Richard R. Davidson, Michael Zoccola, Barney Davis, John W. France
Seepage barriers have become an essential element of dam safety upgrades for many aging dams. Our construction technical specifications are generally written to achieve a degree of perfection that may not be possible or practical. Many practitioners believe that grouting alone can achieve an acceptable seepage barrier through a pervious rock foundation. However, precedent from many Corps of Engineers dams has revealed that grouting can only treat those open features that the grout holes intersect. What about clay filled fractures or solution features that resist grout penetration but then erode over time? Can any grout treatment ever be considered as a permanent seepage barrier? Cutoff walls through embankment dams and their foundations are generally considered as a more permanent seepage barrier. However, do we have the means to construct a perfect seepage barrier wall, or are defects to be expected. Do these defects represent fundamental flaws that require risk mitigation? How can we verify that we have built an acceptable seepage barrier that meets the design intent?
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Asset management and particularly dam safety management at Snowy Hydro is a continually evolving process and at the heart of the program is our desire maintain the legacy of the Snowy dams and to do everything required to meet our obligations and duty of care. There is a significant shift underway from a schedule-based maintenance to a condition-based maintenance plan. The advantage of this is that the right maintenance is delivered at the right time and resources can be efficiently allocated to the right maintenance.
Condition based maintenance is not driven by a desire to cut maintenance or surveillance spend. A key part to this change is determining the condition of an asset or dam. To achieve this, reliability centred maintenance principles have been applied to dam structures through the use of failure mode effects and critically assessment (FMECA) tool, this differs to a traditional failure mode assessment, as it looks at functional failure of individual structures or equipment rather than partial or catastrophic failure of the dam. The outcome of a FMECA is a detailed maintenance and inspection plan that targets the individual functional failure modes.
One of the outputs of this process is a condition assessment methodology which is used as a trigger for corrective and preventative maintenance activities and a tool for the justification of installing performance measuring instrumentation. Condition assessment is therefore the process where asset performance data is assessed against specific criteria to determine its present state. Currently, comparing condition and performance of multiple dams is reliant on a practitioner’s experience and subjective assessment to determine whether an asset’s condition is fit for service. The condition assessment process; reduces subjective data, provides real-time health assessment, highlights performance issues, continuously identifies and updates priorities and provides justification for capital investment.
Peter Buchanan, Malcolm Barker, Paul Maisano, Marius Jonker
Kangaroo Creek Dam located on the Torrens River, approximately 22 km north east of Adelaide, is currently undergoing a major upgrade to address a number of deficiencies, including increasing flood capacity and reducing its vulnerability to major seismic loading.
Originally constructed in the 1960s and raised in 1983, recent reviews have indicated that the dam does not meet modern standards for an extreme consequence category dam.
The original dam was generally constructed from the rock won from the spillway excavation. This rock was quite variable in quality and strength and contained significant portions of low strength schist, which broke down when compacted by the rollers. The nature of this material in places is very fine with characteristics more akin to soil than rock. Review of this material suggests that large seepage flows (say following a major seismic event and rupture of the upstream face slab) could lead to extensive migration of the finer material and possible failure of the embankment. However, it is also envisaged that the zones of coarser material could behave as a rockfill and therefore transmit large seepage flows, which may result in unravelling of the downstream face leading to instability.
This paper addresses the design of the embankment raising and stabilising providing suitable protection against both these possible failure scenarios, which tend to lead to competing solutions. The final solution required the embankment to be considered both as a CFRD and a zoned earth and rockfill embankment.
Extending the useful life of a dam to an extent well beyond what was envisaged by the original designer poses diverse challenges. In this paper, three case studies are described, one involving strengthening of two similar dams and two cases involving raising. In all three cases, the dams continue to provide a reliable source of supply in a water scarce country.
The Woodhead and Hely-Hutchinson Dams have substantial historical significance which guided the selection of restressable post-tensioned anchors as the preferred method of strengthening.
The Stettynskloof Dam was almost doubled in height by constructing a clay core rockfill embankment abutting the downstream face of the existing concrete gravity dam. The new structure was well instrumented to cover areas of concern but the dam was found to perform as largely predicted by the designers.
Keerom Dam faced both technical and regulatory challenges that were eventually overcome and the raising of the dam was able to proceed. A further raising will increase the utilisation of this valuable resource still further.
Peter Foster, Bob Wark, David Ryan, John Richardson
Fairbairn Dam is a zoned embankment dam completed in 1972 and located in central Queensland near the town of Emerald. The spillway, which is located toward the left abutment, consists of a 168 metres wide concrete ogee crest, converging concrete chute and dissipater basin. The overall length from the ogee to the downstream end of the concrete spillway is approximately 195 m. The chute and dissipater basin are underlain by a matrix of longitudinal and transverse drains for pressure relief of the anchored concrete slabs.
Minor repairs to damaged chute slabs were undertaken following the 2011 flood event. During these rectification works, large voids up to 0.3 metre in depth were found under sections of the concrete chute slabs as well as damage and blockage to the sub-surface drainage system. Discoloured water was also observed discharging from sections of the sub-surface drainage system. Some of the 24 mm diameter bars designed to anchor the slabs to the foundation were found to have corroded at the concrete/foundation interface and subsequent pull-out tests showed that the anchors had minimal or no structural capacity.
These investigations led to a review of the hydraulic design of the spillway, upgrade to the sub-surface drainage system and apron slabs, and installation of replacement anchor bars. An understanding of the transmission of pressures and dynamic pressure coefficients resulting from spillway discharge and the effects of the hydraulic jump was an essential component of the design for the new anchor and drainage system.
This paper provides detail on the investigations undertaken, the hydraulic modelling that is underway including physical hydraulic and computational fluid dynamics (CFD) and the design approach for what is described in this paper as the Stage 1 component of works.
Amanda Ament, Thomas Ewing, Frank Nitzsche
The automatic operating buoyancy type spillway gates at Lenthall Dam did not operate properly since installation. This paper discusses the problems encountered, the investigation conducted using computational fluid dynamics to quantify the problems and develop solutions. It describes the design of the modifications to the gate and flow regime and results after construction.