is that there have been no unexplained failure events. If onebecomes a student of tailings dam failure case histories, and all designers and regulatorsshould indeed do just that, a single conclusion arises. These failures, each and everyone, were entirely predictable in hindsight. There are no unknown loading causes, nomysterious soil mechanics, no "substantially different material behaviour" and definitely noacceptable failures. In all of the cases of the past thirty years, the necessary knowledgeexisting to prevent the failure at either the design and/or operating stage. There is lack of design ability, poor stewardship (construction, operating or closure) or a combination of the two, in each and every case history. If basic design and construction considerationsare ignored, a tailings dam’s candidacy as a potential failure case history is immediate.

Table 3 summarizes the main contributory failure mode(s) for a few examples fromthe tailings dam failure database. In each case, and for all other significant failures in thedatabase, elementary engineering issues and/or basic operating issues have beeninvolved. As shown by the examples in Table 3, there is no need for exotic explanationsfor the failures and no need to question the fundamental principles of engineeringmechanics/hydraulics as the latter have governed in each failure case but were seeminglylost along the way.


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Table 3 - Examples of Tailings Dam Failure Causes

Case Reasons for Failure

Baia Mare Pond water management - maintenance of adequate freeboardLos Frailes Non-recognition of brittle foundation conditions and limitations of

surveillance for such conditionsStava Water management / dam design with no soil mechanicsEl Cobre Earthquake / dam design incompatible with seismic loadingSullivan Foundation / static liquefactionMerriespriut Pond water management / static liquefactionOmai Filter design/construction issuesNasca Earthquake / dam design incompatible with seismic loading

Tailings Dam Failure Axiom - Tailings dam failures are a result of design and/or construction/operation management flaws - not "acts of god".

As a positive corollary to the axiom, if the reasons for tailings dam failures arereadily identifiable, there is the potential to essentially eliminate such events with anindustry-wide commitment to correct design and stewardship practices. The necessaryknowledge exists; there just has to be used.

Concluding Remarks

Figure 1 presents a summary of sufficiently well documented "significant" tailingsdam failures over the 20

th century. From the summarized information in Figure 1, two

possible trends are shown and are labeled A and B. Using Figure 1 as a barometer, what

is the likely future for tailings dam performance? Is the trend to be like A; either remainingat roughly 2 or more significant failures per year with a gradual increase and perhaps alsohaving the occasional particularly "bad" decade (like the 1970's)? Alternatively, will thetrend of an apparent decrease in failures since the 1970's suggested by line B continueinto this new century?

An optimistic response, e.g. a B trend, is possible with a commitment from theentire industry to an adherence to fundamentally sound design and operating concepts;the authors are cautiously optimistic, as this commitment appears to be growing. Theoptimism would be further increased if those in the industry who believe there has notbeen a significant problem from tailings dam failures would take the time to review andacknowledge the less than perfect history. These individuals should also understand that

the current scrutiny under which the industry currently finds itself is largely a result of thishistory.This conference is a unique event with the representation of owners, designers and

regulators. The authors suggest some minimum expectations for each of the four mainparticipants in the tailings dam life cycle to provide the best opportunity for animprovement in the tailings dam performance record. These participants are:


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accounts of incidents to develop and maintain credibility. Avoid supporting non-government organizations that endorse actions against corporations committed to a highdegree of environmental stewardship and who operate their mines accordingly.

References

1. Agricola, G. (1556). De Re Metallica. 1st Edition.2. Bruce. I., C. Louge and L. Wichek (1997). "Trends in Tailings Dam Safety".

Proceedings to CIM Annual General Meeting, Vancouver.3. Davies, M.P. and T.E. Martin (2000). "Upstream Constructed Tailings Dams - A

Review of the Basics". In proceedings of Tailings and Mine Waste '00, Fort Collins,January, Balkema Publishers, pp. 3-15.

4. Davies, M.P., B.B. Dawson and B.G. Chin (1998). “Static Liquefaction Slump of MineTailings – A Case History”, Proceeding of 51st Canadian Geotechnical Conference,Edmonton.

5. Haile, J.P. (1997). "Discussion on the Failure of the Omai Tailings Dam". GeotechnicalNews, Volume 15, No. 1, March, pp. 44-49.

6. Leonards, G.A. (1982). "Investigation of Failures". Sixteenth Terzaghi Lecture. ASCEJournal of Geotechnical Engineering, Volume 108, Number GT2, February, pp. 224-283.

7. Martin, T.E. and M.P. Davies (2000). "Trends in the Stewardship of Tailings Dams", Inproceedings of Tailings and Mine Waste '00, Fort Collins, January, Balkema Publishers,pp. 393-407.

8. Morgenstern, N.R. (1998). "Geotechnics and Mine Waste Management - An Update",in proceedings of ICME/UNEP Workshop on Risk Assessment and ContingencyPlanning in the Management of Mine Tailings, Buenos Aires, November, pp. 172-175.

9. United Nations Environment Programme (1996). " Environmental and SafetyIncidents concerning Tailings Dams at Mines: Results of a Survey for the Years 1980-

1996", Industry and Environment. Paris, 1996, 129 pages.10. U.S. Environmental Protection Agency, (1997). "Damage Cases and Environmental

Releases from Mines and Mineral Processing Sites", Office of Solid Waste,Washington DC, 231 pages.

11. USCOLD (1994). Tailings Dam Incidents, a r eport prepared by the USCOLDCommittee on Tailings Dams, November.

12. Vick, S.G. (1997). "Failure of the Omai Tailings Dam: Closure". Geotechnical News,Volume 15, No. 1, March, pp. 49-55.

13. Vick, S.G. (1997). “. Proceedings to CIM Annual General Meeting, Vancouver.

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MINE TAILINGS DAMS: WHEN THINGS GO WRONG