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
The design of tailings dams under earthquake loading is quite challenging due to the nature of the tailings materials which are generally liquefiable under earthquake shaking. The design will be more complicated when the dam foundation is also liquefiable material. While assessment of liquefaction potentials is well developed in practice, assessment of liquefaction induced deformation varies from the simplest Newmark’s displacement method to the more complex effective stress dynamic analysis approach. It is generally accepted that the simplified method can be used for cases involving non-liquefiable materials. However, the use of this method for cases involving liquefaction may generally result in overly conservative designs to cater for the many simplified
assumptions in the method. With the advance of computer technology, time and cost are no longer obstacles for using the more appropriate method for estimating liquefaction-induced deformations of a tailings dams and achieving an optimum dam design.
This paper attempts to critically discuss issues in seismic design of tailings dams and provide an example of the use of the effective stress dynamic analysis method to estimate the liquefaction-induced deformations of a tailings dam and its importance in optimizing the design. The approach used is capable of estimating pore pressure response of liquefiable materials at any given time during the shaking. The effective stress analysis method used herein is embedded in FLAC software using a specially written FISH routine. Using this method, it can be demonstrated that although liquefaction is an issue, it does not necessarily mean that we must prevent its occurrence. As long as the deformation is acceptable, liquefaction is not necessarily a ‘show stopper’ for the project.
Keywords: liquefaction, seismic deformation, tailings dam design.
Karen Riddette, Chee Wei Tan, Alan Collins, David Ho
Due to a number of historical stilling basin slab failures around the world, modern basin slab stability assessment approaches now require allowance for hydrodynamic pressure fluctuations. Extreme fluctuations in uplift pressures have been found to occur in hydraulic jumps and plunge pools resulting in high-pressure pulses being transmitted via joints and drainage openings to the underside of the slab. If, peak uplift forces beneath the slab coincide with minimum pressure fluctuations on the top of the slab, the resulting pressure differential can be sufficient to lift a slab. As a result, simple static design based on tailwater depth and mean floor pressures is now considered highly non-conservative.
Through a case study on the Waipapa Dam spillway stilling basin, this paper examines the use of CFD modelling to compute mean hydrodynamic slab pressures taking into account the location of the hydraulic jump and the effect of the impact blocks on the pressure distribution over the slab. By combining the CFD results with empirically-derived pressure fluctuations, uplift scenarios are applied in a FEA model to compute the maximum load in the slab anchors and examine the sensitivity of the stilling basin slabs to uplift failure.
Keywords: Stilling basin, hydrodynamic modelling, CFD, pressure fluctuation, slab stability.
Tim Griggs and Richard Herweynen
The river diversion is an important aspect to be considered in the design of a dam. It generally consists of an upstream cofferdam, river diversion conduit and downstream cofferdam and allows the dam to be constructed in a dry section of river.
This paper reviews the diversion design adopted at three recent Australian roller compacted concrete (RCC) dams and comments on the effectiveness of the design in providing risk mitigation during the construction of each of these dams. The dams considered are Paradise Dam (2005), Meander Dam (2007) and Wyaralong Dam (2011).
Rather than selecting an arbitrary design flood for the diversion, a risk-based assessment was used that generally resulted in a relatively low design capacity. Even though there were cases where the diversion capacity was exceeded, it is considered that the risk based design process provided an economical diversion design for these recent Australian dams.
Keywords: Diversion, roller compacted concrete dam, RCC.
P C Styles, A L Garrard
The Victorian town of Nathalia was surrounded by flood water during the March 2012 floods in Northern Victoria.
Nathalia is protected by earthen levees of various sizes and age. Portable aluminium levees were installed during the March 2012 flood event, generally in areas where a permanent levee would restrict access to a park and views. The flood level came within 200mm of the crest of many of the levees and remained at a high level for nearly 2 weeks.
The paper describes the emergency management issues and procedures which relied on engineering advice to provide targeted and relevant remedial works on the levee system as potential problems arose. Engineers worked alongside the SES, CFA, Victoria Police, ADF and other volunteers to monitor, repair and reinforce the levee system on a 24 hour basis. The engineering support continued over a period of approximately 2 weeks, from the time the flood waters commenced rising until they had receded sufficiently for the orders for evacuation of the town to be rescinded.
Keywords: Nathalia, floods, levees, emergency management
Zhenhe Song, Arjuna Dissanayake, Shunqin Luo
One of the potential tailing dam failure modes that is commonly evaluated is for prediction of earthquake induced crest displacement in relation to available freeboard. The prediction of seismic induced displacement for tailing dams can be evaluated using simplified approaches, i.e. analytical methods by Newmark (1965), Makdisi and Seed (1978), Bray and Travasarou (2007) and empirical method by Swaisgood (2003) and Pells and Fell (2003).
Seismic induced displacements have been estimated using these simplified methods and numerical methods by FLAC and PLAXIS. The results from the numerical modelling were compared with results derived from the simpler analytical and empirical methods. The results indicate the numerical analysis results agrees reasonably well with empirical methods by Swaisgood (2003) and Pells and Fell (2003) and can be used to provide additional confidence in the seismic stability of tailings embankments. However, simplified analytical methods by Newmark (1965), Makdisi and Seed (1978), Bray and Travasarou (2007) could underestimate the seismic induced displacements.
Keywords: Tailing dam, Seismic analysis, numerical analysis, simplified analysis, liquefaction.