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Past present future tailings dams behaviour

Past present future tailings dams behaviour

Jun 1st, 2016

Three papers we have published in the past three years deal with Past present future tailings dams behaviour. Here we propose a summary and our conclusions.

Past present future tailings dams behaviour summary

Past tailings dams behaviour

At TMW 2013 (Oboni, Oboni, 2013) we attempted an estimate of the rate of failure of major tailings dams failures and to compare their risks to human life to well known social tolerance. After clearly stating the limitations of the available data and the lack of clear definition of what constituted a major failure in commonly available statistics, the values we found varied between 10-3 (decade around ’79) to 2*10-4 (decade around ’99). In the same paper we showed quantitatively how, over time, multiple hazards hit would significantly increase the probability of failure of a dam and lead to intolerable risks. The paper concluded “… Especially in the case of TDs located in areas where demographic pressure leads to settlements in the downstream areas, social and legal consequences of a failure will dramatically increase. This will particularly be the case if the methodologies used to perform the risk assessments prove to be in disconnect with the needs of our modern society.”

Past present future tailings dams behaviour

Mount Polley Tailings Dam Breach

The at TMW2015 we presented the results of a new study of tailings dam historic failures (Bowker, Chambers, 2015) which used detailed data and actuarial techniques to define historic rate of failures of tailings dams after attempting to define what constitutes “serious” and “very serious” TD failures. We stated that “… The common practice approach of using oversimplified consequence functions (with “and/or” clauses as just defined above) is often used in research papers because of scope/budget limitations, but should not be accepted for a rational world-wide approach to decision making and tailings risks management for an industry that has significant societal impacts like mining. Tailings accidents generate multiple direct and indirect consequences on the environmental, human, H&S, operational and reputational areas and we believe it is time for the mining industry as a whole to adopt a uniform consequence function.” Our paper concluded that “… It is comforting that the results of the “quick and dirty” 2013 (Oboni & Oboni) study reached globally comparable results to the 2015 (Bowker & Chambers) very deep and solid analytical approach. We note that the selection of the time frame has a large influence on the conclusions of the 2015 study and therefore we recommend these comparative studies to be performed with constant duration (for example decade by decade) to avoid the hazard of drawing misleading conclusions. “Averaging” over 70 years, during which so many conditions have changed, may indeed mask decennial spikes. To prove this it is enough to look at the Table above which shows that the accident rates have actually decreased by 15%-24% from the 1990-1999 to the 2000- 2009 decades using the 2015 study’s own data.” The paper concluded “… We reiterate that the aim of zero tailings failures is impossible to achieve.

There will continue to be tailings failures. In fact, in the long term all tailings facilities will spiral toward significant increases of their pf and when they fail the tailings will go to downstream rivers, lakes, and the ocean as they did at every failure to date.

Present tailings dams behaviour

At TMW2015 (Caldwell, Oboni, Oboni, 2015) we reviewed a number of very recent (2014) tailings dams failures, noting their common traits:

  • Existing facilities that had been used for many years, reportedly following the “as it is impossible to anticipate everything, why bother” principle described in the first half of past century (Merton, 1936).
  • Antiquated design and management practices, meaning poor initial conditions and overall structure category, as shown above for the cases where some data were available leading to substandard pf.
  • Absence of risk assessment or even of common health and safety programs. Let’s note that typical, common practice, risk matrices can only correctly and unambiguously compare a small fraction, reportedly less than 10%, of randomly selected pairs of hazards. Furthermore, they can assign identical ratings to quantitatively very different risks, a phenomena often referred to as “range compression” and can mistakenly assign higher qualitative ratings to quantitatively smaller risks and vice versa. These inaccuracies can lead to mistaken resource allocation. (Oboni, Oboni, 2012)
  • No peer review. Independent peer review of water dams is a long-standing practice. It is disgraceful that all regulators do not insist on it. Until they do, and peer reviews are performed very seriously, we see no hope of reducing the incidence of tailings facility failure (Morgenstern, 2010, Caldwell, 2011)
  • Limited engineer involvement that appears to have been aware of potential problems but not heard or acted on. Alarming disconnect comes from the poor definition of potential consequences of mishaps and their societal ripple effects. This aspect is indeed mostly ignored in codes, leaving professionals ample room to biases and censoring applied to potential losses (Oboni at Al., 2013, CDA, 2014).
  • Overconfident mining companies that did not act when prudence may have so dictated. Risk assessments are almost always censored and biased towards “credible events”. However history, even recent, has shown that major failures occur when “incredible events” occur, or long chains of apparently benign events are produced and the public has now got that clearly in mind, generating widespread controversy and projects’ opposition (Oboni at Al., 2013).
  • Absence of significant regulatory oversight or involvement. Risks assessments are “at risk” if plagued by conflict of interest or overly optimistic cognitive biases, or censure. (Oboni, Oboni, 2014).

