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.
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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.
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
Alice Lecocq, David Brett, Mike Rankin
Tailings Dams class amongst the world’s largest man made structures. They are interactive structures that evolve over time, with tailings discharge, water management, embankment raising and finally closure and abandonment. Understanding of the design, the impact of operations and regular, committed surveillance is essential to ensure the safety and performance of a tailings dam. Dam Safety Management Plans should be developed to optimise these parameters. These plans should include Operation, Maintenance and Surveillance (OMS) manuals, emergency response plans and monitoring databases. They should be managed by the mine management and implemented by the operations personnel.
The tailings dam operators are the key to a successful dam safety management program. It is imperative that the tailings dam management and operators appreciate the risks inherent with the facility, their role and their responsibilities. They also need to have an appropriate understanding of the tailings dam design features, failure modes and safety triggers. With training it is expected that personnel will be better able to recognise and act on safety issues arising.
The paper presents case histories of tailings dam failures due to poor operation and management and outlines the operational requirements and risks inherent with tailings dams. The paper discusses the training approach and criteria to be adopted, and describes a training course developed by the authors for mine management and operators. The paper examines the feedback collected from the courses held at several mines. A model to successfully implement a surveillance program with the involvement and leadership of the operators is proposed.
Keywords: TSF failures, surveillance program, OMS manuals, training of personnel.
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
Rob Ayre, Simone Gillespie
The Burdekin Falls Dam (BFD) is a SunWater-owned dam completed in 1987. BFD is located in North Queensland, approximately 180 kilometres south of Townsville. BFD is the largest dam in Queensland having a storage capacity of 1,860,000 ML and it has the largest spillway capacity in Australia. The Burdekin River basin drains an area of about 114,770 km2 which is nearly twice the size of Tasmania. Runoff in the catchment is very reliable and flows have overtopped the spillway every year, except one, since it was built. The volume of inflow into the dam during a flood event is considerable, and water spills from the dam for an average of three months each year.
SunWater is investigating the raising of BFD, to increase the storage capacity of the dam by two metres or approximately 30% of its current storage capacity to 2,446,000 ML. In addition SunWater are investigating provisions to further stabilise the concrete gravity main dam to improve dam safety performance by ensuring it complies with current guidelines. Design flood estimation has advanced since BFD was constructed, as the techniques for determining extreme rainfall have been progressively refined. To meet current Acceptable Flood Capacity (AFC) guidelines, the flood discharge capacity at BFD must be increased by 35%. However, whilst this estimate was derived in accordance with current relevant guidelines (ARR, Book VI, 2001) the size of the BFD catchment means that this particular catchment lies on the fringe of the applicability of these guidelines.
Of particular concern is the assignment of the Annual Exceedance Probability (AEP) of the Probable Maximum Precipitation (PMP), which is based upon catchment area. The adoption of the AEP of the PMP for BFD at 1 in 9,000 has implications for the application of risked based approaches for the design.
This paper discusses the existing methodology of design flood hydrology used in Australia and identifies areas of concern for the application of such techniques for large catchments. It also discusses the methodology SunWater utilised in an attempt to meet existing guidelines within these limitations.
Keywords: Burdekin Falls Dam, Flood Hydrology, Probable Maximum Precipitation, Annual Exceedence, Probability