Dr Andy Hughes
Tailings dams continue to undergo failures at an unacceptable rate compared to water storage dams, including failures at operations owned by high profile mining companies.Tailings dams have often a different form and method of construction than water storage dams in that tailings dams continue to be raised over time as part of the mine operations and rise to considerable heights. These failures are often the result of a combination of design, construction and operations actions that are controlled by humans and must be better coordinated and managed in the future. The consequence of failure can be widespread flows of tailings and water over the landscape and water courses. This can have extreme consequences in terms of life loss, environmental damage, social license to operate, company value, and mining industry sustainability. Therefore,it is necessary that the mining industry strive for zero failures of tailings facilities. Any additional technology and information that enables an owner of a tailings dam to be more certain of its condition and thereby reduce the risk of failure is of tremendous value to reliable tailings and mine water management.The Willowstick method uses low voltage, low amperage, and alternating electrical current to directly energise the groundwater by way of electrodes placed in wells or in contact with seepage or leaks. This approach has been successfully used to identify water flow paths through, under and around tailings dam in plan and elevation.The Willowstick technology provides additional information to supplement the geological, geotechnical and hydrological, evaluations analyses and designs, and to further improve tailings dam safety by more robust designs if necessary. This paper, using several tailings dam case studies, illustrates the procedure, findings, and the benefits of the Willowstick methodology. The findings of many Willowstick surveys range from tailings dams where the methodology has confirmed the design evaluations, to tailings dams where new groundwater and leakage flow paths were identified. In the latter case, the dam designers were able to update the designs, based on the new information,to mitigate the identified risks and to improve the overall safety of the tailings dams in accordance with the goal of zero failure.
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Now showing 1-12 of 47 2981:
T. Allen, J. Griffin, M. Leonard, D. Clark and H. Ghasemi
Geoscience Australia (GA) has embarked on a project to update the seismic hazard model for Australia through the National Seismic Hazard Assessment (NSHA18) project.The draft NSHA18 update yields many important advances on its predecessors, including: 1) calculation in a full probabilistic framework using the Global Earthquake Model’s OpenQuake-engine; 2) consistent expression of earthquake magnitudes in terms of moment magnitude, MW; 3) inclusion of epistemic uncertainty through the use of alternative source models; 4) inclusion of a national fault-source model based on the Australian Neotectonic Features database; 5)the use of modern ground-motion models; and 6)inclusion of epistemic uncertainty on seismic source models, ground-motion models and fault occurrence and earthquake clusteringmodels.The draft NSHA18 seismic design ground motions are significantly lower than those in the current (1991-era) AS1170.4–2007 hazard map at the 1/500-year annual ground-motion exceedance probability (AEP) level. However, draft values at lower probabilities (i.e., 1/2475-year AEP) are entirely consistent,in terms of the percentage area of land mass exceeding different ground-motion thresholds,with other Stable Continental Regions(e.g.,central & eastern United States). The large reduction in seismic hazard at the 1/500-year AEP level has led to engineering design professionals questioning whether the new draft design values will provide enough structural resilience to potential seismic loads from rare large earthquakes. This process underscores the challenges in developing national-scale probabilistic seismic hazard analyses (PSHAs)in slowly-deforming regions, where a 1/500-year AEP design level is likely to be much lower than theANCOLD Maximum Credible Earthquake (MCE) ground motions. Consequently, a robust discussion among the Standards Australia code committee, hazard practitioners and end users is required to consider alternative hazard and/or risk objectives for future standards.Site-specific PSHAs undertaken for owners and operators of extreme and high consequence dams general-ly require hazard evaluations at lower probabilities than for typical structural designas recommended in AS1170.4.However, modern national assessments, such as the NSHA18, can provide a benchmark in terms of recommended seismicity models, fault-source models, ground-motion models, as well as hazard values, for low-probability site-specific analyses.With a new understanding of earthquake processes in Australia leading to lower ground-motion hazard values for higher probability events (e.g.,1/500-year AEP), we should also ask whether the currently recommended design probabilities provide an acceptable level of seismic resilience to critical facilities (such as dams)and regular structures.
