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.
Glen Hobbs and Danny Azavedo
Recent years have seen a growing awareness and understanding of the factors that contribute to the reliability of spillway gates and the incorporation of reliability data into overall dam risk studies.
The study of a number of spillway gate failures shows that no one component or incident leads to gate failure, but rather a combination of factors have resulted in gate failure. A rigorous reliability assessment should consider all factors, not only the equipment condition and performance but the complete system, from the receipt of data through to the actuation of the gates. It should take into account issues such as human factors, poor design, maintenance history and policy. Unfortunately one of the main hindrances to quantifying gate reliability is the lack of information on spillway gate equipment and system performance and failures.
This paper considers a number of gate failures, then looks at some of the tools of reliability assessment and the role of human factors in gate reliability.
The paper then discusses a recent study of four gated dams. For this study a systems approach was adopted and human factors were considered. The results compare favourably with other similar critical structures, and show that for these well designed and maintained structures human factors are the limiting criteria in multiple gate operations. The study also shows that the probability of opening all the spillway gates at a dam improves with time (2-4 hours) during the flood operation, and it is considered that time based reliability provides a more meaningful and useful assessment of overall spillway gate reliability.
B. A. Cole
In October 2000 ANCOLD published a history of dam technology in Australia covering the 150 years in which large dams have been constructed in this country. The paper describes how this project began, the search for authors, the way the authors tackled their tasks, the peer reviews which resulted in additional chapters being written, and the archive searches for interesting photographs to illustrate the text. All this was accomplished by dam engineers including the editor. Then follows an account of the professional publication process: sub-editing, desktop publishing, proof-reading, the preparation of an index, the cover design and the printing process. Some conclusions are drawn from this first experience of book publishing.
This paper describes the use of a high strength woven geotextile and preloading to stabilise the surface of a very low strength tailings pond, and the incorporation of a geosynthetic clay liner (GCL) within the final capping design to complete closure. The pond, which contains tin and copper tailings, formed the lower tailings containment area of a three-tiered tailings storage, located directly above the Wild River in North Queensland. Stabilising the lower pond (area 2,500 m2), which contained tailings of “zero strength” in the central area involved the placement of a woven geotextile over the surface, which was anchored around the perimeter. The placement of finger berms (preloading fill) on the geotextile was successful without exceeding the bearing capacity of the tailings overall. Settlements of the berms were closely monitored to allow the system to support construction plant. After the finger berms were joined, they were widened until the area was covered. A sand layer was then placed over the area followed by a GCL to form an impermeable barrier prior to the placement of clay and topsoil.
A. Ash, D. S. Bowles, S. Abbey and R. Herweynen
A preliminary risk assessment was undertaken of its three dams by the South East Queensland Water Board (SEQWB) in 1999. The risk assessment process used included a series of workshops that proved to be an important part of ensuring a worthwhile result. The combined expertise of the consultants together with that of staff from the Board and the Queensland’s Department of Natural Resources were used to improve the outcome. The results of the assessment showed that the process had both advantages as well as difficulties in comparison to a standards based approach for making dam safety decisions. Risk Assessment was seen to be a useful management tool for managing dam safety. It gave the owner the ability to quickly rank upgrade requirements or maintenance options on the basis of probability of failure, life safety risks and financial risks to the owner or economic risks to all stakeholders.