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.
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Kristen Sih, Richard Rodd
Melbourne Water currently manages over 235 stormwater retarding basins. The process of assessing the risk posed by these assets began in 2006, and at the end of 2015 full risk assessments were completed for around 30 of the basins that were estimated to pose the highest societal risk. However, when analysing the results of these risk assessments, there was some concern that the results were inconsistent and often too conservative, given the few incipient or actual failures that had been experienced.
It was found that one of the key areas causing the conservatism was poor documentation of design and construction details, and the fact that the tools used for assessing the Potential Loss of Life (PLL) were aimed at larger storages that cause much higher depths and velocities in dambreak events than these (generally) small storages. To remedy this situation, advice was sought from specialist practitioners to develop guidance notes on the assessment of PLL and failure likelihoods for retarding basins.
On the back of these guidance notes, Melbourne Water initiated an accelerated program of assessing the risk associated with 78 retarding basins over a 6 month period. This paper describes the key recommendations from the guidance notes, compares the results of the risk assessments performed pre- and post-guidance notes and provides a summary of the portfolio risk assessment outcomes, what they mean for Melbourne Water and what the organisation intends to do to manage this risk into the future.
Kate Brand, Matthew Ind
Failure impact assessments of tailings dams are largely pre-determined by the input assumptions and, due to lack of supporting data, the results can be highly subjective. Despite numerous guidelines available for undertaking failure impact assessments of water dams, there are very few technical guidelines on how to form the above assumptions and how to undertake dam breach modelling of a tailings storage facility (TSF).
Tailings dam failure databases are limited, with the available information generally not analogous with the TSF under assessment, especially given the rising volume and height of modern tailings dams. ‘Rule of thumb’ methods are often referred to, with a percentage of tailings and water assumed to be discharged along with assumptions of the breach height and width made.
Using a case study, this paper compares a range of potential failure impact assessments generated using typical methods of analysis and runout modelling to demonstrate the reliance on engineering judgement in failure impact assessments. Given the subjectivity observed within the results, consideration should to be given to the level of reliance on tailings dam failure impact assessments in formulating emergency action plans. It is recommended that regulators take an active role in formulating tailings dam impact assessment guidelines.
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.
Paradise Dam is located on the Burnett River 20 km northwest of the town of Biggenden in Queensland. It is a gravity dam with a height of 37 metres and a total capacity of 300,000 ML. It was primarily constructed to service local agriculture.
The dam features a complex outlet works contained within a tightly constrained footprint. It provides for irrigation releases, fish passage and power generation. Additionally, the outlet is required to pass very high environmental flows of up to 270 m3/s.
The dam was subjected to major flooding in 2013 resulting in significant damage to the mechanical equipment associated with the outlet works, and severe scour downstream of the spillway.
Since construction, the operating range for the environmental outlet has been restricted. A rough operating zone has been identified through which the gates are quickly moved through. It is believed to be caused by the dynamics of the gates and the upstream conduit arrangement. Failure of the downstream stainless steel liner associated with the conduit has also occurred. The environmental outlet lacks the ability to be isolated from the storage, complicating the maintenance / modification of the gates. At the time of design, it was agreed by the alliance partners that major maintenance of the gate would be planned for when the reservoir was low, being below the intake bellmouth.
The irrigation release valves suffer from high vibration levels during operation. Component failure and severe corrosion have also been experienced.
This paper details:
Operational and maintenance experiences and restrictions since commencing operation including the impact of flooding;
Investigation and testing of environmental gate dynamics and the impact of these on the intake tower;
Failure of the environmental conduit liner, investigation and proposed rectification;
Proposed method to enable servicing of environmental gates without the use of a bulkhead and without draining the storage;
Proposed enhancements to irrigation valves to reduce vibration and extend service life.
Alan P. Jeary, James O’Grady, Thomas Winant
Mainmark are introducing the STRAAM system of full scale non-destructive testing for dams into Australia and New Zealand. Advances in measuring extremely low amplitude vibrations combined with methods for extracting the unique dynamic signature have now enabled the rapid measurement of the response of earthen and concrete dams. This ability allows the quick calibration of Finite Element Models that can be used to accurately assess the strength of a dam. Furthermore, this information allows dam owners to efficiently track changes in the capacity of their dams due to aging, earthquake or flood activity through changes in the dynamic.
The STRAAM system measures the vibration of the dam structure to establish the natural frequencies, mode shapes and associated damping ratios of the dam. The field measurements are correlated with a three-dimensional finite element model to fine tune the effects of abutments and foundations on the three dimensional model. Because of the sensitivity of the instrumentation and the novelty of the analysis techniques, the information available to dam managers allows information-based decisions to be made in a way that optimizes the financial implications. In addition, the techniques are non-invasive and non- destructive and they give additional information about the connectivity of the dam with the surrounding terrain, and whether that connectivity is compromised by water seepage.
This paper discusses the results obtained from field measurements from four dams located in Switzerland, USA and Scotland.