Abstract
Recent major upstream raised tailings dam failures have led to a reopening of the discussion of the validity of some of the existing routine practices within the profession. Despite its many shortcomings, deterministic slope stability limit equilibrium analysis is and will continue to be, at least for some time ahead, an important tool for tailings dams’ safety assessment. Within this context, this paper presents a contribution to the postulation of minimum factors of safety required for tailings dams’ slope stability analysis. A recent review and discussion of limit equilibrium analysis and the guidelines of international standards and current trends, with focus on tailings dams, are presented. Based on this review, and the authors’ academic and professional experience, minimum required factors of safety recommendations are proposed. The framework of the recommendations strives to conciliate, in a simple manner, the deterministic minimum required factors of safety with concepts of consequence, uncertainties, risk and characteristics of loose tailings behaviour as a material.
Highlights
The recently reported failures of major tailings dams raised from an initial conventional earthwork structure, starter dam, by the upstream method, Mount Polley, Fundão, Cadia and Brumadinho, all owned by high standard mining companies and subjected to inspections and safety assessments following local standards and legislation, have led the profession to open the discussion on the validity of the existing routine practices
Morgenstern (2018) stated in the written version of his Victor de Mello Lecture: “At this time, there is a crisis associated with concern over the safety of tailings dams and lack of trust in their design and performance” as well as emphasized during the lecture itself that engineers have to consider that a tailings dam will liquefy if the material deposited is in a contractive condition: “if it can it will [liquefy]”
These uncertainties inherently increase the probability of failure in each stage decommissioning scenario, which is partially compensated by the higher factors of safety listed in Table 23, i.e. factors of safety in limit equilibrium analysis, if numerical simulations cannot be performed, larger than the minimum value of 1.5 for both short and long term conditions are necessary to minimize the likelihood of accidents during the process of decommissioning or decharacterization, and avoid major human consequences if they occur
Summary
The recently reported failures of major tailings dams raised from an initial conventional earthwork structure, starter dam, by the upstream method, Mount Polley, Fundão, Cadia and Brumadinho, all owned by high standard mining companies and subjected to inspections and safety assessments following local standards and legislation, have led the profession to open the discussion on the validity of the existing routine practices. Taylor uses the wording of a margin of safety being specified so that “the working values must be smaller than those given above”, those being related to the available shear strength of the soil in a slope He proposes that the margins of safety may be different for the two components of the shear strength, recommending a value of 1.5 in the cohesion component, and 1.26 in the tangent of the friction angle, and postulates that it is usually preferred that the two factors have the same value. It is worth mentioning that, according to Meyerhof (1994), the concept of factor of safety was first used in geotechnical design by Bélidor and Coulomb in the 18th Century This brief historical introduction highlights the fact that selecting the safety conditions and reducing the risk of failure of a structure is a controversial topic since the early works of soil mechanics, requiring continuous scrutiny and judgment. A conceptual view of required factors of safety for slope stability shared by the authors is put forward for discussion and debate, based upon academic and professional experience and recently revised guidelines of international standards on tailings dams
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