Zivko R. Terzic, Mark C. Quigley, Francisco Lopez
The Mt Bold Dam, located in the Mt Lofty Ranges in South Australia, is a 54m high concrete arch-gravity dam that impounds Adelaide’s largest reservoir. The dam site is located less than 500m from a suspected surface rupture trace of the Willunga fault.Preliminary assessments indicate that Mt Bold Dam is likely to be the dam with the highest seismic hazard in Australia, with the Flinders Ranges-Mt Lofty region experiencing earthquakes of sufficient magnitude to generate shaking damage every 8-10 years on average. Prior evidence suggests that the Willunga Fault is likely capable of generating M 7-7.2 earthquakes.As part of the South Australia Water Corporation (SA Water) portfolio of dams, Mt Bold Dam is regularly reviewed against the up-to-date dam safety guidelines and standards. SA Water commissioned GHD to undertake detailed site-specific geophysics, geotechnical and geomorphological investigations, and a detailed site-specific Seismic Hazard Assessment (SHA) of the Mt Bold Dam area. The results of this investigation will be used to inform decisions related to planned upgrade works of the dam.Geomorphological mapping of Willunga Fault, detailed geological mapping, analysis of airborne lidar data, geophysical seismic refraction tomography and seismic reflection surveys,and paleoseismic trenching and luminescence dating of faulted sediments was conducted to obtain input parameters for the site-specific SHA.Discrete single-event surface rupture displacements were estimated at ~60 cm at dam-proximal sites. The mean long-term recurrence interval (~37,000 yrs) is exceeded by the quiescent period since the most recent earthquake (~71,000 yrs ago) suggesting long-term variations in rupture frequency and slip rates and/or that the fault is in the late stage of a seismic cycle. The length-averaged slip rate for the entire Willunga Fault is estimated at 38 ± 13 m / Myr. Shear wave velocity (Vs30) of the dam foundations was estimated based on geotechnical data and geological models developed from geophysical surveys and boreholes drilled through the dam and into the foundation rock. The nearest seismic refraction tomography (SRT) lines were correlated with the boreholes and those velocity values used in the Vs30 parameter determination. All relevant input parameters were included into seismic hazard analysis with comprehensive treatment of epistemic uncertainties using logic trees for all inputs.Deterministic Seismic Hazard Analysis (DSHA) confirmed that the controlling fault source for the Mt Bold Dam site is Willunga Fault, which is located very close to main dam site (420m to the West).For more frequent seismic events (1 in 150, 1 in 500 and 1 in 1,000 AEP), the probabilistic analysis indicates that the main seismic hazard on the dam originates from the area seismic sources (background seismicity).Based on deaggregation analysis from the site specific Probabilistic Seismic Hazard (PSHA), the earthquakes capable of generating level of ground motion for the 1 in 10,000 AEP event can be expected to occur at mean distances of approximately 22km from the dam site(with the mean expected magnitude atMt Bold Damsite estimated at Mw >6).For less frequent (larger) seismic events, the contribution of the Willunga Fault to the seismic hazard of Mt Bold Dam can be clearly noted with Mode distance in the 0-5 km range, which indicates that most of the seismic hazard events larger than the 1 in 10,000 AEP comes from the Willunga Fault. The Mode magnitudes of the events are expected to be Mode Magnitude at Mw= 6.6 for a segmented Willunga Fault scenario, and Mw= 7.2 for a non-segmented fault scenario.Consideration was also given to the upcoming update of the ANCOLD Guidelines for Earthquake, which calls for the determination of the Maximum Credible Earthquake (MCE) on known faults for the Safety Evaluation Earthquake (SEE) of “Extreme” consequence category dams. The MCE for Mt Bold Dam was estimated from the DSHA; in terms of acceleration amplitude, the MCE event approximately equals the 1 in 50,000AEP seismic events.
