Jonathon Reid, Mark Sinclair
Large concrete piers supporting gates on spillways can be impacted by earthquake loading and often found deficient in cross-valley loading. However, piers can also be susceptible to damage from upstream-downstream seismic loading and from additional pressures from increased flood loading requirements. This can have detrimental impacts on the connection of the pier with the main dam body. In recent years, a number of projects have required spillway piers to be upgraded by post-tensioning into the dam body with large multistrand anchors, up to 65 strands, to prevent failure mechanisms (bending, shearing, rocking) from forming and causing serious damage. These, often very short anchors, present special design and construction challenges that are explained in this paper. This situation also often requires the anchors to be very closely spaced with bond zones overlapping. Case examples demonstrating potential limitations in the design of spillway pier anchors and the different requirements to the more usual vertical post-tensioning of a dam body are presented.
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Lindsay Millard, David T Roberts, Steven Cox, Andrew Berghuis, Anna Hams
Addressing historical impacts of waterway barriers on regional fisheries values is a major focus for fisheries regulators when assessing proposed water infrastructure projects such as dam safety improvements. To inform prudent investment decisions, it is essential to quantitatively determine the feasibility and benefits of various fish passage options to mitigate barrier effects. In Queensland, the regulatory frameworks require consideration of multiple options to achieve mitigation with the overarching goal to support and restore regional fish productivity. Addressing multiple objectives on large water infrastructure projects can be challenging, particularly for existing assets requiring retrofit solutions. There is a need to balance the requirements for dam safety, water supply reliability, while also mitigating the loss of fish habitat access upstream of barriers. Finding optimal fish passage solutions requires consideration of multiple options and using objective approaches that can weigh up the many aspects. The best solution may not always be the most obvious. Here we describe an approach that addresses multiple objectives through a novel off-site solution that provides increased benefit to the impacted fish community. Seqwater, Queensland,
The approach involved weighing up various fish passage options, informed by stochastic hydrologic
modelling to produce a range of probabilistic scenarios. 120 years of modelled water levels and discharges from the study site and the broader catchment, enabled an evaluation of the benefits and dis-benefits of different options in relation to dam safety, water supply reliability and fish migration opportunities. Inputs to the assessment process included fish habitat availability and migratory needs, capital and operational feasibility considerations. Numerous modelling scenarios were produced to assess a range of possible solutions, both on and off-site, to provide an objective weighting of the relative strengths of each scenario.
In this instance, while an onsite option could be feasibly engineered, it would be costly and given the
hydrology of the system, would operate so infrequently as to provide limited opportunities for fish passage and minimal regional fisheries productivity benefits. The optimal solution found was to provide fish passage on a higher order stream within the same catchment area that has impacted fish migration and access to upstream habitats for the same fish community. This option improves fish habitat access to a larger proportion of the catchment and over a wide range of flow conditions, thus providing greater regional fisheries productivity outcomes.
Our method demonstrated an objective approach to balancing multiple project objectives for dam
improvements. The use of hydrologic modelling combined with fish migration and habitat information, found an optimal solution for regional fisheries productivity goals, while also balancing the dam safety and water supply reliability goals.
Anna Hams, Lindsay Millard, Elizabeth Jackson, Zara Bostock, Helena Sutherland
The Queensland dam regulator requires that dam safety risk during construction must not increase from its existing profile. The Stage 2A upgrade of Ewen Maddock Dam required excavation of its homogeneous embankment to retrofit chimney and filter blankets, and also the construction of a concrete parapet wall. Due to the constraints of the embankment profile and a constricted site, it was necessary to excavate the downstream face of the embankment. This excavation increased the risk of embankment failure due to overtopping, piping and instability. This paper discusses the measures taken to manage those dam safety risks, and includes:
● use of a temporary system consisting of six large siphons to regulate the lake level to a Restricted Full Supply Level (Restricted FSL). This encompassed the optimisation of lake level and capacity of siphons required to balance competing risks; dam safety, environmental, community and water security. This optimisation was based on a probabilistic assessment of hydrological inflows and lake levels, the development of a flow management plan;
● implementation of a Dam Safety Management Plan which outlined the roles and responsibilities for
managing dam safety during construction at each pre-determined lake level trigger levels. This includes how the contractor was involved to ensure quick response from the “eyes and ears on the ground”; and,
● development of recommended construction methodologies including a “rolling front” and placing
filters vertically to increase production, maintain quality and limit the extent of embankment excavation underway.
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,
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.
Rachel Jensen, Adam Broit, Chriselyn Kavanagh
Downstream emergency response is a critical driver in the consequences and potential life loss associated with dam flooding and failure. This response is highly varied between stakeholders, communities and the nature of the flooding or dam threat. As assessments on dam failure consequence and potential loss of life become increasingly important in understanding holistic dam risk, they are also becoming increasingly complex.
As part of a portfolio wide Comprehensive Risk Assessments, Sunwater have undertaken workshops with a wide range of stakeholders to better understand downstream emergency response and the warning timeline. The workshops have been aimed at facilitating better downstream stakeholder engagement, obtaining key data for consequence assessments and developing consistency in assumptions for potential life loss.
This paper presents the standardised methodology undertaken for warning time workshops, the outcomes for a range of downstream stakeholders and correlations between stakeholder groups which influence warning time response. These outcomes may be used by practitioners in the absence of catchment specific warning time data and provide a counterpoint to international standard warning time assumptions.