ORE2_Tailings case history

ORE2_Tailings case history

Oct 30th, 2019

ORE2_Tailings case history looks at a real tailings dam. It shows how to interpret ORE2_Tailings probabilities ranges predictions, benchmarks and other useful indications.

 

Description of the real-life active tailings dam case history

The case history is a 72.5m high dam with a modified centerline cross section.

Construction has stopped, meaning there is no active deposition at this time. Therefore the dam is inactive. The dam stores tailings and a supernatant pond. It has a weir in fair state of maintenance, fair design for a 1,000-years flood. A diversion ditch is not present, thus the pond is unprotected. The water balance has not seen any near misses in the last decades. The dam has a penstock reportedly in a fair state of conservation.

ORE2_Tailings case history causality, probabilities and benchmark analyses

Riskope gathered information by reading the various extant and available reports, examining available plans, Google Earth “time” exploration, and other documentation. It was therefore possible to generate optimistic and pessimistic evaluations for the thirty ORE2_Tailings diagnostic nodes necessary to the analysis.

This led to the single-dam ORE2_Tailings dashboard in the figure below.

ORE2_Tailings Dashboard

Failure Causality pie-diagram

The causality pie-diagram in the figure shows that the two strongest predicted causalities for a potential failure, are geotechnical investigations and geomechanical tests. These two sum-up at 48% of causality. Operations, monitoring and maintenance have the lowest causality, due to the quality of the work performed by the owner, at 15%.

Moreover, this means that the potential failure of the dam arises from congenital deficiencies that the work of the present owner cannot correct, unless drastic capex expenses are allowed.

Probability of Failure and benchmarking chart

The main rectangular chart in the figure displays a wealth of information about the estimated dam’s behavior as follows:

  • Starting from the left we notice two green rectangles. This is the benchmark generated by hundred years of history of the world tailings dams portfolio major failures and a theoretical approach.
  • The red rectangle is the benchmark constituted by the recent catastrophic failures in Canada and Brazil.
  • The salmon colored bars correspond to the ranged of estimated probabilities of failure for:
    • effective stress analyses (ESA),
    • undrained strength analyses (USA) and finally
    • pseudo-static analyses carried out to date by the engineers of the project.
  • The basis for these estimated probabilities are the usual deterministic factors of safety (FoS) determined by the engineers. Of course, the rapid draw-down case is not applicable, hence not evaluated here.
  • NB: Because we noted that the engineers had considered tailings at peak in all their analyses, and neglected to consider potential liquefaction, an extra set of analyses was run at our request. We integrated these results in the prediction.
  • The salmon bars include the “ancillary water management” effects such as, for example the missing diversion ditch and the penstock.
  • Finally, to the extreme right, the blue bar shows how the maximum probabilities would reduce if the ancillary structures were all in “excellent state” and proper design. The figure also delivers the approximate result in words and numbers in the darker area at the bottom left.

It immediately appears that this dam has serious deficiencies, even in the ESA case, with respect of the world portfolio. Additionally, should liquefaction occur or the dams conditions even slightly evolve toward undrained conditions the dam would be in the range of recent catastrophic failures probabilities.

ORE2_Tailings case history conclusions

Again, even if the owners allotted funds to better the water management ancillary structures, the situation would not significantly better in this ORE2_Tailings case history.

The congenital deficiencies of the dam, which include:

  • overestimating the contribution of tailings to the stability,
  • neglecting the liquefaction potential,

limit the mitigative options and point toward heavy capex solutions we will not discuss here.

For owners and operators responsible for large portfolios of tailings dams it is paramount to be able to prioritize these actions in a rational and consistent way. Riskope designed ORE2_Tailings for that purpose. We have deployed the methodology for this purpose for medium-sized dams inventories.

Finally, we will be discussing ORE2_Tailings at our book presentation at CIM-MES in Vancouver on November 12th and will be giving a presentation at the coming TMW2019, also in Vancouver, on Monday Nov. 18th.

Contact us to lean more about the advantages ORE2_Tailings can bring to your company.

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Category: Consequences, ORE2_Tailings, Risk analysis

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