Mark Locke, Buddhima Indraratna, Phillip Cummins and Gamini Adikari
ABSTRACT: Australia has a large number of older embankment dams, which have been in service and performed adequately for over 50 years. However, current industry practice in embankment dam design predicts that the granular filters within these dams may not be adequate. This may require refurbishment of the dam by retro-fitting a new filter to ensure the continued safety of the structure. This paper outlines the potential problems with older embankment dam designs, and the reasons for constructing a new filter. Potential problems may include inadequate or non-existent filters, risk of failure due to earthquake, piping, or excessive foundation seepage. Design methods for granular filters are described briefly, concentrating on whether an existing filter is adequate, and the potential improvement by constructing a new filter. Construction issues for placing filters on existing dams are also discussed.
A new analytical method, developed to describe the time dependent erosion and filtration within embankment dams, is described briefly. The model predicts particle erosion, transport and retention based on fundamental fluid mechanics and geotechnical concepts. The application of this model to the design of filters for new and existing dams will be described. The predictions of such analytical modelling can give a designer a significantly clearer picture of the purpose of a granular filter, the extent of core erosion that can be expected, and the effect of retrofitting a new filter to an existing dam.
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I. R. Forster
Lyell Dam is a concrete-faced rockfill dam, located on the Coxs River, near Lithgow, NSW. The dam forms part of the Coxs River Water Supply Scheme, which supplies water to Delta Electricity’s Wallerawang and Mount Piper Power Stations. In 1994, the spillway capacity of the dam was upgraded, and the storage augmented with the addition of two 40 m long by 3.5 m high inflatable rubber dams to the spillway crest. An automatic deflation system, controlled by a programmable logic controller, was installed to provide a staged bag deflation sequence during flooding, and hence minimise the downstream impact of rubber dam operation.
Although the rubber dams and control system initially operated as designed, more recently, two uncontrolled bag deflations have occurred, which have caused flooding downstream and loss of significant storage volumes. In the first incident, a spontaneous uncontrolled deflation of the rubber dams released about 1600 ML, before the bags re-inflated automatically. An investigation revealed that the incident was most likely the result of design deficiencies in the control system. Recommendations were made for improvements to the system.
During the most recent deflation, one of the rubber dams failed by spontaneous rupture, and approximately 6000 ML of water was released from the dam. The Dam Safety Emergency Plan was activated to ensure persons at risk downstream were notified of the impending flood wave. A post- failure inspection of the ruptured bag suggested that the likely cause of failure was a manufacturing defect, which allowed air to penetrate the layers of rubber forming the bag. The rupture most likely occurred when the resulting air pocket expanded on exposure to the sun.
The paper examines the two deflation incidents in detail, and analyses the emergency response to the second incident.
Pieter van Breda, Peter Walton, Kate Lenertz and Tim Sheridan
The Warragamba Dam Auxiliary Spillway Project, designed to manage floodwaters up to a Probable Maximum Flood event, was approved by the NSW Minister for Urban Affairs and Planning on February 12, 1998. An Environmental Impact Statement prepared for this project predicted that noise, dust (suspended and deposited), blasting, vibration, water quality and revegetation would be the significant environmental issues requiring management throughout the construction phase.
The closest residents are approximately 200m from the construction activity. The works must not interfere with the operation of the Dam, which stores 80% of Sydney’s drinking water and the integrity of the existing infrastructure must be maintained at all times. The approved proposal was to emplace the 2.2Mm3 of spoil excavated to create the spillway in an area 25 ha by 20m high on top of a ridge on the left bank adjoining the Blue Mountains National Park. This created visual impact and rehabilitation challenges.
Although the contract for this project was primarily performance based, strict environmental clauses were incorporated to manage these priority issues. Noise and dust modelling were required from each pre-qualified Tenderer, to demonstrate capability of compliance with NSW Environment Protection Authority requirements. This formed part of the tender assessment. Criteria were also developed for revegetation, specifying numbers of endemic trees, shrubs and grasses per 400m2 of spoil emplacement in order to create a floral community similar to the existing adjacent National Park.
The implementation of these requirements and the development of a site Environmental Management Plan by the Sydney Catchment Authority, Australian Water Technologies and Abigroup Contractors, whilst maintaining productivity, has proven to be a working example of the benefits of Partnering.
Michael Somerford, Michelle Northover and Steve Wilke
Western Australia’s Water Corporation is constructing the Stirling-Harvey Redevelopment Scheme, a $275 million scheme to supplement Perth’s public supply. A major component of the scheme is the construction of the Harvey Dam, a 45 metre high, earth core rockfill dam.
The main environmental issues associated with the construction of the Harvey Dam are related to construction and traffic noise, blast vibration and dust generated during the construction period. Appropriate environmental management is required to minimise noise and dust emissions because of nearby schools, town site, residences and horticultural activities.
The new reservoir will commence filling in 2002. It will inundate several private properties, farming land, an area of pine plantation and six sites of cultural and heritage significance.
This paper discusses the management and monitoring strategies associated with the construction of the new dam. It also describes the initiatives that the Water Corporation has undertaken to ensure that adverse impacts of the project on the environment are minimised.
D. S. Bowles
Portfolio risk assessment (PRA) can now be considered to be a standard of practice in Australia. In this paper various advances in the state-of-the-practice for performing PRA’s are reviewed, including some pitfalls and limitations. The uses of PRA outcomes by owners are discussed, along with some ways to improve the value derived from PRAs. The challenges that are common in seeking to achieve an integration of the PRA process into the owner’s dam safety management program and with broader business processes, and the importance of targeting PRA outcomes to an owner’s specific business needs, are emphasised.
P.W. Heinrichs and R. Fell
Ben Boyd Dam, a 29 m high earthfill embankment built in 1978, has had an unusual history. In 1979, a number of seeps developed during first filling with water 5 m below FSL indicating unexpectedly high pressures. Investigations concluded the coarse filter permeability was very low due to excess fines. Remedial works in 1982 included a drainage filter beyond the toe and a new stability berm above. New piezometers were installed, including several in the blanket filters in the existing dam. These later indicated up to 10.5 m head in isolated areas within the filter. Pump out tests partially lowered the water level in the standpipes but in 1995 the water level rose by 4 m back to its previous high level. All this during a period of relatively low rainfall. Stability analyses were carried out and further investigations in 1999 concluded that apart from general leakage from the foundation abutment into the filters, the rise in pressures was due to leakage from a riser hole from one of the nearby foundation piezometers. A potential for piping along the piezometer tubes within the dam was also identified.
This situation was managed without resort to costly capital works, because it was concluded that the pressures from the vertical riser were not a potential failure mode, and potential piping failure would be adequately handled by the existing chimney drain, intersecting the piezometer tubes trench. Any potential piping failure would also give warning signs which increased frequency of monitoring (now in place) would pick up in time to allow lowering of the storage.