Raymond A. Stewart
On I7 June 1996 while investigating a small pothole on the crest 183 m high Bennett Dam an unexpected crest collapse occurred resulting in a large sinkhole. Following this incident the safety status of the dam was uncertain. The reservoir was lowered by 2 m over a six week period by spilling up to 5,000 m 3 over the spillway and through the turbines.
An unprecedented dam investigation commenced immediately and was completed December 1996. During drilling a second sinkhole was discovered at another location on the dam.
A sophisticated compaction grouting technique was developed to remediate the sinkholes to the depth of 5 m and the work was successfully completed by 1997. -The reservoir was returned to service in time to collect the freshet in spring 1997. This event was the most dam safety concern in the history of BC Hydro operations.
This paper describes how B.C. Hydro managed the crisis, and the subsequent safety assessment.
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Now showing 1-12 of 31 2948:
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”.
P.I. HILL, R.J. NATHAN, P.E. WEINMANN, J.H. GREEN
The assessment of flood risk is important to the safe design, maintenance and operation of dams. Traditionally, a standards-based approach has been adopted, in which the adequacy of a spillway was assessed by its ability to pass the whole, or a specified fraction, of the Probable Maximum Flood (PMF). More recently, however, the ANCOLD Position Paper on Guidelines for Selection of Acceptable Flood Capacity for Dams has moved towards a risk-based approach, in which attention is focused on establishing the exceedance probability of the maximum flood that can be safely passed by the spillway.
The move to a risk-based approach has led to an increasing focus on the derivation of floods with very low probabilities of exceedance. The chapter in Australian Rainfall and Runoff that gives guidance on the estimation of extreme floods has recently been revised and issued as Book VI. The new guidelines reflect the move from a standards, to a risk based approach and also include recent developments in extreme flood estimation. These recent developments result in an improved estimate of floods in the large to extreme range and hence a more reliable estimate of hydrologic risk.
This paper illustrates the impacts of the new flood guidelines by summarising the results for 7 dams in Southeastern Australia. For the examples presented in this paper the impact of the new guidelines is to reduce the estimated hydrologic risk. The new guidelines have an important effect on the estimation of hydrologic risk and therefore the assessment and management of dams in Australia.
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.
One of the most important issues during design and construction of an earthfill dam is how to secure a dam against unwanted events which may occur as a result of water flow (uncontrolled seepage, leakage & piping) through the dam.
Although earthfill dams are the largest by volume compared with other types of dams and they are designed to cope with seepage, their integrity is most sensitive to the effects which may be caused by it. The reason being that the earthfill materials are generally extremely heterogeneous and only one “unwanted” pocket is enough to create problems.
Another critical area is the foundation. In many situations it is not possible to avoid the complex geology which includes faults and joints as part of the foundation. An additional complication may be the presence of dispersive clay in the foundation.
In the area of tailings dams, the problems with seepage are slightly reduced as in most cases, tailings provide a degree of sealing. Tailings dams are very often designed as leaky dams. However, there is a hidden danger in approaching the design this way as at any stage of their lives they can retain water.
This paper presents two case histories of repairs carried out to tailings dams suffering leakage. One case describes leakage through the embankment wall while the other describes seepage through the foundation which contains dispersive soil.
Steven Rosin and Chen Han Shan
Tailings dam rehabilitation is a major challenge for the mining industry in China requiring a multi-disciplinary approach. A team of Chinese and Australian professionals have recently completed a 4-year project to develop guidelines for rehabilitation procedures for tailings dams in China.
This paper focuses on the geotechnical stability issues that were required to be addressed as part of dam rehabilitation assessment. It also discusses Chinese practices in tailings dam design and operation considered for the assessment. Three case studies are presented from sites in various parts of China.