Fabric, rheology, and shear strength for risk reduction in tailings impoundment management

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The failure of tailings dams with catastrophic results continues to occur with regularity. The recent tailings dam failures and associated environmental disasters in Brazil in 2015 and in British Columbia (Mount Polley) in 2014 are only two of the most well-publisced occurrences: the failure in Brazil resulted in excess of $7 billion dollars in damages, with the tailings from Mount Polley flowed into Quesnel lake, one the largest pristine freshwater bodies in the world, and an important component of the water way for British Colombia's salmon fishery. These failures continue to occur despite several previous initiatives to improve tailings dam safety and tailings impoundment management.The catastrophic consequences of dam failure, namely the distance the tailings travel after the breach, or runout, can be attributed to at least two factors: i) the presence of water on top of the tailings may push the tailings, and ii) the susceptibility of the tailings themselves to strength loss and high flowability upon the sudden change in stress state following dam breach. Much speculation exists in practice concerning the relative contribution of these two factors. Recent research has shown that tailings are susceptible to collapse up to high stress levels and relatively high densities. Also, it is known that the rheology of the tailings, the material properties that control how far the tailings flow after failure, are susceptible to degradation following high rates of shear, as might be expected during dam failures. Further, the fabric of the tailings, or the arrangement of tailings particles, influences collapse behaviour. For instance, it is known that desiccation induces a fabric change that leads to higher strength.Therefore, the proposed research program investigates the various factors that influence the deformation behaviour of tailings post-dam collapse or dam breach. This will involve: i) use of an existing large earth testing facility to physical simulate dam failures, and ii) numerical modelling, using the large-scale physical tests as a calibration / verification tool, iii) varying several factors in the simulated deposits (climate freeze/ thaw or desiccation), degree of consolidation, age, type of deposition, use of amendments (waste rock, layers of cemented tailings), and iv) to conduct measurements of fabric and shear strength test with multiple load paths to interpret large-scale tests . The proposed research will lead to improved understanding of the factors contributing to tailings runouts and therefore to improved methods in practice to reduce or eliminate the catastrophic consequences of such events.