Abstract
Mine tailings impoundment failures continue to occur at unacceptable rates. The worldwide mining industry has experienced roughly one significant impoundment failure per year over the past 30 years. Many of these failure events have resulted in massive damage, severe economical impact and, in several cases, loss of life.
A tailings impoundment failure case history database has been developed. In addition to an overview of this database, the basic features of a number of specific case histories are presented that provide valuable lessons to the industry. From the overall database, failure modes, failure impacts, and failure frequency are identified.
The review of failure modes shows that most events can be attributed to easily preventable causes - a disappointing conclusion but one that offers a readily identifiable solution. The review of failure impacts indicates the large scale of immediate economic losses and expensive longer-term harm resulting from tailings dam failures.
The paper shows there are clear trends that arise from objectively reviewing tailings dam failure case histories. Understanding these trends greatly assists in enhancing design, construction, operation and closure stewardship of mine tailings facilities. As demonstrated by a review of case histories, an ignorance of past failure events and the lessons 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 thousands of years. More recently, dams have been developed for both hydroelectric power generation and the retention of industrial byproducts such as mine tailings. Mine tailings dams, which really became recognized as "structures" near the beginning of the 20th century, rival or in many cases exceed the scale of conventional water supply, flood control or hydroelectric dams. Despite their size, and despite tailings impoundments representing some of the largest man-made structures, tailings dams have only gained recognition as "dams" in the last few decades.
Conventional dams have had a generally good safety record although catastrophic failures have occurred. Examples from each of the past three centuries include:
• The 46-m high Estrocho de Rientes dam in Spain breached in April 1802 following first filling of the reservoir. The town of Lorca was inundated and approximately 600 people lost their lives.
• On May 31, 1889, the 22-m high South Fork dam in Pennsylvania initially overtopped and, 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 Italy occurred as a result of a reservoir landslide. The resulting landslide induced wave passed over the dam roughly 250 m above the crest and swept down more than 500 m into the valley below killing about 2500 people in the villages of Longarone, Pirago, Villanova, Rivalta and Fae. The actual dam structure was essentially undamaged by the overtopping event.
Conventional dams continue to be constructed to greater heights with greater storage volumes. However, the safety record of conventional dams has been steadily improving over the past 40 years to the point that the probability of a conventional dam failure in any given year is roughly 1 in 10,000. As will be shown in this paper, this safety trend is not the case for mine tailings dams which appear to be failing at a rate at least ten times 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 dams and 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 a complacent attitude. It also does not appear to account for the fact that tailings dams can undergo environmental failures while maintaining physical integrity - an issue not readily associated with conventional dams.
The authors support efforts to show the mining industry in a good light with respect to the tailings dam performance history. Recent trends and initiatives in tailings dam stewardship, spearheaded by the mining industry, are extremely positive and encouraging (Martin and Davies, 2000), though these initiatives tend to get ignored by a relatively biased 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 to adequate standards. Moreover, the safety record of tailings dams cannot be considered acceptable given the tremendous damage to the overall mining industry that every new failure provides.
Tailings dams currently have a higher profile in the mining process than at any previous period. There has been a dramatic increase over the past ten years in the number of regulatory agencies involved in setting prescriptive and/or rigid guidelines. The number of mining companies with internal programs aimed specifically at assessing current and planned tailings dams likely outnumbers those who do not have such programs; 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 roughly doubled 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 commissioned in the past 5 to 20 years - supposedly the "modern age" of tailings dam engineering. The first step in evaluating the reasons for continued tailings dam failures comes from 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 is reliance on periodic design and perhaps construction monitoring from consulting engineers. Most large-scale water supply and/or hydroelectric agencies more often than not have very capable dam designers and surveillance engineers/technicians in-house. Owners of large conventional dams also typically retain an independent board of eminent consultants to provide expert third-party review. This is not a typical practice in mining at this 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 appropriate external consultants and/or the reliance on third-party consultants without adequate review of 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 engineering and these requirements need to be followed.
