2021 – Holistic tailings management and governance
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.
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2021 Papers
2021 – Some considerations when using deformation monitoring to manage the static liquefaction risks of TSFs
Learn moreDavid Reid, Andy Fourie, Riccardo Fanni, Cristina Vulpe, Alexandra Halliday
Recent failures of a number of tailings storage facilities (TSFs) has highlighted the need for better
governance and operational management of these structures. One means to improve their safety is clearly better and more focussed monitoring. Significant efforts are underway in this area, with a number of technologies being deployed. In particular, the monitoring of deformations through a variety of means (direct, satellite inferred) is increasingly being applied. While deformation monitoring to warn against failure has a long history in geotechnical engineering, some aspects of the rapid triggering and resulting flow of some TSFs may not be amenable to deformation monitoring, in the sense that actionable warning of an impending failure is not assured.To examine this issue, a series of numerical models of an idealised TSF are carried out. This idealised TSF is brought to failure by means of a rising phreatic surface – often referred to as the constant shear drained (CSD) stress path. Deformations of the outer slope and crest of the numerical model – i.e. those that could be monitored for a real TSF – are tracked and analyses for the models carried out. It is seen that under CSD loading distinct deformation patterns indicative of impending failure are not always clear. Rather, minimal deformations and indeed swelling of the crest is seen leading to failure. The importance of recognising the minimal pre-failure deformation patterns that may manifest with a rising phreatic surface is noted.
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2021 Papers
2021 – Accounting for brittleness in tailings storage facilities
Learn moreJiri 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.
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2021 Papers
2021 – Paradise Dam and its present and future impact on dam projects
Learn moreChris Nielsen, Ron Guppy, Donna Dunn, David Murray
Following several years of investigations and analysis a serious safety issue with the stability of the primary spillway during major flood events was identified at Paradise Dam that required urgent risk reduction works. The response to this safety issue was significant.
The Inspector General Emergency Management conducted a review into the effectiveness of emergency response if a dam safety event were to occur, taking into consideration process and communications to manage around 40,000 population at risk, comprised mostly of residents within the city of Bundaberg.
An essential works program to reduce the risk was urgently prepared then executed effectively within a calendar year. This short timeframe required significant and novel amendments to Queensland’s laws to bypass normal legislated process for such a major project.
The Paradise Dam Commission of Inquiry was established to identify the root cause of the issues, the facts and circumstances that contributed to them and recommendations to consider for future dam projects. All recommendations from the commission were accepted by the Queensland government and, following an extensive stakeholder engagement exercise, have been implemented through changes in policy and methodology and described in published guideline revisions.
For future dam projects the lessons learnt highlighted the need for early and ongoing engagement of
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independent technical review, project governance that is cognisant of risk and the ownership and capacity to bear of that risk, the need to consider testing to confirm critical design parameters and the need for an effective regulator. The essential works program has established a precedent for the timely and appropriate application of risk reduction measures. -
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2021 Papers
2021 – Importance of adequate characterisation of fissured clay in dam foundations, a case study
Learn moreYuqi Tan, Behrooz Ghahreman-Nejad, Keith Seddon
Inadequate geotechnical investigation and hence undetected issues within the dam foundation have been responsible for many dam failures in the past. Fissured clay in the foundation poses a significant risk to the stability of the dam if it is not adequately detected and characterised. This paper presents a framework to evaluate the strength characteristics of fissured clay and its effect on the stability and performance of an embankment dam. The strength of fissured clay can be characterised from conventional triaxial test result based on the dip angle of the fissure plane. A design chart for the strength of the fissure has been developed based on the dip angle. The stability assessment for a tailings dam indicated that the dip angle of the fissure has significant impact on the overall stability of the embankment when the angle of the fissure aligns with the angle of the critical failure plane. Both fissure strength and fissure angle should be carefully evaluated for a site where fissured clay is observed.
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2021 Papers
2021 – Risk analysis and safety upgrade of Mangrove Creek Dam; SFAIRP assessment under Dam Safety NSW regulations
Learn moreVicent Espert, Peter Buchanan, Colleen Baker, Malcolm Barker, Mark Locke
Mangrove Creek dam is an 80 m high CFRD constructed between 1976 and 1982 for water supply to the NSW Central Coast area, and is currently operated by Central Coast Council (CCC). The dam is classified as a ‘High A’ Consequence Category dam for both Sunny Day and Flood breach in accordance with ANCOLD guidelines.Previous assessments of the dam identified that it would not be able to safely pass the ANCOLD Fallback flood capacity of the PMP flood in its current configuration. As such, the dam has been operated at a restricted full supply level for many years.
In 2020, GHD was engaged by CCC to develop a concept and detailed design to increase the spillway capacity using a standards-based approach to achieve the flood capacity fallback position. The first phases of this contract also required GHD to undertake additional investigations and analyses of various aspects of the dam and spillway to confirm the scope of works for the upgrade. During this review, it became evident that although the spillway capacity does not meet the ANCOLD fallback position, the Annual Exceedance Probability (AEP) of the existing capacity was relatively low and could potentially be deemed acceptable from a risk-based position.
A Risk Assessment was subsequently undertaken, with a SFAIRP assessment developed based on the new Dam Safety NSW guidelines. This assessment may be the first one to be completed for a major dam using the Dam Safety NSW guidelines. This paper discusses the different outcomes for a standards-based ‘Fallback’/’Simplified’ criteria and risk criteria based on DS NSW regulations, as well as the investigations developed to maintain confidence in the assessment. In addition, it describes a practical case for the application of SFAIRP criteria to a major dam.
In the case of Mangrove Creek Dam, the application of the new DS NSW Guidelines resulted in the dam being assessed as acceptable in its current state, with the FSL returned to the original design level. The outcome provided significant savings to the client, by avoiding costly upgrade works and avoiding disruption to the operation of the storage – a real success story.
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