2012 – Estimating Individual Risk
Kelly Maslin, Mark Foster, Len McDonald
A key requirement of assessing the tolerability of dam safety risks is the assessment of individual risk. The ANCOLD Guidelines on Risk Assessment provides guidance on acceptable levels of individual risk and some general guidance on the calculation of individual risk.
Individual risk is a key measure in the consideration of the tolerability of risk, ALARP and development of risk mitigation works. It is essential that there is consistency in the approach to estimating individual risk used across the dams industry.
This paper reviews the approaches taken to estimating individual risk across the dams industry both locally and internationally as well as the experience of other industries.
The paper includes a review of the various methods for estimating the vulnerability of individuals subjected to flood inundation based on historical fatality rates as well as identification of the individual most at risk
The paper then describes a method that has been developed based on the principles used for assessing individual risk due to other hazards, such as landslides. The method includes consideration of a range of factors such as warning time, temporal variation and vulnerability of the individuals most at risk. The method developed provides a transparent, defensible and pragmatic approach to estimating individual risk. Practical guidance and examples are also provided on the application of the method.
Keywords: individual, risk, exposure, fatality
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Papers 2012
2012 – 15 Years of Experience with a Dam Improvement Program
Learn moreShane McGrath, Andrew Reynolds, Garry Fyfe, Chris Kelly, Steven Fox
Goulburn-Murray Water is a rural water corporation located in Northern Victoria. It has responsibility for 12 State dams and is also the constructing authority for the Murray Darling Basin Authority’s Victorian assets.
Over the past 15 years G-MW has been engaged in a dam improvement program across its portfolio. To date 14 individual projects have been undertaken at 11 dams. The total expenditure is $125 million.
Starting from a base level of data at its inception in 1997, the program has encompassed all facets required for a dam improvement program. From early prioritisation to set the investigation program, through design reviews and risk assessments to develop the upgrading program and subsequent implementation. Some elements of the program were at the leading edge of practice at the time and a range of experiences along the way were character building as dam safety investment challenged other corporate priorities.
This paper sets out the lessons learned in developing the methodology and implementing the program of works, particularly relating to corporate adoption of the program, organisational capability, investigations, risk assessments, design and implementation.
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Papers 2012
2012 – Innovation in the Design and Construction of the Enlarged Cotter Dam
Learn moreMartin Weir
The Enlarged Cotter Dam project was selected as a key component in securing the future water supply for Canberra and the ACT region. The RCC gravity dam, when completed, will stand 84m high and will be the largest of its kind in Australia.
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The dam was designed, and is currently being constructed, under the Alliance contract model. The collaboration this model brings between the owner and the design and construction teams facilitated a drive in innovation from the design through to the construction stages of the project. The focus of this paper is on some of the key innovative aspects of the project, for consideration on future RCC and dam projects.
Investigation was made into the placement of RCC in 400mm layers, compared to the industry adopted standard of placement in 300mm thick layers. Whilst full scale trials demonstrated that placement in 400mm thick layers was not detrimental to the quality of the RCC, the benefits in terms of increased production were never fully realised due to adverse weather and the geometry of the dam placement area. Some issues were also encountered with regards to the compaction of the GERCC on the dam faces. The results do however suggest that the method warrants consideration on future RCC projects.
The construction of the dam’s secondary spillway included a waterstop installation in a constrained RCC placement zone. By developing an arrangement that could hold the waterstop in place and induce the movement joint in the correct location, this arrangement simplified what could have been a complicated procedure in an already time consuming placement area.
The start of RCC placement was at risk of further delay on account of the extensive mass concrete pours required to level the dam foundation. A conventionally vibrated concrete mix, made from the existing site won RCC materials, was designed so that it could be produced from the RCC batch plant. This method of concrete production, combined with an efficient means of delivering the concrete to the pour area, accelerated the placement process and reduced the cost of construction.
