P Amos, N Logan and J Walker
There are a number of geological faults in close proximity to Aviemore Power Station in the South Island of New Zealand, including a fault in the foundation of the 48m high earth dam component of the power station. Possible movement of the Waitangi Fault in the earth dam foundation is of particular concern for dam safety, and the effects on the dam of a fault rupture has been the subject of detailed investigation by the dam’s owner Meridian Energy Ltd. These investigations have concluded that the dam will withstand the anticipated fault displacement related to the Safety Evaluation Earthquake without catastrophic release of the reservoir.
The identification of damage to the dam following an earthquake and monitoring of the dam to identify the development of potential failure mechanisms are important for determining the post-earthquake safety of the power station. The first stage of the post-earthquake response plan is the quick identification of any foundation fault rupture and damage to the dam to enable immediate post-earthquake mitigation measures
to be initiated, such as reservoir drawdown. Following initial response, the next stage of the postearthquake monitoring programme for the embankment dam is longer term monitoring to identify a changing seepage condition due to damage to the dam that might lead to a piping incident. Such an incident may not occur immediately after an earthquake, and it can be some time before the piping process becomes evident.
This paper presents some key instrumentation installed at Aviemore Dam and included in the emergency response plan for the post-earthquake monitoring of the embankment dam.
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Manuel G. de Membrillera, Ignacio Escuder, David Bowles, Eduardo Triana, Luis Altarejos
The work herein presented is an application of the risk assessment process to retroactively estimate the justification of an operating restriction implemented on a Spanish Dam. Since the risk approach is not yet an established practice in Spain, the main objective of this case study is to show, the utility that risk assessment can have as a decision support tool for decisions on dam safety risk reduction investments.
An operating restriction has been imposed at this dam since its first impoundment. All studies, analysis and documents related to the safety of the dam and reservoir have been completed, as required by the Technical Regulation on Dam and Reservoir Safety (Spanish legislation, 1996). In addition, the structural corrective actions recommended in these evaluations are being implemented, so it is expected that the operating restriction can be removed in the near future.
In this context, the problem that has been formulated and solved comprises an evaluation, after more than 30 years since construction, of the operating restriction justification in terms of risk mitigation. In order to achieve the objective of the work, ANCOLD guidelines on Risk Assessment (2003) have been followed in addition to tolerable risk guidelines from several other countries and organizations.
Peter J Burgess, Delfa Sarabia, John Small, H. G. Poulos and Jayanta Sinha
The assessment of settlement behaviour of clay core rock fill dams has always been a challenge for dam designers and geotechnical engineers. The method of construction and the material properties of the clay and rock fill materials used in the dam construction have a significant influence on the inter-zonal interaction and the load transfer that occurs within the dam. At times this load transfer can lead to excessive differential and total settlements. The paper presents a case study of a major dam that experienced large settlements during and after construction. An elaborate analysis has been carried out by modelling the sequences of construction by using a finite element program (PLAXIS).
The paper describes the influence of the degree of compaction and moisture control on non-linear deformation characteristics of clay core. High vertical strains in the wet placed region of the core and low strains in the dry placed regions were analysed for possible shear development between the core and shell. The rock fill for the dam embankment consists of quartzite, metasiltstone and phyllite material. These materials have apparently undergone deformation with increasing height of the dam due to softening and crushing as saturation of the embankment took place. The effect of soil consolidation and strength gains have been considered in the analysis and are discussed. The settlement behaviour of the dam including these effects has been analysed, and compared with the historical post-construction settlements.
This paper is intended to provide valuable information for dam engineers handling clay core rock fill dams – especially where there is excessive settlement of the core.
Nerida Bartlett, David Scriven, Peter Richardson
The failure of a number of consecutive wet seasons has resulted in storage levels in Eungella Dam being at dangerously low levels such that supply could be exhausted by June 2007. Eungella Dam supplies bulk water to the Bowen Basin coal fields as well as the Collinsville power station and the Collinsville township.
The Collinsville township, power station and coal mine as well as the Newlands mines take water from the Bowen River Weir which is supplied from Eungella Dam some 95 kilometres upstream. Transmission losses of the order of 25 to 50% have been experienced for releases from Eungella Dam to Bowen River Weir.
The Eungella Dam catchment area is 142 square kilometres. Significant flows occur in the Bowen River downstream of Eungella Dam, the catchment area above Bowen River Weir being 4,520 square kilometres. The topography in the surrounding area (near Collinsville) is not suitable for dam construction.
The opportunity existed for the construction of an offstream storage adjacent to the Bowen River Weir so that the downstream flows could be captured reducing the demand on Eungella Dam thus making more water available for upstream users.
A 5,200 ML offstream storage, associated pump station and rising main was designed, constructed and filled within a period of 12 months.
Foundations at the site are highly permeable sands. Marginally suitable clay for a seal was in short supply as was suitable rock for slope protection. A fixed price budget had been set by the contributing customers.
This paper describes the hydrology, site conditions, design and construction of the project.
David S. Bowles
Portfolio Risk Management is a risk-informed approach for improved management of dam safety for a portfolio of dams in the context of the owner’s business. It can be used to identify ways to strengthen technical and organisational aspects of a dam safety program, and to provide valuable inputs to various business processes. Portfolio Risk Assessment is a decision-support tool, which is incorporated in Portfolio Risk Management. It can combine engineering standards and risk assessment approaches to provide a systematic means for identifying, estimating and evaluating dam safety risks, including comparisons with other industries. It should be periodically updated to provide a basis for managing prioritised queues of investigations and risk-reduction measures to achieve more rapid and cost-effective reduction of both knowledge uncertainty and risk.
Portfolio Risk Assessment is a standard of practice in Australia and is being applied by the US Army Corps of Engineers and others. When properly conducted and used within its limitations, the Portfolio Risk Assessment process is generally considered to be robust, adaptive, defensible for corporate governance, and to justify its cost through such benefits as increased dam safety funding, identification of failure modes that were not previously recognised, identification of opportunities for improved risk management, and more rapid “knowledge uncertainty” and risk reduction.
The Koralpe hydropower scheme is a major development on the Feistritzbach tributary of the River Drau to utilize water in a 50 MW powerhouse located in the south-eastern Carinthia, Europe. The Soboth reservoir is situated 735 m higher in a narrow valley and is created by the 85 m high Feistritzbach dam which was constructed near the border of Austria and Slovenia between 1988 and 1990. This rockfill dam is the latest addition to KELAG’s more than 15 structures and is sealed by an asphaltic core. The excellent deformability and impermeability of the asphaltic core is able to follow the deformation of the compacted rock-fill material best during construction, initial filling and operation period without any seepage. The asphaltic core was placed in three 20 cm layers per day by a specially developed placing unit from a contractor. The upstream and downstream filter zone was placed at the same time with the same machine and compacted carefully by vibrating rollers. The dam is curved in plan with a radius of 650 m and contains about 1.6 million m³ rock fill material. The surface of the downstream side was built exceeding the environmental standards of the time.The most important indicator of the normal function of a dam is the behaviour of seepage. A monitoring system of seepage, piezometers, earth pressure cells and deformation has been installed. The seepage water is monitored online at seven points of the dam base and at the access tunnel to the bottom outlet valve. Geodetic measurements on and inside the dam are done once a year. Several additional pieces of surveillance equipment were installed to observe the behaviour of the asphaltic core. The paper concentrates on the design, construction and performance of the dam with the asphaltic core.