David Watson and John Adem
For several years risk management has been promoted by the Victorian Department of Natural Resources and Environment – Water Agencies as the key mechanism for the effective and efficient business management of dams. As part of an extensive water reform program, the Victorian Government announced in October 1997, a financial assistance package for the water industry which included funding for dam improvements covering a majority of large dam owners in the State. One of the conditions for receipt of these improvement funds was the need for each water authority to undertake a Business Risk Assessment of all significant and high hazard dams under its responsibility.
This paper discusses the Business Risk Assessment document based on a framework developed by Water Agencies after consultation with the industry and expands on the following reasons why the document was produced:
Trevor Daniell, David Kemp and Jenny Dickins
Early February 1997 saw the occurrence of heavy rainfalls over a wide area of South Australia’s north. One of the worst hit areas was near Olary, in eastern South Australia, where over a three day period, rainfall totals up to 320 mm were recorded. Within this period, localised, short duration intense rain occurred. In one four hour period on 7 February, about 200 mm fell.
The rain produced floods that washed away large sections of the main Sydney to Perth railway and inundated long sections of the Barrier Highway. Repair costs were of the order of $6 m for the railway and $1.5m for the road. Damage to rural infrastructure in the region was substantial. Flows within the catchment would have been sufficient to wash away most stream gauging stations.
The airmass over much of South Australia was of tropical origin, contained a high amount of moisture and was unstable. Thunderstorms were the main rain producer, consequently the event was characterised by localised, very intense rain episodes. This contrasts with the March 1989 floods, where it rained at a fairly steady rate over large areas for durations up to 24 hours, as a monsoon low tracked across the state.
Analysis of the depth-area relationship for the Olary storm indicates that the relationship to be used for design purposes should be the humid area relationship of Australian Rainfall and Runoff, not the arid area. This is reinforced when it is considered that the 1997 rainfall was localised, not general rain as in 1989.
Investigation of the event indicates that the Olary Creek catchment experienced overland flow, resulting in much higher peak flows than would occur with more frequently occurring “normal” processes. It is possible that any catchment may change its behaviour with extreme rainfall, and produce flows well in excess of those predicted with currently available runoff routing models, or flood frequency analysis of “normal” events.
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.
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.
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.
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.