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.
Now showing 1-12 of 42 2979:
Bronson L McPherson, Eric J Lesleighter, David C Scriven, Erik F R Bollaert
A number of medium to major floods in Queensland caused substantial scour around spillway structures. This included the Paradise Dam primary spillway which experienced significant scour of the rock body below the spillway during flooding in January 2013. The occurrence has led to a series of evaluations of the geology, and the prevailing hydraulics behaviour as part of a process to determine the scour mechanism, and to determine the response of the spillway and areas downstream to future floods of larger magnitude. Part of the process has been to utilise a large-scale physical model to obtain transient data which together with the detailed geologic assessment would be incorporated into the comprehensive scour modelling procedures developed by Dr Erik Bollaert, AquaVision Engineering, Switzerland.
The paper will describe the design and construction of the physical model with special features to obtain pressure transients from more than 60 transducers, and velocity transients in more than 40 locations using Acoustic Doppler Velocimeter (ADV) instrumentation. The features of the rock scour will be discussed and the geology of the area below the spillway apron will be described. The range of discharges, and the model’s results including the pressure and velocity characteristics will be described in detail to illustrate the violent nature of the turbulence in the energy dissipation zone. The paper will go on to describe the computational scour modelling procedures of calibration and application, demonstrating a “system” approach to spillway scour analysis for plunge pools and similar situations with energy dissipation on natural materials.
Keywords: Spillways, flood hydraulics, hydraulic modelling, rock scour, transients, numerical analysis, energy dissipation.
J.H. Green; C. Beesley; C. The and S. Podger
Rare design rainfalls for probabilities less frequent than 1% Annual Exceedance Probability (AEP) are an essential part of spillway adequacy assessment as they enable more accurate definition of the design rainfall and flood frequency curves between the 1% AEP and Probable Maximum events.
Estimates for rare design rainfalls were previously derived using the CRC-FORGE method which was developed in the 1990s. However, as the method was applied on a state-by-state basis, there are variations in the approach adopted for each region. Differences in the cut-off period for data, the amount of quality controlling of the data undertaken, the base used for the 2% AEP estimates, gridding settings and smoothing processes have created inconsistencies which are particularly apparent in overlapping state border areas.
The Bureau of Meteorology has derived new rare design rainfalls for the whole of Australia using the extensive, quality-controlled rainfall database established for the new Intensity-Frequency-Duration (IFD) design rainfalls. These data have been analysed using a regional LH-moments approach which is more consistent with the method used to derive the new IFDs and which overcomes the limitations of the spatial dependence model in the CRC-FORGE method. In particular, the selection and verification of homogenous regions and the identification of the most appropriate regional probability distribution to adopt relied heavily on the outcomes of the testing of methods undertaken for the new IFDs. However, to focus the analysis on the rarer rainfall events, only the largest events have been used to define the LH-moments.
Keywords: Rare design rainfalls; Intensity-Frequency-Duration (IFD); Annual Exceedance Probability
Susantha Mediwaka, Nihal Vitharana, Badra Kamaladasa
Nalanda dam is the oldest concrete gravity dam on the Island built in the 1950s by the Ceylon Department of Irrigation. The dam was built in 9 monoliths having a dam crest length of approximately 125m and a maximum height of about 36m. The spillway consists of: (1) a low-level uncontrolled ogee-crested horse-shoe section with a crest length of 46m, and (b) a high-level broad crested weir with a crest length of 43m.
It was designed and constructed according to the then standard practices adopted throughout the world. Over the years, Nalanda dam has been showing signs of deterioration which is suspected to be Alkali-Aggregate Reaction (AAR). The dam was also shown to be deficient with respect to the stability levels required by modern standards. Under a program of dam safety improvement of the dams throughout Sri Lanka, it was decided to stabilise Nalanda dam as the first step in addressing a series of issues affecting the dam.
This paper presents the construction history, current issues, design assumptions and salient construction features in the upgrading of the dam to modern dam safety requirements.
Keywords: Concrete dams, dams Sri Lanka, concrete buttressing, upgrade, horse-shoe spillway
Maree Dalakis, Dr Saman de Silva, Siraj Perera and Dr Gamini Adikari
This paper describes the results of a statistical and qualitative analysis on historical dam safety incidents in Victoria, the first study of its kind conducted in the State. The study investigates trends arising from qualitative dam safety incident data collected by the Department of Environment, Land, Water and Planning since the year 1996. The reported incidents are categorised based on their severity and statistical trends are identified in relation to the types of incidents common to regulated and unregulated dams, as well as common responses to incidents, including their post-incident operation. The geographical distribution of incidents across the State is also analysed to determine the effects of seismicity on dam safety incident rates. Furthermore, the unique Victorian conditions of sustained drought and subsequent flooding and their impact on incident rates are investigated through the combined analysis of geographical incident distribution and streamflow data. The incident data is further assessed according to the frequency of visual inspection and reporting of the structures in order to gauge the relative influence of these practices, and dam regulation in general, on mitigating incident risk in dams. An understanding of dam safety incident trends and the impact of inspection and reporting practices is increasingly important given the increasing expectation for dam owners to properly operate and maintain their assets with minimal resources and finances.
Keywords: dam, safety, incident, historical, failure.
Robert Kingsland, Michelle Black, Andrew Russell
Managing the vibration impacts associated with blasting is a challenge for mine planners and operators. In an open cut mining environment production blasting is often an integral part of operations. The management of surface water is a key operational requirement for open cut pits and mine water dams are often a part of the water management infrastructure. Consequently, mine water dams are often subject to blasting impacts.
For the mine operator the foremost questions are, “how close can mine blasting progress towards the dam?” and “what is the maximum vibration that the structure can be safely subjected to?” For the dam safety regulator the key concerns are around potential modes of failure, consequence of failure, the likelihood of failure and the management of risk.
With reference to case studies, this paper will discuss the acceptable blasting limits for earth dams, impacts on various dam elements and failure mode analysis. Failures modes discussed include embankment cracking, slope failure and deformation, foundation cracking and outlet structure cracking. Risk mitigation measures will be presented including design, operation and monitoring controls.
Keywords: blasting impacts, embankment dams, coal mine.