Deryk Forster and Manoj Laxman
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.
Barton Maher, Richard Rodd
Changes to the estimation of extreme rainfall events resulted in significant increases in the estimates of the PMF since the original design of Wivenhoe Dam. To upgrade the dam to meet these new requirements, SEQWater (owner and operator) formed an Alliance with Leighton Contractors, Coffey Geosciences, MWH and the NSW Department of Commerce.
The option selected for the upgrade works included the construction of a new secondary spillway, upgrade of the existing gravity section, radial-gated spillway, and strengthening of the dam crest.
Value management was key throughout the project ensuring the Alliance was continually looking to
improve practices, increase cost-effectiveness and create innovative solutions for design elements of the project.
On numerous occasions when the design was challenged, the Alliance made ‘best for project’ decisions to carry out additional investigations or design work to pursue alternatives. As an example, the powerful tool of Computational Fluid Dynamics was used in the analysis and design of flow deflector plates on the existing spillway, which were an alternative to the originally designed gate locking pins. The investigation and development of this alternative resulted in significant cost savings and a more effective design solution.
This paper presents aspects of the design carried out by the Wivenhoe Alliance, lessons learned, and the way continual investigations during construction provided value for money solutions.
Martin Pinkham, Robin Dawson, John Grimston
Resource consents for Christchurch’s existing solid waste disposal facility at Burwood expire in May 2005 and the landfill must close. A new, state-of-the-art regional landfill is under construction at Kate Valley, which will accept solid waste from Christchurch and surrounding districts. Investigations and studies for the landfill have attracted considerable public attention, engaging public groups in discussions through resource consent hearings in 2002 and 2003.
The proposed landfill includes two embankment dams in a cascade arrangement below the landfill. The first is a 19m high sedimentation dam designed to retain silt runoff from the earthworks associated with landfill construction and operation, protecting the health of the stream and environment below the dam. The second is a 9m high dam performing dual roles of storing and supplying water for the landfill earthworks activities, and providing an additional safety buffer for silt control and containment of any accidental release of leachate at the landfill.
While the dams are relatively modest in size, they are being built to very high standards with strict peer review as a result of their association with the landfill project, and to minimise any community and environmental impacts. The design and construction of the landfill and dams is being completed using an innovative modified alliancing arrangement which provides the close working relationship that alliances are renowned for, while minimising up-front financial risk to the owner.
This paper deals with key aspects related to the landfill dams, such as community consultation and expectations, environmental impacts as well as the technical features. Construction is underway for the dams and the landfill at the time of writing of this paper.
Warragamba Dam supplies up to 80% of Sydney’s water needs and is currently undergoing a range of major infrastructure upgrades. The outlet works upgrade is one of these projects. The outlet works of the dam were constructed in the 1950s and consisted of four 2100mm pipes with isolating gate valves and needle control valves feeding two large above ground pipelines running 27 kilometres east to Prospect Reservoir in Sydney’s western suburbs.
In the 1990s the then dam owner (Sydney Water) undertook a detailed and extensive risk analysis of the outlet works. The study resulted in a recommendation to remove the existing valves and replace them with a combination of emergency closure (guard) valves and isolating valves. Under the Sydney Catchment Authority (the present dam owner) work subsequently proceeded in 2004 as a design and construct contract with all aspects of construction and water supply risks identified. Stringent controls were developed and placed on work programs and pipeline shutdowns to ensure the safety of all involved and the integrity of the supply to Sydney.
The four outlets required eight large valves, which were manufactured in Germany and were required to meet stringent operational requirements.
At the time of writing three of the four outlets have been successfully upgraded and commissioned. Work has commenced on upgrading the fourth outlet, which is due for completion by the time of the conference, approximately 20 months ahead of schedule.
This paper discusses the project from the initiation of the risk analysis study, through the
consideration of options, development of the contract, and the supply, installation and commissioning of the large valves and pipe work. It highlights the role of risk assessment in selection of the preferred option and addresses some of the engineering challenges faced during the project.
The Wivenhoe Dam Spillway Augmentation Project involved the construction of an additional spillway on the right abutment of the main dam. The right abutment is located in massive sandstones and siltstones of Jurassic and Upper Triassic age.
Seismic refraction surveys and borehole drilling conducted at the design stage for the project
indicated that part of the spillway area was likely to be marginally rippable to unrippable using a
Caterpillar D9 bulldozer or equivalent. Further assessment and rock strength testing was conducted during the initial stages of excavation where D9 and D10 bulldozers were in operation. The results from this further work indicated that a section of the spillway extending from the proposed position of the ogee crest to approximately 100m further upstream were unlikely to be unrippable for a D9 dozer and marginally rippable for a D10.
Excavation options considered for this section included full scale blasting and load out, limited small scale ‘popping’ combined with ripping or the use of larger ripping equipment. Based on an
assessment of cost-benefit, and given the availability of larger ripping equipment, it was decided to
use a combination of D10 dozers and a Komatsu 475A bulldozer (D11 equivalent) equipped with
single tine ripping tools. The use of this equipment proved successful with better than anticipated
production rates being achieved. This resulted in significant cost and time savings for the project and reduced the likelihood of potential adverse impacts on the existing dam grout curtain, environment, travelling public and residents that may have occurred during blasting.
SunWater has completed a portfolio risk assessment (PRA) on its 25 major dams and has identified a number of dams that do not currently satisfy the ANCOLD fallback position on spillway capacity. It has taken an initiative to target these dams for spillway upgrades to ultimately achieve the ANCOLD fallback standard and has prioritised these upgrades in a preliminary program for action in the short to medium term.
As background to this PRA, SunWater has developed and implemented a dam safety program which has successfully updated all necessary flood hydrology and dam break analyses and reassessed the consequences and hazards associated with dam failures. It has also completed within the last eight years, dam safety reviews on all its dams in preparation for a comprehensive risk assessment process which is now well in-hand. This process will identify and evaluate all other risks, in addition to floods, that should be addressed or at least considered in the planning and design of these spillway capacity upgrades.
This paper describes SunWater’s experience and approach to PRA and discusses the controlling factors considered in prioritisation. It shows the results and trends of a number of risk ranking methods, provides details of the current level of societal risks in respect of the ANCOLD tolerability limits and outlines SunWater’s current strategy for the timing and staging of spillway upgrades to achieve compliance and an optimum level of risk reduction.