2.1 Mount Polley
17
dam. At the breach section, water was in direct contact with the upstream zone
of tailings fill when failure occurred. This increased the piezometric level in the
upstream zone above what it would have been had a wide tailings beach been
present. The Panel’s analyses show that this had some influence on dam stability,
although it was not the dominant factor.
• Water impoundment volume. The panel was of the opinion that:
1. “Had the water level been even a meter lower and the tailings beach commensurately wider, this last link might have held until dawn the next morning, allowing timely intervention and potentially turning a fatal condition into
something survivable.”
2. “It was water erosion that transported the bulk of the tailings, and these fluvial
processes ended when the supply of water was exhausted. Had there been less
water to sustain them, the proportion of the tailings released from the TSF
would have been less than the one-third that was actually lost.”
3. “It is not clear to the Panel why it took so long to design and implement a
water treatment strategy that would provide for a significant reduction in the
amount of surplus water stored on the TSF.”
• Downstream dam slope: The adoption of the steeper 1.3H:1V downstream slope
may have been due to limited material availability or other aspects related to mine
planning.
• Other concerns related to management practices:
1. Departures from intended designs related to filter and transition construction.
2. Failure of much of “the as-placed filter material…to meet applicable filter
criteria and requirements for internal stability of its grading.”
3. Chronic problems with maintaining the tailings beach continued.
4. Related absence of a well-developed tailings beach violated the fundamental
premise of the design as a tailings dam, not a water-storage dam.
5. Even if not contributing directly to the failure, some design details were problematic. Already thin to begin with, reducing the core width from 8 to 5 m
made it even more vulnerable to differential settlement and cracking. Both the
filter and transition zones were just 1 m wide, placing great demands on their
performance.
Panel Recommendations
In response to its mandate to identify any changes that could be considered to reduce
the potential for future such occurrences, the Panel’s recommendations pertaining to
risk identification and management are summarized as follows:
1. 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 the following:
17
dam. At the breach section, water was in direct contact with the upstream zone
of tailings fill when failure occurred. This increased the piezometric level in the
upstream zone above what it would have been had a wide tailings beach been
present. The Panel’s analyses show that this had some influence on dam stability,
although it was not the dominant factor.
• Water impoundment volume. The panel was of the opinion that:
1. “Had the water level been even a meter lower and the tailings beach commensurately wider, this last link might have held until dawn the next morning, allowing timely intervention and potentially turning a fatal condition into
something survivable.”
2. “It was water erosion that transported the bulk of the tailings, and these fluvial
processes ended when the supply of water was exhausted. Had there been less
water to sustain them, the proportion of the tailings released from the TSF
would have been less than the one-third that was actually lost.”
3. “It is not clear to the Panel why it took so long to design and implement a
water treatment strategy that would provide for a significant reduction in the
amount of surplus water stored on the TSF.”
• Downstream dam slope: The adoption of the steeper 1.3H:1V downstream slope
may have been due to limited material availability or other aspects related to mine
planning.
• Other concerns related to management practices:
1. Departures from intended designs related to filter and transition construction.
2. Failure of much of “the as-placed filter material…to meet applicable filter
criteria and requirements for internal stability of its grading.”
3. Chronic problems with maintaining the tailings beach continued.
4. Related absence of a well-developed tailings beach violated the fundamental
premise of the design as a tailings dam, not a water-storage dam.
5. Even if not contributing directly to the failure, some design details were problematic. Already thin to begin with, reducing the core width from 8 to 5 m
made it even more vulnerable to differential settlement and cracking. Both the
filter and transition zones were just 1 m wide, placing great demands on their
performance.
Panel Recommendations
In response to its mandate to identify any changes that could be considered to reduce
the potential for future such occurrences, the Panel’s recommendations pertaining to
risk identification and management are summarized as follows:
1. 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 the following: