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A. Ash, D. S. Bowles, S. Abbey and R. Herweynen

A preliminary risk assessment was undertaken of its three dams by the South East Queensland Water Board (SEQWB) in 1999. The risk assessment process used included a series of workshops that proved to be an important part of ensuring a worthwhile result. The combined expertise of the consultants together with that of staff from the Board and the Queensland’s Department of Natural Resources were used to improve the outcome. The results of the assessment showed that the process had both advantages as well as difficulties in comparison to a standards based approach for making dam safety decisions. Risk Assessment was seen to be a useful management tool for managing dam safety. It gave the owner the ability to quickly rank upgrade requirements or maintenance options on the basis of probability of failure, life safety risks and financial risks to the owner or economic risks to all stakeholders.

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2000 Papers

2000 – Risk Assessment: A Complex Exercise But A Worthwhile Tool
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A. Ash, D. S. Bowles, S. Abbey and R. Herweynen

A preliminary risk assessment was undertaken of its three dams by the South East Queensland Water Board (SEQWB) in 1999. The risk assessment process used included a series of workshops that proved to be an important part of ensuring a worthwhile result. The combined expertise of the consultants together with that of staff from the Board and the Queensland’s Department of Natural Resources were used to improve the outcome. The results of the assessment showed that the process had both advantages as well as difficulties in comparison to a standards based approach for making dam safety decisions. Risk Assessment was seen to be a useful management tool for managing dam safety. It gave the owner the ability to quickly rank upgrade requirements or maintenance options on the basis of probability of failure, life safety risks and financial risks to the owner or economic risks to all stakeholders.
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2001 Papers

2001 – On the Art of Event Tree Modelling for Portfolio Risk Analyses
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P.I. Hill, D.S. Bowles, R.J. Nathan, R. Herweynen

With the growing emphasis on a risk-based approach to dam safety management, event tree models are increasingly being used as an analysis tool. The simple structure of event trees belies some of the more complex issues associated with their application to dam safety risk analysis. This paper outlines some of the basic principles of event tree analysis and then demonstrates how inappropriate construction of event trees, and particularly oversimplification, can result in a bias in the estimated risk and produce misleading results when used to assess the dam against various risk criteria. Issues considered include the partitioning of the loading event and the impact of conservative assumptions such as assuming the reservoir is initially at full supply level.
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2002 Papers

2002 – Assessing The Likelihood Of Concentrated Leaks Leading To Failure In Concrete Faced Rockfill Dams
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P. H. Southcott, R. Herweynen and R. Fell

Hydro Tasmania is in the process of undertaking a Portfolio Risk Assessment of its 54 referable dams, of which 14 are concrete faced rockfill dams. One of the potential failure modes identified during the study so far is a concentrated leak developing in the face slab or joints of the slab, leading to failure of the dam. Current methodologies for assessment of piping failures through embankment dams are considered inadequate for this failure mode. This paper discusses an event tree methodology developed from the work of Foster and Fell (1999) and Foster et al (2001) to address this failure mode. The key aspect of this method is identifying the factors that influence the likelihood of initiating a concentrated leak through the perimetric, vertical and crest wall joints and through the face slab concrete. It is concluded that for the vast majority of well designed and constructed concrete faced rockfill dams that a concentrated leak leading to failure is very unlikely.
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2003 Papers

2003 – CAIRN CURRAN DAM UPGRADE – INVESTIGATIONS AND GEOTECHNICAL MODELLING
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Cairn Curran Dam is a 44m high zoned earthfill embankment located near Bendigo in central
Victoria. The dam is owned and operated by Goulburn-Murray Water.

A risk assessment had identified that the junction between the embankment and spillway wall was a weakness in regard to the potential for piping. Initial geotechnical investigations indicated a softened zone adjacent to the foundation.
The conceptual upgrade design was to excavate the downstream slope and place filter material and a rockfill weighting berm. A 2-D slope stability analysis gave unacceptably low factors of safety for this excavation. The three dimensional nature of the embankment/spillway interface and excavation geometry was identified as an important factor in the upgrade design.

A detailed geotechnical assessment was undertaken and a geotechnical model developed that
accounted for potential softened zones adjacent to the spillway wall, along the foundation, and within the embankment.

A 3-D limit equilibrium slope stability program was utilised to analyse the 3-D factors of safety. The
program employed an extension of Bishop’s method of slices to a 3-D ‘method of columns’. A 3-D
finite element analysis was also undertaken to estimate likely deformations of the embankment and cut slope during construction.

The development of the geotechnical model and subsequent analysis allowed the upgrade works to be undertaken with confidence.
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2004 Papers

2004 – BURNETT RCC DAM DESIGN – AN INNOVATIVE APPROACH TO SITE SPECIFIC CONDITIONS
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The Burnett Dam is a 50m high Roller Compacted Concrete Dam on the Burnett River, located approximately 50km inland from the town of Childers in Queensland. The design and construction of the Burnett Dam is being undertaken by the Burnett Dam Alliance (BDA) consisting of Walter, Macmahon, Hydro Tasmania, SMEC and Burnett Water. The design of the dam commenced in 2003, construction started in November 2003 and the planned completion date is mid 2005.

The Alliance Contract was awarded through a competitive process and therefore innovation was a key aspect of the design and construction methodologies. This paper will look at a number of specific aspects of the design where innovation delivered true value, namely:
• Allowing the right abutment to act as a secondary spillway by utilising the high tailwater levels during the extreme flood events.
• Developing a river diversion strategy, which had the added benefit of addressing a foundation issue and providing source material for aggregate.
• Creating an economical RCC mix design, which uses the available materials effectively to achieve the required properties.
• Designing a dam cross-section with an upstream membrane to eliminate the need for a gallery and assure watertighness and uplift control at the upstream face.

The Burnett Dam design has clearly demonstrated that the unique site conditions have had a significant impact on the final design and construction methods.
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2004 Papers

2004 – Burnett RCC Dam Design – An Innovative Approach to Site Specific Conditions
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R. Herweynen, T. Griggs, E. Schrader and D, Starr

The Burnett Dam is a 50m high Roller Compacted Concrete Dam on the Burnett River, located approximately 50km inland from the town of Childers in Queensland. The design and construction of the Burnett Dam is being undertaken by the Burnett Dam Alliance (BDA) consisting of Walter, Macmahon, Hydro Tasmania, SMEC and Burnett Water. The design of the dam commenced in 2003, construction started in November 2003 and the planned completion date is mid 2005.

The Alliance Contract was awarded through a competitive process and therefore innovation was a key aspect of the design and construction methodologies. This paper will look at a number of specific aspects of the design where innovation delivered true value, namely:
- Allowing the right abutment to act as a secondary spillway by utilising the high tailwater levels during the extreme flood events.
- Developing a river diversion strategy, which had the added benefit of addressing a foundation issue and providing source material for aggregate.
- Creating an economical RCC mix design, which uses the available materials effectively to achieve the required properties.
- Designing a dam cross-section with an upstream membrane to eliminate the need for a gallery and assure watertighness and uplift control at the upstream face.
The Burnett Dam design has clearly demonstrated that the unique site conditions have had a significant impact on the final design and construction methods.
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