Monique Eggenhuizen, Peter Buchanan, Reena Ram, Tusitha Karunaratne
The Department of Environment, Land, Water and Planning (DELWP) has a regulatory role for the safety of dams under the Water Act 1989 (Act) and is the control agency for dam related emergencies. Local Government in Victoria is divided up between 79 LocalGovernment Authorities (LGAs), each responsible for administering local infrastructure and community services such as roads, drainage, parks etc. Current records indicate that 42 of the 79 LGAs own or manage up to 435 dams and retarding basins.Many of these assets, which include a mix of old water supply dams, ornamental lakes and retarding basins, have been accumulated by LGAs over many years as a result of asset transfers and conversions, land development projects, flood mitigation programs and opportunistic acquisitions by the transfer of land. DELWP engaged GHD to assist and provide advice to the LGAs to significantly improve and update knowledge on LGA dams and retarding basins. The objective of this project is to ascertain where the State’s LGA dams and retarding basins are located, what risks they might pose to communities and infrastructure, what to consider during emergency management planning and response, and to provide owners with the essential management tools and procedures to effectively manage these assets, if these are not in place already.The outcome of this project was to support LGAs to improve management of their dams and retarding basins. It aimed to do this by assisting LGAs with the development of basic dam safety programs that will enable LGAs to more effectively manage their portfolios of dams and retarding basins in terms of ongoing maintenance, dam surveillance and emergency planning and response, and demonstrate due care.This project had a number of key challenges. These included the requirement to process and assess a large number of sites within a small timeframe whilst achieving good value for money,without compromising DELWP’s objectives. A number of efficient methods were adopted during this project particularly during the initial data gathering process, identifying those dams which needed to be inspected based on embankment heights, reservoir capacity and consequences, rapid preliminary assessment of consequences, the development of effective templates for the site inspections, and a method of applying qualitative risk assessments, applicable to the majority of the dams, allowing a consistent assessment of the status of each dam and damsafety documentation.The methods discussed(although developed specifically for the Victorian LGA dams portfolio)provide a sound basis for a screening tool to assess a large number of smaller dams in an efficient manner and quickly identify higher consequence category dams requiring attention. This method could easily be modified and adapted to be applied to similar portfolios of dams.
James Stuart, Michael Hughes
Several recent rain events in Australia have resulted in impoundment flood levels where there was a surprising variability between the Annual Exceedance Probability (AEP) of the flood level and that of the rainfall. The issue was highlighted during the Queensland Flood Commission of Inquiry (QFCI, 2011) by the Queensland Dam Safety Regulator who suggested there may be a problem with design hydrology after a dam safety event that saw impoundment levels of around 1:9000 AEP with a 1:200 AEP catchment rainfall at North Pine Dam, north of Brisbane in 2011. Wide disparities have occurred at Wivenhoe Dam west of Brisbane, at Callide Dam, west of Gladstone and at other locations.
This paper examines the Generalised Short Duration Method (GSDM) (BoM, 2003) and the Revised Generalised Tropical Storm Method (GTSMR) (BoM, 2003) typically used for dam flood capacity assessments in an attempt to explain the variability outlined above and whether it is, in part, exacerbated by the methods themselves.
It finds that processes of generalising rainfall depth, intensity, temporal and spatial characteristics are working together with adopted hydrological methods to contribute to such variability, that in the worst case could lead to PMF levels in dams with much less rainfall than the associated PMP would infer.
Moreover, two key assumptions; that of AEP neutrality (AEP of rainfall is equal to that of the flood) and frequency of PMP based on catchment area, which are the foundations stones of our understanding of flood frequency for large structures, are found to be untested or simply interim advice. This leads to the conclusion that the likelihood of floods in the range 2000 year AEP to PMF may continue to show surprising variability, potentially of an order of magnitude or more, compared to the rainfall AEP.
There is a need for a review of these methods and potentially provision of interim guidance as these methods are currently being used in dam upgrade programs throughout Australia and are also the basis for emergency planning. The identification of these issues concerns current methods and are independent to any discussion on climate change.Prior to commencing, it is worth defining two terms that re-occur throughout the document:
Annual Exceedance Probability (AEP): The probability that a given rainfall total accumulated over a given duration will be exceeded in any one year. AEP Neutrality is the theory that assumes the probability of the rainfall can be transferred to the resulting flood.
Average Variability Method (AVM): Technique for estimating design temporal pattern of average variability to ensure AEP Neutrality in transition from PMP to PMP design flood
On Hampstead Heath in Central London, just 3 kilometres from the centre of the city, there are more than 20 dams and reservoirs set within the landscape setting of Hampstead Heath. A number of dams were built in the 16th century and formed the original water supply to the City of London. They are set in a landscape laid out by the world renowned Humphrey Repton.Three of the embankments which are laid out in two chains of reservoirs across the Heath are subject to safety legislation in the UK. As such they were identified as being deficient in spillway capacity and thus fairly significant works were required to be carried out in this sensitive setting.The Heath is protected by the Hampstead Heath Act of 1870 which seeks to prevent significant changes to the Heath and thus it was quite clear that there would be opposition to any works on the Heath, even though they were required by law to protect persons and property downstream. In fact a significant lobby group formed which challenged the need for the works and also the legislation of the UK via a judicial review. This paper will describe the process by which significant stakeholder consultation was undertaken (costing more than £2M), the judicial review that took place in the Royal Courts of Justice, the option study and the major engineered elements carried out on the Heath.