This paper examines the phenomenon of tailings dam failure, or, “when things go wrong." The paper is not geared at assigning blame for dam failures but takes the approach that most, if not all, of the failures that have occurred fit into a very consistent set of trends. This consistency is emphasized with the clear premise that if one becomes familiar 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 perhaps wane considerably as the safety record for tailings dams improves to the standard demanded 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 in operation, neglect, not succeeding, becoming bankrupt. All of these have some appropriateness with tailings dam incidents. Leonards (1982) in his Terzaghi lecture defines 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 flow failures for those who wish to learn the most from the errors of others. In fact, there are dramatically more "mundane" failures to learn from (e.g. compare the USEPA "failure" case histories, USEPA, 1997, with the USCOLD, 1994, "failure incident" summary document). While the more catastrophic failures gather the most attention and certainly dominate the typical failure databases that get developed, the same trends and lessons are available from the lesser failures (also called "upsets" by many in the industry). As the lesser 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 an extensive literature review and discussions with regulatory officials worldwide, it is estimated that there are somewhat more than 3500 tailings dams worldwide. This total is made up of contributions that include the following where relatively good inventory lists exist: 350 in Western Australia, 65 in Quebec, 130 in British Columbia, 400 in South Africa and 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 include the four most often referenced:
1. 1994 USCOLD database of tailings dam failure incidents
2. 1996 UNEP database on mine waste incidents
3. 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 a significant 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. This does 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 entire suite of information available on tailings dam failures. A great many failures (and the valuable lessons associated with them) go unpublished due to sensitivity and legal implications.
The developed database includes a compilation of available case histories published as single events or in compilations such as those noted above. The database has been further augmented with largely unpublished information gathered by the authors over time. From the overall developed database, it can be concluded that for the past 30 years, 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 a worldwide 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. This rate of failure does not offer a favorable comparison with the 1 in 10,000 figure that appears representative for conventional dams. The comparison is even more unfavorable if less "spectacular" tailings dam failures are considered.
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 not thought 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 on mined products, public perception of the industry should be commensurate with the value of the industry. Tailings dams, particularly the well-publicized failure events, are lightening rods for public scrutiny of the industry. However, as summarized below, this is not as new a public sentiment as many would believe:
“The strongest argument of the detractors of mining is that the fields are devastated by mining operations…further, when the ores are washed, the water used poisons the brooks and streams, and either destroys the fish or drives them away…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 - 1556
The 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 events are on the rise due to the increase in publicity each successive event receives. The influence public sentiment can have on the viability of a proposed or existing mining project has never been higher. Public, and some regulatory, perception considerations are now largely driving project design decisions, as opposed to appropriate experience and technical logic.
Failures of tailings dams tend to get viewed as events caused by the collective mining 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 deserves the 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 officers
For 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).
Table 1 - Approximate Costs Associated with Tailings Impoundment Failures1 (all costs x 106 US Currency)
1 Data partially from Vick, 1997.
Tailings dam failures have also resulted in loss of life during extreme events.
Table 2 presents a list of the case histories involving fatalities. There are several other incidents, several in the former Soviet Union, where fatalities have occurred but the details of the event and/or the actual number of fatalities are difficult to ascertain.
Table 2 - Examples of Fatalities from Tailings Impoundment Failures
Finally, as society becomes more litigious, there are increasing legal ramifications for company owners and, in some recent cases, their design consultants. These legal considerations can be more than purely financial as criminal charges were considered in at least two tailings dam failures in the past fifteen years (e.g. involuntary manslaughter). There is no reason to expect this litigious trend to subside in at least the foreseeable future.
Tailings Dam Failure Modes - Example Case Histories
To better illustrate the nature of tailings dam failures, and hence their impacts, a few examples are briefly introduced. In each case, the likely cause of the failure is suggested along with information indicating factual versus perceived impact and lessons that can be learned from the event.
Aurui Gold Plant - 2000
On January 30, 2000, a spill of cyanide laden supernatant tailings water was released from a tailings dam at the Aurui SA Tailings Retreatment Plant in Baia Mare, Romania. The overflow was caused by a build up of surface water and occurred over an approximately 25-m length of the more than 3800 m long tailings dam. The period of December 1999 and January 2000 included abundant snowfall and subsequent melting combined with heavy rainfall during January 26-30, 2000. The overflow from the tailings dam entered the adjacent Lapus River 5.2kms away, then entered the Somes River which flows 75kms to the border of Romania and Hungary. This failure event did not include a massive structural failure of the tailings dam and no tailings were released in the event.
