David Brett, Robert Longey, Jiri Herza
The independent expert review panel for the Mount Polley Tailings Storage Facility failure came out strongly recommending changes to the technology of tailings dams in British Columbia (and by inference, world-wide). The Panel had examined the historical risk profile of tailings dams in British Columbia and recommended, amongst other things, that best available technology (BAT) be adopted for tailings disposal. Examples of BAT, described by the panel, included “dry-stacking” of filtered, unsaturated, compacted tailings and reduction in the use of water covers in a closure setting. The recommended technologies would require a major shift in current practice and raises many questions, such as:
– Are these recommendations appropriate in Australia?
– Does this signal the end of the tailings dams as we know them?
– Do the current Australian National Committee on Large Dams Guidelines (ANCOLD) apply to these new BAT technologies?
– If not, is there a role for ANCOLD in setting standards for the future?
This paper discusses the Mt Polley tailings dam failure and searches for answers to these questions. In particular, this paper reviews the background to “dry-stacking’, to explore the implications for the Australian mining industry.
Keywords: Tailings Dam, Dry Stacking, Best Available Technology
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
Bob Wark, Louise Thomas, Andrew Peek
Alkali Silica Reaction (ASR) has been by far the dominant cause identified in the deterioration of concrete caused by expansion of the pastes from an interaction with the aggregates. However the path to the identification of the presence of the deleterious effects of ASR is not always straightforward. In a recent example, the concrete spillway slabs and walls at South Dandalup Dam exhibited classic craze cracking symptoms of ASR. However when subjected to more detailed analysis the driving process was found to be delayed ettringite formation (DEF).
ASR and DEF are chemically different concrete deterioration mechanisms with physically similar manifestation, causing slow concrete expansion in the presence of moisture. ASR has been reported mostly in concrete structures constructed prior to the early 90’s when the DEF deterioration mechanism was not fully recognised. However it is possible that ASR and DEF can take place simultaneously and more extensive damage due to DEF could have occurred and remain undetected.
The paper will also describe a recent case using basalt aggregate for Stirling Dam in which the use of an accelerated mortar bar test gave an extreme reaction but the ASTM concrete prism expansion test gave a negative result. Further detailed petrographic examination provided the clues to the real cause.
The paper will describe the occurrence of the problems, compare the causes and outline the methods undertaken to investigate the issues. Alternative concrete mix designs, incorporating a high flyash content to replace ordinary Portland cement as the main pozzolanic material, have been investigated and successfully implemented. This paper describes the investigations undertaken to develop these alternate mixes, the resultant properties of the concrete and its resistance to deterioration.
2011 – Searching for Solutions to ASR
Mark R. Sinclair & Richard J. Rodd
Over the last six years there have been ongoing significant developments in the design, fabrication and particularly of the corrosion protection details for high capacity ( >13,500kN MBL ) re-stressable ground anchors used to improve stability of gravity dams. These Australian based developments and the resultant specifications and details have now become the de-facto standards adopted.
The ANCOLD Register dams to have had this generation of cables installed have included; Ross River Dam, Lake Manchester Dam, Catagunya Dam, Tinaroo Falls Dam and Wellington Dam. These projects include the highest capacity permanent ground anchors installed to date worldwide. Some smaller capacity anchors installed into dams have also benefited from this technology.
The Recent Developments and Application of Large Ground Anchors for
Robert Keogh RPEQ, CE Civil (Hon), Mal Halwala, Peter Boettcher, Renee Butterfield
SunWater is a Government Owned Corporation (GOC), operating in a competitive market on an equal commercial footing with the private sector. SunWater owns 23 referable dams. Over the last fifty years there has been significant development of the methodologies used to estimate extreme rainfall events. These have resulted in substantial increases in probable maximum flood (PMF) estimates for most of SunWater’s dams.
SunWater has undertaken a Comprehensive Risk Assessment program across its portfolio. SunWater now has a good understanding of the deficiencies and available risk reduction options for each dam under all load conditions. The total cost to rectify all deficiencies is several hundred million dollars and well beyond the financial capacity of the organisation in the short term.
ANCOLD and Regulators have different published opinions on decision making criteria for dam safety upgrades. Once the conditions for the tolerability of Societal and Individual Risk are satisfied the onus remains with the dam owner to meet the ALARP principle. The decision making process is complicated by uncertainties in inputs to risk assessments. The authors have considered these uncertainties as well as the legal implications, differing ANCOLD and Regulator requirements, and business and economic loss, in formulating the decision making process. The methodology is simplified but effective. If the process is followed the dam owner’s investments will meet ANCOLD, Regulatory, legal and business requirements.
This Paper details a logical decision making process designed to allow a non technical Board to balance social, legal and financial objectives. The process considers overall risk, tolerability, the ALARP principle, and project prioritisation. The process is being used by SunWater to determine the Acceptable Flood Capacity of each dam, which dams will be upgraded, priorities and scheduling of each upgrade.
How SunWater, as a commercial dam owner makes investment decisions for dam safety upgrades