Andrew Pattle and Bram Knoop
This paper provides an outline of a process that can be used to optimise regular dam surveillance and monitoring activities. The process is applicable for a wide range of dam types that an owner/operator may be responsible for. Basic assessments are made of inherent reliability and potential consequences of failure using key factors such as construction features, foundation conditions and observed performance. The key factors are combined to give a relative risk ranking for each dam. These rankings are used to determine specific dam monitoring schedules. The process focuses the monitoring effort on those dams that are perceived to constitute the greatest portion of the overall risk. The methodology is simple and provides a cost-effective framework for setting appropriate resourcing levels for dam monitoring.
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D.N.D. Hartford and R.A. Stewart
It seems perfectly logical, obviously desirable and apparently sensible to prioritise dam safety reviews, deficiency investigations and capitalised remediation projects in terms of risk. However, our experience in attempting to apply the various deterministic and risk- based approaches in prioritising dam safety activities has revealed that, while it may appear to be quite logical and desirable to prioritise in terms of risk, it is rather less feasible than it appears.
This paper explores why different prioritisation processes can lead to different priority rankings across the same portfolio of dams. B.C. Hydro’s Preliminary Risk Exposure Profile process, which utilises the best and most robust attributes of risk analysis process at the preliminary level but avoids the pitfalls associated with estimating risks which will often have little or even no basis is presented. The paper explains how this process provides a “fail-safe” backup which will identify non-conservative and erroneous facility risk estimates; thereby allowing for correction in a timely fashion. The paper also raises some awkward philosophical issues which the profession will have to address in order to permit confident dam safety decision-making on the basis of risk analyses. Not the least of these is the following issue – “If preliminary estimates of risk are reasonably good, then there should be little need for more detailed risk analysis for confident and defensible decisions concerning making or not making dam safety improvements”.
Richard I Herweynen
For concrete gravity dams, when the foundation’s value of cohesion is low, it is very difficult to meet the sliding criteria proposed by ANCOLD. Low cohesion is generally associated with serious foundation defects. This was the case for Meadowbank Dam, with a foundation having persistent horizontal seams containing material of a clayey silt size classification. By adopting the ANCOLD strength reduction factors, it was found that a large number of ground anchors would be required to meet the ANCOLD sliding criteria. During original design, extensive laboratory and insitu testing was performed on the seam material. This paper proposes a methodology for arriving at less severe strength reduction factors based upon a statistical analysis of the strength parameters measured in the Meadowbank Dam foundation.
Additionally, a probabilistic approach using a Monte Carlo simulation is used to give further weight to this argument. This paper concludes that the probability of Meadowbank Dam failing due to sliding is very low and within acceptable limits.
D. B. Edwards, B.H. Jackson & R. H. Wright
Ground anchorages are installed to support structures such as dams, slopes and tunnels. Failure of anchorages could be serious.
The condition of these critical supports is currently assessed by monitoring the load in the anchorages by either load cells or lift-off testing (jacking). Both methods are expensive and testing may damage the corrosion protection beneath the anchorage head.
A non-destructive testing method for ground anchorages needed developing and the UK Universities of Aberdeen and Bradford developed a testing system called GRANIT with patent applications on the system filed world-wide.
Full scale measurements were conducted during the construction of Penmaenbach and Pen y Clip Tunnels on the UK’s A55, where rock support was provided by prestressed rock anchorages. In all 9000 records of anchorage response were analysed.
A major finding from the research was that the response of the anchorages to the dynamic impulse motion produced by the blast loading depended on how the anchorage had been constructed and on the nature of the surrounding rock mass. If the prestress load in the anchorage was changed, or the free length increased, a noticeable change was observed in the response ‘signature’ as monitored by an accelerometer located at the anchorage head.
Applying a known impulse load to the anchorage head immediately after construction and measuring the response, provides a datum response signature for the intact anchorage. If the anchorage was to deteriorate in any way, eg loss of prestress, this should be noticeable on subsequent response signatures. This approach is the basis of the GRANIT system.
A short programme of anchor calibration testing for bolts was conducted in Hawkesbury sandstone in Sydney during March 1998 and developments in Australia and UK are proceeding.
Leonard A McDonald and Chi Fai Wan
A risk assessment has been undertaken as part of a comprehensive review of the safety of Hume Dam. Use of risk assessment techniques, to assist in evaluating the safety of existing dams, is a relatively recent trend. Hume Dam was a particularly challenging subject for the application of risk assessment techniques at their present stage of development. The challenge lay in the number and diversity of dam elements to be analysed, in the number and complexity of the potential failure modes and in the fact that there were significant safety issues under normal operating conditions.
This paper outlines some of the key lessons learned from that phase of the risk assessment that was concerned with estimating the chance of dam failure. Some of the issues discussed have not previously been addressed in the literature and some demonstrate a clear need for improved analysis procedures.
Alkali-aggregate reaction (AAR) is a potentially deleterious process in concrete containing reactive aggregates, and can lead to varying degrees of cracking in structures, and differential movement and misalignment of concrete elements and mechanical installations. The rehabilitation of affected structures would require information on the extent of current damage and possibility of on-going damage that could be caused by AAR.
Information on the characterisation of concrete components of an AAR-affected dam and estimation of their future potential for further expansion and cracking are provided and repair options discussed in this paper.