Flow liquefaction can be triggered by either cyclic or static loading and is a major issue for tailings and mine waste structures.  Flow liquefaction can occur in any saturated or near saturated contractive soil such as very loose sands and silts as well as very sensitive clays. For failure of a soil structure, such as a slope or embankment, a sufficient volume of material must strain soften and show strength loss. Case histories have shown that when significant strength loss occurs in critical sections of a soil structure, failures are often rapid, occur with little warning, and the resulting deformations are often very large.  Case histories also show that most flow liquefaction failures occur in young, uncemented non-plastic or low-plastic soils that have a brittle strain softening response where peak strength is reached at small strains followed by rapid strength loss. Methods have been developed to evaluate if soils are either dilative or contractive at large strains based on Critical State concepts.  However, not all contractive soils are strain softening and not all soils that are strain softening have high brittleness.  Laboratory data, supported by field observations, shows that contractive sand-like soils become progressively more ductile with increasing stress.  In recent years, mine waste has been placed in an unsaturated state and evaluation of liquefaction in these deposits adds additional uncertainty.  This presentation will provide an overview on soil liquefaction and an update on the influence of high stress.  Case histories will be used to illustrate the main points.

The risks associated with tailings storage facilities (TSFs) are often thought of technical issues, which for the most part is true (although there are social, environmental, and financial risks to be considered). However, the way we approach and manage risks requires us to adopt less prescriptive methods in favor of risk-based processes for the design, construction, and operation of TSFs. Intuitively, risk-based approaches should be more rigorous than their prescriptive counterparts, however adopting risk-based processes introduces the human element in the form of biases. These biases can complicate the risk-based processes resulting in a potentially flawed risk assessment.