Matthew Ind, Kate Brand and Mark Ferrier
The framework for undertaking a dam breach analysis for water dams is reasonably well established with a depth of information and software available to guide practitioners on a consistent approach to undertaking failure impact assessments. In contrast, dam breach modelling for tailings dams is currently a developing field with a wide range of modelling approaches taken and an inconsistency in the quality of the failure impact assessments undertaken. Recent tailings dam failures at the Mt Polley Mine in British Columbia, Canada and the Fundäo and Santarém dams at the Samarco iron ore operation in Minas Gerais, Brazil have provided a sobering reminder of the hazards presented by tailings dams and the clean-up challenges that are significantly more complex than a similar failure of a water dam.
Current guidelines and approaches to dam breach modelling are often done assuming the run-out material from the breach is just water without due consideration of the impact from tailings loss. There is limited analysis undertaken on credible failure modes of tailings dams with an assumption that the embankment just “breaks” at some random point without appreciation of the failure mechanism. The misunderstanding of failure modes leads onto inconsistencies with application on whether a ‘sunny-day’ or extreme flood event modelling should be applied, with one or the other selected without explanation.
This paper outlines a framework that can be applied when undertaking a dam breach study for tailings dams to enable a consistent and credible assessment of potential failure impacts. The following tasks are discussed in detail in support of this framework:
— OR —
Now showing 1-12 of 46 2980:
Kristen Sih, Richard Rodd
Melbourne Water currently manages over 235 stormwater retarding basins. The process of assessing the risk posed by these assets began in 2006, and at the end of 2015 full risk assessments were completed for around 30 of the basins that were estimated to pose the highest societal risk. However, when analysing the results of these risk assessments, there was some concern that the results were inconsistent and often too conservative, given the few incipient or actual failures that had been experienced.
It was found that one of the key areas causing the conservatism was poor documentation of design and construction details, and the fact that the tools used for assessing the Potential Loss of Life (PLL) were aimed at larger storages that cause much higher depths and velocities in dambreak events than these (generally) small storages. To remedy this situation, advice was sought from specialist practitioners to develop guidance notes on the assessment of PLL and failure likelihoods for retarding basins.
On the back of these guidance notes, Melbourne Water initiated an accelerated program of assessing the risk associated with 78 retarding basins over a 6 month period. This paper describes the key recommendations from the guidance notes, compares the results of the risk assessments performed pre- and post-guidance notes and provides a summary of the portfolio risk assessment outcomes, what they mean for Melbourne Water and what the organisation intends to do to manage this risk into the future.
Russell Mills PhD,Rebecca Freeman, Malcolm Barker
The global mining industry lives with the risk of catastrophic events such as water storage or tailings dam failures as part of its daily operations, and has developed a number of approaches to enable mine management to understand the nature of the risks and the ways in which they are being managed. One such approach involves the use of bowties for the understanding of the hazards and risks. Building from bowties, the second approach involves the selection and management of controls critical to the prevention or mitigation of the catastrophic event. The Australian mining industry is a world leader in this regard and the purpose of this paper is to illustrate how bowties are constructed, how risks can be semi-quantitatively estimated, how critical controls are selected and managed, and how, if all this is done well, risks can be demonstrated to be as low as reasonably practicable (ALARP).
This paper sets out key themes and presents an example for a tailings dam failure to illustrate the role of bowties and critical controls in management of catastrophic events. It will also highlight the role of bowties in the anticipated introduction of a Safety Case approach to dam risk management. Bowties provide a useful tool for the transfer of risk management knowledge from the designer, to allow dam owner / operators to better understand their risks and to recognise the link between design and operational controls and how they are used to manage those risks to ALARP.
Jamie Cowan, Chris Kelly and Gavan Hunter
Dam safety upgrade works were undertaken at Tullaroop Dam in 2015/16 to reduce the risk of piping through the main embankment. Unexpected cracking and elevated pore water pressures were observed within this earthfill embankment over a period of 10 to 15 years. In 2005/06 a filter and rockfill buttress local to the embankment was constructed on the left abutment after a 60 mm wide diagonal crack opened up on the downstream shoulder from crest to toe.
Similar to the 2005/06 upgrade works, the 2015/16 embankment works were direct managed by Goulburn-Murray Water. Filter and rockfill materials were sourced from commercial quarries previously used for dam upgrade projects and for which significant testing of materials had been undertaken, especially on the fine filter.
Mid project it became clear that the fine filter was breaking down under handling and compaction such that several in-bank gradings fell outside the specified fine limit. Further testing of quarry surge piles, site stockpiles and in-bank placed filters was undertaken to understand the extent of the breakdown. It was assessed that the breakdown was occurring on the 0.5 to 2.0 mm fraction, generating finer sizes in the 0.1 to 0.6 mm fraction. The increase in fines content (minus 75 micron) was less than 1% and met specification. The in-bank material was accepted as placed and the specified filter envelope adjusted to allow for the observed breakdown.
Difficulties were experienced with compaction of the fine filter in the inclined chimney filter to achieve the target density in the range 65% to 80% Density Index when the layer width reduced to 0.75 m for a 0.5 m compacted lift thickness. No difficulties were experienced when the layer width was 1.5 m or in trenches. Further trials were undertaken on the embankment to better understand the compaction issues and used different roller types. It was assessed that an important factor was the arching effect of the adjacent coarse filter. Going forward thinner lifts were used and smaller width rollers to achieve the specified minimum density.
The paper provides details on the embankment construction works, focusing on the fine filter breakdown and compaction issues. Details of the testing undertaken, the actions to resolve the issues and interactions with the supply quarry and construction team are provided.
Robert Shelton, Jako Abrie, Matt Wansbone
The Mahinerangi dam – arguably the most valuable in Trustpower’s portfolio of 47 large dams – is over 80 years old and needs a plan of work to confirm it meets current design standards.
The dam was completed in 1931, subsequently raised in 1944-1946, and strengthened with steel tendon anchors in 1961.
A comprehensive safety review (CSR) in 2007 noted a potential deficiency in the fully grouted anchors and a program of work commenced to re-evaluate the overall stability of the dam.
A potential failure mode assessment revealed that the dam may need upgrading to meet the criteria for maximum design earthquake (MDE). Areas of uncertainty were identified and a significant programme of survey, geological mapping, concrete testing and site specific seismic assessments have been carried out to reduce risk and uncertainty in design.
The paper discusses the dam’s history, current condition, and describes the ongoing programme of work planned to extend the life of the dam for another 80+ years.
Asset management and particularly dam safety management at Snowy Hydro is a continually evolving process and at the heart of the program is our desire maintain the legacy of the Snowy dams and to do everything required to meet our obligations and duty of care. There is a significant shift underway from a schedule-based maintenance to a condition-based maintenance plan. The advantage of this is that the right maintenance is delivered at the right time and resources can be efficiently allocated to the right maintenance.
Condition based maintenance is not driven by a desire to cut maintenance or surveillance spend. A key part to this change is determining the condition of an asset or dam. To achieve this, reliability centred maintenance principles have been applied to dam structures through the use of failure mode effects and critically assessment (FMECA) tool, this differs to a traditional failure mode assessment, as it looks at functional failure of individual structures or equipment rather than partial or catastrophic failure of the dam. The outcome of a FMECA is a detailed maintenance and inspection plan that targets the individual functional failure modes.
One of the outputs of this process is a condition assessment methodology which is used as a trigger for corrective and preventative maintenance activities and a tool for the justification of installing performance measuring instrumentation. Condition assessment is therefore the process where asset performance data is assessed against specific criteria to determine its present state. Currently, comparing condition and performance of multiple dams is reliant on a practitioner’s experience and subjective assessment to determine whether an asset’s condition is fit for service. The condition assessment process; reduces subjective data, provides real-time health assessment, highlights performance issues, continuously identifies and updates priorities and provides justification for capital investment.