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There is a large stock of embankment dams throughout the world needing the assessment of their
safety as required by modern dam safety regulations. Due mainly to economic and site constraints
associated with potential dam upgrading work, it is imperative that a rational approach be adopted in
assessing their safety and in designing the remedial works. One of the most important criteria is the
selection of appropriate geotechnical parameters under different conditions. Predominant loading
conditions in a dam are much different from those in other structures such as bridge and building
foundations and therefore the direct adoption of traditional approaches may not always be valid. This
paper presents the various aspects of issues associated with the stability assessment of dams including
the rational selection of the parameters and numerical codes available to dan/geotechnical engineers
to assess their safety.

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2005 Papers

2005 – ASSESSMENT CRITERIA FOR EMBANKMENT DAMS AND THE ROLE OF NUMERICAL MODELS
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There is a large stock of embankment dams throughout the world needing the assessment of their
safety as required by modern dam safety regulations. Due mainly to economic and site constraints
associated with potential dam upgrading work, it is imperative that a rational approach be adopted in
assessing their safety and in designing the remedial works. One of the most important criteria is the
selection of appropriate geotechnical parameters under different conditions. Predominant loading
conditions in a dam are much different from those in other structures such as bridge and building
foundations and therefore the direct adoption of traditional approaches may not always be valid. This
paper presents the various aspects of issues associated with the stability assessment of dams including
the rational selection of the parameters and numerical codes available to dan/geotechnical engineers
to assess their safety.
Learn more
$15.00

2005 Papers

2005 – Assessment Criteria for Embankment Dams and the Role of Numerical Models
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N. Vitharana and S. Terzaghi

There is a large stock of embankment dams throughout the world needing the assessment of their
safety as required by modern dam safety regulations. Due mainly to economic and site constraints
associated with potential dam upgrading work, it is imperative that a rational approach be adopted in
assessing their safety and in designing the remedial works. One of the most important criteria is the
selection of appropriate geotechnical parameters under different conditions. Predominant loading
conditions in a dam are much different from those in other structures such as bridge and building
foundations and therefore the direct adoption of traditional approaches may not always be valid. This
paper presents the various aspects of issues associated with the stability assessment of dams including
the rational selection of the parameters and numerical codes available to dan/geotechnical engineers
to assess their safety.
Learn more
$15.00

2005 Papers

2005 – Dam design, Geotechnical Parameters, Numerical Codes, Hydraulic Fracturing, Seismic Analysis.
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P Maisano, M Taylor , M Barker and A Parsons

South Para Dam, completed in 1958, is located on the South Para River, 38 km north of Adelaide. The embankment is 45 m high and comprises compacted crushed phyllite with rockfill toes. The 13 m high rock fill toes are protected with three-stage filters but the remaining 32 m of embankment height has no downstream filter protection.

The South Australian Water Corporation (SA Water), the owner and operator of the dam, is considering modifications to the dam, to augment its flood mitigation role. The proposed works, while not affecting the full supply level, involve a modification to the spillway crest and raising of the embankment crest to accommodate increased flood levels. SA Water therefore commissioned a dam safety review to assess the need for any piping or overtopping protection that may be required. This was followed by concept designs to ensure that flood mitigation work is compatible with any required dam safety upgrade work.

The results of a detailed dam failure risk analysis using event trees showed that the Societal Risk for the existing dam needed to be reduced, and that the proposed spillway modifications increased the Societal Risk due to the increased risk of piping failure with higher flood levels.

The risk analysis showed that eliminating the overtopping modes of failure by raising the dam crest is not sufficient in itself to achieve the required reduction in risk. The provision of filter protection to reduce the risk of piping failure is required, but it was shown that it is not necessary to provide full height filters as the provision of filters only above full supply level would be sufficient to achieve the required reduction in risk.

The recommended upgrade works, in addition to the proposed spillway modification for flood mitigation purposes, consist of filter protection and a weighting fill above the top berm (4.4 m below FSL) to facilitate connection to a possible full height filter in the future, and a parapet wall to provide overtopping protection.The resulting cost saving compared with the installation of full height filters is in excess of $2 Million.
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2006 Papers

2006 – Recent Advances in the Seismic Analysis of Intake Towers
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John Bosler and Francisco Lopez

The ANCOLD “Guidelines for the Design of Dams for Earthquake” were published in August 1998. The guidelines contain a brief outline of the performance requirements and recommend, in general terms, a method of analysis for intake towers.

