Remediation of a National Icon
Scrivener Dam forms Lake Burley Griffin through Canberra’s Central National Area constituting a key landscape feature in accordance with Walter Burley Griffin’s vision for the city. The design of the Dam, incorporating 5 over flow fish-belly flapgates is also thought to be unique in Australia. Construction of the Dam took place between 1960 and 1964 in a time of rising population and increasing incidence of civil infrastructure to increase the productive capacity of Australia.
It is argued that Scrivener Dam can be seen as a metaphor for all aging assets delivered during the post war boom years and that their true heritage value is as a gift from a previous generation for the benefit of a future generation. The principles developed for the remediation of Scrivener Dam can be applied to all existing infrastructure assets and are consistent with Australia’s National Infrastructure Objectives: what is good for Scrivener Dam is good for the nation.
During the Dam’s 2011 annual safety and surveillance inspection it was discovered that small exposed sections of many of the hammerhead anchor bolts exhibited visible evidence of corrosion. Non-destructive testing was undertaken to assess the extent of corrosion of all 120 bolts. Initial physical inspections via endoscope of the top 300 to 500 millimetre non-concrete embedded sections of the bolts found corrosion in varying degrees. Corrosion was attributed to exposure to air, water penetration around the bolts and microbiological action. Subsequent ultrasonic testing suggested a likely reduction of the ‘effective diameter’ of many of the anchor bolts.
Given the level of corrosion identified, the estimated diameter reduction, and the consequent estimated reduction in the load bearing capacity of the hammerhead bolts, the owner acted promptly to temporarily lower the lake water level by 500 millimetres to maintain a level of safety considered appropriate prior to the bolts being remediated.
There remained, however a great deal of uncertainty regarding the real extent of the corrosion and the most cost effective way to remediate the problem. Early remediation approaches considered included undertaking exploratory in-situ demolition and testing and possible dismantling of all or a section of the flap gate to improve access for investigation and subsequent works. Following more detailed analysis, these options were found to present considerable risk and additional uncertainty; creating the potential to develop a large and complicated solution to what could in fact have been a much more modest problem.
Following advice that the risk of dam failure was adequately and effectively managed in the short term by the reduction in lake level, we undertook further research and investigation of the problem and a review of possible solutions before the construction scope was finalised.
To develop the optimum solution to an apparent problem requires intimate knowledge of the built environment. The legacy of the original designer is a fundamental part of this knowledge. We undertook an extensive review of the available records on the construction history of Scrivener Dam. This included the original contract material held in the National Archives and the site foremans diary sheets still held at the Dam maintenance office. A complete set of Works as Executed drawings of the flap gates and the Designers original calculations were also collated from a number of sources. This review was of great benefit to the project team. It created an understanding of the context of the corrosion problem in relation to the broader embedment of the gates within the body of the dam, the installation sequence of the gates, problems encountered during construction and an appreciation of the original designers response to providing access for maintenance and repair. It became clear that the original installation of the gates had not gone smoothly and that there was some uncertainty about the anchorages irrespective of the level of corrosion observed.
When industry considers problems identified with existing infrastructure, whether it is of heritage significance or not, it is natural that current thinking and solutions will be applied to historic problems. This is often desirable but can also lead to solutions growing far beyond the scope of the original problem. This phenomenon was encountered throughout the design development of this project in the following two ways.
Firstly, it is difficult to determine the extent of corrosion on buried components which are difficult to inspect. A high degree of conservatism is required on any assessment of residual capacity and engineering professionals are rightly cautious about providing definitive opinions regarding the safe loads that can be applied to these components. Clearly, the critical question that needed to be answered in developing a solution was ‘to what extent can we rely upon the existing anchorages?’
The legal principle of ‘beyond reasonable doubt’ has relevance when considering problems of this nature. In this application we might ask; is it reasonable to assume that the corroded bolts have 70% of their original strength remaining? 50%? 20%? And what if corrosion should continue? And what would the wider profession consider reasonable? What quickly becomes clear, is that whilst opinions may be supported by calculation, the inputs to the calculations are subjective and uncertain. This in turn leads to cascading levels of conservatism until the only acceptable solution is to remove all doubt and not rely on the original anchorages at all. This in turn is clearly inconsistent with the observed facts; we know that the hinges have satisfactorily supported the gates since installation.
Secondly, the development of engineering knowledge over the last 50 or so years has led to greater understanding of the unusual or exceptional loads which may be applied to a structure like Scrivener Dam. More specifically, the disciplines of seismology and flood hydrology have enabled the development of methods of calculating loads which were not available to the original designer. Whilst it is appropriate to design remediation works for loads calculated by current methods, care needs to be taken that new elements are not installed beyond the capacity of the rest of the structure to withstand the same loads. This phenomenon can cause identified problems to grow well outside their root cause.
Careful management is required to ensure that the project scope does not exceed the available budget. Early Design meetings considered these issues in some depth in developing the preferred solution.
Scrivener Dam was constructed between 1960 and 1964 at a time of increasing population and associated demand for civil infrastructure works. Whilst Scrivener Dam is an infrastructure asset of national heritage significance because of the Lake that it forms, it can also be seen as an icon of the civil infrastructure of its time.
The concept of Moral Rights is one which concerns the rights of the original designer to the protection of their creative works. We argue that this can be interpreted as a function of inter-generational equity. Is it appropriate that today’s society consider the rights of the society which funded the construction of a particular piece of infrastructure? Is aging infrastructure an asset provided by a previous generation? Or a liability we will pass to a future generation?
The twin problems of cascading levels of conservatism, and growth in scope, can lead to solutions which are only affordable when money is abundant. Owners are faced with difficult choices between increasingly expensive maintenance works and more expensive remedial works to bring the asset back to compliance with current standards. The temptation to defer major works in to the future is very real and ultimately means that problems are left for future generations to address. Unless the industry develops better methods to understand and embrace the challenges faced when dealing with ageing infrastructure then there is a very real danger of squandering the legacy passed from previous generations, and leaving a significant liability for future generations. Infrastructure Australia (1) recently reported ‘That there is a fundamental disconnect between the infrastructure we want and our willingness to pay for it’. To be able to increase productivity growth with a growing, but aging, population will require asset owners and the engineering profession to develop better ways to maintain and enhance existing assets. Infrastructure assets designed and constructed in the post war period are now 50 – 60 years old and will progressively require remedial works if they are to continue providing the functions for which they were originally designed. It is considered that the methodology followed for the remediation of Scrivener Dam could benefit work on other ageing infrastructure assets.
The concept of ‘heritage’ is often seen as little more than a cost multiplier when remediating existing assets. It is argued in this paper that respecting existing assets as a gift from previous generations to be protected for the benefit of future generations is appropriate irrespective of the assets ‘heritage’ significance. Detailed knowledge of the original designers legacy can result in real cost savings and risk reduction today. Lessons from this Project have resonance not only in dealing with heritage infrastructure but also respond to our National Infrastructure Objectives to make better use of what we have and develop more cost effective ways to deliver infrastructure capacity. The investigations, design and guiding philosophy outlined above ultimately led to a design solution which was of a clearly defined scope, which minimised uncertainty regarding latent conditions to the lowest possible extent, increased the capacity and improved the serviceability of the hinge anchors, and was delivered well below initial budget expectations. In addition the detailed appreciation of the site through extensive research led to a solution which both respects the integrity of the original design and is efficient in its use of materials.
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1. Infrastructure Australia “National Infrastructure Plan” Report to COAG, June 2013.
