Tommie Conway, Katherine Miller, Peter Hill
The ‘Black Saturday’ fires of the 7th of February 2009 and the continuation of fires over the following weeks had devastating human, environmental and financial costs for Victoria. Many of Melbourne’s water supply catchments and assets were burnt and the major harvesting catchments were seriously threatened. This paper highlights the need for owners and managers of catchments, dams and associated infrastructure to better understand and plan for the potential impacts of fire, given its predicted increased likelihood and severity due to climate change.
The paper will share Melbourne Water’s recent experiences of the fire, the scale of the impact to the business in terms of assets damaged and catchments affected, the extent of the burn and the threat that was faced. It will describe Melbourne Water’s experience with the United States Burnt Area Emergency Response (BAER) team to expediently map the severity of the fires, to identify areas of concern and prioritise fire recovery works. Of interest to those involved in risk management will be the discussion of the construction flood risk analysis at Tarago Reservoir which was revisited due to severe fire damage to the catchment.
Keywords: fire impact, Melbourne’s water catchments, BAER team, hydrology, Tarago, construction flood risk analysis
Jonathon Reid, Chris Kelly, Bob Wark
One of the most important aspects in the construction of an embankment dam is to be confident that the filter materials placed meet the design intent. The design methodology for filters is now well documented.
However, all too often during construction the filter material, as placed, does not comply with the specified requirements and all parties are faced with costly decisions and delays to the works to determine correction measures and whether the work completed meets the design intent. This paper shares the knowledge gained over a number of projects the authors’ have been involved in and the methods used to improve the properties of the placed filters taking into account some of the practicalities of having these materials produced and placed in a commercial environment
Keywords: filters, specifications, manufacturing, construction, quality assurance.
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.
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.
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
Australia’s prosperity is closely linked to the development of mining. Tailings production has always been associated with mining and acceptable management strategies of tailings have progressively developed to meet ever changing community expectations. In the late 1800’s, tailings were typically dumped into streams or onto land as mullock heaps, resulting in severe pollution. Practices gradually changed so that by the 1920’s tailings were often held in dams or ponds. However failures were common with slugs of slimes and contaminants moving down watercourses. For the purpose of protecting life and property, States started regulation of the management of tailings under various dam safety umbrellas in the late 1980’s. In 1995, Queensland, in consultation with stakeholders, produced tailings management guidelines, which enunciated good tailings management principles. Later guidelines have incorporated many of these principles. In 2002, the regulation of tailings disposal in Queensland moved into the Environment and Resource Management framework, where the emphasis is on obtaining a sustainable environment. Emerging practices are seeking better ways of incorporating mine tailings into the environment with minimal impact. Backfilling of mine workings, integration of mine waste facilities and beneficial use are some of the methods now used for tailings disposal. This paper looks at the historical management of tailings, the evolving regulatory framework, and the emerging practices for protecting the environment while allowing for development that improves the total quality of life, both now and in the future, in a way that maintains the ecological processes on which life depends.
Keywords: Dams, Tailings, TSF, Community, River Pollution, Cleanup, Risk, Mining