2.1 Mount Polley
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2.1.1 Mount Polley Panel Report
The government of British Columbia established an independent expert engineering
investigation and review panel (Panel 2015) to investigate and report on the Mount
Polley breach. The primary purpose of the Panel was to investigate and report on the
cause of the failure of the tailings storage facility, including the identification of any
mechanism(s) of failure. The Panel was also asked to identify or comment on what
actions could have been taken to prevent the failure, any technical, management or
other practices that may have enabled or contributed to the mechanism(s) of failure,
and to identify any changes that could be considered to reduce the potential for future
such occurrences.
The Panel concluded: “The breach of the Perimeter Embankment on 4 August
2014 was caused by shear failure of dam foundation materials when the loading
imposed by the dam exceeded the capacity of these materials to sustain it”. The Panel
further stated: “The dominant contribution to the failure resides in the design” in that
the design did not take into account the complexity of the sub-glacial and pre-glacial
geological environment associated with the Perimeter Embankment foundation. In
this regard it was also stated: “The type and extent of pre-failure site investigations
were not sufficient to detect this stratum or identify its critical nature”.
In making the above statements the Panel noted that:
… in conducting its inquiry, the Panel limited itself to relying on interviews and on the
documents that it received from the various stakeholders, which were sufficient to determine root cause of the breach. The Panel did not conduct its process according to formal
legal procedures. To do so would have extended the length of this investigation and would
have entered into an assessment of roles and responsibilities, which is beyond the Panel’s
authorization. As a result, the Panel is not able to offer an adequate assessment of the role
of management and oversight in its contribution to the cause of the failure. In particular,
the Panel has not explored the relationship between the designers and owner, contractual or
otherwise. Accordingly, the Panel is unable to ascertain the circumstances that contributed
to key decisions.
However, in keeping with its terms of reference the Panel was able to offer valuable
insights by identifying the hazards that were unique to the moment of the failure and
commenting on those aspects of the design, management and government processes
that allowed those causal or contributing factors exist in the first place.
Design References
With regard to design, the Panel was of the opinion that:
• “There were ambiguities in the governing factor of safety, adapted from Canadian
Dam Association (CDA) Guidelines never intended for tailings dams. An FoS =
1.3 design criterion using peak effective-stress strength left little margin for error,
and trigger-level factors of safety for critical piezometric conditions were even
lower at FoS = 1.1.”
• “Looking specifically at the failure, it was deemed desirable to increase the target
FoS to 1.5 since the TSF was operating more or less continually at full capacity.”
15
2.1.1 Mount Polley Panel Report
The government of British Columbia established an independent expert engineering
investigation and review panel (Panel 2015) to investigate and report on the Mount
Polley breach. The primary purpose of the Panel was to investigate and report on the
cause of the failure of the tailings storage facility, including the identification of any
mechanism(s) of failure. The Panel was also asked to identify or comment on what
actions could have been taken to prevent the failure, any technical, management or
other practices that may have enabled or contributed to the mechanism(s) of failure,
and to identify any changes that could be considered to reduce the potential for future
such occurrences.
The Panel concluded: “The breach of the Perimeter Embankment on 4 August
2014 was caused by shear failure of dam foundation materials when the loading
imposed by the dam exceeded the capacity of these materials to sustain it”. The Panel
further stated: “The dominant contribution to the failure resides in the design” in that
the design did not take into account the complexity of the sub-glacial and pre-glacial
geological environment associated with the Perimeter Embankment foundation. In
this regard it was also stated: “The type and extent of pre-failure site investigations
were not sufficient to detect this stratum or identify its critical nature”.
In making the above statements the Panel noted that:
… in conducting its inquiry, the Panel limited itself to relying on interviews and on the
documents that it received from the various stakeholders, which were sufficient to determine root cause of the breach. The Panel did not conduct its process according to formal
legal procedures. To do so would have extended the length of this investigation and would
have entered into an assessment of roles and responsibilities, which is beyond the Panel’s
authorization. As a result, the Panel is not able to offer an adequate assessment of the role
of management and oversight in its contribution to the cause of the failure. In particular,
the Panel has not explored the relationship between the designers and owner, contractual or
otherwise. Accordingly, the Panel is unable to ascertain the circumstances that contributed
to key decisions.
However, in keeping with its terms of reference the Panel was able to offer valuable
insights by identifying the hazards that were unique to the moment of the failure and
commenting on those aspects of the design, management and government processes
that allowed those causal or contributing factors exist in the first place.
Design References
With regard to design, the Panel was of the opinion that:
• “There were ambiguities in the governing factor of safety, adapted from Canadian
Dam Association (CDA) Guidelines never intended for tailings dams. An FoS =
1.3 design criterion using peak effective-stress strength left little margin for error,
and trigger-level factors of safety for critical piezometric conditions were even
lower at FoS = 1.1.”
• “Looking specifically at the failure, it was deemed desirable to increase the target
FoS to 1.5 since the TSF was operating more or less continually at full capacity.”