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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:Learn more
– 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
Jiri Herza and John Phillips
The design of dams for mining projects requires processes and technology that are unfamiliar to many mine owners and managers. Dam designers rely on ANCOLD assessments of Consequence Category, commonly leading to a High rating for mining dams due to a combination of potential loss of life, impact on environment and damage to assets such as mine voids, process plants, workshops, offices, roads, railways etc.Learn more
From this High Consequence Category the relevant annual exceedance probabilities for design parameters and loading conditions such as earthquakes and floods are selected.
Mining companies have sophisticated methods available for assessing risk, yet for their assets they often adopt an order of magnitude lower security for earthquake and floods even though the consequences in terms of lives at risk and impact on project are similar.
The discrepancies in the design standards lead to situations where extreme dam loads are adopted to prevent damage and loss of life in assets that theoretically would have already collapsed under much lower loads.
One difference may be that some mining dams exist in an environment which is controlled by a single entity. Unlike other dams, failure of these mining dams would therefore impact only individuals and assets which fall under the responsibility of the same entity.
This paper discusses the discrepancies between the design of mining dams and the design of other mine infrastructure. The paper considers the impact of discrepancies on the overall risk to the mine and compares the degree of protection offered by a factor of safety and the influence of reliability of design input parameters, alternate load paths and design redundancy.
Keywords: Dams, tailings dams, mining, acceptable risk, factors of safety
Jiri Herza, Michael Ashley, James Thorp
The principle of minimum acceptable factors of safety has been used to assess the stability of embankment dams for decades. The commonly applied minimum acceptable factors of safety remain very similar to those recommended in the early 1970’s, despite the development of new design tools and better understanding of material behaviour. The purpose of factors of safety is to ensure reliability of the dam design and to account for uncertainties and variability of dam and foundation material parameters, uncertainties of design loads and limitations of the analysis method used. The impact of uncertainties and reliability of input values into stability analyses was recognised many decades ago, and the factor of safety was recommended depending on the loading conditions and the consequences of failure or unacceptable performance. Interestingly, the minimum recommended factors of safety used today do not take into account the potential consequences of dam failure or the uncertainties in input values, and are based on the loading conditions only. Yet, several authors have demonstrated that a higher factor of safety does not necessarily result in a lower probability of failure, as the analysis also depends on the quality of investigations, testing, design and construction. This paper summarises the history of the factor of safety principle in dam engineering, discusses the calculation of the factor of safety using commonly used analytical tools, demonstrates the impact of uncertainties using a case study and provides recommendations for potential improvements.Learn more
Ryan Singh, Jiri Herza, Emily Taylor, William Kettle
As predicted by Powel (2000) claims for professional negligence are very common and their frequency is increasing due to the increasing demand for professionals’ services, specialisation, higher standards, intolerance of poor performance by societies and the increasing litigious nature of business.
The increasing expectations of the society are reflected in the changing attitude of the authorities and courts towards professionals when things go wrong. The changing attitude is fuelled by the unprecedented media coverage of failures of structures with human and environmental losses. This is particularly relevant to the tailings industry, which is marked by the recent dam failures in Canada, Brazil, Mexico, China, Australia and India.
The far reaching expectations for duty of care of professionals has been strikingly illustrated from the fallouts from recent major and widely publicised TSF failures such as Mt Polley (three consultant engineers accused of unprofessional conduct), the 2015 Samarco failure (22 individuals charged with various criminal offences including homicide) and the recent Brumadinho failure (charges of false representation have also been brought against the consultant engineers).
This paper examines the responsibilities and duties of engineers operating in the tailings industry with respect to the professionals’ duty of care and the consequences of breaching those responsibilities and duties. This paper also discusses the potential conflicting interests of consulting engineers and proposes that engineers are, in the vast majority, ill-prepared for navigating the changing waters of professional negligence.
The authors of this paper believe that a better understanding of the professional duty of care could reduce the number of claims for professional negligence. As a corollary, the reduced rate of professional negligence could result into fewer tailings failures in the future.
Professional industry bodies such as Engineers Australia should act to clarify the legal obligations and duties of engineers, as they are the best placed institutions to do so for the whole industry. In addition, consideration should be given to inclusion of a discussion of the aforementioned obligations and duties into relevant ANCOLD Guidelines.Learn more
Dr. Mark Locke, Jiri Herza
Gördes Dam is a nickel and cobalt mine tailings dam situated in a seismically active zone in Manisa Province, Western Turkey. The dam is a conventional cross valley earthfill structure with a fully lined storage basin. The starter embankment with a maximum height of 50 m will be raised in downstream lifts to an ultimate height of 90 m. The total storage capacity is 19 million m3. Construction of the starter embankment is planned to commence in late 2012 and the dam will be commissioned in June 2013.
