Sarah McComber, Peyman Bozorgmehr
Boondooma Dam is a concrete-faced rockfill dam with an unlined, uncontrolled spillway chute. Construction was scheduled for completion in 1983; however a spill event occurred during the last stage.of construction Following this spill event an Erosion Control Structure (ECS) was built across the spillway chute to help mitigate any future scouring.
The spillway performed as expected during minor spill events in the 1990s and early 2000s. During the significant rainfall event of 2010/11, significant scour occurred to the spillway chute and downstream of the ECS, as a result of the spillway operation.
Following the 2010/11 flood, emergency repairs were made and long term repair solutions were investigated. However, during Tropical Cyclone Oswald in January 2013, the dam experienced the flood of record, and further scour occurred in the spillway chute.
The long term repair solution was reviewed in light of the 2013 damage. A solution is required that would satisfy the engineering problem and prevent further damage, while satisfying the commercial considerations faced by dam owners, insurers, customers and downstream stakeholders.
Keywords: Boondooma Dam, flood damage, scour damage, commercial engineering solutions.
Now showing 1-12 of 42 2979:
R. Nathan, P. Jordan, M. Scorah, S. Lang, G. Kuczera, M. Schaefer, E. Weinmann
This paper describes the development and application of two largely independent methods to estimate the annual exceedance probability (AEP) of Probable Maximum Precipitation (PMP). One method is based on the Stochastic Storm Transposition (SST) approach, which combines the “arrival” and “transposition” probabilities of an extreme storm using the total probability theorem. The second method – termed “Stochastic Storm Regression”(SSR) – combines frequency curves of point rainfalls with regression estimates of areal rainfalls; the regression relationship is derived using local and transposed storms, and the final exceedance probabilities are derived using the total probability theorem. The methods are used to derive at-site estimates for two large catchments (with areas of 3550 km2 and 15280 km2) located in inland southern Australia. In addition, the SST approach is used to derive regional estimates for standardised catchments within the Inland GSAM region. Careful attention is given to the uncertainty and sensitivity of the estimates to underlying assumptions, and the results are compared to existing AR&R recommendations.
Keywords: Annual exceedance probability, Probable Maximum Precipitation.
Nihal Vitharana, Nuno Ferreira
The raising and/or stabilising of existing concrete gravity dams by continuous concrete buttressing is a viable solution and, in some cases, it is the only solution available. There are few medium-large dams in Australia currently under consideration for raising with continuous buttressing.
Two of the major issues to be surmounted are: (a) the existing dam should not be subjected to cracking (particularly on the upstream face) due to heat-hydration effects, and (b) the requirement for the two dam bodies to resist the hydrostatic and other loadings as a monolith (unified dam).
However, there is great need for understanding the mechanisms involved in selecting an appropriate heat-of-hydration model and in calculating thermal stresses rationally. Due to such lack of understanding, expensive precautions, mostly with compounding conservatisms, would be adopted in concept and detailed designs eg. shear-keys on the interface, artificial cooling, post-grouted interface, anchor bars at the interface, concrete with high cement contents. On the other hand, unsafe designs could be the result.
The paper discusses these issues highlighting that a rational approach can be adopted to economise the design and construction processes. An example is also presented to demonstrate how the potential for temperature-induced cracking in new and old dam bodies can be evaluated with reduced uncertainty by considering all the mechanisms involved in a holistic way.
