There is a large stock of embankment dams throughout the world needing the assessment of their
safety as required by modern dam safety regulations. Due mainly to economic and site constraints
associated with potential dam upgrading work, it is imperative that a rational approach be adopted in
assessing their safety and in designing the remedial works. One of the most important criteria is the
selection of appropriate geotechnical parameters under different conditions. Predominant loading
conditions in a dam are much different from those in other structures such as bridge and building
foundations and therefore the direct adoption of traditional approaches may not always be valid. This
paper presents the various aspects of issues associated with the stability assessment of dams including
the rational selection of the parameters and numerical codes available to dan/geotechnical engineers
to assess their safety.
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P Maisano, M Taylor , M Barker and A Parsons
South Para Dam, completed in 1958, is located on the South Para River, 38 km north of Adelaide. The embankment is 45 m high and comprises compacted crushed phyllite with rockfill toes. The 13 m high rock fill toes are protected with three-stage filters but the remaining 32 m of embankment height has no downstream filter protection.
The South Australian Water Corporation (SA Water), the owner and operator of the dam, is considering modifications to the dam, to augment its flood mitigation role. The proposed works, while not affecting the full supply level, involve a modification to the spillway crest and raising of the embankment crest to accommodate increased flood levels. SA Water therefore commissioned a dam safety review to assess the need for any piping or overtopping protection that may be required. This was followed by concept designs to ensure that flood mitigation work is compatible with any required dam safety upgrade work.
The results of a detailed dam failure risk analysis using event trees showed that the Societal Risk for the existing dam needed to be reduced, and that the proposed spillway modifications increased the Societal Risk due to the increased risk of piping failure with higher flood levels.
The risk analysis showed that eliminating the overtopping modes of failure by raising the dam crest is not sufficient in itself to achieve the required reduction in risk. The provision of filter protection to reduce the risk of piping failure is required, but it was shown that it is not necessary to provide full height filters as the provision of filters only above full supply level would be sufficient to achieve the required reduction in risk.
The recommended upgrade works, in addition to the proposed spillway modification for flood mitigation purposes, consist of filter protection and a weighting fill above the top berm (4.4 m below FSL) to facilitate connection to a possible full height filter in the future, and a parapet wall to provide overtopping protection.The resulting cost saving compared with the installation of full height filters is in excess of $2 Million.
The Stage I construction of the Ross River Dam was completed in December 1973. The reservoir
reached full supply level (FSL) and then spilled in January 1974. In 1976, the left embankment was
raised to Stage II level. Spillway gates were installed in February 1978 with full supply level for
Stage 1A (FSL).
In the years following the first filling of the reservoir after the raising of FSL, salt scalding
downstream of the northern portion of the left embankment occurred. This was attributed to
foundation seepage. Investigations started in 1978 to define what remedial measures were required to ensure the safety of the left embankment. Fissured clays were first discovered in the foundations of the Ross River Dam during these investigations.
Fissures could substantially reduce the overall strength of the soil foundations. Therefore the effect of these fissures needs to be considered when evaluating the acceptable levels of reliability against embankment failure. More extensive fissuring was discovered during the current investigations and a cataloguing system was employed to characterise the foundation conditions.
A simplified layer model was adopted early on in the design but did not fully demonstrate the
complexity of the subsurface conditions. Extensive use was made of historical geological data,
current investigation data and the application of GIS systems. The resulting model more clearly
represents the foundation conditions and high degree of variability and was used in subsequent risk assessments for the upgrade design.
J S Marsden, P H Jacob, R Nathan and L A McDonald
This paper relates to the conference sub-themes of Dam Safety Upgrades – Management of Risk and Due Diligence and Dam Construction.Specifically, it relates to the changing willingness of governments to fund risk reduction indams compared with risk reduction in other areas.
The cost of dam safety upgrades is just one of a portfolio of risk reduction strategies affecting the general community that governments are required to underwrite.
This paper examines the variation in acceptable risk standards between dam safety and other areas. This may be explained in terms of what is acceptable to the community and the courts. For instance, a corporation is likely to attempt to minimise its liability (which may differ to minimising risk for the community).
We also examine:
There is an increasingly well-established literature on the value of a human life and increasingly systematic approaches to the evaluation of risk and the setting of risk standards. Risk standards are particularly explicit in the area of dam safety – they set limits of tolerable risks for large-scale loss of life (eg. for existing dams, a loss of life of more than 10 persons with a probability of more than one in a ten thousand per annum is regarded as unacceptable under the Australian guidelines).
However, there are significant contrasts in what is tolerated as acceptable risk between different areas of government activity. To date,there appears to be no systematic evaluation of the portfolio of risks or a common view on what is acceptable levels.
Graeme Hannan, David Jeffery
Lake Mokoan is a 365 GL capacity off-stream storage in the Broken River basin in northern
Victoria. Lake Mokoan will be decommissioned to provide 44 GL of water savings to benefit the
River Murray and the Snowy River. The Victorian Government has committed to maintain
reliability of supply in the Broken River supply system by implementing a package of offset projects.
The paper describes the community engagement process implemented by Goulburn-Murray Water
to steer the selection and implementation of the offset projects.
A reference committee of Broken systems irrigators was established in late 2004 to provide advice
to Goulburn-Murray Water and the Department of Sustainability and Environment on the package
of offset measures to be implemented to maintain the supply reliability once the 365 GL capacity
Lake Mokoan was decommissioned, leaving the 40GL capacity Lake Nillahcootie as the sole
storage in the Broken River irrigation system.
A REALM based system simulation model was refined to test the sensitivity of the parameters
defining the system reliability and to assess proposed offsets measures. The paper describes the
modelling which was undertaken and the evaluation and ranking of offset projects priorities.
The community engagement process is described. The paper concludes with commentary of the
lessons learned from this process.
Don Macfarlane; Nick Eldred; Sigi Keis
Project Aqua was planned to be a major hydropower development along the lower Waitaki Valley, New Zealand. Geotechnical investigations for the project were conducted in two main stages – from the late 1970’s to mid-1980’s, and again in the period from 2002 to 2004.
Community consultation was an important part of the 2002-2004 investigations, and was a key risk management issue for Meridian Energy. The proposed scope of the work included 512 drillholes and 734 test pits spread along the 60km project corridor. All proposed drillholes and test pits were subject to the Resource Management Act 1991 and needed Resource Consent applications, which required consultation with landowners, territorial authorities, and community and cultural groups including three Maori tribes.
A number of proposed investigations could not be undertaken because the landowner would not allow land access, but over 70% of the proposed work was completed with community support.