Craig Johnson, Mark Arnold
Toorourrong Reservoir is a small storage reservoir which was constructed in 1885 and forms an important part of Melbourne’s water supply network. As part of Melbourne Water’s dam safety upgrade program, remedial works at Toorourrong Reservoir were identified to address deficiencies in flood capacity, embankment stability and to provide protection against piping. These works included an engineered filter system, downstream stabilising berm and raising of the dam crest level by 2.3m through a combination of earthfill and a concrete parapet wall. The existing spillway also required substantial enlargement and the existing scour and outlet structures were to be reconfigured. These works were designed and undertaken by the Water Resources Alliance (WRA).
Preliminary geotechnical investigations indicated the dam was founded on soft alluvial deposits, with the potential for foundation liquefaction under earthquake loading. During the course of subsequent investigations, the full complexity of the dam foundation was realised using numerous techniques including geophysics, CPT
u probes and seismic dilatometer testing. The results of these investigations were used to develop a detailed geotechnical model and embankment design sections. A range of analytical methods were utilised to characterise the liquefaction potential of the foundation, with these making reference to recent developments in this area of practice. Through an extensive assessment and review process, the design soil properties for the foundation were established and the liquefaction potential determined.
Based on these assessments, it was found that the potential for liquefaction existed across the majority of the dam foundation, with discrete soil layers liquefying depending on the intensity of the design seismic event. Strain-weakening (sensitive) soils were also identified in the foundation. A quasi risk-based stability assessment was undertaken for a range of post-liquefaction strength parameters and FoS to determine the sensitivity of the foundation response. Stability analyses were performed which indicated that additional stabilising berms were required at several locations. However, even with these berms, the extremely low post-liquefaction strengths meant that further ground improvement was required. This was assessed further and Grouted Stone Columns (GSC) were ultimately selected as the preferred foundation improvement method for the critical design sections with GSC to be installed both upstream and downstream to reinforce the dam foundation. This is the first time GSC have been used in Australia and some key “lessons learned” will be discussed.
2011 – Toorourrong Reservoir – Small Dam, Big Problems
The application of maintenance to mechanical assets is a mature and well understood discipline. Although techniques and methods differ from industry to industry, the experiences and knowledge of practitioners should be transferable, but are they?
A challenge faced by all engineers is finding the appropriate balance between obtaining detailed physical evidence of asset condition through invasive disassembly and inspection against assessing condition based on monitoring and past performance.
This paper describes the experiences of the author in applying mechanical maintenance skills in the dams environment and shares some important lessons learned along the way.
2011 – If it ain’t broke don’t fix it – or should you? Lessons learned from relying on past performance of mechanical assets.
Malcolm Barker, Toby Loxton
The Gladstone Area Water Board (GAWB) owns and operates Awoonga Dam, which is a concrete-faced rock fill embankment with a fixed crest concrete spillway on the left bank impounding a storage volume of 770,000 ML.
The current arrangement can accommodate the Probable Maximum Flood, allowing for flow over Saddles 3, 4 and 6 on the left abutment. A comprehensive study was carried out to evaluate the erosion potential downstream from Saddles 3 and 6 as well as other spillway options adjacent to the existing dam. One option was a radical approach including the removal of the Saddle Dam 3 and provision of downstream erosion protection works. This reduced the PAR and improved the overall dam flood capacity; however concerns were expressed about the environmental impact of possible erosion downstream from Saddle 3 for relatively frequent events.
A risk assessment showed that the erosion protection works downstream from the Saddle 3 or 6 were not cost effective and the preferred option for the upgrade was the closure of the Saddle Dam 3 with an auxiliary spillway created in Saddle 6,
This paper summarises the methods used and the outcomes from this study.
2011 – Awoonga Dam Acceptable Flood Capacity design – the anguish of erosion risk and implications for design
Robert Keogh, Rob Ayre, Peter Richardson, Barry Jeppesen, Olga Kakourakis
SunWater owns 23 referable dams and operates a further two dams for other owners. The dams are located across Queensland from Texas and St George in the South to the Atherton Tablelands in the north to Mt Isa in the west.
During the period December 2010 to February 2011 there were several significant rainfall events across Queensland. The first occurred in late December 2010, the second in mid January 2011 and third in early February 2011. Generally it was the most significant rainfall event in Queensland since the 1970’s. 22 Emergency Action Plans were activated simultaneously by SunWater. Eleven dams experienced a flood of record during the events.
This paper will discuss what has been learnt from these events including the optimisation of management structures for a dam owner with a large portfolio of dams: review of O&M Manuals including the adequacy of backup systems: relationships with the State disaster management framework: the value of rigorous communication protocols: managing fear and a general lack of understanding in the community: and the value of being prepared.
Workshop paper – Robert
George Bolliger and Clare Bales
Traditionally, the dams engineering profession has been a career path for engineers of civil/structural or geotechnical persuasion. As dams are constructed there is understandably a predominate focus on the civil requirements. Beyond the first few years of the dam’s life, effective operation and maintenance becomes increasingly important. A number of mechanical/electrical components and plant items form part of the critical infrastructure of the dam. A good maintenance routine is an essential requirement of the dam safety management program.
State Water Corporation, as the owner of 20 large dams and over 280 weir and regulator structures, runs a dam safety management program that is in line with the Australian National Committee on Large Dams Guidelines and NSW Dams Safety Committee requirements. The maintenance procedures and outcomes are audited through an internal maintenance audit program.
The maintenance audits form an integral part of the total asset management plan as well as the dam safety program. They are used to identify areas of strength as well as common errors or defects. Using State Water’s internal maintenance audits as case studies, the paper elaborates the role of maintenance audit program in enabling a cultural change to further include mechanical/electrical aspects and thereby enhance the longevity and safety of the assets.
Cultural Change – A Mechanical Perspective on Dam Safety Management
B. Ghahreman Nejad, H. Taiebat, M. Dillon and K. Seddon
One of the causes of tailings dam failure has been seismically induced liquefaction during earthquakes. Liquefaction, if mobilised, significantly reduces the stiffness and strength of affected soils in the embankment dam or its foundation and may lead to large deformations and dam failure. This paper reports the results of seismic liquefaction assessment and deformation analyses of Bobadil tailings dam located in Tasmania. The tailings dam consists of a perimeter rockfill starter dam which has been raised in stages using the “upstream” construction method. The embankment raises (formed by clay or coarse tailings) are constructed over a foundation of previously deposited tailings in the impoundment which is potentially susceptible to liquefaction. Extensive field and laboratory tests were carried out to assess the tailings liquefaction potential and also to determine the material properties required for seismic stability and deformation analyses. Numerical modelling of seismic liquefaction and deformation analyses were carried out to predict the magnitude and pattern of deformations that may lead to uncontrolled release of tailings. The results of these analyses are presented and compared with literature report of those observed during past earthquakes.
2011 – Numerical Modelling of Seismic Liquefaction for Bobadil Tailings Dam