The public outcry was immediate and worldwide. Television, radio, newsprint media and the Internet were quick to condemn the 50% owners from Australia and, in the two weeks following the failure to the time of finalizing this paper, the following headlines could be found from "reputable" news agencies during the week ending February 11, 2000:
• (United Press) A 100,000-gallon spill of cyanide-contaminated water from an Australian-owned gold mine has caused an ecological disaster in Romania, the British newspaper Guardian reported Friday. The EU and national environmental officials say there is a major threat to the purity of drinking water supplies for at least 2.5 million people.
• (ABC Internet) A massive cyanide spill has occurred from an Australian-owned gold operation in Romania, killing fish and rendering water undrinkable. The 100,000 cubic metres of cyanide solution has entered River Szamos, a river which flows from Romania to Hungary. The cyanide concentration is between 325 and 700 times the legal limit.
• (The Guardian - London) February 10, 2000. The spill from the Aurul gold mine, near Baia Mare in the north of the country, began 10 days ago. The affected water is reported to be headed downriver toward the Danube and has already reached Yugoslavia. Some sections of the river Tisza, up to 80 percent of the fish have been killed. The Perth, Australia company Esmeralda Exploration, owns 50 percent of the mine and 45 percent is owned by the Romanian government. The remaining 5 percent is owned by foreign investors. Esmeralda Exploration denies responsibility for any spill and claims reports of a disaster were "grossly exaggerated".
• (CNN Internet) ABC Radio reports from Hungary - suggestions that 95% of marine life dead in upper section of river. Hungary's thirst prolonged by environmental disaster.
• (ABC National Radio) - Gold firm suspended from ASX trade, company plays down damage.
• (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 occurred literally during final edits to this paper. Experience would tend to indicate that the environmental impacts have been overstated and cyanide degradation will be rapid with little, 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 the world, can cause immediate and devastating damage to the mining company and its shareholders. The failure also serves to graphically illustrate the need to maintain adequate flood storage volume in tailings impoundments - storage that is based upon appropriate design criteria.
Sullivan Mine, Canada - 1948 and 1991
Davies et al. (1998) describe the static liquefaction event that occurred to the Active Iron Pond tailings impoundment at the Sullivan Mine in August of 1991. The event resulted in a flowslide but, fortunately, another tailings dyke contained the flow and no offsite 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 the upstream constructed facility by the initiation of shear stresses in the foundation tailings in excess of their shear strength. As the material strained, the pore pressures rose and drainage was impeded leading to liquefaction event. The downstream slope of the dyke was roughly 3H:1V, imposing stresses in excess of the collapse surface for the foundation tailings 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 a recognized consulting organization. This event served to demonstrate that "a well intentioned corporation employing apparently well-qualified consultants is not adequate insurance 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 the designers 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 that very 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.
Merriespruit, South Africa - 1994
TDA struck again but in a slightly different form with the Harmony mine adjacent to Merriespruit, South Africa. The Bafokeng tailings dam in South Africa, also a paddock upstream facility, failed in almost the same manner (static liquefaction involved) with a similar result; e.g. downstream fatalities. At Bafokeng in 1974, seepage/piping introduced the retrogressive liquefaction flowslide whereas at Merriespruit, overtopping due to
inadequate freeboard was ample trigger for liquefaction once enough toe material was eroded away.
The Merriespruit failure occurred on February 22, 1994 in the evening. A massive failure of the north wall occurred following a heavy rainstorm. Over 600,000 m3 of tailings and 90,000 m3 of water were released. The slurry traveled about 2 km covering nearly 500,000 m2. 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 fail suddenly and release approximately 240,000 m3 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 at Tesero, at the junction of Stava Creek and the Avisio River at the 4 km point from the mine.
The tailings dams were both nearly 25 m high with one directly upstream of the other. The failure mechanism began with failure of the upper dam that in turn overtopped and failed the lower dam as well. The dams were upstream constructed with outer slopes from 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 with even the most elementary of engineering principals available at the time. There are a number of "rules" for upstream tailings dam engineering (Davies and Martin, 2000) that were understood for many years prior to the Stava failure. The Stava dams both broke far more of these rules than they followed.
Los Frailes, Spain - 1997
Possibly the most publicized tailings dam failure in history was the 1997 Los Frailes event in Spain. A shallow foundation failure led to the release more than 3 x 106 m3 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 a candid assessment of how such incidents can have immediate, and dramatic, impact on a mining company's finances. While other events were certainly at play in 1998, the failure triggered an immediate negative market response. The event occurred at only one of a number of mines for a relatively major mining company. The dramatic share devaluation in 1998 demonstrated the collective impact a single tailings failure event can have on at least medium-term investment confidence in a given corporation.
