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
David Brett, Bruce Brown, Imran Gillani, David Williams
This paper reports the direction of a current review of the 1999 ANCOLD Guidelines on Design, Construction and Operation of Tailings Dams. A sub-committee has been formed and has determined that the majority of the current guidelines need only minor editing but that additional attention is required to the concepts of risk and design for closure.
Major mining companies recognise that effective operation and closure of their tailings facilities are fundamental to their continued business from financial and political aspects. Risk needs to be managed throughout the life cycle of a TSF through planning, design, operation, closure and post-closure. Various methods are used to assess the “consequence category” of a TSF. This then determines design and operational criteria. Risks are identified and controls developed to limit these to acceptable levels.
The involvement in the sub-committee of representatives of the mining industry gives an industry perspective to this issue. This includes determination of acceptable risk levels and how to manage operations to achieve them.
The current ANCOLD Guidelines are very limited in terms of guidance for closure and possible abandonment of TSFs. However this area is perhaps the most critical from an economic and environmental perspective. The issues to be faced at closure and post-closure should be considered at the planning and design phases. The paper outlines some of the post closure cases that might need to be considered in design.
Keywords: guidelines, tailings dams, ANCOLD
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.
A. Tahershamsi , H. Bayat , J. Mahboubi , M. Geranmehr
Rockfill dams with inbuilt spillways are capable of passing surplus discharge by through-flow and overtopping and exhibits many advantages including reduced diversion costs, smaller terminal structures, etc. Further clarification of discharge coefficient (Cd) for this type of spillways is required for engineering purposes due to the complicated nature of turbulent flow through coarse porous media and boundary conditions. An experimental program was conducted to determine effects of uniformity coefficient (Cu), curvature coefficient (Cc), void ratio(e) and a dimensionless grain diameter defined by (d50⁄d100) on the discharge coefficient (Cd) . Observations indicated a significant reduction of discharge coefficient by increasing Cu and decreasing e and d50⁄d100. However, probable effects of (Cc) on (Cd), were not significant in these investigations.
Keywords: Internal spillway, discharge coefficient, uniformity coefficient, curvature coefficient
Angus Swindon, Tony Ang
Managing risk lies at the heart of the asset management processes. All utilities and large asset owners strive to ensure their asset management practices meet the needs of the business. To be able to demonstrate this to internal and external stakeholders including Boards, Regulators and Insurers, some form of bench marking or comparison with industry practice is often undertaken. This inevitably leads to a discussion about what good or indeed best practice might look like and a somewhat philosophical discussion about what aspects of these might be appropriate. If an organisation chooses to define “World Class Asset Management” as “the level of practice that no one else could exceed given the same internal requirements and constraints and external operating environment” then that organisation must develop a framework such that a pathway to achieve this can be defined and an assessment made. This paper will present such a framework and describe a pathway to demonstrate progress towards asset management maturity. Actively managing surveillance practices to monitor dam condition and performance is presented as an example within such a framework.
Keywords: World Class Asset Management, Dam Safety, Surveillance, ANCOLD Guideline.