T. I. Mote, M.L. So, N. Vitharana, and M. Taylor
This paper explores the sensitivity of selection of earthquake design magnitude to liquefaction triggering in Australia for ground motions typically used for dams. The low seismicity of Australia creates a situation where liquefaction triggering is marginal at design hazard levels and this low level of seismic hazard makes the liquefaction trigger analysis very sensitive to the derivation of the seismic inputs. A methodology is presented that couples the probability of liquefaction triggering with the distribution of earthquake contribution to the hazard from the magnitude-distance deaggregation. The results show that for the “typical” soil profile and input ground motions approximately equivalent to the maximum design earthquake for Australia, the probability of liquefaction triggering varies significantly with the design magnitude selected. Using the maximum credible earthquake or mean magnitude may provide significantly different liquefaction triggering implications. Combining the probability of liquefaction triggering with the contribution of varying magnitudes to calculate liquefaction probability is a useful method to understanding the sensitivity of liquefaction to design magnitude.
Keywords: Liquefaction Assessment, Design Magnitude, Probability of Liquefaction, Magnitude-distance deaggregation, Australia
Simon Lang, Chriselyn Meneses, Kelly Maslin, Mark Arnold
It is now common practice for dam owners in Australia to take a risk based approach to managing the safety of their large dams. Some dam owners are also using risk based approaches to manage other significant assets. For example, Melbourne Water manage the safety of their retarding basins in a manner similar to their water supply dams.
Assessing the risks posed by retarding basins using methods developed for larger dams can raise challenges. For example, the Graham (1999) approach to estimating potential loss of life (PLL) is generally applied when estimating the consequences of dam failure. However, Graham (1999) may not be the most suitable model for estimating PLL downstream of structures with relatively low heights and storage volumes (e.g. retarding basins), given the characteristics of the case histories used to develop the method.
In this paper six potential methods for estimating PLL are tested on four retarding basins in Melbourne. The methods are Graham (1999), the new Reclamation Consequence Estimating Methodology (RCEM), the UK risk assessment for reservoir safety (RARS) method, a spreadsheet application of HEC-FIA 3.0, and empirical methods developed by Jonkman (2007) and Jonkman et al. (2009). Results from the methods are compared, and comment is made about which is most suitable.
Keywords: potential loss of life, dam safety, risk analysis, retarding basins.
A. Scuero, G. Vaschetti, J. Cowland, B. Cai , L. Xuan
Nam Ou VI rockfill dam is part of the Nam Ou VI Hydropower Project under construction in Laos. The scheme includes an 88 metres high rockfill dam, designed as a Geomembrane Face Rockfill Dam (GFRD), which when completed will be the highest GFRD in Laos. The only element providing watertightness to the dam is an exposed composite PVC geomembrane, installed according to an innovative design now being increasingly adopted to construct safe rockfill dams at lower costs. The same system will shortly be installed on a water retaining embankment for a coal mine in NSW, Australia, and has been approved for a tailings dam in Queensland, Australia. At Nam Ou VI the geomembrane system is being installed in three separate stages, following construction of the dam. The first two stages have been completed, and the last stage will start in November 2015. The paper, after a brief discussion of the adopted system’s concept, advantages and precedents, focuses on the construction aspects.
Keywords: GFRD, PVC geomembrane, waterproofing, rockfill dam.
Paul Southcott, Tim Griggs & Jamie Campbell
Suma Park Dam is the principal water supply dam for the City of Orange in central NSW. The 30m high single curvature concrete arch dam has a High A consequence category and required an upgrade due to an inadequate spillway capacity. To maximise the benefits of this major capital works, the Council also sought to increase the storage capacity and modernise the outlet works to help supply the rapidly growing population of the city.
Challenges that needed to be overcome to develop an affordable and safe solution included: very high flood inflows; limited freeboard; a highly stressed arch with a narrow crest width; poor access to the toe and right abutment; and a saddle dam located on a deeply weathered foundation.
Innovations incorporated into the design of the works included: Monte-Carlo based modelling of the flood hydrology that better estimated the design inflows resulting in a significant reduction in flood upgrade requirements; precast parapet crest units that incorporated crest widening to improve constructability; an anchored toe block to ensure the toe of the arch is stable; an upgrade to the stilling basin; and an auxiliary spillway incorporating Fusegates at the saddle location designed only to operate in floods in excess of the 1;1,000 AEP event with minimal loss of storage.
Construction of the works is now well underway. A number of challenges have been overcome during the construction stage including a re-design of the auxiliary spillway to use Fusegates and discovery of Naturally Occurring Asbestos (NOA) on site. Construction of the upgrade works is expected to be completed by the end of 2015.
Keywords: Concrete arch dam, flood upgrade, pre-cast, fuse gates, anchoring.
Makeena Kiugu, Siraj Perera
Dam owners are influenced by drivers such as ensuring economic efficiency, achieving industry good practice, and meeting regulatory or due diligence obligations when making decisions on how to manage their dams. While these drivers can be inter-related, the decisions finally made by dam owners are reflected in planned and completed dam safety activities.
In Victoria, dam owners update the regulator on the status of their dam safety management programs every year. Victorian dam safety regulation is underpinned by risk management principles. Benchmarking of dam safety management practices is also promoted within the industry. The information provided to the regulator includes risk levels of dams, scheduled upgrades and associated cost estimates, interim risk reduction measures, and details of surveillance, emergency management and operation and maintenance programs. A considerable amount of information has been collected over the past few years allowing trends in dam safety management activities to be examined at a State-wide level.
This paper will consider how dam safety management decisions, and the drivers behind those decisions, are reflected in the dam safety practices of Victorian dam owners. Trends in dam safety activities will be observed and linkages made to prevailing industry-wide challenges.
Dam owners are increasingly being required to address a wider range of issues in an environment of limited resources. Ensuring due diligence and improving emergency preparedness are some current challenges facing dam owners. This paper also examines how these emerging drivers may influence dam safety activities into the future.
Keywords: Dam safety management
David Brett, Robert Longey, Jiri Herza
The independent expert review panel for the Mount Polley Tailings Storage Facility failure came out strongly recommending changes to the technology of tailings dams in British Columbia (and by inference, world-wide). The Panel had examined the historical risk profile of tailings dams in British Columbia and recommended, amongst other things, that best available technology (BAT) be adopted for tailings disposal. Examples of BAT, described by the panel, included “dry-stacking” of filtered, unsaturated, compacted tailings and reduction in the use of water covers in a closure setting. The recommended technologies would require a major shift in current practice and raises many questions, such as:
– Are these recommendations appropriate in Australia?
– Does this signal the end of the tailings dams as we know them?
– Do the current Australian National Committee on Large Dams Guidelines (ANCOLD) apply to these new BAT technologies?
– If not, is there a role for ANCOLD in setting standards for the future?
This paper discusses the Mt Polley tailings dam failure and searches for answers to these questions. In particular, this paper reviews the background to “dry-stacking’, to explore the implications for the Australian mining industry.
Keywords: Tailings Dam, Dry Stacking, Best Available Technology