Extending the useful life of a dam to an extent well beyond what was envisaged by the original designer poses diverse challenges. In this paper, three case studies are described, one involving strengthening of two similar dams and two cases involving raising. In all three cases, the dams continue to provide a reliable source of supply in a water scarce country.
The Woodhead and Hely-Hutchinson Dams have substantial historical significance which guided the selection of restressable post-tensioned anchors as the preferred method of strengthening.
The Stettynskloof Dam was almost doubled in height by constructing a clay core rockfill embankment abutting the downstream face of the existing concrete gravity dam. The new structure was well instrumented to cover areas of concern but the dam was found to perform as largely predicted by the designers.
Keerom Dam faced both technical and regulatory challenges that were eventually overcome and the raising of the dam was able to proceed. A further raising will increase the utilisation of this valuable resource still further.
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Peter Woodman, Andrew Northfield, Tim Kallady
Currently there is little guidance available on how itinerants on roads should be included in a consequence assessment. The methods available are often subjective which can lead to itinerants on roads either being ignored or insufficiently considered. A fact that can in turn lead to consequence categories being inappropriately assigned to the asset being assessed or risks being under or over estimated. Consideration of these itinerants is especially important for smaller dams or retarding basins in urban areas where often the Potential Loss of Life (PLL) in buildings is small but there are major roads carrying a large Population At Risk (PAR) through the inundation extent, which experience flooding of sufficient severity to pose a threat to life.
This paper looks at how the method used to assess itinerants on roads can affect the consequence category assigned to an asset and/or the risk of the dam or retarding basin. It will draw on a number of recent assessments undertaken for retarding basins within Melbourne and make comment on a possible approach to consider itinerants on roads in the future.
David Laan, Kim Matsen
A slip on the upstream face of Hedges Dam was observed during an annual site inspection in late March 2016. At that stage the slip appeared to be largely contained within the right hand third of the embankment.
By early April, the slip area had developed into a head scarp across the entire central portion of the embankment. Multiple other head scarps were observed, indicating multiple or segmented slips. Several tension cracks were also visible on the face of the dam. The toe of the slips was indicated by a poorly defined bulge.
The most recent drawdown of the reservoir level was identified as a potential driver for the initiation of the slip failure. During the most recent drawdown the maximum drawdown rate was approximately 0.6 m/day whereas in the previous 17 years the maximum drawdown rate was approximately 0.2 m/day.
The remedial works proposed are to place a rockfill weighting zone on the upstream face to stabilise the embankment. The strength of the materials along the sheared surface was back calculated from the mechanics of the failure surface. This data was then used to calculate the shape of the weighting zone required to stabilise the slope.
James Penman, Terence Jibiki, Len Murray, and Mark Rynhoud
Two earthfill embankments are being constructed to form an impoundment in a mountainous region with a tropical climate. The embankment abutments are underlain by tropical weathered rock/soil including a significant thickness of residual soil. Previous slope failures within the area, including a 150 m wide failure of a construction access road, have been potentially due to weakness of the residual soil. In order to quantify the potential risk to the embankments, a geotechnical characterisation program consisting of in situ and laboratory testing was completed to determine the shear strengths and loading response within the residual soil material. This paper summarises the geotechnical investigation program and characterisation of this tropical residual soil in the context of the embankment stability.
Results of laboratory direct simple shear testing are presented and compared to common empirical methods for estimating the undrained shear strength of both over-consolidated and normally-consolidated materials using index properties and/or over-consolidation ratios. Methods used for comparison include those proposed by Skempton (1952 & 1957), Bjerrum-Simons (1960), Lambe & Whitman (1969) and Wroth & Houlsby (1985) for normally consolidated material and Ladd (1977) and Jamiolkowski et al. (1985) for over-consolidated material.
The results of in situ testing, including pocket penetrometer data and field shear vane data, are also presented.
Mark Arnold, Gavan Hunter and Mark Foster
Following the dam safety risk assessment for Greenvale Dam in 2008, Melbourne Water implemented a 3.0 m reservoir level restriction on the operation of the storage as an interim risk reduction measure. The 3.0 m restriction coincided with the ‘as constructed’ top of the chimney filter in the main embankment. This interim action reduced the dam safety risk to below the ANCOLD limit of tolerability.
Dam safety upgrade works were undertaken in 2014/15 to bring the dam in-line with current risk based guidelines and to enable the removal of the interim reservoir restriction, bringing the storage back to full operating capacity. Greenvale Dam was required to remain operational throughout the works and this required careful consideration of the dam safety risk during construction.
Deep excavations were required within the crest and downstream shoulder of the embankments, that,, without adequate management, had the potential to increase risk to the downstream population. Excavations up to 18 m depth were required into the wing embankments for construction of full height filters from foundation to crest, excavations up to 7 m deep were required in the main embankment to expose and connect into the existing filters and secant filter piles up to 13 m deep were used to connect the new chimney filter of the wing embankments with the original chimney filter of the main embankment.
A key element of the design and construction of the upgrade works was managing dam safety during construction. Dam safety considerations included (i) design based decisions to manage the level of exposure; (ii) implementation of further restrictions on reservoir level by the owner Melbourne Water; (iii) construction methods to manage exposure; (iv) an elevated surveillance regime during the works and (v) emergency preparation measures including emergency stockpiles and 24 hour emergency standby crew. The construction based dam safety requirements were focused on early detection and early intervention, and were managed via the project specific Dam Safety Management Plan.
This paper focuses on dam safety management including the decisions made, actions taken and construction requirements and touches on how these relate to the key project features.
Shane McGrath, Stuart Richardson, Mark Arnold
Melbourne Water Corporation has recently completed a complex safety upgrade of Greenvale, an extreme consequence category dam. An assessment concluded that the residual risks were As Low as Reasonably Practicable (ALARP). However, given the uncertainty associated with the calculations the estimated residual societal risk was not comfortably below the limit of tolerability. Melbourne Water has experience with preparing hazardous industry safety cases for its water treatment chemical storages and decided to trial the methodology for Greenvale Dam. This paper describes the approach taken in hazardous industries to construct safety cases and how his was adapted to demonstrate that dam safety risks are ALARP.