Robert Fowden, Peter Allen, John McKenna
The Large Referable Farm Dam Assessment Program commenced in early 2006 after inspections identified a significant number of Queensland dams that were unknown to the Department of Environment and Resource Management (DERM) and could potentially threaten life if they were to fail. The program is unique given the number of structures under consideration and is understood to be the first widespread, systematic search for dams with a population at risk in the world. The Dam Safety (Farm Dams) team has developed many original solutions to allow the majority of investigations to be undertaken in-house, thus minimising the potentially higher cost and timeframe issues associated with obtaining external engineering and surveying support.
Keywords: Queensland, dam safety, dam failure, regulation, farm dams, surveying, modelling
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Thomas Vasconi, Glen Fergus
Abstract: This paper describes the design of an 80 m-high stepped chute spillway, in gabion material, that will be constructed on a tailings storage facility dam of a mine in South East Asia. This dam, constituted of two cells, will be raised progressively via a series of intermediate crest elevations as mining proceeds, and each lift will be equipped with an operational spillway. The design of such spillways was challenging since it had to integrate local topography configuration, dam design, water balance, wall raise sequence and structure interdependency parameters. The design included flood routing, spillway sizing, stepped spillway design, followed by hydraulic and civil/geotechnical computations. Challenging design aspects included optimizing the stepped spillway structure costs in light of the structure’s short service (estimated to be less than 5 years), and ensuring the stability component. The design incorporates an innovative solution which allows reduction in the rockfill quantity of up to 40% with associated cost benefits, and sustainability in terms of material usage. The lessons learnt in applying this innovative design are useful for other sites requiring adaptive construction and short service life spillways.
Keywords: tailings dam, stepped spillway, hydrology, hydraulics, mine water management, gabions.
Many earthen dams and embankments throughout the world are in need of remediation to address seepage or other issues and ensure structural integrity. Borehole drilling plays a vital role in facilitating implementation of remedial designs, both in the initial information gathering stage and the actual construction of a chosen remedy.
Within the past six to eight years Sonic drilling has become recognized within the geotechnical community as a viable method to meet overall project objectives and address site specific issues for a variety of projects. Key aspects of Sonics include: the ability to efficiently penetrate difficult subsurface conditions, provide a continuous core sample of unmatched quality, and minimize or eliminate risk to the structure from the drilling process. This paper focuses on the application of Sonic in support of a remedial effort at Wolf Creek Dam, including information on the background and overall objectives of the project, a brief explanation of the Sonic method, the scope of services required at the site, and the specific reasons for utilizing Sonic in this case.
Keywords: sonic drilling, grout curtain, Wolf Creek Dam, dam remediation
Amanda Barrett, Mike Marley, Tariq Rahiman
The site of the Wyaralong Dam, west of Beaudesert, Queensland, has been investigated in progressive stages since 1991. The first stage of the investigation was a siting study and the second in 2006 was designed to gather sufficient geotechnical information to develop a preliminary design and provide input into the environmental approval process. The third stage of the investigation was designed to gather further information to allow the detailed design of the dam to commence. This iterative investigation approach has ensured confidence in the site geology and geotechnical model.
The site investigations have included diamond drilling, piezo-cone penetration testing, geophysics techniques, hydraulic conductivity testing, groundwater pumping tests, costean excavation, geological mapping and accompanying laboratory test programs. Investigations have been targeted to assess the foundation conditions for the proposed engineering structure and have been refined to the needs of the dam design as it has developed.
With sufficient data, a 3-dimensional geotechnical model has been developed using the computer modelling program Vulcan, to assess the position and influence of a number of key geological features observed in the site investigation. Assessment of engineering properties based on in situ and laboratory testing were then extrapolated across the site through application of the geotechnical model.
Keywords: Wyaralong Dam, geotechnical investigation, geology, 3-D Vulcan model, foundation.
Bob Wark, Alex Gower. Graeme Mann
Stirling Dam is a 53 m high extreme hazard zoned earthfill dam located in south west WA. Construction was completed in two phases between 1939 and 1947. Recent safety reviews confirmed that the societal risk exceeded the ANCOLD guideline tolerable limit due to inadequate spillway capacity and the lack of embankment filters. Remedial work would involve: widening the spillway; removing the downstream shoulder of the dam; adding downstream filters; and reconstructing the downstream shoulder fill. Rock from the spillway excavation would be used to provide the fill for the downstream shoulder. The works optimisation involved a 3 m raising of the embankment to provide the required spillway capacity.
The design criteria included: ensuring the risks of failure during construction were to be no higher than the risks prior to remedial works; maintaining reservoir operation during construction; and no river releases based on median monthly inflows. This required the spillway crest to be temporarily lowered during construction to provide adequate flood capacity while the embankment height was reduced. A key feature of the design had also been the scheduling of the storage drawdown and remedial works to manage the failure risk and probability of river releases during construction. Higher than average inflows after contract award resulted in water levels above the scheduled drawdown curve. This lead to river releases to prevent spillway flows and rescheduling the onstruction over two seasons.
Keywords: Stirling Dam, water conservation, embankment filters, spillway capacity, construction scheduling
Giovanni De Cataldo
The ANCOLD Guidelines on Dam Safety Management August 2003 were formulated to ensure that dam owners adopt a responsible approach towards the safe operation and maintenance of their dams.
Is it possible to safely, responsibly and acceptably work outside the regulatory Guidelines/Requirements?
The challenge for dam owners now and into the future in meeting stringent standards, is to cost effectively manage their assets within available financial constraints whilst minimising risks and maintaining acceptable levels of safety.
With the continuing drought and suppressed storage levels in most dams, the risk to downstream communities and to the environment from dam failure is significantly reduced.
Based on various studies, investigations, internal workshops and external “Expert Panel” reviews, this paper puts forward a case for a sound and responsible risk-based approach to routine visual and surveillance monitoring frequencies at varying storage levels for “Sunny Day” conditions and compares it against traditional ANCOLD standards which are based solely on consequences.
Keywords: State Water Corporation, ANCOLD guidelines, risk-based approach, dam safety, regulator.