Rick Friedel, Len Murray, Gerrad Suter, James Penman, James Watt, Hendra Jitno
The Hidden Valley tailings storage facility (TSF) has set a new precedent in environmental management of tailings in Papua New Guinea (PNG). Modern mining in PNG arguably began with the development of Bougainville Copper in the late 1960s, and continued through to Ok Tedi, Porgera, Lihir, Misima (and others). These mines have proceeded with deep sea or riverine tailings deposition, rather than construction of a tailings dam to retain the mine waste within an impoundment; as is the practice throughout the majority of the mining industry.
The Hidden Valley TSF is comprised of two large earth and rock fill dams, raised by the downstream method. Starter dam construction was completed in 2009. At final height the Main Dam will be one of the highest tailings dams in the world. The dams are constructed of pit waste and therefore have the dual function of storing tailings and waste rock.
Construction of the starter dams and subsequent raises is complicated by conditions at the site. Water management was, and remains, the dominant issue. High rainfall, weak erosive soils, material availability, dense vegetation and remoteness of the site provide constant challenges to construction. The Observational Approach to construction was recommended by the designers and adopted by the mine operator. This involves a knowledgeable pre-assessment of what is likely to change and having contingency plans to deal with possible major issues. This approach allows changes to the design during construction so the “as-built” product is suited for the site, fit for purpose, and remains consistent with the overall intent of the design.
The TSF has been in operation since August 2009 and monitoring data of the structures has been collected during construction and operation. This data is reviewed to confirm design assumptions and assess dam performance.
Personnel involved with this project combined their experiences working in the PNG environment and dam building from other locations. This process led to close interaction between the mine operators, designers and construction teams. Team work and diligent construction practices were and will continue to be necessary to construct and operate the pioneering TSF in PNG.
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David Ryan, Peter Richardson, William Steen
Ibis Creek Dam, a referable dam and classified as a mass concrete gravity structure, was constructed in 1906 to supply water for both tin ore processing and the local township of Irvinebank. Irvinebank is a small township near Atherton in North Queensland and is situated about 3 km downstream of the dam. The mill ceased operation in 1990 but the township of Irvinebank remains reliant on the dam for water supply.
In 1996 the dam was raised about 1 m and strengthened by the addition of mass concrete on the crest and downstream face.
One recommendation of the Safety Review conducted in 2009 was that an investigation be made of the strength of the lift joints and the shear capacity of the connection between the Stage I and Stage II concrete sections. The investigations revealed that the structure was not constructed as had been originally assumed and the overall stability of the structure had been overestimated.
This paper details the investigations and remedial works proposed to strengthen the structure so that it complies with current design standards.
Glen Hobbs, Robert Rigg, Alan Hobbs, Adam Butler
Maintenance errors and associated non-conformances are becoming increasingly recognised as a source of system failures in a wide range of industries. Research in other industries has shown that errors often arise in response to local factors beyond the control of the maintainer. Various dam ‘incidents’ have been attributed to maintenance errors. In Australia we have been fortunate with few serious dam safety events. However, the dam operating and maintenance environment is changing dramatically.
A survey of dam maintenance personnel was recently undertaken in Australia. The survey was in the form of 49 questions that asked participants to state how frequently a situation occurred. This survey format has previously been used in other industries; thus allowing a comparison of dam maintenance with other high-risk industries such as rail infrastructure, oil and gas, and airline maintenance.
A number of ‘error-producing’ conditions have been identified and survey results indicate a high level of poor procedures/documentation and supervision; highlighting the need for accurate and appropriate manuals and supervision of tasks. These and other factors are leading to instances of maintenance non-compliance, which may threaten the reliability and safety of equipment. The survey has revealed that trade training needs to be addressed. However, occupational safety issues are low; indicating a positive approach to a safe working environment. The paper also discusses the responses to specific maintenance questions relevant to the dam industry.
Cubit T, Swindon A, Tanner D
Catagunya Dam is located on the Derwent River in Tasmania’s south east. During construction of the dam in early 1960’s 412 post-tensioned anchors were installed, however the integrity of the original anchors can no longer be assured. The stability of the dam was restored between 2008 and 2010 using 92 modern, large diameter, load monitorable and corrosion protected post-tensioned anchors. These are the most highly stressed anchors applied to a dam at this time.
Some of the key construction challenges included installing 53 anchors within an operating spillway, utilising a very limited construction window and replacing severed surface reinforcement using carbon fibre rods.
This paper details how these challenges were resolved and presents a number of innovative solutions developed along the way.
Paul C. Rizzo, Ph.D., P.E.; Carl Rizzo; John Bowen
The Authors served in key roles for the design and rebuild of the Dam for the Taum Sauk Rebuild Project between 2007 and 2009. Taum Sauk is the largest RCC Dam in the United States and has a symmetrical cross-section with conventional concrete faces upstream and downstream. The curvilinear shape and the cross-section presented a number of placement issues. In addition, a large number of “Lessons” were learned because of the rapid construction schedule, highly variable temperatures, highly confined working space, numerous details related to waterstops, construction joints and crest-to-gallery drains, foundation preparation, lift maturity, bedding mixes, crack repairs and the conventional concrete upstream face. The authors discuss these issues from the perspective of the Designer, Contractor and Construction Manager.
Ben Greentree, David Bamforth, Matthew O’Rourke and James Willey
A series of relatively small floods occurring between end of construction in 1978 and late 1980s caused extensive and dramatic rock erosion to the very steep unlined section of the Googong Dam spillway. Following a review of hydraulic performance at larger floods, the spillway’s future erosion potential was evaluated and it became clear that extensive remedial work was required. A detailed design was developed comprising the retro-fitting of a full concrete-lined chute, the raising and extension of the spillway chute walls, strengthening of the upstream training walls and excavation of a large plunge pool. The Googong Dam has an ANCOLD hazard rating of ‘extreme’ because of its location upstream of Queanbeyan and Canberra.
In early 2008, the Bulk Water Alliance (BWA), comprising ACTEW Corporation Ltd, (in cooperation with ActewAGL) (the Owner), GHD Pty Ltd (the Designer) and Abigroup Contractors Pty Ltd in joint venture with John Holland Pty Ltd (the Constructors) was formed to deliver a package of water security projects for the ACT, one of which is the Googong Dam Spillway Upgrade.
After preparation of a construction methodology and target outturn cost (TOC), the project was approved by the Actew Board and construction commenced in February 2009. Completion is due in late 2010. A number of significant geotechnical, structural and logistical challenges were encountered during construction, resulting in major changes to the construction methodology necessitating design changes. The changes were incorporated within the original TOC, without instigating scope change contractual claims and while still maintaining spillway functionality in line with Owner operational requirements.
This paper presents delivery phase challenges that necessitated construction methodology and design changes to achieve best for project outcomes; how these challenges were overcome through genuine innovation reliant on a collaborative effort by all the Alliance partners; and how the contractual framework of the Alliance was essential for the change management process to be successful.