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6 Justifying the Need for New Approaches
6.1.1 Factor of Safety (FoS)
The first observation is that just because a given design has been created by a
geotechnical firm using a stipulated FoS, and then adopted by a mining company
and approved by a government, it is not in itself sufficient in itself to ensure that both
the consequences and likelihood of failure meet the tests of materiality to a company
and tolerance for the public.
With specific reference to the Mount Polley breach, it is noted that the FoS
approach used for the first design did not lead to the recognition of the full nature of
the foundation geology in the breach area. Furthermore the FoS approach was used
to justify decisions to defer construction of the buttress and steepen the downstream
slope from 2H:1V to 1.3H:1V. Both these alterations have been shown by the Panel
to have had a significant effect on the robustness and resilience of the dam and, as
such, significantly increased the likelihood of failure.
Another hazardous element that does not seem to have been adequately addressed
by the FoS approach was related to the volume of impounded water, which had
increased from 3.6 to 10.2 million m
3 in the three-year period prior to the breach.
Less hazardous operating practices that would have reduced the volume of water
would have, according to the Panel, possibly allowed for “timely intervention”, thus
reducing the likelihood of failure, or would have reduced the consequences of failure
by “less than one third of what was actually lost”. Both the expert panel and the
Chief Inspector found that the Mount Polley tailings dam failed because the strength
and location of a layer of clay underneath the dam was not taken into account in the
design or in subsequent dam raises. The Chief Inspector summed up this situation by
observing that other factors, including the slope of the perimeter embankment, inadequate water management, insufficient beaches and a sub-excavation at the outside
toe of the dam, exacerbated the collapse of the dam and the ensuing environmental
damage.
In its report, the Panel recognized limitations presented by the use of FoS as
included in current Canadian Dam Association (CDA) Guidelines and the use of
these guidelines as sole statutory requirements “… intended to be protective of public safety, environmental and cultural values …”. The Panel also stated that tailings
dam guidelines and criteria tailored to conditions in the Province of British Colombia
would more effectively meet the needs of the province and added that this would “result in more prescriptive requirements for site investigation, failure mode recognition,
selection of design properties, and specification of factors of safety.”
6.1.2 Design Decisions and Approvals
The Panel rightly pointed out that key design and approval decisions must be based on
a high level of detail that consider all technical, environmental, social and economic
aspects of the project in sufficient detail to support an investment decision. In this
6 Justifying the Need for New Approaches
6.1.1 Factor of Safety (FoS)
The first observation is that just because a given design has been created by a
geotechnical firm using a stipulated FoS, and then adopted by a mining company
and approved by a government, it is not in itself sufficient in itself to ensure that both
the consequences and likelihood of failure meet the tests of materiality to a company
and tolerance for the public.
With specific reference to the Mount Polley breach, it is noted that the FoS
approach used for the first design did not lead to the recognition of the full nature of
the foundation geology in the breach area. Furthermore the FoS approach was used
to justify decisions to defer construction of the buttress and steepen the downstream
slope from 2H:1V to 1.3H:1V. Both these alterations have been shown by the Panel
to have had a significant effect on the robustness and resilience of the dam and, as
such, significantly increased the likelihood of failure.
Another hazardous element that does not seem to have been adequately addressed
by the FoS approach was related to the volume of impounded water, which had
increased from 3.6 to 10.2 million m
3 in the three-year period prior to the breach.
Less hazardous operating practices that would have reduced the volume of water
would have, according to the Panel, possibly allowed for “timely intervention”, thus
reducing the likelihood of failure, or would have reduced the consequences of failure
by “less than one third of what was actually lost”. Both the expert panel and the
Chief Inspector found that the Mount Polley tailings dam failed because the strength
and location of a layer of clay underneath the dam was not taken into account in the
design or in subsequent dam raises. The Chief Inspector summed up this situation by
observing that other factors, including the slope of the perimeter embankment, inadequate water management, insufficient beaches and a sub-excavation at the outside
toe of the dam, exacerbated the collapse of the dam and the ensuing environmental
damage.
In its report, the Panel recognized limitations presented by the use of FoS as
included in current Canadian Dam Association (CDA) Guidelines and the use of
these guidelines as sole statutory requirements “… intended to be protective of public safety, environmental and cultural values …”. The Panel also stated that tailings
dam guidelines and criteria tailored to conditions in the Province of British Colombia
would more effectively meet the needs of the province and added that this would “result in more prescriptive requirements for site investigation, failure mode recognition,
selection of design properties, and specification of factors of safety.”
6.1.2 Design Decisions and Approvals
The Panel rightly pointed out that key design and approval decisions must be based on
a high level of detail that consider all technical, environmental, social and economic
aspects of the project in sufficient detail to support an investment decision. In this