2016 – Warning and Mobilization of Populations at Risk of Dam Failure
Jason Needham, John Sorensen, Dennis Mileti, Simon Lang
The potential loss of life from floods, including those caused by dam failure, is sensitive to assumptions about warning and evacuation of the population at risk. Therefore, the U.S. Army Corps of Engineers engaged with social scientists to better understand the process of warning and mobilizing communities that experience severe flooding. This improved understanding enables dam owners to better assess the existing risk posed by their assets and investigate non-structural risk reduction measures alongside structural upgrades.
In this paper, the U.S. Army Corps of Engineers research is summarised to provide general guidance on the warning and mobilization of populations at risk for practitioners assessing the potential loss of life from dam failure. This includes commentary and quantification of three primary timeframes: warning issuance delay, warning diffusion, and protective action initiation. A questionnaire for estimating these parameters is also introduced, alongside a case study application for an Australian dam.
This paper also summarises the current understanding of how to reduce delays in determining when to issue warnings, increase speed at which warnings spread through communities, and decrease the time people spend before taking the recommended protective action. These insights will help all people involved with emergency management, including those tasked with developing Dam Safety Emergency Plans.
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Papers 2016
2016 – Constructions of a High Capacity Post Tensioned Permanent Carbon Fibre Ground Anchor
Learn moreDr Matthew Sentry, Nabeel Elias
Although permanent ground anchor technology has advanced in leaps and bounds over the past two decades, the focus of anchor technology has been on developing techniques to minimise the risk of component and system failure due to corrosion. The advancements in structural materials available in the market in recent years have enabled research into alternative materials for permanent ground anchor systems.
Carbon fibre has become a significant structural alternative throughout North America for bridge and building construction as well as repair and structural strengthening of deteriorated/corroded structures. These advancements and the necessity to investigate alternative materials for anchor systems have led to research in understanding the long term performance effects of using carbon fibre products as an alternative to steel tendons in permanent ground anchors.
Following on from the advanced research works at Monash University and Geotechnical Engineering which investigated the durability performance of various available CFRP strands when used as an alternative to conventional steel tendons in permanent ground anchor systems, Geotech developed the first post tensioned ground anchor system using CFRP strand.Following laboratory based trials and small scale bun barrel tests, Geotech was able to successfully design, construct, install and stress the first 27 strand post tensioned CFRP ground anchor installed into Yass Dam. The CFRP strand was stressed and locked off at 4,000kN. Real time monitoring has been installed to monitor the load throughout the anchors service life.
This paper provides the details of the construction, installation and stressing of the first CFRP anchor installed into a dam structure.
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Papers 2016
2016 – Investigation of the Foundation, Sub-surface Drainage and Slab Anchor Degradation of a Concrete-lined Spillway: Fairbairn Dam, Queensland
Learn morePeter Simson, Deryk Foster
Fairbairn Dam is an earth and rockfill embankment dam with an ungated, concrete-lined, spillway, located at AMTD 685.6 km on the Nogoa River, approximately 16 km south of Emerald in Central Queensland.
Following the flood of record in 2011 it was decided to repair a number of areas of spalling concrete which uncovered a collapsed transverse drain and a large void beneath the chute floor. The spillway chute is designed with subsurface drainage system of floor slabs consisting of alternate strips of concrete footing and gravel bed to aid in the control of uplift. The gravel was flushed from under the spillway floor into collapsed earthenware pipes of the drainage system resulting in an unsupported floor slab. Further investigation was carried out using Ground Penetrating Radar (GPR) which identified additional locations of possible voids. Concrete coring was carried out at selected locations to confirm the voids with some being over 250 mm in depth.
Investigation of the sub-surface drains was carried out using CCTV and showed many of the open jointed earthenware collector pipes had cracked and/or collapsed causing the drainage gravel and founding sedimentary rock to be scoured out by spillway flows entering the system through open contraction joints.
Following the discovery of the foundation scouring it was decided to expose a number of anchor bars in the chute floor to undertake a pull-out testing program. Of the ten anchor bars that were exposed, six were found to have corroded completely with the remaining four noted to be partially corroded and subsequently failed under loading.A geotechnical investigation of the foundation materials was planned to determine the condition and strength of the founding sedimentary rock. In addition, the investigation also included sampling of seepage and reservoir waters to characterise the hydro-geochemistry and its contribution to the deterioration of the anchors.
Artesian conditions also occur within the spillway area, driven by the reservoir, with water passing through an extensive network of pervasive defects in addition to permeable flat-lying strata.Coal seam gas is also known to occur, providing a further contribution to aggressive water geochemistry.
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Papers 2016
2016 – Hedges Dam – Upstream Face Slip from Rapid Drawdown and Subsequent Remedial Works
Learn moreDavid 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.
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Papers 2016
2016 – Comparing CRCFORGE Estimates and the New Rare Design Rainfalls
Learn moreJ.H.Green, C.Beesley, C.The, S.Podgerand, A.Frost
The ability to estimate design rainfalls for probabilities rarer than 100 years or 1% Annual Exceedance Probability (AEP) is an essential part of dam hydrology. The earliest means of estimating rare events consisted of a pragmatic curve fitting procedure between the 50 and 100 year design rainfalls and the Probable Maximum Precipitation. In the 1990s a more rigorous method of estimating design rainfalls as rare as 2000 years was developed – the Cooperative Research Centre – FOcussed Rainfall Growth Estimation (CRC-FORGE) method. CRC-FORGE estimates were derived for Victoria in 1997 followed progressively by each of the other states. Over the subsequent two decades CRC-FORGE estimates were an integral part of the risk assessment of large dams – being used to determine the AEP of the Dam Crest Flood.
The Bureau of Meteorology will soon release new rare design rainfall estimates for probabilities to 2000 years. The new rare design rainfalls are a significant improvement on the CRC-FORGE estimates as they have been derived using up to date data; contemporary analytical techniques and a method that is consistent across Australia.
However, there are differences between the CRC-FORGE estimates and the new rare design rainfalls. These differences do not constitute a systematic change to the CRC-FORGE estimates but rather vary with location; duration and probability. The results of a detailed comparison between the CRC-FORGE estimates and the new rare design rainfalls are presented together will an assessment of the possible impacts on previous estimates of the AEP of the Dam Crest Flood.
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Papers 2016
2016 – Dam Safety Management for Upgrade of an Extreme Consequence Storage
Learn moreMark 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.
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