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2 Two Recent Catastrophic Tailings Dams Accidents
• “Mount Polley illustrates that dam safety guidelines intended to be protective of
public safety, environmental and cultural values cannot presume that the designer
will act correctly in every case. To do so defeats the purpose of FoS criteria as
a safety net. In this, the CDA Guidelines are unable to achieve their intended
purpose. Neither is the Province well served, to the extent that the Ministry of
Energy and Mines (MEM) has incorporated compliance with these guidelines as
a statutory requirement.”
• Had a planned buttress “… been in place… failure would have been prevented.”
• “Had the downstream slope in recent years been flattened to 2.0 horizontal to 1.0
vertical, as proposed in the original design, failure would have been avoided.”
• “The Observational Method was adopted as a design philosophy, but misapplied.
For reasons not unrelated to planning shortcomings, instrumentation was relied
upon to substitute for definitive input parameters and design projections. The
Mount Polley dam was ill-suited to this approach, for both practical and strategic reasons. The steep slopes and constant construction activity on the Perimeter Embankment prevented installation of instruments at optimal locations. More
importantly, the instrumentation program was incapable of detecting critical conditions because, once again, the critical materials and their critical mode of undrained
behavior were not recognized.”
• “The Panel found it disconcerting that, notwithstanding the large number of experienced geotechnical engineers associated with the Mount Polley TSF, the overall adequacy of the site investigation and characterization of ground conditions
beneath the Perimeter Embankment went unquestioned. This may reflect a regional
issue, or possibly one of wider extent. Regardless, it calls for a concerted effort to
improve professional practice in this area.”
Management References
With regard to management practices, the Panel observed or was of the opinion that:
• Mine planning: “A lack of foresight in planning for dam raising contributed to
the failure. Successfully executing the raising plan required intimate coordination
of impoundment water-level projections, production and transport of mine waste
for raising, and seasonal constraints on construction. This made the tailings dam
contingent at the same time on the water balance, the Mine plan, and the weather.
But instead of projecting these interactions into the future, they were evaluated a
year at a time, with dam raising often bordering on ad hoc and only responding
to events as they occurred. The effects were twofold: a near over-topping failure
in May 2014, and restrictions on mine waste availability that produced the oversteepened slopes and deferred buttress expansion.”
• Water balances: The water balances used did not “…provide a reliable approach to
establishing adequate capacity for tailings and water storage” and “uncertainties
in the water balance input parameters combined with uncertainties in climatic
conditions and construction schedules cannot provide a robust design for water
containment”.
• Water impoundment levels: High impoundment water levels were a major cause
of chronic problems in maintaining a tailings beach around the perimeter of the
2 Two Recent Catastrophic Tailings Dams Accidents
• “Mount Polley illustrates that dam safety guidelines intended to be protective of
public safety, environmental and cultural values cannot presume that the designer
will act correctly in every case. To do so defeats the purpose of FoS criteria as
a safety net. In this, the CDA Guidelines are unable to achieve their intended
purpose. Neither is the Province well served, to the extent that the Ministry of
Energy and Mines (MEM) has incorporated compliance with these guidelines as
a statutory requirement.”
• Had a planned buttress “… been in place… failure would have been prevented.”
• “Had the downstream slope in recent years been flattened to 2.0 horizontal to 1.0
vertical, as proposed in the original design, failure would have been avoided.”
• “The Observational Method was adopted as a design philosophy, but misapplied.
For reasons not unrelated to planning shortcomings, instrumentation was relied
upon to substitute for definitive input parameters and design projections. The
Mount Polley dam was ill-suited to this approach, for both practical and strategic reasons. The steep slopes and constant construction activity on the Perimeter Embankment prevented installation of instruments at optimal locations. More
importantly, the instrumentation program was incapable of detecting critical conditions because, once again, the critical materials and their critical mode of undrained
behavior were not recognized.”
• “The Panel found it disconcerting that, notwithstanding the large number of experienced geotechnical engineers associated with the Mount Polley TSF, the overall adequacy of the site investigation and characterization of ground conditions
beneath the Perimeter Embankment went unquestioned. This may reflect a regional
issue, or possibly one of wider extent. Regardless, it calls for a concerted effort to
improve professional practice in this area.”
Management References
With regard to management practices, the Panel observed or was of the opinion that:
• Mine planning: “A lack of foresight in planning for dam raising contributed to
the failure. Successfully executing the raising plan required intimate coordination
of impoundment water-level projections, production and transport of mine waste
for raising, and seasonal constraints on construction. This made the tailings dam
contingent at the same time on the water balance, the Mine plan, and the weather.
But instead of projecting these interactions into the future, they were evaluated a
year at a time, with dam raising often bordering on ad hoc and only responding
to events as they occurred. The effects were twofold: a near over-topping failure
in May 2014, and restrictions on mine waste availability that produced the oversteepened slopes and deferred buttress expansion.”
• Water balances: The water balances used did not “…provide a reliable approach to
establishing adequate capacity for tailings and water storage” and “uncertainties
in the water balance input parameters combined with uncertainties in climatic
conditions and construction schedules cannot provide a robust design for water
containment”.
• Water impoundment levels: High impoundment water levels were a major cause
of chronic problems in maintaining a tailings beach around the perimeter of the