Jon Williams and Chi Fai Wan
The Ross River Dam was first commissioned in 1974 and raised in 1976. The 8200 m long embankment was not fitted with chimney filters and has suffered extensive desiccation cracking since it was raised. A significant component of the dam upgrade is the retrofitting of filter zones to ensure the embankment meets current dam safety guidelines.This paper describes the process of investigation of the existing desiccation cracks and the use of Hole Erosion Tests (HET) and No Erosion Filter (NEF) tests to validate the design of the retrofitted filter.
A significant challenge in the design is to provide a cost effective solution given the 7500 m length of embankment requiring treatment. Assessment of flow rates within cracks and expected piping erosion along the cracks was used to assess the required drainage capacity. This assessment of expected flow capacity allowed the deletion of the coarse filter inthe design reducing the filter requirement from a triple filter to a single fine filter. Results of this assessment were incorporated into the Risk Assessment based design validation process
Maz Mahzari and Chi-Fai Wan
Upgrading of an existing dam often faces challenges in both static and seismic safety assessment. The use of new hydrological and seismological data and improved design methods often mean more severe loading which outdates the original design and demands expensive upgrade works. Establishing the design criteria for checking the structural adequacy of an existing dam for multiple unusual load events occurring within a relatively short time frame presents another challenge.
A probabilistic approach is presented to rigorously address the effects of multiple load events while maintaining a consistent risk of failure for the structure. This is based on a probabilistic conditional combination where probability of each event is defined and used to develop a joint probability distribution. For instance if an earthquake occurs following a severe flood, the seismic hazard curve of the site can be used to adjust the seismic loading with shorter average recurrence interval to be used in conjunction with the pre-earthquake flood when assessing the structural adequacy of the dam. With this method of adjustment, the design can benefit from the choice of a reduced seismic design loading and hence a more cost effective design solution.
The proposed method is straightforward and can be effectively used in most engineering practices, including the design of hydraulic structures such as dams.
Keywords: Dams, Seismic Hazard, Post-earthquake, Risk analysis
Leonard A McDonald and Chi Fai Wan
A risk assessment has been undertaken as part of a comprehensive review of the safety of Hume Dam. Use of risk assessment techniques, to assist in evaluating the safety of existing dams, is a relatively recent trend. Hume Dam was a particularly challenging subject for the application of risk assessment techniques at their present stage of development. The challenge lay in the number and diversity of dam elements to be analysed, in the number and complexity of the potential failure modes and in the fact that there were significant safety issues under normal operating conditions.
This paper outlines some of the key lessons learned from that phase of the risk assessment that was concerned with estimating the chance of dam failure. Some of the issues discussed have not previously been addressed in the literature and some demonstrate a clear need for improved analysis procedures.
The Tarong coal-fired power station near Kingaroy in southern Queensland discharges ash to a storage facility of 42,000 ML capacity, impounded by a 48 m high-zoned earth and rockfill dam embankment. The embankment was constructed in 1980–81. In recent years, Tarong Energy Corporation (TEC) has investigated a number of options for a new storage facility as the remaining capacity of the existing ash dam storage diminishes. TEC determined that the existing facility should be upgraded to provide additional storage capacity for the short term. At the same time, there emerged a requirement to improve the long-term seismic resistance of the embankment. Enlarging the existing spillway cut provided the material for a 400,000 m3 weighting zone and, by reducing the design flood freeboard, extended the ash disposal capacity by several years without a need to raise the embankment. Challenges included significant foundation seepage and deteriorated riprap. The paper describes the issues, risks, adopted criteria, investigation undertaken, and implementation of the upgrading works. Innovative approaches to the provision of future storage capacity are outlined.
Chi Fai Wan, Tom Haid, Jim McClain, Kelly Rodgers
The dams market in California is alive again with phenomenal growth driven by an increasing need for storage to hedge against ongoing water scarcity due to climate change and a growing demand for reliable water supplies driven by population and economic needs. To meet the region’s water supply needs the San Diego County Water Authority has launched the Emergency Storage Project (ESP), an extensive program to create a system of reservoirs, interconnected pipelines and pumping stations to provide more flexibility for water deliveries to the San Diego region, especially in the event of an emergency, such as a devastating earthquake. The Water Authority has planned for water needs in an extended drought by creating the Carryover Storage Project (CSP), which provides additional storage to capture water in wet periods for use in dry periods.
Up to 90 percent of the region’s water supply is imported by pipelines travelling hundreds of kilometres across earthquake fault lines from Northern California and the Colorado River. The major component of the fourth and final phase of the ESP is the San Vicente Dam Raise. The project includes raising the existing dam by 35 m to increase the reservoir and provide an additional 187 million m3 of water storage for the region. This will be the largest dam raise in the United States and the largest roller compacted concrete dam raise in the world. The Water Authority has contracted with Parsons/Black & Veatch Joint Venture to provide construction management services for this vital project. The dam raise is another one of the marquee water supply dams and reservoir projects that the Joint Venture members have been involved in Southern California, after successful completion of the Diamond Valley Lake for the Metropolitan Water District of Southern California and the Olivenhain Dam for Water Authority.
This paper presents a brief description of the San Vicente Dam Raise, the underlying water shortage and the emergency backup needs, and the need for carryover storage. The dam raise design has been previously presented in numerous papers and publications. Therefore, following an overview and general project description, this paper focuses on the critical role that effective construction management plays in implementation of a dam construction project of this size and complexity. Key construction management activities that are discussed in the paper include engineering design constructability reviews, independent cost estimation and scheduling, on-site laboratory management and quality control, and contractor oversight. The construction manager will be involved in this project through final design and construction over a five-year period.
Keywords: Water Scarcity, San Diego County Water Authority, Emergency Storage Project, San Vicente Dam Raise, Roller-Compacted-Concrete (RCC), Construction Management, Climate Change.