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Within about ten years of the 1991 edition of the ANCOLD Guidelines on Design Criteria for Concrete Gravity Dams being published the Limit State design approach it proposed fell out of favour with Australian dam engineers.  The ANCOLD Executive recognised this situation and resolved that the Guidelines be revised to reflect the preferred design approach amongst Australian dam engineers.  A Working Group and a separate Expert Review Panel were formed in 2005 to review the Guidelines with the intention of preparing a document that was not only more in line with the current thinking but would also be widely adopted and used by practicing dam engineers.

It has not been an easy task for the Working Group and the Expert Review Panel.  Although the working stress method, with associated Factors of Safety, approach was agreed amongst the team, it took considerable time and effort to reach consensus on the details, in particular chapters four through to six.  The issue that took most effort to resolve was how the Factors of Safety relate to the level of certainty of the inputs, including knowledge of the foundations, loads, strength of materials and assumed mechanically feasible failure surfaces.  The notes to Table 6.4.1 and the discussion in Appendix A highlight this issue.

One thing the drafting team agreed on early, and unanimously, was the importance of the concrete gravity dam design team having a thorough understanding of the foundation conditions, the development of a detailed geological model of the foundations and an understanding of the kinematically feasible failure mechanisms. In keeping with this principle, this revision of the guidelines has the section on foundations at the front of the document.  There is a strong recommendation that the design team has expertise covering foundation geology and rock mechanics as well as the dam engineer.

These Guidelines provide a basis for design of concrete gravity dams in most situations. However, they require that the user will be a professional dam engineer with significant experience who is able to use sound engineering judgement in the application of the guidelines. It is recommended that less experienced dam engineers using these guidelines do so under the advice and guidance of an experienced dam engineer. It is the dam engineer who is responsible for the design.

On behalf of ANCOLD I would like to thank the members of the working group and all the other contributors that persevered in developing and bringing these Guidelines to fruition. They will be of great assistance to dam engineering professionals both within Australia and internationally.

ANCOLD has prepared these important Guidelines to provide a consistent method of categorising the consequences of dam failure so that resources can be allocated according to the potential effects of failure on the general community.

These Guidelines replace the ANCOLD Guidelines on the Assessment of the Consequences of Dam Failure issued in May 2000 and follow a similar approach.  However, they include quite significant changes aimed at providing enhanced description of Consequence Categories and making them easier and more consistent to use.

The Technical Working Group has made every effort to make these Guidelines straightforward to use by professional engineers and consistent with other ANCOLD guidelines.

However the complexity of determining the various parameters that make up each Consequence Category means that only experienced dam engineering professionals should interpret and use these Guidelines when making decisions that could impact on community safety, community cost and services, infrastructure, natural environment, heritage, and the owner’s and other businesses.

In Australia we have a very diverse portfolio of dams, some regarded as being designed and constructed before there was a good understanding of the loading conditions that could apply.  Sometimes there is limited knowledge of the actual details of construction and always there is a desire to ensure that the community’s resources are properly applied.  The difficulty facing those who allocate funds for the benefit of the community measure the cost against the benefit achieved and to compare that with opportunities available.

Risk Assessment gives us the tools to estimate the likelihood of a circumstance occurring and of its consequences. It also gives us the tools to estimate what these will be after any improvement.  These tools are diverse and can be imprecise.  As time goes on our methods will improve but chance and circumstance will always be with us.  But, if we use consistent methods, we will generally be able to rank the possible outcomes and compare them, in scale, with others.

ANCOLD is aware of the diversity of dams and the circumstance associated with them and offers this document merely as an indicator of the approach that may be taken.  Nevertheless we hope that its adoption will lead to greater comparability between individual workers.