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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.
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1998 Papers
1998 – Risk Assessment for Hume Dam – Lessons from Estimating the Chance of Failure
Learn moreLeonard 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.
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2003 Papers
2003 – Experimental Investigation of Internal Erosion by the Process of Suffusion in Embankment Dams and their Foundations
Learn moreThis paper presents the findings of experimental investigation of internal erosion by the process of suffusion within embankment dams and their foundations.
Suffusion is the process by which finer soil particles are moved through constrictions between larger soil particles by seepage forces. Soils susceptible to suffusion are usually described as internally unstable. Understanding of the suffusion process is important to the assessment of the risk of internal erosion in an embankment dam and its foundation. Suffusion results in a coarser soil structure, leading to increased seepage, progressive deterioration of the dam or its foundation, and a higher risk of toe instability. Suffusion within the protective filter of a dam may result in a coarser filter, rendering it ineffective in protecting the core materials from erosion.
Two types of suffusion tests, namely the downflow test and the upflow test, have been conducted at the University of New South Wales. The downflow test aims at identifying the types of soils that are susceptible to suffusion, whereas the upflow test aims at identifying the hydraulic gradient at which suffusion is initiated. This paper presents the initial findings of the downflow test. Eighteen downflow tests have been carried out on fourteen clay-silt-sand-gravel soils. The Kenney and Lau (1985, 86) method, which is commonly used for assessing the internal stability of coarse-grained soils, appears to be too conservative when used to predict the internal stability of silt-sand-gravel or clay-silt-sand- gravel soils, whereas the Burenkova (1993) method appears to provide better predictions. Further testing is required to define more accurate criteria for determining the internal stability of broadly-graded clay-silt-sand-gravel soils.
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2003 Papers
2003 – Experimental Investigation of Internal Erosion by the Process of Suffusion in Embankment Dams and their Foundations
Learn moreChi-fai WAN, Robin FELL
This paper presents the findings of experimental investigation of internal erosion by the process of suffusion within embankment dams and their foundations.
Suffusion is the process by which finer soil particles are moved through constrictions between larger soil particles by seepage forces. Soils susceptible to suffusion are usually described as internally unstable. Understanding of the suffusion process is important to the assessment of the risk of internal erosion in an embankment dam and its foundation. Suffusion results in a coarser soil structure, leading to increased seepage, progressive deterioration of the dam or its foundation, and a higher risk of toe instability. Suffusion within the protective filter of a dam may result in a coarser filter, rendering it ineffective in protecting the core materials from erosion.
Two types of suffusion tests, namely the downflow test and the upflow test, have been conducted at the University of New South Wales. The downflow test aims at identifying the types of soils that are susceptible to suffusion, whereas the upflow test aims at identifying the hydraulic gradient at which suffusion is initiated. This paper presents the initial findings of the downflow test. Eighteen downflow tests have been carried out on fourteen clay-silt-sand-gravel soils. The Kenney and Lau (1985, 86) method, which is commonly used for assessing the internal stability of coarse-grained soils, appears to be too conservative when used to predict the internal stability of silt-sand-gravel or clay-silt-sand- gravel soils, whereas the Burenkova (1993) method appears to provide better predictions. Further testing is required to define more accurate criteria for determining the internal stability of broadly-graded clay-silt-sand-gravel soils.
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2005 Papers
2005 – Use of No Erosion Filter Design for Control of Seepage in Desiccation Cracks for the Ross River Dam Upgrade
Learn moreThe Ross River Dam was first commissioned in 1974 and raised in 1976. The 8200 m long
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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 in the 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. -
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2005 Papers
2005 – Use of No Erosion Filter Design for Control of Seepage in Desiccation Cracks for the Ross River Dam Upgrade
Learn moreJon 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
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2005 Papers
2005 – Use of Risk Based Design Validation for Justification of the Ross River Dam Upgrades
Learn moreThe Ross River Dam, designed in the early seventies, does not meet current dam safety criteria for overtopping and piping within the embankment or the foundation. The dam comprises a 40m long concrete overflow spillway flanked by a central core rockfill embankment of 130 m on the right bank and 170m on the left bank with a 7620 m long left bank earth fill embankment, which has no internal filter zones for piping protection. The embankment was extensively assessed and treated forfoundation deficiencies in 1982, and further assessed in 20002002 for appropriate upgrade options.
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This paper describes the process of validation of the detailed design using Risk Based Design Criteria. This process included data mining for historical performance and original design intention,
comparison of the original design against current and historical investigations and assessment of the upgrades using the large volume of data available from previous work. A design team comprising specialist hydrologists, hydrogeologists, geologists, geotechnical and dams engineers worked within a risk assessment framework at all stages of the design to ensure the design was validated using the design Validation Model. This process incorporated assessment of crest level based on flood risk and wave overtopping, review of 2D and 3D seepage models to assess piping and foundation erosion potential, assessment of fissured soils within the embankment foundation for structural stability and evaluation of spillway model testing for potential spillway failure modes. -
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2005 Papers
2005 – Use of Risk Based Design Validation for Justification of the Ross River Dam Upgrades
Learn moreMalcolm Barker, Jon Williams and Chi Fai Wan
The Ross River Dam, designed in the early seventies, does not meet current dam safety criteria for overtopping and piping within the embankment or the foundation. The dam comprises a 40m long concrete overflow spillway flanked by a central core rockfill embankment of 130 m on the right bank and 170m on the left bank with a 7620 m long left bank earth fill embankment, which has no internal filter zones for piping protection. The embankment was extensively assessed and treated for foundation deficiencies in 1982, and further assessed in 2000-2002 for appropriate upgrade options.
