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Mark Foster, Robin Fell and Matt Spannagle

This paper describes a method for estimating the probability of failure of embankment dams by piping. The so called “UNSW method” is based on the results of an analysis of historic failures and accidents of embankment dams. An estimate of the probability of failure of a dam by piping is made by adjusting the historical rates of failure by piping by applying weighting factors which take into account the dam zoning; filters; age of the dam; core soil types; compaction; foundation geology; dam performance; and monitoring and surveillance. The method is intended for preliminary assessments only and is ideally suited as a risk ranking method for portfolio type risk assessments to identify which dams to prioritise for more detailed studies and as a check on event tree methods.

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1998 Papers

1998 – Risk Assessment – Estimating the Probability of Failure Embankment Dams by Piping
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Mark Foster, Robin Fell and Matt Spannagle

This paper describes a method for estimating the probability of failure of embankment dams by piping. The so called “UNSW method” is based on the results of an analysis of historic failures and accidents of embankment dams. An estimate of the probability of failure of a dam by piping is made by adjusting the historical rates of failure by piping by applying weighting factors which take into account the dam zoning; filters; age of the dam; core soil types; compaction; foundation geology; dam performance; and monitoring and surveillance. The method is intended for preliminary assessments only and is ideally suited as a risk ranking method for portfolio type risk assessments to identify which dams to prioritise for more detailed studies and as a check on event tree methods.
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2000 Papers

2000 – The Dam Safety Upgrade at Lake Eppalock
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Richard R. Davidson, Roger Vreugdenhil and Mark Foster

Significant cracking was observed on the crest of the main embankment at Lake Eppalock for many years, but in recent years increasing movement upstream during low reservoir levels indicated a progressively deteriorating stability situation. Investigations also revealed cohesive filter material that would allow a crack to propagate. A fast-tracked remedial works program was completed in 1999 to rebuild the highly vulnerable upper rockfill shells and filters, both upstream and downstream. To manage construction risk, the works were carried out directly by G- MW with innovations in removal, protection and replacement of the downstream shoulders, and placement of a new multi-zone filter.
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2001 Papers

2001 – Risk and Standards Based Assessment of Internal Erosion and Piping Failure – A Convergence of Approaches
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Graeme Bell, Robin Fell and Mark Foster

Standards based, dam safety management has always been about managing risks. Risk based approaches attempt to quantify the risks in a formal manner, but are based on the same requirement for good investigation and engineering, and understanding of the physical processes, as standards based methods.

This is demonstrated by the assessment of the potential for internal erosion and piping of Eucumbene Dam. The assessment is a combination of semi-quantitative risk based, and standards based approaches, and considers the likelihood of initiation, continuation, progression to form a pipe, and breach. The filter transition zones are coarser than required to meet modern filter design criteria, but it has been demonstrated by laboratory testing, and relation to the performance of other dams, that if, in the low likelihood erosion of the core initiates, it will after some erosion, seal on the filter transition zone. The downstream zone of rockfill has sufficient permeability to discharge any potential leakage which might occur, so the likelihood of breach of the dam by piping is
extremely low.
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2005 Papers

2005 – LAKE BUFFALO DAM RISK REDUCTION UPGRADE
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Construction of the Lake Buffalo Dam was completed in 1965. It was to be a temporary dam, required to operate for several years, then act as a cofferdam for the construction of a much larger dam downstream. This larger dam was never built and a risk assessment completed by Goulburn Murray Water (G-MW) in 2001 identified several dam safety deficiencies at Lake Buffalo were among the highest priorities for risk reduction measures across the G-MW dams portfolio. Specifically it identified Lake Buffalo as having inadequate flood capacity and there were also concerns about transverse cracking within the embankment.

This paper describes the detailed investigation and analysis of the embankment cracking including assessing the potential for piping through an embankment having deficient filters and known transverse cracking. The design features of the upgrade are also described including the design of the a filter buttress, a parapet wall raise, Computational Fluid Dynamics (CFD) modelling and spillway anchoring. Construction was completed in 2003.
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2005 Papers

2005 – Lake Buffalo Dam Risk Reduction Upgrade
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Stephen Newman, Mark Foster

Construction of the Lake Buffalo Dam was completed in 1965. It was to be a temporary dam, required to operate for several years, then act as a cofferdam for the construction of a much larger dam downstream. This larger dam was never built and a risk assessment completed by Goulburn Murray Water (G-MW) in 2001 identified several dam safety deficiencies at Lake Buffalo were among the highest priorities for risk reduction measures across the G-MW dams portfolio. Specifically it identified Lake Buffalo as having inadequate flood capacity and there were also concerns about transverse cracking within the embankment.

This paper describes the detailed investigation and analysis of the embankment cracking including assessing the potential for piping through an embankment having deficient filters and known transverse cracking. The design features of the upgrade are also described including the design of the a filter buttress, a parapet wall raise, Computational Fluid Dynamics (CFD) modelling and spillway anchoring. Construction was completed in 2003.
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2016 Papers

2016 – Dam Safety Management for Upgrade of an Extreme Consequence Storage
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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.
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2019 Papers

2019 – Estimating the Individual Risk From Dam Failure
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Simon Lang, Mark Foster

