A design flaw led to the catastrophic breach of a tailings pond at Mount Polley Mine last summer.
That is the conclusion of an independent review panel tasked with investigating the cause of the August 4 failure, which resulted in the release of 17 million cubic metres of water and eight million cubic metres of tailings into Hazeltine Creek, Polley and Quesnel Lakes.
The review panel said 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.
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 best available technology using a phased approach:
a. For existing tailings impoundments. Rely on best practices for the remaining active life.
b. For new tailings facilities. Best available technology should be actively encouraged for new tailings facilities at existing and proposed mines.
c. For closure. Best available technology 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 per cent. More explicitly it should contain the following:
a. A detailed evaluation of all potential failure modes and a management scheme for all residual risk
b. Detailed cost/benefit analyses of best available technology 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
c. 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:
a. 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
i. Undrained shear failure of silt and clay foundations
ii. Water balance adequacy
iii. Filter adequacy
b. 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.