Keywords: Heat-of-Hydration modelling, raising concrete dams, thermal stresses, concrete buttressing
Chris Topham, Andrew Pattle, David Tanner, Oliver Giudici
Many owners around the world have dams that rely on grouted, post-tensioned rock anchors for stability. The anchors were installed during the original construction of the dams or retrofitted to improve stability during their operational life. The use of fully grouted post-tensioned anchors spanned the period of the 1960’s to 1980’s. The main issue with these un-sheathed grouted rock anchors is the question of integrity of the grout column protecting the anchor and concerns about possible corrosion of the high tensile wires from which the cables are constructed. While some of these anchors have corrosion monitoring systems installed, it is difficult to validate such data and there is considerable uncertainty over the condition of such anchors. To compound the problem, replacement of the anchors is technically complex, extremely costly and difficult to justify in the absence of known condition. For example, Hydro Tasmania has recent experience of work to cease reliance on such anchors at Catagunya Dam that cost $41m in 2009. With fifteen dams relying on some form of post-tensioned anchors, Hydro Tasmania has recently taken the unusual step of over-coring and extracting three post-tensioned rock anchors from operating dams in order to assess their condition. In what is believed to be a world first, a 42m long 70 strand high tensile anchor was overcored and removed from Meadowbank Dam in 2014. A further two anchors were successfully extracted from Repulse Dam in 2015, in conjunction with a group of international sponsors with similar anchors. This paper uses the 2015 work to illustrate the methodology used to extract the anchors, outlines the information gained from this unusual work, and presents the results of the condition of the extracted anchors. The paper concludes with some inferences for other owners with similar anchors and suggestions for further work.
Keywords: Grouted, post-tensioned rock anchor, ground anchor, corrosion, over-coring, extraction, dam safety.
Jiri Herza and John Phillips
The design of dams for mining projects requires processes and technology that are unfamiliar to many mine owners and managers. Dam designers rely on ANCOLD assessments of Consequence Category, commonly leading to a High rating for mining dams due to a combination of potential loss of life, impact on environment and damage to assets such as mine voids, process plants, workshops, offices, roads, railways etc.
From this High Consequence Category the relevant annual exceedance probabilities for design parameters and loading conditions such as earthquakes and floods are selected.
Mining companies have sophisticated methods available for assessing risk, yet for their assets they often adopt an order of magnitude lower security for earthquake and floods even though the consequences in terms of lives at risk and impact on project are similar.
The discrepancies in the design standards lead to situations where extreme dam loads are adopted to prevent damage and loss of life in assets that theoretically would have already collapsed under much lower loads.
One difference may be that some mining dams exist in an environment which is controlled by a single entity. Unlike other dams, failure of these mining dams would therefore impact only individuals and assets which fall under the responsibility of the same entity.
This paper discusses the discrepancies between the design of mining dams and the design of other mine infrastructure. The paper considers the impact of discrepancies on the overall risk to the mine and compares the degree of protection offered by a factor of safety and the influence of reliability of design input parameters, alternate load paths and design redundancy.
Keywords: Dams, tailings dams, mining, acceptable risk, factors of safety
Monique Eggenhuizen, Eric Lesleighter, Gamini Adikari
St Georges Dam is located on Creswick Creek approximately 2km southeast of the township of Creswick and 135km northwest of Melbourne. The reservoir, located within the Creswick Regional Park and originally constructed to supply water for the Creswick quartz crushing plant in the 1890s, has since been established as a popular recreational storage and is the responsibility of Parks Victoria. The dam is approximately 16m high and located across a relatively steep gully. The embankment consists of earthfill with an upstream face of rock beaching and a grass covered downstream face. The primary and secondary spillways are cut into the right and left abutments respectively.
At the completion of a detailed design review, St Georges Dam was assessed to be within the top three of Parks Victoria’s dams portfolio in regards to Public Safety Risks. The detailed design review assessed that the risk position for the dam plotted within the unacceptable region of the ANCOLD Guidelines for the static, earthquake and flood failure modes. As such, upgrade measures were considered to be required. In 2010 and 2011, a number of significant flood events emphasised the importance of upgrade works at this dam, particularly in regards to upgrading the spillway capacity, and consequently Parks Victoria assigned these works a high priority.
SMEC was engaged to design the upgrade works for the dam. A number of arrangements to increase the spillway capacity of the dam were considered, with the most cost effective option being assessed to be a secondary spillway over the dam embankment in the form of a rock chute.
This paper describes the decision making process associated with the option selection and the methodology for designing the overbank spillway which utilised the findings in ‘Riprap Design for Overtopping Flows (Abt & Johnson, 1991), and US Army Corps of Engineers, Waterways Experiment Station, publications of standard riprap gradations and computer program CHANLPRO.
Keywords: Embankment Dams, Spillway, Rock Chute, Erosion Protection