The tailings will be discharged from the dam crest and return water will be collected by a floating decant pump at the opposite site of the storage. Decant water has high calcium sulphate levels and will require treatment before re-use in the plant or release. The tailings contain about 33 % of solids and are classified as high plasticity silts and clays with more than 90 % of particles passing the 0.075 mm sieve.
The dam is founded on a complex formation of altered sedimentary and metamorphic rocks including mudstones, siltstones, limestones and serpentines. The mudstone blocks, the predominant foundation materials, are juxtaposed with siltstones and serpentines via a complex arrangement of faults. Where exposed, the mudstones are highly to completely weathered with a well-developed structure of smooth bedding surfaces leading to anisotropic strength characteristics. Several landslides, likely associated with the anisotropic character of the mudstones, were identified within the area including a significant landslide under the upstream shoulder of the dam.
Mining development in Turkey has a complex legislative environment. There is also standard practice which is not legislated but expected, this can be considerably different to normal design practice in Australia. The Turkish legislation is based on waste management guidelines and may be more appropriate to landfills than large tailings storages. The legislation is very prescriptive in some aspects and silent in others, with little consideration of risk or consequence based design.
This paper discusses the design difficulties associated with the challenging foundation conditions, which have been magnified by the requirements and limitations embedded in the approval documentation and the legislative environment in Turkey. It will also address some of the key differences between the design philosophy in Australia and in Turkey with a focus on the major risk elements of the design.
Keywords: Tailings, Turkey, Liner, HDPE, Nickel laterite
Jiri Herza, Graeme Maher, Nihal Vitharana, Megan Evers, Michael Somerford
Abstract: Drakesbrook Dam is a 17m high earthfill embankment constructed in 1931 to provide water for the Waroona Irrigation District, 80km south of Perth. The dam has a storage capacity of 2.30 GL and is classified as a “High A” hazard dam according to ANCOLD Guidelines on Dam Safety Management.
A Dam Safety Review, undertaken in 2001 identified a number of deficiencies associated with main embankment, outlet works and the main spillway requiring remedial works. Detailed design of these remedial works is currently underway. Challenging features of this project are the design of a liner for the existing curved conduit and the design of a new spillway with an usual drop-type stilling basin.
This paper presents the salient aspects of the remedial works design along with the various design and construction criteria adopted to achieve an economical design of a new spillway and outlet conduit sleeve without compromising the safety of the dam.
Keywords: remedial works, earth embankment, conduit lining, spillway replacementLearn more
Jiri Herza, Nihal Vitharana, Alex Gower
The Western Australia Water Corporation plans to increase the storage capacity of Millstream Dam, which is located near Bridgetown in the south west region of WA. The existing dam is an 18 m high zoned earthfill embankment constructed in 1962. The dam suffered a block heave of the foundation at the downstream toe during the first filling, probably attributable to high foundation pore water pressures. The dam upgrade will be challenging due to complex and unfavourable foundation soils coupled with these artesian pressures.Learn more
The dam is founded on lateritic soil, which is a common weathering profile throughout the region. These soils formed in a tropical environment of fluctuating water tables, severe leaching and translocation of iron oxides over many millions of years. As a consequence some of the lateritic horizons at Millstream Dam have been modified such that they exhibit behaviours that are not consistent with conventional constitutive models and correlations. These are attributed to a complex structure of the soil microfabric, which comprises clay particles bonded together into larger aggregates. The clayey aggregates are also bonded to each other, forming a porous matrix of silty or sandy appearance characterized by low dry density and high void ratio, which may nevertheless disintegrate on working.
Comprehensive geotechnical investigations and extensive laboratory testing have revealed that the foundation materials display characteristics of clayey and granular soils. Under shearing, these soils demonstrate high initial strength, which gradually reduces as the inter-aggregate bonds are broken and the relative position of the aggregates changes. Several soil samples also exhibited significant contractive behaviour on shearing generating high pore pressures under undrained conditions.
This paper presents the investigation and design methods used in the foundation design of the Millstream Dam upgrade with emphasis on unusual behaviour of the foundation media.