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.
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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.
This paper discusses the common environmental issues and requirements project lenders have when financing hydropower dam projects in developing countries. The environmental specialist’s role, as part of the Lender’s Technical Advisor team, is discussed throughout the main phases of project finance (credit approval, financial close, lending/construction and loan repayment/operation). Further, how environmental issues are reviewed and monitored, thereby minimising reputational risks to the lenders are outlined.
Lenders typically consider hydropower dam financing, especially reservoir schemes, as high reputational risk loans. Finance is usually syndicated and although most international lenders are Equator Principles signatories or use the International Financing Corporations (IFC) Performance Standards, some lenders have additional environmental guidelines and requirements to enable financing. These differences are discussed.
Common environmental concerns include loss of habitat of endangered and/or threatened species, changes to river flows, erosion and sediment control during construction, and the minimisation and disposal of project wastes.
These issues are discussed drawing on the author’s experience in monitoring environmental issues of hydropower projects in Asia Pacific and Africa, including both smaller run-of-river schemes and larger storage reservoir projects.
Keywords: Environment, impacts, project financing, concerns, lenders, lenders technical advisor.
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.
Thomas Ewing, Marius Jonker & James Willey
The use of Computational Fluid Dynamics (CFD) modelling techniques is gaining broad acceptance in the dams industry as an important design tool for hydraulic structures. This is particularly so in the earlier stages of analysis and design where the construction of physical models would be prohibitive on the basis of cost and time. Current CFD techniques allow users to produce a rapid evaluation of the existing conditions, which when coupled with the ability to quickly test an array of potential scenarios, enables the incorporation of innovative design solutions that may otherwise not have been considered during the design selection process prior to the advent of CFD capabilities.
Details of a recent case study are presented to illustrate the broad capabilities and benefits of CFD modelling techniques and their application in engineering analysis and design. The case study involves modelling of the Somerset Dam, a 50 m high concrete gravity dam with a gated overflow spillway including overtopping of the spillway bridge, gates and complex flow conditions in the abutment sections, which individually and collectively could not be accurately analysed with the traditional, simplified methods. The CFD study enabled an understanding of the hydraulic behaviour including discharge efficiency, jet impact loads on the gates and gate operating equipment and bridge structure; extent of potential erosion as a result of jet impingement on the abutments; loads on sluices and behaviour of the stilling basin. In addition to being a very large and complex model, the modelling involved several novel technical aspects.
The case study clearly highlights the benefits of the CFD modelling in understanding the complex hydraulic conditions and delivering cost effective solutions.
Keywords: Computational Fluid Dynamics, Somerset Dam.
Richard R. Davidson, P.E., CPEng Kenneth B. Hansen, P.E.
Early in the twentieth century, placing concrete core walls within embankment dams was a popular construction technique for small to medium height dams. It became in vogue as a replacement for the popular British dam construction technology of puddle clay core dams which were used between the 1860’s and 1920’s. It avoided the many problems with semi-hydraulic / manned placement methods of the puddle clay cores within narrow trenches. However, after the mid 1930’s this concrete core wall construction fell out of favour because of the improvements made in embankment compaction methods and the difficulties in building reinforced concrete core walls to more significant heights.
Today concrete core wall embankment dams are now reaching an age where their continued performance is being questioned. This dam building technology has become extinct and is unknown to the last few generations of dam engineers. Therefore, it is relevant to re-examine this dam building technology in a modern context and work on answering the following questions. How have these dams performed after almost a century of service? Are there unanticipated performance features that have produced positive results when subjected to extreme flood and seismic events? Does the concrete provide enhanced performance over time? What role does steel reinforcement play in the performance of the core wall? Are there lessons here that can be applied to the more common concrete cutoff wall solutions being applied to embankment dams with seepage problems? This paper examines these questions with a number of illustrative case histories to provide a retrospective illumination of this forgotten dam building technology.
Keywords: Embankment dams, Concrete core walls, Dam construction history.