Over the last three decades there has been considerable research on the seismic performance of intake towers as they move into their inelastic range. In the years following the publication of the ANCOLD guidelines, some of the findings from this research have been incorporated into revised design procedures issued by the US Army Corps of Engineers. These procedures, if embraced by ANCOLD and the local dam engineering community, are likely to have a significant impact on how the structural adequacy of existing towers under seismic loading are assessed.

Rocking behaviour in which the tower becomes unstable as a transient condition has long been recognised as acceptable under certain conditions. Attempts to prevent tower rocking by measures such as retrofitting tensioned ground anchors may, in some situations, be of limited value in improving the seismic performance of a tower and could result in an increase in bending moments in the tower stem. Guidance is now available on the amount of rocking behaviour that is tolerable.

For seismic events greater than the Operating Basis Earthquake most towers will start to exhibit inelastic behaviour. Specific guidance is also now available on the length of time during an earthquake that bending moments in excess of the elastic capacity can be tolerated, the amount by which these moments can exceed the nominal bending moment capacity and the vertical extent of the tower stem that can be stressed beyond its elastic limit.

The paper discusses the different approaches taken by ANCOLD and the Corps of Engineers. Key differences in outcomes are highlighted using a worked example for a typical reinforced concrete tower and the ANCOLD approach is found to be generally, but not always, more conservative. The paper concludes with recommendations for dealing with these differences.
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2007 Papers

2007 – Current international state of practice for the seismic design and analysis of appurtenant structures at dams
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Steve O’Brien

Appurtenant structures associated with a dam play and important part to the dam’s operation. For these structures it may be important that their functional and structural integrity is retained in the event of a notable earthquake, particularly when they are required to release water from the reservoir in a controlled manner to lower the storage following an earthquake. Research has been conducted into the current state of practice for the seismic design and analysis of these structures, including review of the main issues for seismic effects, documentation of case histories and review of current research, international guidelines and standards. The general assessment philosophy was found to be relatively consistent internationally, however, the adopted assessment procedures were found to vary. The status of the current ANCOLD earthquake guidelines has been provided in relation to the current international state of practice for various types of appurtenant structures.

Keywords: Appurtenant structures, performance criteria, seismic performance, seismic analysis.
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2012 Papers

2012 – Seismic Induced Slope Displacement of a Tailings Embankment – Comparison of Numerical and Simplified Analysis Methods
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Zhenhe Song, Arjuna Dissanayake, Shunqin Luo

One of the potential tailing dam failure modes that is commonly evaluated is for prediction of earthquake induced crest displacement in relation to available freeboard. The prediction of seismic induced displacement for tailing dams can be evaluated using simplified approaches, i.e. analytical methods by Newmark (1965), Makdisi and Seed (1978), Bray and Travasarou (2007) and empirical method by Swaisgood (2003) and Pells and Fell (2003).
Seismic induced displacements have been estimated using these simplified methods and numerical methods by FLAC and PLAXIS. The results from the numerical modelling were compared with results derived from the simpler analytical and empirical methods. The results indicate the numerical analysis results agrees reasonably well with empirical methods by Swaisgood (2003) and Pells and Fell (2003) and can be used to provide additional confidence in the seismic stability of tailings embankments. However, simplified analytical methods by Newmark (1965), Makdisi and Seed (1978), Bray and Travasarou (2007) could underestimate the seismic induced displacements.
Keywords: Tailing dam, Seismic analysis, numerical analysis, simplified analysis, liquefaction.
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2008 Papers

2008 – Inelastic seismic analysis of Dartmouth Intake Tower
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Francisco Lopez, John Bosler

Abstract: A study has been undertaken to determine the structural adequacy of the Intake Tower for the Dartmouth Dam Low Level Outlet Works under the Maximum Design Earthquake (MDE). The tower is a reinforced concrete frame structure. The study included the definition of appropriate seismic inputs and nonlinear time-history analyses for different levels of ground shaking.

The behaviour of the tower was assessed in terms of material strains. The seismic acceleration-time history was applied to the model and the nonlinear analysis identified the locations in the structure which reached the following performance thresholds: cracking of core concrete, yielding of reinforcing steel, fracture of reinforcing steel and crushing of core concrete.

The results of the study showed that the tower was able to resist, without collapse, the loadings imposed by three different MDE acceleration time-histories. The predicted level of damage consisted of generalised yielding of reinforcement, formation of numerous plastic hinges at different locations in the tower and generalised spalling of cover concrete.

Overall, the structural response of the tower under the MDE events was considered satisfactory and met the performance requirement that structural collapse should not occur.

Keywords: intake tower, nonlinear, inelastic, time-history, seismic analysis.
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