The National Seismic Hazard Assessment 2018 (NSHA 18) project intends to revise the existing seismic hazard map (AS1170.4 2007) for Australia. Geoscience Australia (GA) are leading the project along with a consortium of seismologists, geologists and earthquake engineers.
The NSHA 18, due to be released in 2018 is of great importance to dam owners and operators. The project intends to incorporate a comprehensive approach to seismic hazard, particularly in modelling uncertainty and variability.
The Global Earthquake Model (GEM) is an international consortium of scientists, engineers and policy makers. One of the primary aims of GEM is to provide a uniform set of tools for analysis in seismic hazard and risk. GEM was established to provide a framework for global standards in comparing risk analysis, awareness and actions in an effort to increase resilience to vulnerable communities.
The NSHA 18 will use the GEM framework in order to meet its own objectives for the new upcoming hazard map. The Seismology Research Centre will contribute to the NSHA 18 in three areas. Firstly, to produce a unified earthquake catalogue where GA will homogenise magnitudes to a uniform scale. Secondly, to produce a number of applicable alternate seismotectonic models, and thirdly, through the contribution of ground motion data collected over the last forty years within Australia.
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Kate Brand, Matthew Ind
Failure impact assessments of tailings dams are largely pre-determined by the input assumptions and, due to lack of supporting data, the results can be highly subjective. Despite numerous guidelines available for undertaking failure impact assessments of water dams, there are very few technical guidelines on how to form the above assumptions and how to undertake dam breach modelling of a tailings storage facility (TSF).
Tailings dam failure databases are limited, with the available information generally not analogous with the TSF under assessment, especially given the rising volume and height of modern tailings dams. ‘Rule of thumb’ methods are often referred to, with a percentage of tailings and water assumed to be discharged along with assumptions of the breach height and width made.
Using a case study, this paper compares a range of potential failure impact assessments generated using typical methods of analysis and runout modelling to demonstrate the reliance on engineering judgement in failure impact assessments. Given the subjectivity observed within the results, consideration should to be given to the level of reliance on tailings dam failure impact assessments in formulating emergency action plans. It is recommended that regulators take an active role in formulating tailings dam impact assessment guidelines.
The key differences between probabilistic seismic hazard analysis (PSHA) and deterministic seismic hazard analysis (DSHA, preferably referred to as a scenario-based analysis) are that, unlike DSHA, PSHA takes account of all magnitudes on all earthquake sources that may affect the site, including the frequency of occurrence of each earthquake scenario that is considered, and fully considers the random variability (epsilon) in ground motion level. The result of a DSHA is the ground motion at the site resulting from a single earthquake scenario (or a few scenarios) having a preselected value of epsilon (usually 0 or 1), and the annual frequency of exceedance (or return period) of this ground motion level is undefined. In contrast, the hazard curve produced by PSHA yields the mean annual rates of exceedance (or return period) for each ground motion level. The complementary nature of PSHA and DSHA is manifested in the fact that practical application of PSHA, especially using ground motion time histories, results in scenario earthquakes that resemble the products of DSHA. Application of the period dependence of epsilon using the conditional mean spectrum (CMS) avoids the inaccurate and overconservative representation of the hazard by the uniform hazard spectrum (UHS) obtained in PSHA.
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.
This paper describes the unique characteristics of near-fault ground motions for use in developing ground motions for the design and evaluation of dams that are located close to identified active faults. These characteristics include near-fault rupture directivity effects, permanent ground displacements, and hanging wall effects. In Australia, active faults make a significant contribution to the Maximum Credible Earthquake (MCE) only at near-fault sites when Probabilistic Seismic Hazard Analysis (PSHA) is used. However, some sites may be close enough to nearby or even more distant identified active faults that a Deterministic Seismic Hazard Analysis (DSHA) produces MCE ground motions that are for larger than those obtained using a probabilistic approach even for very long return periods. Knowledge of the unique characteristics of near-fault ground motions should be applied to the development of ground motions for the design and evaluation of dams that are located close to identified active faults.
Peter Foster, Bob Wark, David Ryan, John Richardson
Fairbairn Dam is a zoned embankment dam completed in 1972 and located in central Queensland near the town of Emerald. The spillway, which is located toward the left abutment, consists of a 168 metres wide concrete ogee crest, converging concrete chute and dissipater basin. The overall length from the ogee to the downstream end of the concrete spillway is approximately 195 m. The chute and dissipater basin are underlain by a matrix of longitudinal and transverse drains for pressure relief of the anchored concrete slabs.
Minor repairs to damaged chute slabs were undertaken following the 2011 flood event. During these rectification works, large voids up to 0.3 metre in depth were found under sections of the concrete chute slabs as well as damage and blockage to the sub-surface drainage system. Discoloured water was also observed discharging from sections of the sub-surface drainage system. Some of the 24 mm diameter bars designed to anchor the slabs to the foundation were found to have corroded at the concrete/foundation interface and subsequent pull-out tests showed that the anchors had minimal or no structural capacity.
These investigations led to a review of the hydraulic design of the spillway, upgrade to the sub-surface drainage system and apron slabs, and installation of replacement anchor bars. An understanding of the transmission of pressures and dynamic pressure coefficients resulting from spillway discharge and the effects of the hydraulic jump was an essential component of the design for the new anchor and drainage system.
This paper provides detail on the investigations undertaken, the hydraulic modelling that is underway including physical hydraulic and computational fluid dynamics (CFD) and the design approach for what is described in this paper as the Stage 1 component of works.