11.2 Semi-empirical Approach
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the dam slope for high frequency, less intense events as well. That may be a way to
avoid what happened to many dams that actually failed under “weak” earthquakes.
11.2.4 A Recent Example
Let’s look at a recent dam failure example, namely the Cadia Ridgeway Mine in
Australia already referenced in Sect. 9.2.1.
What We Do and Do Not Know about the Tailings Dam Failure
We have not had access to any information about this dam except what was available
in the media before the Expert review, but it is not important for this discussion as
we want to discuss tailings dams’ seismic probability of failure. We do not know
how the engineers evaluated the dams at this mine from a seismic point of view. We
do know (because it is public knowledge) that the mine temporarily closed last year
after a magnitude 4.3 earthquake hit. Furthermore, a couple seismic events reportedly
occurred in the region prior to the recent failure.
We also know that forensic investigators/independent expert panels oftentimes
invoke the possibility of “minor quakes” as triggers of tailings dams failures. For
example for the Samarco tailings dam, the panel found that three small seismic shocks
“triggered” the collapse. The panel also indicated the dam’s failure was reportedly
“already well advanced”, caused by a string of design and maintenance failures.
Common practice for seismic pseudo-static analyses for tailings dams would be
to:
• follow jurisdictional codes;
• look at the Maximum Credible Earthquake (MCE) or some code imposed level;
• select the corresponding Peak Ground Acceleration (PGA);
• perform pseudo-static stability analyses with that PGA (or a fraction of it, as
defined by codes);
• ensure the dam attains a code-imposed FoS under those conditions.
Let’s note the minimum FoS under those conditions may be as low as 1.1 following
some national codes.
Tailings Dams Risk Management Considerations
The approach described above is absolutely common and unhesitatingly trusted
by basically all engineers and regulators we know. Let’s take a big risk here (pun
intended) and question that credo using a hypothetical dam.
Assume for a minute that the hypothetical dam is designed for a 1/2500 earthquake, with a corresponding PGA of 0.075 (g) and the selected 50% PGA value
as horizontal acceleration. (NB: The 50% “rebate” is allotted in some jurisdictions
because the foundation soils are of good quality without really wondering how localized geological variations may influence that quality.) For the sake of this discussion
we have calculated the dam’s downstream slope FoS for the static case, ensuring it
was code compliant; a 1/2500 earthquake ensuring again it was code compliant; and
165
the dam slope for high frequency, less intense events as well. That may be a way to
avoid what happened to many dams that actually failed under “weak” earthquakes.
11.2.4 A Recent Example
Let’s look at a recent dam failure example, namely the Cadia Ridgeway Mine in
Australia already referenced in Sect. 9.2.1.
What We Do and Do Not Know about the Tailings Dam Failure
We have not had access to any information about this dam except what was available
in the media before the Expert review, but it is not important for this discussion as
we want to discuss tailings dams’ seismic probability of failure. We do not know
how the engineers evaluated the dams at this mine from a seismic point of view. We
do know (because it is public knowledge) that the mine temporarily closed last year
after a magnitude 4.3 earthquake hit. Furthermore, a couple seismic events reportedly
occurred in the region prior to the recent failure.
We also know that forensic investigators/independent expert panels oftentimes
invoke the possibility of “minor quakes” as triggers of tailings dams failures. For
example for the Samarco tailings dam, the panel found that three small seismic shocks
“triggered” the collapse. The panel also indicated the dam’s failure was reportedly
“already well advanced”, caused by a string of design and maintenance failures.
Common practice for seismic pseudo-static analyses for tailings dams would be
to:
• follow jurisdictional codes;
• look at the Maximum Credible Earthquake (MCE) or some code imposed level;
• select the corresponding Peak Ground Acceleration (PGA);
• perform pseudo-static stability analyses with that PGA (or a fraction of it, as
defined by codes);
• ensure the dam attains a code-imposed FoS under those conditions.
Let’s note the minimum FoS under those conditions may be as low as 1.1 following
some national codes.
Tailings Dams Risk Management Considerations
The approach described above is absolutely common and unhesitatingly trusted
by basically all engineers and regulators we know. Let’s take a big risk here (pun
intended) and question that credo using a hypothetical dam.
Assume for a minute that the hypothetical dam is designed for a 1/2500 earthquake, with a corresponding PGA of 0.075 (g) and the selected 50% PGA value
as horizontal acceleration. (NB: The 50% “rebate” is allotted in some jurisdictions
because the foundation soils are of good quality without really wondering how localized geological variations may influence that quality.) For the sake of this discussion
we have calculated the dam’s downstream slope FoS for the static case, ensuring it
was code compliant; a 1/2500 earthquake ensuring again it was code compliant; and