M C N Taylor, Dr H E Cherrill, S F Croft, S F Eldridge
The Stuart Macaskill Lakes are two raw water storage lakes with a combined storage of approximately 3280 ML supplying Wellington City, New Zealand. The lakes are High Potential Impact Category (PIC) earth embankment dams constructed on terrace gravel deposits adjacent to the Hutt River and located within approximately 20 to 50 metres of the Wellington Fault Deformation Zone. Construction of the lakes began in 1982 and they were commissioned in 1985.
In early 2008, the lake’s owner Greater Wellington Regional Council (GWRC), embarked on a programme to supplement Wellington City’s water supply storage. Whilst that study is ongoing, GWRC engaged Tonkin & Taylor (T&T) to investigate the feasibility of increasing the Stuart Macaskill Lakes capacity as an interim measure.
The feasibility study concluded in late 2009 that the lake dam embankments could be raised by up to 1.3 metres in height to gain an approximate additional 450 ML of water storage. An important finding of that feasibility study has been that the seismic requirements have increased significantly since the construction of the lakes. To address this issue GWRC is currently constructing Stage Two of a two stage construction programme to both raise the lakes and to incorporate seismic resistant features into the lakes.
The primary design features are downstream rock buttressing in the critical areas of the lakes and synthetic lining the inside of the lake embankments. The buttressing works were completed in early 2011 and the lining and crest raising works are due for completion in 2013.
This paper summarises the design, laboratory testing and construction to enhance the lakes performance during very strong seismic accelerations (Peak Ground Accelerations of up to 1.08g) expected during a maximum design earthquake originating from the Wellington Fault.
Keywords: Water Reservoir, Seismic Design, Geomembrane, Rock Buttressing, Seismic Risk Assessment, Wellington Fault
Simon Lang, Peter Hill, Wayne Graham
The empirical method developed by Graham (1999) is the most widely used in Australia to estimate potential loss of life from dam failure. It is likely to remain that way while spatially based dynamic simulation models are not publicly available (e.g. LIFESim, HEC-FIA and LSM). When the Graham (1999) approach was first developed the prevalence of spatial data and the speed of computers was much less. In addition, most people did not have mobile phones, social media was in its infancy, and automatic emergency alert telephone systems were 10 years from being used in Australia. Graham (1999) was intended to be applied to populations at risk (PAR) lumped into a discrete number of reaches. The selection of fatality rates for the PAR in each reach was based on average flood severity and dam failure warning times. Today, there is typically much more spatially distributed data available to those doing dam failure consequence assessments. Often a property database is available that identifies the location of each individual building where PAR may be, along with estimates of flood depths and velocities at those buildings. News of severe flooding is likely to be circulated by Facebook, Twitter and e-mail, in conjunction with official warnings provided by emergency agencies through radio and television and emergency alert telephone systems.
This raises the question of how Graham (1999) is best applied in today’s digital age. This paper explores some of the issues, including the estimation of dam failure warning time, using Graham (1999) to estimate loss of life in individual buildings and the suitability of Graham (1999) for estimating loss of life for very large PAR.
Keywords: loss of life, dam safety, risk analysis.
Robert Kingsland, Jamie Anderson, Andrew Russell, David Brooke
This paper presents the methods, observations and results from a programme of No-Erosion Filter (NEF) testing for the evaluation of a manufactured filter aggregate product that did not conform to normally accepted D15F grading limits. Base materials tested include both dispersive and non-dispersive soils. The results are compared against published no-erosion, excessive erosion and continuing erosion thresholds. The paper comments on the validity of the adopted thresholds and the effectiveness of the NEF test as a filter evaluation method.
Keywords: dam, filter, test, no-erosion
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.
Gavan Hunter, Robin Fell, Chris Topham
Backward erosion piping is a failure mode that can affect water retaining structures with earthen cores of very low or no plasticity. Backward erosion involves the progressive detachment of soil particles as seepage through a core material exits to a free surface or unfiltered zone. In contrast to other piping failure mechanisms, backward erosion does not require a defect to be present for initiation, and is heavily influenced by the inherent characteristics of the core materials and the available hydraulic head. For dams with non-plastic or very low plasticity core materials, backward erosion can be a material contributor to the overall piping risk and warrants careful consideration during quantitative risk assessments of such dams. However, there is very little published literature for evaluating the potential for backward erosion piping, particularly in broadly graded soils. This paper concerns one such dam where backward erosion of the glacial till core needed to be assessed in the context of a detailed risk assessment for the facility. The backward erosion mechanism was tested in laboratory tests set up to model the situation in the core of the dam at a range of hydraulic heads. The paper describes the core material and objectives for the testing, presents the apparatus used, summarises the findings, and explains how they contributed to the risk assessment for the dam. Recommendations are also made for future similar testing and research needs.
Keywords: Backward erosion, piping, embankment dam, laboratory testing, quantitative risk assessment, glacial till.
Shane McGrath, Andrew Reynolds, Garry Fyfe, Chris Kelly, Steven Fox
Goulburn-Murray Water is a rural water corporation located in Northern Victoria. It has responsibility for 12 State dams and is also the constructing authority for the Murray Darling Basin Authority’s Victorian assets.
Over the past 15 years G-MW has been engaged in a dam improvement program across its portfolio. To date 14 individual projects have been undertaken at 11 dams. The total expenditure is $125 million.
Starting from a base level of data at its inception in 1997, the program has encompassed all facets required for a dam improvement program. From early prioritisation to set the investigation program, through design reviews and risk assessments to develop the upgrading program and subsequent implementation. Some elements of the program were at the leading edge of practice at the time and a range of experiences along the way were character building as dam safety investment challenged other corporate priorities.
This paper sets out the lessons learned in developing the methodology and implementing the program of works, particularly relating to corporate adoption of the program, organisational capability, investigations, risk assessments, design and implementation.