Chris Topham, Andrew Pattle, David Tanner, Oliver Giudici
Many owners around the world have dams that rely on grouted, post-tensioned rock anchors for stability. The anchors were installed during the original construction of the dams or retrofitted to improve stability during their operational life. The use of fully grouted post-tensioned anchors spanned the period of the 1960’s to 1980’s. The main issue with these un-sheathed grouted rock anchors is the question of integrity of the grout column protecting the anchor and concerns about possible corrosion of the high tensile wires from which the cables are constructed. While some of these anchors have corrosion monitoring systems installed, it is difficult to validate such data and there is considerable uncertainty over the condition of such anchors. To compound the problem, replacement of the anchors is technically complex, extremely costly and difficult to justify in the absence of known condition. For example, Hydro Tasmania has recent experience of work to cease reliance on such anchors at Catagunya Dam that cost $41m in 2009. With fifteen dams relying on some form of post-tensioned anchors, Hydro Tasmania has recently taken the unusual step of over-coring and extracting three post-tensioned rock anchors from operating dams in order to assess their condition. In what is believed to be a world first, a 42m long 70 strand high tensile anchor was overcored and removed from Meadowbank Dam in 2014. A further two anchors were successfully extracted from Repulse Dam in 2015, in conjunction with a group of international sponsors with similar anchors. This paper uses the 2015 work to illustrate the methodology used to extract the anchors, outlines the information gained from this unusual work, and presents the results of the condition of the extracted anchors. The paper concludes with some inferences for other owners with similar anchors and suggestions for further work.
Keywords: Grouted, post-tensioned rock anchor, ground anchor, corrosion, over-coring, extraction, dam safety.
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
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
Steven E Pells, Philip J N Pells, William L. Peirson; Kurt Douglas and Robin Fell
The method of Annandale (1995) is widely used by Australian practitioners for the assessment of erosion in unlined spillways. This method is based on comparison to various case studies, where the geology at each site is characterised using the Kirsten index (a rock mass index previously developed to assess the rippability of rock), and the hydraulic conditions are characterised using the unit stream power dissipation. In this paper, the historical development of this comparative design technique is traced and is critically reviewed against the original geotechnical and hydraulic data, and against a new, independent, dataset gained from unlined spillways in fractured rock in Australia, South Africa and the USA. It is shown that, while erosion can be usefully correlated against rock-mass indices and hydraulic indices, this ‘comparative’ design technique has been promoted beyond its reach – the data do not support the inference of an erosion ‘threshold’ as presented by Annandale (1995). It is argued that this type of analysis should be used only as an initial ‘first indication of erosion potential’, as originally proposed by van Schalkwyk (1994b).
Keywords: scour; erosion; spillways.
Chriselyn Meneses, Simon Lang, Peter Hill, Mark Arnold
Risk is the product of likelihood and consequences. Much effort is put into the risk assessment process for large dams to ensure there is a consistent approach to estimating failure likelihoods across an owner’s portfolio. For example, the use of common peer review teams and methods like the ‘piping toolbox’ allow the risk assessment team to apply repeatable logic and processes when estimating failure likelihoods. However, the methods for estimating life safety consequences are often not applied consistently. This inconsistency leads to estimates of potential loss of life (PLL) that vary between dams in unexpected ways, because results from the most commonly applied method (Graham, 1999) are sensitive to threshold changes in flood severity and dam failure warning time.
The recently released Reclamation Consequence Estimating Methodology (RCEM) is intended to supersede Graham (1999). RCEM varies fatality rates continuously with DV, and is therefore less sensitive to changes in flood severity. In this paper, estimates of PLL from RCEM are compared with results from Graham (1999) for five dams. Results from the latest US Army Corps of Engineers model for estimating the consequences of dam failure (HEC-FIA 3.0) are also compared with RCEM and Graham (1999) for one dam. Comment is then made about the important considerations for applying RCEM consistently across a portfolio of dams.
Keywords: potential loss of life, dam safety, risk analysis
Vicki-Ann Dimas, Wayne Peck, Gary Gibson and Russell Cuthbertson
Globally, reservoir triggered seismicity (RTS) is a phenomenon sometimes observed in newly constructed large dams worldwide, for over 50 years now. Over 95 sites have been identified to have caused RTS by the infilling of water reservoirs upon completion of their constructions worldwide. In Australia, there are seven confirmed sites with observed RTS phenomenon that are summarized by temporal and spatial means.
With almost 40 years of seismic monitoring, primarily within eastern Australia, several of Australia’s largest dams have monitored and recorded many RTS events. At present, twelve dams are 100 metres and above in height as possible candidates, with seven of these actually causing RTS and a disputed possible eighth dam.
Important factors of RTS are reservoir characteristics (depth of the water column and reservoir volume), geological and tectonic features (how active nearby faults are and how close to the next cycle of stress release they are temporally) and ground water pore pressure (decrease in pore volume under compaction of weight of reservoir and diffusion of reservoir water through porous rock beneath). RTS is an adjustment process often delayed for several years after infilling of reservoir before eventually subsiding within 10 to 30 years, when seismic activity then returns to its prior state of stress.
Generally there are two type of RTS events, either a major fault near the reservoir most likely leading to an earthquake exceeding magnitude 5.0 to 6.0, or more commonly, a series of small shallow earthquakes.
Seismic monitoring of all dams (except for Ord River) are presented with spatial and temporal series of maps and cross sections, showing the largest earthquake, build-up and decay of RTS events.
Keywords: Seismic monitoring, reservoir triggered seismicity (RTS), earthquake cycle