Characterization of Static Liquefaction in Tailings through the Critical State Framework

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Tailings storage facilities (TSF) are man-made earth structures used for storing mining waste, comprised of fine minerals and water left behind during the extraction process. They exist at the intersection of economics, safety, and sustainability. The mining industry needs better engineering practices for assessing and improving existing TSFs, designing new TSFs, and disposal processes motivated by: recent catastrophic failures of TSFs; economic challenges demanding reductions in tailings disposal costs; and evolving regulatory and social pressures to improve the safety and sustainability of tailings storage. Granular soils, such as tailings, can lose all or part of their strength in a process called liquefaction. Liquefaction is the phenomenon that happens when loosely compacted particles "want" to form a denser structure, but presense of water in the voids prevents that, at the cost of increasing pressure in the water. The result is obvious: if the water pressure sufficiently increases, the whole mixture will turn into a "liquid". Soils' resistance to liquefaction mainly depends on how densely-packed its particles are. Liquefaction potential is assessed by estimating soil density from its resistance to a penetrating probe. The Cone Penetration Test (CPT) is the predominant site characterisation tool since it provides a wealth of information with fast and inexpensive application. Assessing the potential threat of liquefaction to a TSF involves three steps: How the CPT estimates the density of tailings, if liquefaction will occur given the density of tailings, and how much strength the tailings will retain should liquefaction occur. A consultant (Golder) and a specialty contractor (ConeTec) who often collaborate on TSF projects in Canada and around the world have come together with University of Toronto to pool their expertise and answer these pressing questions. The proposed partnership will produce a new tailings-specific liquefaction assessment framework. Consequently, Canadian consultants, contractors, and mining companies will obtain a competitive edge leading to added work and employment, reduced cost of operation, and most importantly safer designs.