Peyman Andaroodi, Barton Maher
Seqwater is a statutory authority of the Government of Queensland that provides bulk water storage, transport and treatment, water grid management and planning, catchment management and flood mitigation services to the South East Queensland region of Australia. Seqwater also provides irrigation services to about 1,200 rural customers in the region that are not connected to the grid and provides recreation facilities. Seqwater owns and operates 26 referable dams regulated under Queensland dam safety legislation.
Leslie Harrison Dam is an Extreme Hazard category dam located in the Redland Bay area of Brisbane.A significant portion of Population at Risk is located within a short distance downstream of the dam, reducing the available warning time in the event of a dam safety issue and impacting on the estimated loss of life used to assess risk. Following the Portfolio Risk Assessment undertaken by Seqwater in 2013, a series of detailed investigations were undertaken to confirm the assessed risk and the scope and urgency of the upgrade works.
Before a final decision on the scope and timing of the dam upgrade is made, Seqwater has completed a detailed review of the downstream consequences. This review was intended to update the Population at Risk(PAR) and Potential Loss of Life(PLL) estimates using the latest estimation methods for a range of scenarios. Three life loss estimation methods were used including empirical and dynamic simulation models and the results were compared.
This paper discusses the updated consequences assessment and the impact on the assessed risks, for Leslie Harrison Dam for both the current dam and the proposed upgrade scenarios using the revised Potential Loss of Life estimates.
Conrad Ginther, Colleen Stratford
The Wyaralong Dam Alliance (WDA), a consortium of seven engineering and contracting companies, was contracted to design and construct the Wyaralong Dam, which impounds the Teviot Brook 14 km from Beaudesert in Queensland, Australia. The dam is an approximately 500 metre long, 48 metre high Roller Compacted Concrete (RCC) structure built on a foundation generally consisting of massive sandstone with intermittent conglomerate zones consisting of cemented gravels, mudclasts and sands. Geologic features of note with regard to dam stability and long term seepage at the site are dominated by downstream sloping bedding features and conglomerate zones. In addition to the bedding-related features, two predominant vertical to subvertical fracture sets exist. The condition of the vertical fractures ranges from tight and fresh at depth to highly weathered and filled with dispersive clay and gravels near the foundation surface. To provide a durable and effective long term seepage barrier for the dam, an extensive foundation cleaning and treatment operation was undertaken. This comprised drilling, blasting, and excavation of the majority of the highly weathered rock and dispersive materials supplemented by localized installation of small cut-offs and dental concrete and the construction of a double-line grout curtain installed using real time computer monitoring, the GIN methodology, and balanced, stable grout mixes.
Foundation Preparation and Seepage Barrier Installation at Wyaralong Dam Construction Project
M. A. Hariri Ardebili, M. Akbari and H. Mirzabozorg
This paper presents a study on the effects of incoherence (considering the Harichandran and Vanmarcke coherency model) and wave-passage (considering various wave velocities) on the nonlinear responses of concrete arch dams . A double curvature arch dam was selected as numerical example, the reservoir was modeled as incompressible material and the foundation was modeled as a mass-less medium. Ground motion time-histories were artificially generated based on a Monte Carlo simulation approach. Four different models were considered in the generation of ground motions; Uniform excitation; Just incoherence effect; Just wave passage effect; and finally take into account both incoherence and wave passage effects. It was revealed that modeling incoherency can have significant effect on the structural response of the dam by modifying the dynamic response of uniform excitation and inducing pseudo-static response. Also, it was concluded that incoherency effect overshadow wave passage effect and results caused by wave passage effect are close to the results of uniform excitation.
2011 – Comparison of wave passage and incoherence effects on nonlinear non-uniform excitation of concrete arch dams
Susan Ryanand Siraj Perera
This paper describes the benefits of the statewide risk reporting framework used in dam safety regulation in Victoria and its ongoing development. Key to this approach is a web-hosted reporting system and benchmarking process, established by the Department of Sustainability and Environment in collaboration with the Victorian water industry. This is the first time that such an approach has been used in Australia for publicly owned dams.
Sector-wide reporting on dam safety is central to the objective-based approach used by the Department in the governance and regulation of the water industry. Water corporations submit detailed annual reports on dam safety status. This incorporates ‘self assessment’ against performance criteria based on ANCOLD risk and dam safety management guidelines. These are collated to produce a statewide report of industry-wide results on the progress of dam safety management programs. This benchmarking process is providing a driver for on-going improvement and proving to be an effective tool for regulation of publicly owned dams.
The reporting framework has significantly advanced the understanding of dam safety risk across the water sector, with outputs easily understood by both dam safety practitioners and decision makers. It has improved monitoring and trend analysis of risk management practices, and is informing policy development on demonstration of the ALARP principle and decision-making about appropriate long-term dam safety levels.
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