Final Rpt of Mt Polley Ind. Review Panel released

Courtesy of the Mt Polley Ind. Review Panel:

The Mount Polley Independent Expert Engineering Investigation and Review Panel (the Panel) today delivered a Final Report on its investigation into the cause of the failure of the tailings storage facility at the Mount Polley Mine on August 4, 2014. The report was delivered to the Ministry of Energy and Mines, the T’exelc First Nation (Williams Lake Indian Band) and the Xat’sull First Nation (Soda Creek Indian Band). The report also contains recommendations on actions that can be taken to ensure a similar failure does not occur at other mine sites in B.C.

The Panel concluded that evidence indicates the breach was the result of a failure in the foundation of the embankment, a failure that occurred in a glaciolacustrine (GLU) layer of the embankment’s foundation.

According to the Panel’s report: “The Panel concluded that the dominant contribution to the failure resides in the design. The design did not take into account the complexity of the sub-glacial and pre-glacial geological environment associated with the perimeter embankment foundation. As a result, foundation investigations and associated site characterization failed to identify a continuous GLU layer in the vicinity of the breach and to recognize that it was susceptible to undrained failure when subject to the stresses associated with the embankment.”

The report also indicated that the failure was triggered by construction of the downstream rockfill zone at a steep slope. The Panel concluded that had the downstream slope been flattened failure would have been avoided.  The slope was in the process of being flattened to meet its ultimate design criteria at the time of the accident.

In its report, the Panel also concludes that there was no evidence that the failure was due to human intervention or overtopping of the perimeter embankments and that piping and cracking, which is often the cause of the failure of earth dams, was not the cause of the breach.

In regard to regulatory oversight, the Panel found that inspections of the tailings storage facility would not have prevented failure.  Regulatory staff are well qualified to perform their responsibilities and they performed as expected.

In delivering the Final Report to the government, T’exelc First Nation (Williams Lake Indian Band), and Xat’sull First Nation (Soda Creek Indian Band), the Panel fulfills its mandate as laid out in its terms of reference.  The Report is authored by a review panel of three distinguished geotechnical experts and draws its conclusions and recommendations from an extensive investigation undertaken between August 2014 and January 2015. The investigation and review entailed independent engineering field investigations, data compilation, laboratory testing and analyses. It also involved the inspection of related documents in the files of the Mine, its consultants, and the Ministry of Energy and Mines. The Review Panel also solicited relevant information through a public call for submissions and conducted a number of personal interviews.

The Panel examined the historical risk profile of other tailings dams in B.C. and concluded that to avoid risk in the future requires the adoption of Best Applicable Practices (BAP) and the migration to Best Available Technology (BAT). Examples of Best Available Practices call for improvements of corporate design responsibilities, and adoption of independent tailings dam review boards. Examples of Best Available Technology include filtered, unsaturated, compacted tailings and a reduction in the use of water covers in a closure setting.

The Panel made seven recommendations to improve practice and reduce the potential for future failures. Recognizing that the path to zero failures involves a combination of best available technology (BAT) and best applicable practices (BAP), the Panel recommends the following:

1. To implement BAT using a phased approach:
   • For existing tailings impoundments. Rely on best practices for the remaining active life.
   • For new tailings facilities. BAT should be actively encouraged for new tailings facilities at existing and proposed mines.
   • For closure.  BAT principles should be applied to closure of active impoundments so that they are progressively removed from the inventory by attrition. 
2. To improve corporate governance:
   Corporations proposing to operate a tailings storage facility (TSF) should be required to be a member of the Mining Association of Canada (MAC) or be obliged to commit to an equivalent program for tailings management, including the audit function.
3. To expand corporate design commitments:
   Future permit applications for a new TSF should be based on a bankable feasibility that would have considered all technical, environmental, social and economic aspects of the project in sufficient detail to support an investment decision, which might have an accuracy of +/- 10-15%. More explicitly it should contain the following:
   • A detailed evaluation of all potential failure modes and a management scheme for all residual risk
   • Detailed cost/benefit analyses of BAT tailings and closure options so that economic effects can be understood, recognizing that the results of the cost/benefit analyses should not supersede BAT safety considerations
   • A detailed declaration of Quantitative Performance Objectives (QPOs).
4. To enhance validation of safety and regulation of all phases of a TSF:
   Increase utilization of Independent Tailings Review Boards.
5. To strengthen current regulatory operations:
   • Utilize the recent inspections of TSFs in the Province to ascertain whether they may be at risk due to the following potential failure modes and take appropriate actions
     • Filter adequacy 
     • Water balance adequacy
     • Undrained shear failure of silt and clay foundations
   • Utilize the concept of Quantitative Performance Objectives to improve regulator evaluation of ongoing facilities.
6. To improve professional practice:
   Encourage the Association of Professional Engineers and Geoscientists of British Columbia (APEGBC) to develop guidelines that would lead to improved site characterization for tailings dams with respect to the geological, geomorphological, hydrogeological and possibly seismotectonic characteristics.
7. To improve dam safety guidelines:
   Recognizing the limitations of the current Canadian Dam Association (CDA) guidelines incorporated as a statutory requirement, develop improved guidelines that are tailored to the conditions encountered with TSFs in British Columbia and that emphasize protecting public safety.

In presenting its Final Report, Panel Chair Norbert Morgenstern said on behalf of the Panel, “We have been acutely aware of our responsibilities in conducting this investigation. We set out to be thorough, focusing on the technical issues, and to report our findings in an open, transparent and timely manner.”

Final report can be viewed here

Media briefing video can be viewed here

Video of Aug 4th Tailings Breach (helicopter overview) can be viewed here