Chris Nielsen, Irene Buckman
As individuals, we are concerned about how a risk affects us and the things we value
personally. We may be willing to live with a risk if it secures us certain benefits and if the
risk is kept low and clearly controlled. We are less tolerant of risks over which we have little
ANCOLD’s risk assessment guideline (2003) identifies an individual risk threshold as being
one where “the dam safety risk to an individual should be close to the average background
risk of the population”. This is a principle of equity, where “all individuals have
unconditional rights to certain levels of protection” (HSE, 2001). The definition of
population at risk applied to Queensland’s referable dams (DNRME, 2018), being
individuals within a residence or workplace and typically not participating in any risky
activities such as driving a vehicle or walking through flooded waters, provides further
justification of this right.
In practice addressing societal risk tolerances and duty of care considerations may result in
individual risks being substantially lower than the thresholds. This may not always be the
case and, irrespective, should not distort the purpose of the individual risk tolerance test;
the principle of equity that drives individual risk tolerability has foundations in our societal
values and is easily and widely understood as a core value. This should be succinctly
described when justifying expenditure on risky infrastructure such as dams.
This poster describes aspects to consider when selecting a threshold individual risk
tolerance. Subject to site-specific considerations of the particular age group of individuals
most at risk, the wider benefit of the dam to society and ALARP, a single threshold
individual risk tolerance of less than 10-5 per annum (or 1 in 100,000 years) would appear
The aspects described are elaborated in the revised Guidelines on Safety Standards for
Referable Dams, soon to be published on the Queensland Government website (RDMW,
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Damien Bryan, John Sukkar, Erin Hughes, Michael Cawood
Alert triggers are a critical component of Dam Safety Emergency Management, aligning clearly defined adverse conditions with alert levels to initiate an appropriate emergency response. Early detection of these conditions allows for potential mitigation measures to be undertaken, early engagement of key stakeholders such as emergency agencies, and where necessary, the warning or evacuation of affected downstream communities. The Dam Safety Alert Trigger Framework provides WaterNSW with a consistent, repeatable, and defensible methodology for the determination of appropriate dam safety alert triggers. The framework was developed through the engagement of consultants, emergency and regulatory agencies (NSW SES & DSNSW), and several Australian large dam owners.
The determination of appropriate Dam Safety Alert Triggers is a challenge faced by all dam owners. Through the development and implementation of the Alert Trigger Framework, WaterNSW has achieved the ability to define defensible alert triggers through a consistent and repeatable methodology. This has resulted in an improved dam safety emergency response posture for WaterNSW, key emergency services partner the NSW SES, and greater protection for affected downstream communities. Concepts, processes and methodology covered in this paper could be used by other dam owners in addressing their own dam safety alert trigger challenges.
Jarrad Coffey and John Plunkett
As tailings standards continue to evolve, a greater focus is being placed on the monitoring of tailings storage facilities (TSFs). While this is a positive development for TSF safety into the future, it is only one component of the work required to implement Performance Based Risk Informed (PBRI) management. There is also a significant human element that can be aided by reducing the time spent of personnel sourcing/aggregating data and instead focussing on decision making. It is discussed in this paper how a more holistic approach to monitoring via a dashboard that displays all management data relevant to a portfolio of TSFs can be applied in parallel to risk assessment to work towards the goal of PBRI. The dashboard also facilitates review and governance activities, which are central to the Global Industry Standard on Tailings Management. An example of the dashboard utilised at Rio Tinto Iron Ore is presented to provide an example of such a system and its benefits.
Claudia Smith, Shannon Dooland, Adam Broit, Rachel Jensen, Samantha Watt
The estimation of real consequences from dam failure that directly link to the overall likelihood of the failure is a challenging task, particularly in data sparse locations. Previous regional methods have often relied on simplistic assumptions without consideration of the true joint probability of the volume of flow in the downstream tributaries of concurrent catchments. As a result, concurrent downstream flooding directly impacting the consequence in dam break assessment scenarios may be misrepresented. More recently, the adoption of streamflow-based joint probability has become the standard, particularly where consequence estimation is used within the context of risk assessment. This paper progresses the work completed by others to establish a practical treatment method based on rainfall analysis where suitable streamflow information is unavailable. A case study is also presented where this method has improved the understanding of the risk profile associated with a coastal storage based on a better estimate of the likely flood concurrence within the storage and downstream catchments.
Dan Clark, Joanne Stephenson, Trevor Allen
We present earthquake ground motions based upon a paleoseismically-validated characteristic earthquake scenario for the ~ 48 km-long Avonmore scarp, which overlies the Meadow Valley Fault, east of Bendigo, Victoria. The results from the moment magnitude MW 7.1 scenario earthquake indicate that ground motions are sufficient to be of concern to nearby mining and water infrastructure. Specifically, the estimated median peak ground acceleration (PGA) exceeds 0.5 g to more than ~ 10 km from the source fault, and a 0.09 g PGA liquefaction threshold is exceeded out to approximately 50-70 kilometres. Liquefaction of susceptible materials, such as mine tailings, may occur to much greater distances. Our study underscores the importance of identifying and characterising potentially active faults in proximity to high failure-consequence dams, including mine tailings dams, particularly in light of the requirement to manage tailing dams for a prolonged period after mine closure.
Jiri Herza, Kyle Smith, Ryan Singh
Following the failures of Samarco and Feijão dams, brittle failure has become a frequently discussed topic within the geotechnical community. The post-failure review of the Feijão Dam identified that the sudden failure of the dam was caused in part by tailings exhibiting brittle behaviour. Brittle failure has also been identified to be a contributing factor in many previous tailings storage facilities failures. Of concern to the tailings community was the finding that there were no apparent signs of distress prior to the failures, which characterises brittle failure.
The industry’s concern regarding the presence of brittle materials within tailings storage facilities, particularly when featuring upstream raises is evident in the requirements of the newly published Global Industry Standard on Tailings Management, which includes a requirement to “Identify and address brittle failure modes with conservative design criteria…”. This is also reflected in ANCOLD Guideline on Tailings Dams, which provides recommendations for conservative design assumptions if materials are found to be susceptible to static liquefaction which is noted to be a brittle subset of contractive materials. The ICMM’s Good Practice Guide for tailings management uses the term
brittle on numerous occasions and even refers to “credible brittle failure modes” when discussing the performance based approach. Despite its frequent use, the term brittle failure has not been defined in any of the listed references and the authors of this paper are not aware of the any clear geotechnical definition for brittle embankment failure in literature.
Brittleness, on the other hand, is a well-known geotechnical parameter that describes the degree of reduction of the soil shear resistance after reaching the peak strength. Bishop (1967) described the soil brittleness in the context of progressive failure of clays by means of a brittleness index, which is the ratio of the shear resistance loss to the peak shear strength. In recent years, the brittleness index has become a common soil parameter that is used as an indicator for tailings susceptibility to liquefaction. The brittleness index does not consider the rate at which the soil resistance reduces, and it ignores the stress strain relationship. As a result, the same brittleness index can be calculated for a soil that collapses over a very small strain range and a soil that gradually reduces its shear resistance over extensive strain levels as long as both soils have similar peak and residual shear strengths.
This paper discusses the root causes of brittle behaviour of tailings, summarises the current approach for brittleness assessment and recommends considerations and methods to assess and deal with potentially brittle soils within TSFs.