David Guest, George Samios, Richard Rodd
Tenterfield Creek Dam is a 15m high concrete gravity structure that was constructed in 1930 and raised by 1.83m and stabilised using 97 post-tensioned ground anchors in 1974.Recent stability assessments concluded that the dam does not satisfy the ANCOLD Guidelines for Stability of Gravity Dams and that the situation is likely to deteriorate given the questionable performance of the post-tensioning cables and on the grounds of continuing corrosion and demonstrated loss of load.Tenterfield Shire Council is committed to improving the stability of the dam to meet the requirements of the NSW Dam sSafety Committee and engaged Public Works Advisory to assist them achieve this outcome.
Public Works Advisory prepared a dam upgrade options study which selected two options for further consideration. The estimated costs of the two preferred options were found to be potentially close;therefore Tenterfield Shire Council requested that both options be taken to detail design and tender stage to allow the market to indicate which option was in-fact better value.Factors other than construction costs were also considered in the options evaluation process and these factors influenced the selection outcome. The two upgrade options of lowest cost were the conventional gravity dam strengthen solutions i.e. installation of new post-tensioned ground anchors and downstream mass concrete buttressing. The decision to proceed to tender with two options was supported by the other key funding stakeholder, DPI Water.
This paper provides some unique insight on the comparison of conventional upgrade options for concrete gravity dams and also examines some interesting design aspects encounter edduring the design development process
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Gavan Hunter, David Jeffery and Stephen Chia
The Main Embankment at Tullaroop Dam in central Victoria is a 43 m high earthfill embankment with a very broad earthfill zone and rockfill zones at the outer toe regions. There has been an extensive history of cracking within the Main Embankment since formalisation of visual inspections in 1987.Widespread cracking has been observed on the crest and downstream shoulder. Cracking on the crest has mainly been longitudinal, but transverse cracks have also been observed. Cracking on the downstream shoulder has comprised longitudinal, diagonal and transverse cracking. In April 2004, a 60 mm wide diagonal crack opened on the downstream shoulder of the left abutment (from crest to toe) and Goulburn-Murray Water constructed a local filter buttress in 2005/06 on the left abutment. In 2011/12 a longitudinal crack opened up on the upper downstream berm toward the right abutment. The crack was initially 15m long and 10 to 215 mm wide, then propagated several months later to 70 m in length, 40 to 50 mm width and greater than 3 m in depth.In May 2011 three piezometers within the earth fill core recorded a very rapid rise in pore water pressure equivalent to 12 to 13 m pressure head above their previous readings. The piezometers were located on the same alignment (upstream to downstream) and were located below the crest and downstream shoulder, and the rise was to levels close to and above the embankment surface. The piezometers then showed a steady fall with time returning to the pre rise levels after 4 to 6 weeks.In 2015/16 Goulburn-Murray Water undertook dam safety upgrade works to reduce the risk of piping through the Main Embankment by extension of the filter buttress across the full width of the embankment. During these upgrade works, very deep (greater than 5 m) and extensive transverse cracks were observed in the embankment over relatively subtle slope changes on the right abutment.Thecracking and pore water pressure behaviour in the Main Embankment at Tullaroop Reservoir present an important case study. The paper provides details on the cracking and postulated crack mechanisms, and the rapid pore water pressure rise and postulated mechanisms. A summary of the upgrade works is also provided.
Radin Espandar, Mark Locke and James Faithful
Brown coal ash has the potential to be a hazard to the environment and local communities if its storage is not well managed. The risk of releasing contained ash from an ash tailings dam due to earthquake induced liquefaction is a concern for mining lease holders, mining regulators and the community.Ash tailings dams are typically raised by excavating and compacting reclaimed ash to form new embankments over slurry deposited ash, relying on drying consolidation and minor cementation for stability. Understanding the post-earthquake behaviour of the brown coal ash is necessary to assess the overall stability of an ash tailings dam during and after seismic loading events.A particular concern is the seismic motion may break cementation bonds within the ash resulting in a large reduction in shear strength (i.e. sensitive soil behaviour) and potential instability. There is limited information available for black coal ash however, brown coal ash has different properties to black coal ash and no known work has been carried out to date in this area.The dynamic and post-earthquake behaviour, including liquefaction susceptibility, of the brown coal ash was studied, specifically for Hazelwood Ash Pond No. 4 Raise (HAP4A) in Latrobe Valley, Victoria. In this study, different well-known methods for liquefaction susceptibility, including the methods based on the index parameters, the cone penetration test (CPT) and the cyclic triaxial testing, were used and the results were compared.It was found that the impounded brown coal ash is susceptible to liquefaction and /or cyclic softening. Triggering of the liquefaction or softening was assessed based on the results of cyclic triaxial test.In this methodology, the relationship among axial strain(εa), Cyclic Stress Ratio (CSR) and number of uniform cycles (Nequ) was determined based on the triaxial test results. Then, asite-specific CSR was determined using the ground response analysis. The CSR and number of uniform cycles (Nequ) for each ash layer was calculated and added to the εa-CSR-Nequgraph to determine the expected axial strain during an MCE event. It was found that the calculated axial strain for the ash embankment and ash deposits during site specific Maximum Credible Earthquake (MCE) are less than the axial strain of the ash material required for triggering of liquefaction and the brown coal ash in HAP4A does not liquefy and/or soften the material during an MCE event. Also it was found that the insitu tests which break the cementation between particles(such as CPT)does not provide accurate results on triggering or sensitivity.
Peter Allen and Mark Rhimes
Recent tropical cyclones have had significant impacts on coastal Queensland and produced significant inflows into a large number of major dams with the triggering of a number of Emergency Action Plans for downstream release hazards. While there were several floods of record, there were no significant dam safety incidents. The dams seemed to have been blamed for a lot of this flooding even though they provided significant flood mitigation. This paper will cover the emergency responses to these events, the public perceptions and the associated third party reviews of these events. Community expectations and the ability to undertake post flood event assessments of dam operations is also driving such investigations.This paper will also discuss the consequential updates being made to Queensland Emergency Action Planning Guidelines to encourage effective engagement with local emergency planners and other stakeholders in the development of these guidelines.
Although the total tailings dam failure frequency peaked in 1960s through 1980s, the failure rate of significant tailings dams has not dropped. The significant tailings dam failures the mining industry experienced in the recent history include: Merriespruit, South Africa, 1994; Los Frails, Spain, 1998; Kolontár, Hungry, 2010; Mount Polley, Canada, 2014; and Samarco, Brazil, 2015. The dam failures may be due to inadequate design, poor construction and inappropriate operations.This paper discusses the lessons learned and some recommendations and good practices to reduce the tailings dam failure risks. It addresses existing issues and provides some recommendations in risk based design, water management-integrity of facilities and water balance modelling, loading rates, tailings farming, adequate governance and roles and responsibilities of designers and nominated engineer.
Michael Hughes, James Stuart
Tropical Cyclone Debbie (TC Debbie) formed in the Coral Sea on Saturday 25th March, 2017 and developed into a category 4 system that crossed the coast near Proserpine, Queensland with the eye passing very close to Peter Faust Dam. TC Debbie, later becoming Ex-TC Debbie embarked on a tour of SunWater infrastructure (See Figure 1). Of 23 referable dams managed or owned by SunWater in Queensland, only 3 had no inflows with spills resulting at twelve locations. The paper describes the varied experiences of SunWater with relation to preparation for, and operations during TC Debbie. Some key areas of interest to other dam owners include;