Dams Failure modes and catastrophic failures

Dams Failure modes and catastrophic failures

Sep 23rd, 2020

Today we explore the relationship between Failure modes and catastrophic failures. Catastrophic failure of tailings dam and its water management structures are the likely result of the failure of a number of essential components or any combination of states. We purposely use a different terminology from failure modes because they explain how the dam can fail, but not why. If we look at past events, hydro dams and tailings dam are similar as, for example Oroville and Fundao did fail for complex sets of causes.Failure modes and catastrophic failuresIn other words if the methodology does not address combined states and failure modes it will not depict a fair representation of the risks the dam generates.

Indeed, analyses and the resulting evaluation of probabilities must be repeatable, traceable, defensible, and be supported by relevant data.

Specific requirements for dams analysis

In order to address this dams specific requirements it is important to consider all the analyses the engineers have carried out. These generally encompass static and pseudo-static factor of safety of the slope evaluated using:

  • undrained strength analysis -USA-,
  • static effective stress analysis -ESA-, and finally
  • residual-liquefied strengths,

as appropriate. Additionally, we include uncertainties/variability resulting from the initial geotechnical site characterization/investigations, AND

  1. Construction type and materials AND,
  2. human error AND,
  3. ancillary water management facilities (all the ones that are pertinent for a specific dam) AND,
  4. monitoring and inspections AND finally
  5. management, documentation of changes, maintenance actions.

in the assessment of the probability of failure.

Caveat

We recognize that in doing so the risk analysis involves a large number of considerations. Not all of them are amenable to probabilistic modeling and analysis. The use of expert opinion (subject matter experts/peer review) in the risk analysis allows the inclusion of uncertainties that might otherwise be difficult to calculate or quantify. By the way, expert elicitation is one of several methods acceptable in USACE guidance documents for use in reliability analyses and risk assessments. (SACOE ETL 1110-2-561), U.S. Army Corps of Engineers (2006). Reliability Analysis and Risk Assessment for Seepage and Slope Stability Failure Modes for Embankment Dams.

Conclusion

Using the procedure outlined above, that we have crystallized in the ORE2_Tailings methodology, we ground dams’ risk assessments in defensible data. In addition we incorporate model results (engineering analyses), judgement and expert opinion, as well as benchmarks related to the historical database of failures.  One can consider ORE2_Tailings a quantitative evaluation of the probability of failure and the consequences. We blend that evaluation with semi-empirical rules integrating all the aspects that cannot be quantified in a purely geomechanical approach.

We live in a world where climate change is rapidly altering the environment. As a result it is paramount to use tools that swiftly assess risks and allow users to take non-partisan risk informed decision. Those decisions have to ensure geoethical practices in the form of a fair representation of the risks.

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Category: Hazard, Optimum Risk Estimates, ORE2_Tailings, Probabilities, Risk analysis, Risk management, Uncategorized

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