Anthony Moulds and Anthony M Watson
The selection of lightning protection equipment will always remain within the cost versus benefit, or risk management area. As more and more monitoring equipment becomes electronic and microprocessor based, we need to have a better understanding of the ways to protect it, and maintain the data flow.
Recent experience has shown that utilising the Australian Standard (NZS/AS 1768-1991) Lightning Protection, in conjunction with a six-point plan, will go a long way to providing total integrated protection for both structures and contents. However, no matter how much protection is applied, damage due to lightning may still occur. For dam surveillance instrumentation the aim ultimately is to protect the transducer ‘in the ground’ or ‘in the dam’, because generally these instruments are inaccessible and non-replaceable without prohibitive drilling and retrofitting costs.
The six-point plan was applied initially to designing lightning protection for a large, well- instrumented RCC dam, completed in 1991. The protection proved to be not as good as was hoped. The paper describes how the lightning protection at the dam was subsequently developed. This experience, which has pointed the way to achieving a good level of protection at a reasonable cost, has been applied to a number of other, instrumented dams.
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S. Knight, B. Cooper and P. van Breda
Warragamba Dam was completed in 1960 and impounds Sydney’s main water supply storage. Hydrological studies in the 1980’s showed the existing spillway to be significantly undersized by modern standards. Considering the dam’s High Incremental Flood Hazard category, the current risk of dambreak is unacceptably high. This has resulted in a two-stage program to upgrade the dam to full Probable Maximum Flood (PMF) capability.
The interim (first stage) measures were completed in 1990 and involved a 5.1 metre raising of the dam crest and significant post-tensioning of the dam wall. Following many feasibility/option studies and detailed technical and environmental studies, a contract was let by Sydney Water Corporation (SWC) in late 1998 for the construction of an auxiliary spillway as the major (second stage) component of the flood security upgrading. The spillway will be a large capacity (about 18,000m*/s) concrete lined chute 700 metres long around the dam’s right abutment. In the upper curved section will be the largest fuse plug embankments in Australia (up to 14.5 metres high). The lower straight section will terminate with a flip bucket structure.
The NSW Department of Public Works and Services (DPWS) designed the earlier Interim Works, undertook the subsequent engineering option studies for the Major Works and carried out the concept design and technical specification for the new auxiliary spillway and associated dam modification works. This paper summarises the project, describes the main features of the concept design of the spillway and outlines the associated dam modifications.
Buddhima Indraratna, Mark Locke and Gamini Adikari
The main objectives of the filter are to prevent erosion of the dam core, permit controlled passage of seepage flow through the dam and facilitate dissipation of excess pore pressures in the core. In most designs of dam filters, empirical methods based on particle size ratios have been used. These empirical rules are developed through extensive laboratory tests. Although the empirical rules benefit from directly or indirectly incorporating most factors affecting filtration, they cannot be extrapolated for distinctly different soils and do not describe the time dependent changes that occur within the filter medium.
Mathematical models can be formulated to explain the fundamental physics of particle interaction and migration, within a framework of well defined geohydraulic constraints. Considering the mass flow and momentum conservation principles; time dependent changes in particle size distributions, mass flow rates, retention capacity and base soil erosion rates can be simulated.
This paper reviews various empirical and mathematical models, based on the authors experience. A novel approach to large scale filtration is highlighted based on testing actual soil and filter materials from an Australian dam, in a new 500mm diameter apparatus.
Jim Walker, Murray Gillon and John Grimston
Karapiro Dam is at the end of a cascade of hydropower dams on the Waikato River in New Zealand’s North Island. The 52m high, high hazard, arch dam retains the lake for a 96MW power station at its downstream toe. Safety reviews recommended a re-evaluation of the dam stability under seismic loading.
Dam owner, Electricity Corporation of New Zealand (ECNZ), commissioned consultants Tonkin & Taylor Ltd to carry out a series of studies and investigations which provided better understanding of the dam’s safety status. Investigations located a previously unrecorded continuous low strength thrust fault underlying the left abutment. This provided the potential for movement of the left abutment gravity blocks under earthquake loading, with adverse effects on arch dam and reservoir safety. Investigations showed the abutment cut off walls to be lower than the PMF lake level. High groundwater levels and erodible pumiceous soils were found at the left abutment. These findings prompted ECNZ to implement stability enhancement works.
This paper describes the studies and investigations, peer review process, and design and construction of enhancement works.
Ahmad Shayan, Robert J. Wark and John Waters
The Canning Dam concrete gravity structure located in Western Australia has shown an upward movement of 18.3 mm and lateral upstream movement of 14.2 mm over the past 15 years of monitoring. These movements have been associated with considerable cracking of the upper parts of the dam and the upper gallery. Investigations have shown that the cause of the cracking was a strong alkali-aggregate reaction (AAR) in the concrete, brought about by a deformed granitic rock. Extensive horizontal and vertical cracking in the upper part of the dam wall has necessitated the removal of the section above the floor of the upper gallery level, and construction of a new reinforced concrete section to act as head beam for post-tensioning of the rest of the dam wall.
A set of small diameter cores were taken from the various parts for diagnostic purposes, and a vertical core of 100 mm diameter was taken through the whole thickness of the wall for the determination of the strength properties, alkali content and residual expansion potential. Based on these, a post-tensioning stress of 1.5-2.0 MPa has been calculated for restraining the residual expansion of the concrete. The spillway bridge structure which is part of the dam wall has also shown mild signs of deterioration. The piers and abutment walls and the deck were surveyed for corrosion activity and extent of AAR. This work showed that the spillway bridge structure was sound and only needed maintenance. The performance of a triple blend concrete mix containing a high volume of fly ash (45%) and silica fume (5%) developed for the replacement of the old concrete is also discussed.
The role of judgement in risk assessments as applied in dam safety management has been the source of considerable debate in recent years. With regard to risk analysis of dams, and while there is general agreement that judgement is an essential element of the process, essentially two schools of thought have emerged. One view holds that, in the assignment of probabilities, reliance can be based on collective engineering judgement that is anchored to a knowledge base. The second view holds that judgement should be based on the knowledge that is revealed by an appropriate amount of analysis. The paper, written from the perspective of the latter view, explores some of the underlying issues in this debate.
The role of judgement in risk evaluation, the process of judging the significance of risk, is considered to be equally important. However, the process of making value judgements and statements of principles is complex and often beyond the sphere of engineering. The third issue addressed in the paper concerns the search for answers to the question, “How good is the assessment?”