Chi-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.
Murray Gillon, Robin Fell, Harry Keys, M Foster
Volcanic eruptions at Mt Ruapehu in 1995-96 resulted in the deposition of about 7m of tephra over the rock rim overflow of Crater Lake. There is a long history of lahars (debris flows) associated with releases of water from Crater Lake. The 1995-96 eruptions emptied the lake and it has slowly been refilling from rainfall runoff and snow melt. When the lake level rises above the rock rim the tephra layer will act as a “barrier” or dam. Breaching of the barrier will release water and generate a lahar. The magnitude of the lahar flow will be a function of the lake level at the time of breaching
Extensive studies of the effects of the lahar that would be generated by the failure of the tephra barrier have been undertaken. The studies included a failure modes and likelihood analysis to provide information on the relative likelihood of failure as the lake level rises for the different failure modes applicable to this situation. The paper describes the failure modes considered and the results of the analysis.
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.
The ANCOLD Guidelines for the Design of Dams for Earthquake (ANCOLD 1998) were developed by a working group convened by the author over the period 1993-1998.
Since the release of the guidelines there have been some important developments in the design of embankment dams for earthquakes, and it has become apparent some aspects of the guideline could be explained better.
This paper outlines some of these issues with a view to encouraging the Profession to use up-to-date methods.
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