Many quantified risk assessments finish the failure mode event tree at the estimated occurrence of an embankment breach leading to dam failure outflows and downstream consequences. In some situations, for dams with multiple embankments with potentially different consequences downstream of each embankment, the possibility for further breaches may be pertinent if there may potentially be higher consequences for a multiple breach scenario. The location of an initial breach and sequence of subsequent breaches could also result in different contributions to total risk.
This paper discusses a method applied to investigate the conditional probability of flood overtopping breaches for multiple earth-fill embankments with grass covered downstream slopes.
For the subject dam, preliminary modelling identified that for a flood overtopping breach of an embankment the breach’s development may not be sufficient to reduce the lake level and sustained overtopping flow over the remaining embankment crests could lead to further embankment breaches.
A Monte Carlo dam breach simulation modelling approach was used with a large number of flood events. The simulation modelling considered erosion initiation for a grass slope due to the combination of velocity and duration of flow, and erosion continuing to breach based on duration of flow after erosion initiation. Potential uncertainty of erosion initiation and erosion continuing to breach were represented with probability distributions in the Monte Carlo modelling.
The results from the large number of dam breach simulations were then analysed with post processing to derive conditional probabilities for single or multiple breaches and breach sequence.
Fault displacement can occur due to primary faulting on a main fault intersecting a dam foundation or rim, as well as by secondary faulting. This secondary faulting may be triggered locally by the occurrence of primary faulting on a main fault; its occurrence is conditional on the occurrence of an earthquake on the main fault. A probabilistic approach is most viable for fault displacement hazard analysis. Unlike the case of probabilistic ground motion hazard, which is nonzero even for short return periods due to the occurrence of a broad range of earthquake magnitudes in a wide region around the site, probabilistic fault displacement hazard is zero for return periods less than the recurrence interval of surface faulting earthquakes on the fault. In Australia, these recurrence intervals typically lie in the range of 10,000 to 100,000 years.
Consequently, the fault displacement hazard due to primary faulting may be zero or negligible for return periods shorter than 10,000 or 100,000 years. For longer return periods, the hazard is best evaluated using a risk-based approach, as recommended by ANCOLD (2018); the alternative of using a deterministic approach, which disregards return period, could potentially yield a large fault displacement. The probability of triggered secondary faulting, conditional on the occurrence of a large earthquake on the main fault, is typically one or two orders of magnitude lower than that on the main fault, and so is even more likely to be zero or negligible for return periods shorter than 10,000 to 100,000 years
The ANCOLD Guidelines (2019) require that active and neotectonic faults which could significantly
contribute to the ground-shaking or ground-displacement hazard for a dam should be accounted for in seismic hazard assessments. While geological and geomorphological field investigations along suspected active fault structures are undertaken as a matter of course in New Zealand, this practice is relatively uncommon in Australia. Granted, rates of tectonic processes are greater in New Zealand than in intraplate Australia. However, moderate to large and damaging earthquakes are not uncommon in the Australian record; there have been ~26 earthquakes of magnitude >M6 in the last 150 years (~1 event every ~6 years) and similar events might be expected in the future. We present examples of investigations undertaken to better understand earthquake hazard for two faults – previous studies on the Wellington Fault, New Zealand, and new data from recent investigations of the Avonmore Scarp, southeast Australia. We report the results from these studies and discuss how the collection of similar data on faults proximal to Australian dams would allow dam owners and operators to better quantify seismic hazard and, thereby, more meaningfully comply with the ANCOLD guidelines.
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.
As part of the development of some dams and hydroelectric power schemes, deep infrastructure is often required which requires and understanding of the in situ stresses of the rock mass. Recent works completed in southern Australia and Europe have led to improved methodologies for conducting effective, reliable, and repeatable measurements of in situ strain and/or deformation, as well as the subsequent estimation of in situ stress.
In situ stress testing is generally an item that is specified as part of a geotechnical investigation, however it is not commonly well understood in terms of reliability, repeatability, or, in fact, what the result actually means and its implications to project design. Commonly, a handful of tests are completed, with variable results, which often generates more confusion than answers.
This paper provides a discussion of recent in situ stress testing completed for two deep Australian projects. It summarises the aim of the investigations, test selection process, laboratory testing, data review and model development. This is to illustrate how complex the estimation of in situ stress can be and some of the pitfalls that may be avoided whilst acquiring and assessing the data. It also examines several different testing methods available in the Australian and International industry and some of the analysis techniques available to dam and tunnel projects. Finally, the paper provides an update on topical developments provided at recent workshops in Europe.
The majority of Australian tailings dams over the last 100 years have been successfully built using upstream construction. However, recent major tailings dam failures in some countries have led to a global industry wide review of the design and management of tailings storage facilities, with a focus on the upstream raise method as a common factor for some failures. As a reaction to the recent failures, there is the potential for regulations to become more restrictive and the potential for unjustified pressure on existing and new mines to rule out upstream raising due to possible safety and failure risks.
This paper looks at whether it is the upstream construction method or other more fundamental issues that have led to these failures and examines whether such issues are equally relevant in Australia. Does Australia have a specific advantage in being able to successfully use upstream tailings dam construction or are we fooling ourselves?
The topic of upstream tailings storage is a subject of broad and current interest and the lessons learned from historic failures are rightfully leading to improvements. Implementation of good practice starts with the overall management structure that guides how tailings dams are designed, constructed, operated and closed.
Critical design practice involves understanding the unique site conditions, properties of the tailings and management of tailings placement, as the tailings form part of the overall retaining structure. Good practice during operation of upstream tailings dams is key to reducing the risk of tailings dam failures and the success of safe and sustainable closure.
This paper presents key features of both good and bad practice for the upstream raising of tailings dams and discusses how the design and operation can be made more resilient to ensure the safety of the community and infrastructure. It concludes that upstream raising can be a safe and economical method of tailings disposal if designed, constructed and operated correctly.