Keywords: RCC, dam, construction. -
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Papers 2012
2012 – The QCC Process in USACE Risk-Informed Decisions
Learn moreRichard R. Davidson, Nate Snorteland , Doug Boyer, John France
The US Army Corps of Engineers (USACE) has embarked upon a monumental journey in applying risk-informed decision making in the management of the safety of the 650 major dams for which it is responsible. This process has shifted safety criteria from fully deterministic to a probabilistic basis. There has also been a shift from de-centralized district-based decision-making to centralized management of resources through the new Risk Management Center (RMC) and the Senior Oversight Group (SOG), a group of senior engineers and managers from across the USACE organization. The risk process began about five years ago with a portfolio prioritisation using screening-level risk assessments of the entire dam inventory, culminating in Dam Safety Action Classifications (DSAC) for each of the dams. Based on this risk prioritisation, Issue Evaluation Studies (IES) were initiated for the highest risk DSAC I and II dams, with each study including detailed failure mode and risk analyses for each dam. Because the Corps was relatively new to dam safety risk analyses, and their dam design history was one of following codified manuals of practice, various risk tools were prepared to provide guidance when assessing the risk of potential static, seismic and flood failure modes, as well as life loss and economic consequences of dam failure. Although these tools provided useful guidance to a relative large population of inexperienced risk estimators, many of these early risk assessments were flawed; they provided unrealistically high estimates of failure probabilities and the tools did not help estimators understand or explain each failure mode. To assist the RMC in bringing more defensible risk estimates to the table and improve consistency of the evaluations, the Quality Control and Consistency (QCC) review process was initiated about two years ago. The QCC process provides high level review of IES activities, including detailed reviews of risk analyses, by a small group of experienced dam safety risk estimators. Not only has this brought risk estimates into a more reasonable range, it has provided valuable training for risk estimators, and important checks and balances on the risk-informed decision making process for moving dam safety upgrade projects forward. The justification for a number of very expensive projects has been challenged and, in some cases, re-prioritised, and other projects have risen to the prominence they deserve.
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Papers 2012
2012 – Diversion Design for Dam Construction
Learn moreTim Griggs and Richard Herweynen
The river diversion is an important aspect to be considered in the design of a dam. It generally consists of an upstream cofferdam, river diversion conduit and downstream cofferdam and allows the dam to be constructed in a dry section of river.
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This paper reviews the diversion design adopted at three recent Australian roller compacted concrete (RCC) dams and comments on the effectiveness of the design in providing risk mitigation during the construction of each of these dams. The dams considered are Paradise Dam (2005), Meander Dam (2007) and Wyaralong Dam (2011).
Rather than selecting an arbitrary design flood for the diversion, a risk-based assessment was used that generally resulted in a relatively low design capacity. Even though there were cases where the diversion capacity was exceeded, it is considered that the risk based design process provided an economical diversion design for these recent Australian dams.
Keywords: Diversion, roller compacted concrete dam, RCC. -
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Papers 2012
2012 – Negotiating the Geological Structure of the Enlarged Cotter Dam Site During the Abutment Excavation Period
Learn moreAndrew Barclay, Greg Kotze
The Enlarged Cotter Dam (ECD) is under construction on the Cotter River, 18km west of Canberra. The new dam comprises an 85m high roller compacted concrete gravity dam, located 120m downstream of an existing 31m high concrete dam. This paper describes the geological structures that prevail at the site and their significance with respect to design and construction considerations.
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Geological mapping has confirmed that the abutment slopes are characterised by zones of prominent rock outcrop and thin mantles of colluvial soil that form overall slope angles of 45 degrees. The Cotter River valley in the ECD area has been eroded through a geological sequence of Early to Late Silurian age, comprised predominantly of porphyritic rhyolite and lapilli tuffs of the Walker Volcanics.
Geotechnical investigations for the ECD were extensive and comprehensive. The results obtained have enabled the compilation of a detailed geological model of the dam site. Particular attention was paid to defining, characterising and kinematically analysing prominent geological structures, including intersecting sheared or crushed seams and zones that traverse the dam footprint.
Prominent geological structures that were encountered during the abutment excavation had significant design and construction implications for:
Abutment stripping and foundation preparations;
Rock slope stabilisation;
The foundation of the intake tower that comprises a 66m high concrete structure; and
The foundations for 1 x 56m high and 2 x 78m high tower cranes that required positioning on the steep abutment slopes during construction.
This paper highlights the importance of understanding the geological origin, nature and distribution of rockmass defects within a complex rock foundation. Site specific construction requirements and engineering design solutions used to successfully negotiate adverse geological structures are described.
Keywords: Dam, Roller Compacted Concrete, Geological Structures, Abutment, Foundation.