Mount Polley verdict released

The Mount Polley Independent Expert Engineering Investigation and Review Panel has delivered the final report into the cause of the tailings storage facility (TSF) failure at the Mount Polley mine in British Columbia, Canada, on August 4 last year
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The breach was the result of a failure in a glaciolacustrine layer in the embankment’s foundation

Carly Leonida

The panel concluded that the breach was the result of a failure in a glaciolacustrine (GLU) layer (a layer of sediment deposited by a glacially-fed lake) in the embankment’s foundation.

The report stated 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 characterisation failed to identify a continuous GLU layer in the vicinity of the breach and to recognise that it was susceptible to un-drained 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; 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.

The panel concluded 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 TSF would not have prevented failure. 

The report was authored by a review panel of three geotechnical experts and drew 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 British Columbia's Ministry of Energy and Mines.

The panel examined the historical risk profile of other tailings dams in BC, and concluded that to avoid risk in the future required the adoption of best applicable practices (BAP) and migration to best available technologies (BAT). This calls for improvements of corporate design responsibilities, and the adoption of independent tailings dam review boards, as well as the creation of 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:

To implement BAT using a phased approach:
For existing tailings impoundments, rely on BAP for the remaining active life; for new TSFs, BAT should be actively encouraged for new tailings facilities at existing and proposed mines; and, for closure, BAT principles should be applied to closure of active impoundments so that they are progressively removed from the inventory by attrition.

To improve corporate governance:
Corporations proposing to operate a TSF should be required to be a member of the Mining Association of Canada (MAC) or commit to an equivalent programme for tailings management, including the audit function.

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:

  • 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, recognising that the results of the cost/benefit analyses should not supersede BAT safety considerations; and,
  • A detailed declaration of quantitative performance objectives.

To enhance validation of safety and regulation of all phases of a TSF:
Increase the utilisation of independent tailings review boards.

To strengthen current regulatory operations:

  • Utilise 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 and un-drained shear failure of silt and clay foundations; and
  • Utilise quantitative performance objectives to improve regulator evaluation of ongoing facilities.

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 characterisation for tailings dams with respect to the geological, geomorphological, hydrogeological and possibly seismo-tectonic characteristics.

To improve dam safety guidelines:
Recognising 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 BC and that emphasise protecting public safety.

The full report is available here.