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Paul W. Heinrichs & John Bosler
Spring Creek Dam is a 16m high zoned earthfill dam with a central vertical concrete core wall storing 4700 ML for Orange City Council’s water supply. It was a 14.5m high dam constructed in 1931 and in 1947 was raised by 1.0m. In 1966 after a week of heavy rain following a long dry spell, an 80m section of the downstream face slumped but the dam fortuitously survived. In 1969 the dam was re-constructed but no internal drain/filter was installed.
Following the 1994 dam surveillance report, piezometers were installed in the downstream fill. Drilling for these revealed that a substantial portion of the zone downstream of the core wall was saturated. The piezometers recorded piezometric elevations that closely and rapidly followed the reservoir level. Subsequent site investigations identified pockets of very low strength fill immediately downstream of the core wall. It was concluded that the core wall was seriously compromised and the storage level was subsequently, significantly lowered, as an interim dam safety measure.
Dambreak studies indicated the dam is a high hazard and hydrological studies found that the spillway capacity was inadequate.
This paper details the problems involved, their analyses, and the remedial measures proposed at the concept design stage. These include a chimney filter/drain, a stabilising fill combined with embankment crest raising and the construction of a 3-bay fuse plug auxiliary spillway.Learn more
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.Learn more
Marius Jonker, Francisco Lopez and John Bosler
This paper describes the safety evaluation and development of remediation options for Clover Dam, a 28 m high slab and buttress structure situated in the alpine region in northeast Victoria, Australia. The review was particularly challenging considering the complexity, age and cracked condition of the dam structure, which required the development of an analysis method for this type of dam.
Completed in 1953, Clover Dam is one of five dams in the Kiewa hydroelectric scheme. The 76 m long dam comprises a 45.7 m long covered slab and buttress section, supported on each abutment by concrete gravity sections. The review was undertaken as a result of severe cracking occurring since the early 1970s and because a detailed design review had not been undertaken since its construction.
Current guidelines for the safety review of existing dams provide little detailed information on slab and buttress dams. Consequently, a methodology was developed to analyse Clover Dam. This methodology could also be applied in the review of this type of dam in general, and is currently being used for safety assessments of three other slab and buttress dams.
This paper focuses mainly on the dam structural assessments undertaken during the safety review. The structural analyses involved 3-D finite element analyses for thermal, static and earthquake loading.
The outcome of the review was that both the gravity and buttress portions of the dam do not meet current design standards. The development of practicable remediation options was complicated by the operational constraints and the restricted access to those areas within the dam where remedial works were required.
Keywords: Slab and buttress dam, 3D finite element analysis, seismic assessment.Learn more
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.Learn more