Knowledge gaps and management shortcomings contribute to catastrophic dam failures

Tailings facility failures represent a significant risk to the environment and communities globally, but until now little data was available on the global distribution of risks and characteristics of facilities to ensure proper governance. We conducted a survey and compiled a database with information on tailings facilities disclosed by extractive companies at the request of institutional investors. Despite limitations in the data, this information disclosure request represents the most comprehensive survey of tailings facilities ever undertaken. The compiled dataset includes 1743 tailings facilities and provides insights into a range of topics including construction method, stability, consequence of failure, stored volume, and the rate of uptake of alternative technologies to dewater tailings and reduce geotechnical risk. Our analysis reveals that 10 per cent of tailings facilities reported notable stability concerns or failure to be confirmed or certified as stable at some point in their history, with distinct trends according to construction method, governance, age, height, volume and seismic hazard. Controversy has surrounded the safety of tailings facilities, most notably upstream facilities, for many years but in the absence of definitive empirical data differentiating the risks of different facility types, upstream facilities have continued to be used widely by the industry and a consensus has emerged that upstream facilities can theoretically be built safely under the right circumstances. Our findings reveal that in practice active upstream facilities report a higher incidence of stability issues (18.3%) than other facility types, and that this elevated risk persists even when these facilities are built in high governance settings. In-pit/natural landform and dry-stack facilities report lower incidence of stability issues, though the rate of stability issues is significant by engineering standards (> 2 per cent) across all construction methods, highlighting the universal importance of careful facility management and governance. The insights reported here can assist the global governance of tailings facility stability risks.

The probability of failure of tailing dams and associated risks demand improvements in engineering practice. The critical state line provides a robust framework for the characterization of mine tailings. New experimental data for nonplastic platinum tailings and a large database for tailings and nonplastic soils (grain size between 2 and 500 μm) show that the critical state parameters for nonplastic tailings follow the same trends as nonplastic soils as a function of particle-scale characteristics and extreme void ratios. Critical state lines determined for extreme tailings gradations underestimate the range of critical state parameters that may be encountered in a tailings dam; in fact, mixtures with intermediate fines content exhibit the densest granular packing at critical state. The minimum void ratio emin captures the underlying role of particle shape and grain size distribution on granular packing and emerges as a valuable index property to inform sampling strategies for the assessment of spatial variability. Mineralogy does not significantly affect the intercept Γ100, but it does affect the slope λ. The friction coefficients M of tailings are similar to those of other nonplastic soils; while mineralogy does not have a significant effect on friction, more angular grains lead to higher friction coefficients.

The significant amount of laboratory testing required to determine the steady-state line (SSL) of soils has led to an interest in understanding how the SSL correlates with simpler soil index properties. Correlations proposed so far for non-plastic soils are only applicable to sands with fines content (FC) up to about 35%, but many geotechnical applications involve higher FC values. This work explores FC-independent correlations between the SSL and the limit void ratios by adopting the familiar straight line idealisation of the SSL projection on e–log(p′) space. Two parameters, Γn and λ, are used to represent the vertical location and the slope of the SSL, respectively. Using a database of 149 non-plastic soils, it is shown that there is a functional correlation between Γn and the minimum void ratio (emin) that is independent of particle size distribution, including FC, and particle angularity. A weak λ–emin correlation is also observed which can provide some guidance with respect to compressibility. Although the accuracy of the correlations is insufficient to make reliable quantitative SSL estimations, they are useful in guiding the selection process of samples whose SSLs have to be formally determined in order to characterise the SSL variability of non-plastic soil deposits.

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