Omai, Guyana - 1994
Another highly publicized event, the internal erosion failure of the Omai mine's tailings dam, involved a dam breach and the release of cyanide-laden water to the Omai River and then to the much larger Essequibo River. This event caused debatable environmental damage with reports of downstream devastation far outstripping the ability of the dilute contamination to ever accomplish. technical debate that was part of the aftermath of this failure was as unique as the degree of public outcry in comparison with the actual damage to the environment. Following extensive post-failure investigations, representatives of the original design consultant and the post-incident Dam Review Team strongly disagreed on relatively basic engineering issues 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 through reviews, industry contacts and similar sources, several trends from the tailings dam failure database 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 gains momentum
• upstream constructed dams = more incidents - not quite fair as there are more upstream dams, however upstream dams are more susceptible to liquefaction flow events 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 incidents
The 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 most important conclusion; that is that there have been no unexplained failure events. If one becomes a student of tailings dam failure case histories, and all designers and regulators should indeed do just that, a single conclusion arises. These failures, each and every one, were entirely predictable in hindsight. There are no unknown loading causes, no mysterious soil mechanics, no "substantially different material behaviour" and definitely no acceptable failures. In all of the cases of the past thirty years, the necessary knowledge existing 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 considerations are 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 from the tailings dam failure database. In each case, and for all other significant failures in the database, elementary engineering issues and/or basic operating issues have been involved. As shown by the examples in Table 3, there is no need for exotic explanations for the failures and no need to question the fundamental principles of engineering mechanics/hydraulics as the latter have governed in each failure case but were seemingly lost along the way.
Table 3 - Examples of Tailings Dam Failure Causes
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 are readily identifiable, there is the potential to essentially eliminate such events with an industry-wide commitment to correct design and stewardship practices. The necessary knowledge exists; there just has to be used.
Concluding Remarks
Figure 1 presents a summary of sufficiently well documented "significant" tailings dam failures over the 20th 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 remaining at roughly 2 or more significant failures per year with a gradual increase and perhaps also having the occasional particularly "bad" decade (like the 1970's)? Alternatively, will the trend of an apparent decrease in failures since the 1970's suggested by line B continue into this new century?
An optimistic response, e.g. a B trend, is possible with a commitment from the entire industry to an adherence to fundamentally sound design and operating concepts; the authors are cautiously optimistic, as this commitment appears to be growing. The optimism would be further increased if those in the industry who believe there has not been a significant problem from tailings dam failures would take the time to review and acknowledge 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 this history.
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 main participants in the tailings dam life cycle to provide the best opportunity for an improvement in the tailings dam performance record. These participants are:
1. Owners
2. Designers
3. Regulators
4. Public individuals or collectives
Figure 1 - Historical Trend of Significant Tailings Dam Failure Incidents
Owners – only retain design assistance from reputable designers with track records that can be verified. Have submitted designs checked by independent professionals. Give serious consideration to retaining third-party review as part of a periodic audit process. During operations, have a qualified person charged with tailings dam stewardship and provide that individual with the authority to retain professional assistance as deemed necessary. For older operations, be diligent in assessing the history of the operation - look for forgotten "incidents" involving tailings dam management.
Designers – do not work out of your area of competence and/or experience. This includes not using “off the shelf” designs that may have been successful for you in the past but are possibly woefully inappropriate for the climatic/tectonic/foundation conditions for the project at hand. Welcome independent review - do not view such as an attack on your design and/or competency but a benefit to you as much as your client.
Regulators - establish/maintain a database on all tailings dams, operating and otherwise, within your jurisdiction. Maintain candid assessments of the performance records of owners and designers and share such details with other regulators as appropriate. Facilitate developments where the owner presents an independently reviewed design that is consistent with standard design criteria. Work to repeal regulations that are incompatible with common sense.
Public Participants - continue to expect responsible stewardship of the environment by this necessary industry. However, acknowledge that the vast majority of mining industry operators and operations deserve praise for their efforts. Concentrate on factual accounts of incidents to develop and maintain credibility. Avoid supporting nongovernment organizations that endorse actions against corporations committed to a high degree of environmental stewardship and who operate their mines accordingly.