Gavan Hunter and James Toose
Hinze Dam, an extreme hazard storage, is under the authority of Seqwater (Southeast Queensland) and is the principal potable water storage supplying the Gold Coast. The Stage 3 raise, presently under construction, involves raising the embankment almost 15m to a maximum height of 80m.
The central core earth and rockfill embankment is founded on competent greywacke rock within the valley floor and left abutment. On the right abutment it is founded on extremely weathered greywacke and rockfill stability berms were constructed upstream and downstream on this weak foundation.
Key issues for the design of Stage 3 embankment raise on the right abutment were: 1/ removal of the existing downstream stability berm and deep excavation at the toe of the Stage 2 embankment to connect into the blanket filters under the downstream shoulder; 2/ the soil strength properties of the weathered greywacke foundation and the presence of pre-sheared defects with strength properties significantly below the strength of the soil mass; and 3/ undertaking the works while the reservoir remained in operation and close to full supply level.
It was not possible to undertake large scale excavation at the downstream toe of the right abutment as the factor of safety for the excavation condition was below the design criterion for slip surfaces extending back to the upstream shoulder. The innovative design solution was for a staged excavation and back fill operation up the right abutment. In this ay the stability requirements were achieved by a 3-dimensional buttressing support and reduced the time that critical excavation sections were exposed.
The construction risk is being managed under a dam safety management plan. Key elements of this plan include instrumentation monitoring and increased surveillance for early detection of a potential incident, a series of trigger levels and responses to these levels, a clear hierarchy of contacts and adequate preparation for a dam safety emergency (including materials, personnel and equipment).
The embankment construction is presently in progress. The most critical sections have been successfully completed without incident and displacements are within the predicted range. Communication and planning within the Alliance between the designers and constructors has been a key element in the successful construction works to date.
Keywords: construction risk, embankment design, embankment construction, dam safety management
— OR —
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
There are many international guidelines, state regulations and technical standards relating to tailings disposal. In addition, the larger mining companies have their own in-house standards and design rules with competent personnel in charge of their operations. Sound embankment design methods can be used by most designers familiar with earth dam design.
The paper gives a listing of many of the current sources of information and guidance available, with some comments by the author on their perceived relevance to the Australian mining industry. Despite the availability of a number of other guidelines at the time, the need for Australian Guidelines was recognised in the mid 1990s and the reasons for the development of the 1999 ANCOLD Tailings Guideline are explained.
Perhaps the best recognition of the need for the original ANCOLD guideline is the degree to which it has been adopted since publishing the 1999 edition. It is in almost universal use in the Australian mining industry and is recognised as providing appropriate and acceptable standards by all state governments. Its use is recognised and sometimes even specified by a number of neighbouring countries and it is also recognised internationally when used by Australian companies with overseas operations.
The reasons for this wide acceptance are described. However, there are some areas where more recent developments have led to the Guidelines becoming dated and improved international guidelines have been published since 1999. The need for a revised ANCOLD guideline and its elevance is then described.
Keywords: Tailings, dams, mining, guidelines
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
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.
Dr Azan Khan, Ahmad Nasir, Kumud Kandel, Jaya Kandasamy, Hadi Khabbaz, Mahub Ilahee
Cracking in the clay core of embankment dams is important to dam safety because it can cause seepage through transverse cracks and with excessive seepage cracks may begin to erode the soil on the sides of the crack. If there are no filters to control this erosion, the erosion may progress to form a pipe, eventually leading to breach of the dam. Recent climate change has resulted in long term drought conditions in various parts of Australia, especially west of the Dividing Range. The prolonged drought conditions can lead to the loss of moisture content in the clay core causing cracking of the core material. The current research is investigating a relationship between long term drought condition and loss of moisture content in the clay core. This paper presents the loss of moisture content in the clay core of three dams in Australia due to global warming. A rigorous finite element modelling has been conducted to capture the moisture content changes in a typical large clay core dam.
Keywords: clay core, dams, climate change, moisture content