This paper describes the process of validation of the detailed design using Risk Based Design Criteria.This process included data mining for historical performance and original design intention,comparison of the original design against current and historical investigations and assessment of the upgrades using the large volume of data available from previous work. A design team comprising specialist hydrologists, hydrogeologists, geologists, geotechnical and dams engineers worked within a risk assessment framework at all stages of the design to ensure the design was validated using the design Validation Model. This process incorporated assessment of crest level based on flood risk and wave overtopping, review of 2D and 3D seepage models to assess piping and foundation erosion potential, assessment of fissured soils within the embankment foundation for structural stability and evaluation of spillway model testing for potential spillway failure modes.
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2015 Papers
2015 – Estimation of Seismic Hazard for Dams with a Consistent Risk Approach
Learn moreMaz 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.
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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 -
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2015 Papers
2015 Poster – The human elements of a successful dam portfolio safety review
Learn morePhillip Kennedy, Robert Murphy, Pat Russell, Chi Fai Wan
Central Highlands Water (CHW) owns thirty four dams varying significantly in size, age, and condition. Thirty of the dams are used for water supply purposes with the remainder providing storage for wastewater reuse schemes. Out of the thirty-four dams, eighteen are more than one hundred years old. They are zoned earthfill embankments, some with a puddle clay core. Fourteen of the dams have been assessed as having potentially high to extreme consequences if the dam fails. The key safety issues among these high consequence dams are inadequate flood capacity, slope instability, and high potential for piping.
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CHW’s management policy includes a commitment to identify, assess, prioritise improvements to, and periodically review the safety of its dams, and implement a dam safety upgrade works program. CHW’s Water Plan 3 (2013 – 2018 economic regulatory period) includes nine dam safety upgrade projects, which were identified from risk assessments and investigations carried out over several years.
In 2013, CHW and MWH formed a Delivery and Operational Efficiency Review (DOER) Group to refine and confirm priorities for the proposed dam safety upgrades. The main objectives of the DOER Group were to identify solutions to meet current ANCOLD guidelines and any opportunities to achieve 10% – 20% reduction in capital expenditure costs during planning or delivery of the works for Water Plan 3, while achieving the intended risk reduction. The key elements of the DOER were to (1) form a working group to cover operational, planning and executive management considerations together with dam safety consultants and Victorian dam management experience; (2) closely scrutinise previous assessments; (3) challenge the justification for the project; (4) understand the priorities whilst aiming to deliver a major works program; and (5) identify additional investigations.
Initial investigations of the DOER Group developed a revised program of works allowing confirmed capital works to proceed while investigations into other projects were carried out. The follow-up investigations have identified optimal outcomes through a program of cost-effective solutions for CHW.
This paper aims to share the experience from planning the DOER, and the further investigations that resulted in the development of an optimised delivery strategy for the upgrade projects.
Keywords: Delivery and Operational Efficiency Review, Risk. -
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2012 Papers
2012 – Oberon Dam – Failure hazard of a buttress dam and its vulnerability to earthquake damage
Learn moreChi-fai Wan, Jason Hascall, Andrew Richardson, John Sukkar
Oberon Dam is the major headwork of the Fish River Water Supply Scheme providing bulk water supply to Oberon Shire and Lithgow City Councils, Sydney Catchment Authority, and Delta Electricity. The dam is owned and operated by State Water Corporation (SWC).
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Located on the Fish River 2km south of Oberon in New South Wales, Oberon Dam was completed in two stages in 1946 and 1957. In 1996 the dam was upgraded to pass the 1993 Probable Maximum Flood estimate by raising the dam 1.77m and constructing a 50m wide auxiliary spillway on the left abutment. The upgraded dam comprises a 232m long, 35.3m high concrete slab and buttress section and a 165m long earth embankment section.
A typical buttress dam has its inclined upstream face made up of relatively thin reinforced concrete slabs supported by but not integral with the buttresses, making a relatively flexible dam structure vulnerable to earthquake damage.
As buttress dams evolved from concrete gravity dams, their structural design follows the same principles as applied to gravity dams. However, many buttress dams were designed over 60 years ago using outdated methods that did not consider earthquake loads. Current overseas and local design guidelines do not provide sufficient guidance for checking the seismic stability of existing buttress dams. For instance, the simplified seismic analysis, proposed by Fenves and Chopra to investigate the seismic response of gravity dams to earthquake loads in the upstream-downstream direction, is not applicable to buttress dams which are also susceptible to damage by earthquake loads in the cross-valley direction.
SWC engaged Black & Veatch to carry out a three-dimensional finite element analysis of Oberon Dam to better understand the structural behaviour of the dam under earthquakes. The analysis used both the response spectrum and time history approaches. Due to the uncommon design of Oberon Dam and the limited discussion found in the literature on the dynamic behaviour of buttress dams, the Authors would like to share their experience in the assessment of the hazard, and on the use of modern finite element modelling techniques to investigate the dynamic response of this type of dam.
Keywords: Ambursen dams, Buttress dams, Risk assessment, Time history analysis, Finite element -
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2009 Papers
2009 – Water, Always a Premium in Southern California, Brings Back another Marquee Project, San Vicente Dam Raise
Learn moreChi 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.
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