We concluded: “While there are similarities in root causes, there is an amazing difference in the response of the societies affected by the failures. From swift and dramatic, with new regulations and large fines for the responsible parties, to the usual committee meetings, review reports soon ignored, calls for action yet to be undertaken, finally a sort of void: no reports, no action, and the mere hope of something in the unspecified future.”

Future tailings dams behaviour

The theme of the long term survivability of TD was further detailed at TMW 2014 (Oboni, Oboni, Caldwell 2014) where attention was focused on modelling the aging process of a geo-structure as a series of discrete hits by hazardous conditions (these could be anything, from an earthquake to flooding, to icing, etc.). In that paper an attempt was drafted at multidimensional estimate of future consequences. The paper stated that “… Should the value of consequences increase, … then even an “excellent dam” would soon pose a societally unacceptable risk even for shorter terms. Any dam that starts its life with a small initial FoS or reduced standards of care (…) would see its risk evolve towards intolerable societal risks faster, even if its consequences of failure remain constant. Thus it turns out that it is not necessary, for this discussion, to delve into conjectures related to future consequences.”

Conclusions

Over the years, we have demonstrated that consequences are not necessarily correlated, in one way or another, with dam height or pond volume. As in many industries the “scary stuff” is not necessarily the riskier one. Our practice and research have shown that the probability of failure is, or will be, often way higher in smaller structures than in major ones, simply because more care is taken for larger structures than for “insignificant ones”. And examples like Stava or Bafokeng are there to show that “extreme” consequences can actually occur. We have also demonstrated that the rate of fatalities in the tailings “industry” lies way above the generally accepted “safe” thresholds for hazardous industries. The number of existing, operational, and closed tailings storage facilities around the world makes it necessary to prioritize the mitigation tasks, if we want to achieve a higher quality, be it at corporate or at national levels.”

The methodology developed to date enables us to “measure” and give a sense to a complex problem, to transparently compare alternatives, to discuss rationally and openly the survival conditions, or to evaluate the premature failure of a structure. The only way to slow down the increase of the probability of failure is to repair damage occurring as a result of each hazard hit, or to entirely avoid the damage. The second is generally “not feasible” for economic and constructional reasons. Risks, especially long term ones, can never be reduced to nil.”

At TMW2016 we will give a course on this subject, introducing the latest developments of our research. We have sought to develop a simplified approach to frame the probability of failure of a dams (or a portfolio of dams) based on easy to retrieve archival data.
Come and follow our course or make sure you will get a copy of our paper entitled: “A systemic look at tailings dams failure process”.

 

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Category: Consequences, Probabilities, Risk analysis, Risk management

One response to “Past present future tailings dams behaviour”

  1. Maurizio Boaretto says:

    interesting discussion. However recent failures (Mt.Polley and Bento Rodrigues) lead us to re evaluate the correct procedures and design criteria of tailings dams design e re consider different techiniques of tailings management.
    Indeed one of most interesting is to consider the mechanical forced filtration of fines in order to eliminate the most important factor that is at origin of the failures: the water

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