The ANCOLD (2003) Guidelines on Risk Assessment contain criteria regarding the tolerable level of individual risk from dam failure. Maslin et al. (2012) describe an approach to estimating individual risk from dam failure, using exposure factors, warning and evacuation factors, and fatality factors. These factors vary according to the people at risk, the anticipated warning time, the flood severity and the shelter people are likely to be in. Maslin et al. (2012) provide step-by-step instructions, which means their approach can be applied in a consistent manner from dam to dam. However, the recommended fatality factors are based on Graham (1999) and DHS (2011) definitions of high, medium and low severity flooding which have been superseded by the Reclamation Consequence Estimating Methodology (RCEM). Therefore, in this paper modifications to the Maslin et al. (2012) approach are proposed, so that estimates of individual risk from dam failure are consistent with RCEM-based estimates of societal risk. The paper then concludes with two predictions about how the assessment and use of individual risk in Australian dam safety management may change in future.
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2013 Papers

2013 – Risk-based management of two water bodies at a Queensland coal mine
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Rob Campbell, Christopher Dann and Mark Foster

Queensland contains some of Australia’s most significant reserves of mineable metallurgical coal, which is an essential raw material used in the production of steel. The area also has large deposits of thermal coal, used for electricity generation.
For the many active open cut and underground coal mines in Queensland, the enduring operational focus is to maximise returns and productivity, while still meeting key safety and environmental responsibilities.
Maintaining open cut pits in a dewatered state is often a key factor in achieving optimal productivity of an open cut mine. In Queensland, for many mines it is not always practical to maintain all pits in a dewatered state, given the subtropical climate and significant rainfall that can occur during the wet season, between the months of November and March. In effectively managing mine water while maintaining production, it is not unusual for excess mine water to be temporarily stored in a designated open cut pit.
The typical scale and arrangement of open cut pits at mine sites in Queensland is such that relatively deep and high volume pits can be separated by relatively narrow “landbridges”, consisting of in-situ material or mine spoil. The situation can therefore arise where a significant volume and head of mine water is stored in one pit, with mining operations continuing in an adjacent pit, and the landbridge is required to perform as a water retaining structure. This is a scenario that might not have been considered when the landbridge was originally constructed. This paper presents a study of two such landbridges at either end of a mine pit in Queensland, over a 5 year period from 2008 to 2013, with mining activities in the pit ranging from dragline pre-stripping to open cut mining, to large scale construction works and underground mining. By employing a long term interactive approach with mine operations personnel and utilising quantitative risk management techniques, risks were effectively managed, helping the mine to maintain operations while meeting safety and environmental requirements.
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2012 Papers

2012 – Estimating Individual Risk
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Kelly Maslin, Mark Foster, Len McDonald

A key requirement of assessing the tolerability of dam safety risks is the assessment of individual risk. The ANCOLD Guidelines on Risk Assessment provides guidance on acceptable levels of individual risk and some general guidance on the calculation of individual risk.
Individual risk is a key measure in the consideration of the tolerability of risk, ALARP and development of risk mitigation works. It is essential that there is consistency in the approach to estimating individual risk used across the dams industry.
This paper reviews the approaches taken to estimating individual risk across the dams industry both locally and internationally as well as the experience of other industries.
The paper includes a review of the various methods for estimating the vulnerability of individuals subjected to flood inundation based on historical fatality rates as well as identification of the individual most at risk
The paper then describes a method that has been developed based on the principles used for assessing individual risk due to other hazards, such as landslides. The method includes consideration of a range of factors such as warning time, temporal variation and vulnerability of the individuals most at risk. The method developed provides a transparent, defensible and pragmatic approach to estimating individual risk. Practical guidance and examples are also provided on the application of the method.
Keywords: individual, risk, exposure, fatality

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2009 Papers

2009 – Innovative Solution to foundation piping risk at Hinze Dam
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Gavan Hunter, Chris Chamberlain, Mark Foster

Hinze dam, an extreme hazard storage, is under the authority of Seqwater (Southeast Queensland) and is principle potable water storage supplying the Gold Coast. Hinze Dam Stage 3, presently under construction, involves raising the existing embankment almost 15m to a maximum height of 80m.

The foundation geology on the right abutment of the main embankment comprises of a deeply weathered sequence of greywacke and variably silicified greenstone and chert. The deeply (and variably) weathered soil profile below the right abutment of the existing embankment presented an unacceptable piping risk for the embankment in its existing condition. Contributing factors included: 1/ the highly erodible extremely weathered greywacke and presence of continuous defects in the weathered soil mass; 2/ the extremely weathered greenstone in direct contact with highly fractured, highly permeable silicified greenstone and chert bodies aligned normal to the dam axis which provide continuous seepage paths through the foundation.

Works were required as part of the Stage 3 raise to address the foundation piping risk. Significant issues for design included: 1/ the depth of weathering extended up to 25to 40m into the foundation.; 2/ extremely weathered and highly erodible greenstone was present below the right abutment of the embankment and extended down to the lower abutment some 50 to 60 m below the existing dam crest; 3/ the reservoir level could not be drawn down during construction and the probability it would be near full supply level during the works was high; and 4/ the variability of strength in the greenstone form soil to extremely high strength presented challenges for excavation.

The options assessed to address the piping risk included a plastic concrete cut-off wall and an upstream blanketing option. The plastic concrete cut-off wall (220m long and up to 50m deep) and deep filter trench was the selected option. The cut-off wall had been successfully completed ahead of time and below budget. The innovative design required excavation through earthfill core of the embankment under full reservoir level and use of a purpose built trench cutter (by Bauer Foundations Australia) for the variable excavation conditions.

Keywords: dam safety, piping, risk assessment, cut-off wall.
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Search Results: “” — Page 1

Simon Lang, Mark Foster