Cookie Preferences

Tailings Dams Risk Mitigation Through Risk-Informed Decision-Making

Related Services

Mining Risk Assessment

Tailings Dam Risk Mitigation Through Risk-Informed Decision-Making is the title of our Planning for Closure 2022 paper.

 Risk-informed Decision-making Supports Rational and Sensible Risk Mitigation

This blogpost shows how risk-informed decision-making (RIDM) using ORE2_Tailings™ supports rational and sensible risk mitigation selection. This is a requirement of the Global Industry Standard on Tailings Management (GISTM). We use an anonymized case report to preserve confidentiality.

To support rational risk mitigation roadmaps, we first test each possible single mitigative alternative and chain of possible mitigation steps in comparison to the worldwide benchmark failure rate. Then, we split each alternative’s causality between the dam itself and its ancillary water system, including possible necessary creek protection. The goal is to understand where we can apply optimum actions. This helps to evaluate the ratio between the annualized risk decrease and the mitigative capital expenditures for each mitigation alternative.

Furthermore, for each mitigation alternative and stage, we can evaluate the risk abatement “efficiency.” In the end, one can select the “optimal” roadmap considering constructional and regulatory constraints. The RIDM roadmap allows decisionmakers to select a defensible mitigation level (GISTM As Low As Reasonably Practicable (ALARP) or better). RIDM fosters healthy and rational technical and risk perception discussion related to the desired level of risk mitigation, based on GISTM ALARP. One can use RIDM to foster healthy discussions with regulators and insurers as needed. 

Dam Case Study

The dam we use as an example is inactive and parallel to a valley bottom. An unprotected creek runs at its toe and the engineers considered its bed to offer sufficient protection against a 1/500 flood event. Beyond that flood, there will, of course, be progressive erosion of the dam toe. The dam has an upstream construction method. It has diversion ditches and drainage systems capable of carrying the 1/100 return. The area is not seismic, and there is consensus that the material is draining and not liquefiable. The original engineers designed the spillway for the 1/500 event; however, its present conditions do not allow passing more than the 1/100 return flood, as the dam had to be reinforced after past instabilities.

Nevertheless, the estimated annualized system probability of failure (pf) of 1.07*10-2 (appx. 1.1%) per year places the dam much higher than the worldwide benchmark’s highest value. In other words, the dam is more hazardous than the worldwide benchmark. Estimated failure consequences (C) are evaluated at approximately $2B due to the presence of inhabitants, housing, infrastructure, river and a relatively pristine environment downstream.

The Owner and Dam’s Engineer of Record Request

Given the pf and C, the dam risk is corporately and societally intolerable. In addition, it falls in the “extreme” consequences, according to the GISTM. The dam also represents a corporate strategic risk following the tolerance threshold comparison. Thus, the owner and engineer of record (EoR) want to mitigate its risks. We produced what-if mitigation scenarios to support RIDM.

The EoR developed a number of possible mitigative stages increasing from minimal repairs to significant reinforcement of the overall system. In total, the EoR prepared six alternatives. The process consisted in systematically evaluating the risks for each mitigative stage, the related capital expenditure and deriving risk abatement-mitigative investment graphs to determine the ALARP point.

Tailings Dam Mitigation Risk-informed Decision-making

Figure 1 displays each alternative’s causality split between the dam vs. ancillary water system, including the creek. These jointly cause the annualized system pf. The graph also shows that the ancillary water management facilities and the creek are the preponderant failure causality for alternatives 1 to 4. The dam’s pf would drive alternatives 5 and 6.

Mitigative alternatives system annualized
Figure 1 Causality of dam vs ancillary water system evaluation for different mitigations

Figure 2 (top) shows the system pf’s cumulated decrease as a function of the mitigation alternative. Note the minimal estimated advantage in pursuing mitigation beyond a certain stage, unless it is mandatory for jurisdictional compliance. Remember, Figure 2 (top) shows the changes in terms of relative pf, and not in terms of risk. Once we include the risks and the costs of each foreseen mitigative step, we can activate a true cost-benefit analysis (Figure 2 (bottom)).

The crossing of the two curves, i.e. risk and mitigation cost vs. mitigation level, represents the “theoretical optimum.” The GISTM conformance documents (Figure 9 in that document) indicates that point as the ALARP point. However, we would rather call it the lower bound of the ALARP range. In this specific example, the GISTM ALARP appears with a mitigation level between Stage 1 and 2, so one would select mitigation Stage 2 out of prudence. Jurisdictional compliance requirements (e.g. seismic compliance) and public perception considerations may alter that selection.

System pf cumulated decrease
Mitigative costs vs. annualized risk M$
Figure 2 (top) Pf cumulated decrease (bottom) Risk and mitigation cost vs. mitigation level

Perception discussion

The discussion boils down to: “is selecting mitigation stage 2 enough from a pure risk-engineering point of view, if we do not consider jurisdictional issues?” The perception of some stakeholders could easily be that the dam risks should be mitigated “anyways” to stage 3 or 4 or higher despite the high CAPEX of stages 1, 3 and 5 (Figure 3 (top)). In this example, the mitigation costs follow a steeper than linear evolution across the mitigative stages (Figure 3 top).  

Figure 3 (bottom) shows that after stage 2 the mitigation efficiency strongly decreases. The owner and EoR could easily argue at this point that if there are no specific regulatory issues to be complied with, any mitigation beyond stage 3 would be too inefficient. That also corresponds to the reaching of the horizontal asymptote of the system pf (Figure 2). Indeed, as stated above, if stage 2 complies with the GISTM minimum ALARP criteria, it is also true that the horizontal asymptote of the system pf is only reached at stage 3.

ORE2_Tailings offers solid ground for discussion

Thus, our deployment would offer a solid ground of discussion and negotiation (with the public and regulators, and, if applicable, with insurers). Indeed, one could state that stage 3 represents a possible choice for risk mitigation level. However, going beyond stage 2 requires, based on the costs we have assumed for this example, a sharp investment increase for what seems a modest risk mitigation gain. Note that in some cases, this reasoning may lead to different sequences (as possible and feasible) of the stages to seek a better CAPEX allotment.

Ultimately, this deployment example, using assumed initial knowledge and mitigation costs, shows that our quantitative approach can be used in full conformance with GISTM to foster healthy and rational discussions between the stakeholders. The deployment can give useful indications to select and optimize, if feasible from a constructional point of view, the phasing of the various mitigation stages. It can foster healthy technical and perception-based discussion on the level of risk mitigation that should be attained, based on:

  • GIST ALARP concept,
  • the legal negligence test and finally
  • the mitigative stages “efficiency”.

The first stage in any mitigation approach will always be to build a solid knowledge based on the considered system. Building the knowledge base will reduce uncertainties and hence allows professionals to approach the “base case” with more confidence.

Step by step mitigation costs increase M$
Risk decrease M$/yr mitigative CAPEX
Figure 3 (top) Stepped mitigation increase. (bottom) Efficiency of the mitigation stages: Ratio between the annualized risk decrease and the mitigative CAPEX at each mitigation stage

Closing remarks on Risk Informed Decision Making (RIDM) supports rational and sensible risk mitigation

We showed that one can use our ORE2_Tailings™ approach in full conformance with GISTM. The process gives RIDM indications to optimize, if feasible from a constructional point of view, the phasing of the various mitigation stages and alternatives. It can foster healthy and rational internal and external communication on the:

1.    RIDM level of risk mitigation, conforming with GIST minimum ALARP concept and beyond,

2.    legal negligence test and finally the

3.    mitigative alternative risk informed “efficiency”.