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.
Gavan Hunter, Chris Chamberlain, Mark Foster
Hinze dam, an extreme hazard storage, is under the authority of Seqwater (Southeast Queensland) and is principle potable water storage supplying the Gold Coast. Hinze Dam Stage 3, presently under construction, involves raising the existing embankment almost 15m to a maximum height of 80m.
The foundation geology on the right abutment of the main embankment comprises of a deeply weathered sequence of greywacke and variably silicified greenstone and chert. The deeply (and variably) weathered soil profile below the right abutment of the existing embankment presented an unacceptable piping risk for the embankment in its existing condition. Contributing factors included: 1/ the highly erodible extremely weathered greywacke and presence of continuous defects in the weathered soil mass; 2/ the extremely weathered greenstone in direct contact with highly fractured, highly permeable silicified greenstone and chert bodies aligned normal to the dam axis which provide continuous seepage paths through the foundation.
Works were required as part of the Stage 3 raise to address the foundation piping risk. Significant issues for design included: 1/ the depth of weathering extended up to 25to 40m into the foundation.; 2/ extremely weathered and highly erodible greenstone was present below the right abutment of the embankment and extended down to the lower abutment some 50 to 60 m below the existing dam crest; 3/ the reservoir level could not be drawn down during construction and the probability it would be near full supply level during the works was high; and 4/ the variability of strength in the greenstone form soil to extremely high strength presented challenges for excavation.
The options assessed to address the piping risk included a plastic concrete cut-off wall and an upstream blanketing option. The plastic concrete cut-off wall (220m long and up to 50m deep) and deep filter trench was the selected option. The cut-off wall had been successfully completed ahead of time and below budget. The innovative design required excavation through earthfill core of the embankment under full reservoir level and use of a purpose built trench cutter (by Bauer Foundations Australia) for the variable excavation conditions.
Keywords: dam safety, piping, risk assessment, cut-off wall.
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.
John Prentice, Jim Barrett, Dr Martin Mallen-Cooper
Located in south-eastern Australia, the River Murray has provided almost a century of regulated water supply, bringing immense benefit to human welfare during this period. However a healthy river is recognised as being essential to its sustainable future. With this in mind, in March 2001 the Murray- Darling Basin Ministerial Council approved several actions including “a structural works program to provide passage for migratory fish, from the sea to Hume Dam”. The paper outlines progress to-date on this ambitious fishway construction program.
The program involves building fishways on twelve of the fourteen weirs on the river, and at the barrages. The criteria established, to enable passage for all native fish species known to regularly migrate, are detailed. The important lessons learned from earlier fishways, and confidence gained from the Torrumbarry Weir vertical-slot fishways constructed in the 1990’s, have been applied to the new designs. Part way through the program, important changes were made to the design criteria, and the reasons for doing so are described.The fishways constructed at the barrages near the Murray mouth, and their need to respond to estuarine and lake conditions, are recognised. In addition, Mildura and Euston Weirs vary from the other River Murray structures, and their special fish passage consideration is described.
In achieving a successful outcome to date, the important role of river managers, engineers and scientists working together with design consultants is acknowledged. A tri-State monitoring and assessment program has been established to enable the questions about the short and long term outcomes of the program to be answered. The beneficial results to date are detailed.
This paper outlines the steps and challenges involved in delivering this decade long program, scheduled for completion in 2011.
Keywords: Sustainable, fishways, fish passage, native fish, hydraulics, fish monitoring, Murray.
Peter Cordi, Paul Fuller
Tallowa Dam was completed in 1977 at the junction of the Shoalhaven and Kangaroo Rivers in the southern highlands of NSW to provide a pumping pool for water supply transfers to Sydney. These transfers were made only during drought periods, at which time limited and fixed environmental flow releases from a low level outlet were made to the downstream Shoalhaven River. After extensive consultation with the local community the Government decided in 2006 to commence transfers earlier in the drought cycle, and release variable amounts of surface water to improve river health during transfer periods. In addition, Tallowa Dam was identified as having a significant impact on fish passage, as many species migrate to the estuary during their life cycle, and approximately 75% of the viable fish habitat was upstream of the dam. This project involved the design and construction of works to be retrofitted to the dam to address both issues. A surface water release slide gate in the spillway, a low friction coating on the spillway, and a downstream weir were constructed to release environmental flows and allow safe downstream fish passage. A new fish attraction flow outlet was drilled through the dam wall, and a fish attraction chamber and a travelling bucket fish lift was installed for upstream fish passage.
Keywords: environmental flows, fish passage, Shoalhaven River, construction.
Tim Logan, Angus Swindon, Chris Topham
Edgar Dam is a 17m high saddle dam forming part of the Gordon River Power Development (GRPD) in south west Tasmania; the smallest of three dams, which created the current Lake Pedder. It is essentially a homogeneous embankment, designed and built between 1970 and 1972. It is assigned a “High A” Hazard Category. An unusual feature of the dam design is a reinforced concrete facing on the upstream face, crest and the upper portion of the downstream face provided primarily as protection against wave overtopping.The upstream facing is bedded on drainage material encapsulating a longitudinal drain 1.6m above the level of the bottom of the concrete. This drain is connected to four transverse drains (100 mm diameter PVC) which run through the body of the dam and discharge through the concrete slab on the downstream face. The screening level risk assessment for Edgar Dam identified piping through the embankment as the predominant failure mode, particularly related to the transverse drains and the uncertainty surrounding the competency of the backfill around the pipes. To address this, the condition of drain backfill has been assessed using geophysical logging, supplemented by an internal video inspection. The information has allowed a more detailed risk assessment to be performed and potential mitigation measures to be assessed.
Keywords: Risk Management, Dam safety, Conduits, Geophysical Logging.