Strategic operational and tactical planning for tailings hydro dams

Strategic operational and tactical planning for tailings hydro dams

Sep 16th, 2020

We are learning that structural integrity of hydro dams is also becoming an issue. Thus, similarly to tailings dams, we need a strategic operational and tactical planning for tailings hydro dams!

Strategic operational and tactical planning for tailings hydro dams

Karl Terzaghi, American Society of Civil Engineers, “Terzaghi Lectures, 1974-1982,” American Society of Civil Engineers (1986) ISBN 0-87262-532-X. proposed that a dam rating system should provide ALL of the following:

  • A logical framework for rating the relative safety of a dam
  • Allow for the use of judgment and experience (plus results from analysis and historical benchmarking, when appropriate)
  • Yield quantitative ratings (in this case pf) for use in cost-benefit analyses

Thirty five years later we still have a burning question: how do we do that?

How do we do that?

Tailings dam Benchmarking

Dam probabilities of failure can be benchmarked against historic data, namely hundred years of records, although admittedly they are imperfect and heterogeneous records. Benchmarking illustrates which dams in a given portfolio:

  • are better or worse than selected benchmarks,
  • may represent a clear and present danger and or
  • may be better than average when compared to the world portfolio.

This immediately allows decision-makers to understand if mitigation of key hazards, implementation of identified critical controls, would significantly enhance the dam’s survivability. That means reduce its overall probability of failure.

For Tailings dams the benchmarks we normally use in the integrated approach are the following:

Historic records do include causality comments, but these are generally imprecise, biased and have a major category titled “unknown” which makes them rather imprecise in supporting decisions.

For hydro dams the benchmark are available

 LI et al., 2015 compiled tolerances on societal risk criteria. (a) ANCOLD; (b) NSW; (c) USACE; (d) UK and Hong Kong. These derived from benchmarking exercises.

Strategic operational and tactical planning for tailings hydro dams

Strategic operational and tactical planning for tailings hydro dams

 Terzaghi’s dam rating point can be performed with an integrated causality approach that we presented at a conference in London and summarized in prior writeups on ORE2_Tailings.

Tailings dam and hydro dams Causality

The table below shows the relationship between ICOLD Bulletin 121-2001 (ICOLD, 2001) failure drivers and the ORE2_Tailings integrated causality approach.

Relationship between ICOLD Bulletin 121-2001 (ICOLD, 2001) failure drivers and the integrated approach causalities.
It becomes important at this point to also discuss MAC KPIs (MAC, 2019). MAC designed them to help defining the overarching principles of good/best Tailings Management. As a result, they are more general, and difficult to bring into an analytical context. Below is a list of MAC five KPIs which is quite eloquent in showing the different objectives of MAC’s effort with respect to the aim of the approach described in this paper.
It becomes clear when reviewing the table that there is good agreement between ICOLD failure drivers, for example, and/or failure drivers identified based on forensic review of historical failures and the ORE2_Tailings integrated approach causalities, once the due relationships are discussed.

  • Tailings Management Policy
  • Tailings Management System
  • Assigned accountability for TM
  • TM review
  • Operations Maintenance and Surveillance Manual

We note that the approaches discussed in this section are complementary and have some intersections, but their objectives remain distinct. Furthermore, MAC’s approach is an index-based rating whereas the integrated approach described in this paper transforms the KPI information into quantitative probabilities evaluations, allowing for portfolio prioritization and risk informed decision making, and residual risk assessment.

Example of Integrated causalities approach

We show below a one image summary example of analysis carried out with ORE2_Tailings.

benchmarking likelyhood of failure that allow Strategic operational and tactical planning for tailings hydro dams

In this particular example we see that the probability of failure resulting from surface water management (upstream diversions, spillways) AND the potential triggering of liquefaction significantly exceed the probabilities associated with the drained or undrained stability analyses and lie above benchmarking.

Conclusion on Strategic operational and tactical planning for tailings hydro dams

We see that the ORE2_Tailings Integrated causalities approach does offer a logical framework for rating the relative safety of a dam.

It allows for the use of judgment and experience together with results from analysis and historical benchmarking, when appropriate. It also yields quantitative ratings (in this case probability of failure) allowing rational prioritization of risks.

Finally, risk informed cost-benefit analyses become possible.

Thirty five years later we have an answer to burning question: we know how do we do that!

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Category: Hazard, Mitigations, Optimum Risk Estimates, Probabilities, Risk analysis, Risk management, Tolerance/Acceptability

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