Courtesy of Karen Chovan
A new tool in the tailings risk management arsenal By Karen Chovan
Risk Assessment for Tailings Management, CIM Journal, Vol. 12, No. 1
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n the last decade, there has been a growing demand for transparency with respect to tailings storage facilities (TSF) as well as a need for a means for comparative evaluation of TSF at different sites and among different owners. However, tailings management systems and their risks are very complex, often misunderstood, frequently miscommunicated and sometimes mismanaged as a result. In fact, senior management, mining operators and other stakeholders who are not involved in TSF management often do not have the technical background to appreciate the complexity of TSF risks. TSF come in a variety of sizes, types and designs – each as unique as the sites on which they lie. As such, understanding, evaluating, comparing and communicating the risks of TSF infrastructure and systems do not come easy. Complicating matters further, there are a wide variety of strategies to do so within the technical tailings design community. To help address these challenges, I joined a team of experienced engineers to develop an evaluation process that could be consistently applied to any type of TSF. The process includes a semi-empirical method to estimate an annual probability of failure that is then combined with a quantitative evaluation of the consequences of failure to produce an objective and detailed assessment of the risk. Risk is based on both probability of failure (POF) and consequence of failure, and this is perhaps the most widely miscom-
Risk ratings chart based upon proposed risk evaluation process municated and misunderstood aspect of tailings management. For example, the very informative Global Tailings Portal contains information on over 1,800 TSF, including their owners, locations, heights, volumes, construction types and potential consequence of failure ratings. Key focus areas on the portal are the type of construction and the associated consequence rat-
TSF Risk Primer For people who are not involved in the details of design and management of TSF, here are a few relevant phrases and points to facilitate better understanding of typical risk communications: The Factor of Safety (FS) is often used as an indicator of TSF stability, and regulators often specify that a facility meet minimum FS values. The FS for different scenarios is determined through a series of stability analyses, the results of which are heavily dependent on the assumed material characteristics and geometry of the foundation soils, the embankment and the contained tailings. The analyses also assume distributions of stresses and pressures within the TSF based on the height of tailings and the water level within the TSF at various points in time. In other words, the actual FS will vary slightly over time with changing conditions in the field as the TSF are raised, tailings deposited, water levels vary, and consolidation. For a particular FS, the probability of failure (POF) can range several orders of magnitude, depending on how well the actual field conditions and material characteristics correspond to the assumptions of the stability analyses. Field conditions and material characteristics are often heterogenous as opposed to the homogeneous conditions assumed in stability analysis and design. As such, uncertainties or discrepancies can accumulate quickly. Sources of uncertainty and discrepancy include insufficient site investigation and material testing, variable foundation conditions, variations in the materials used for construction, non-uniform construction techniques, etc. The list is long. This fact is well understood by practitioners of TSF design and is the reason that technical review should be incorporated into design, comprehensive quality assurance and that quality control should be incorporated into construction, and that adequate instrumentation and monitoring should be incorporated into operation. The consequences of failure are not considered in the determination of the FS or POF. They are determined under the assumption that a TSF fails and releases some percentage of the contained tailings and water, regardless of its FS or POF. Consequences are evaluated on several aspects, typically including, and not limited to, potential injury or loss of life, environmental damage, relations with local stakeholders, reputation and social licence to operate, as well as the financial impact. The risk rating of a TSF is the product of the probability of failure and the consequence of failure. The product of the two aspects automatically combines infrastructure-specific elements (i.e., POF based on the FS and level of uncertainty associated with the TSF) and system-based elements (i.e., water management and downstream consequences). – Karen Chovan
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