Following recent failures of tailings dams, dramatic change in the global mining industry has been initiated. Numerous technical publications or guidelines for safer management of TSFs have been released. A key objective of these publications is to ensure mining companies raise the performance bar for designing, constructing, operating, maintaining, monitoring, and closing tailings facilities to minimise the risk of failure.
In addition to implementing comprehensive monitoring systems, developing and maintaining an interdisciplinary knowledge base and implementing a performance based approach during the design, construction, and operation phases of TSFs are strongly recommended. Using a deterministic approach, with fixed and defined load and resistance input parameters to provide a factor of safety (FOS) does not constitute a performance-based approach.
One can argue that performing a probabilistic analysis is not an easy task and to estimate the probability of failure of a system requires distributions describing both the load and the resistance. It is recognised that the distribution describing the resistance of geotechnical material is either unknown or could be largely scattered. As more test results become available, the accuracy of the resistance distribution increases. On the other hand, the distribution describing the load could be defined early in the project and should not vary much as the project advances. In addition, the performance criteria or performance objectives applicable to facilities/embankments are known and established at an early stage in the engineering process. Similar to load distribution, these performance objectives are not expected to fluctuate substantially during the engineering process.
The methodology typically used in the industry to assess the safety of an embankment is to confirm the FOS under various loading conditions meets the pre-defined FOS criteria. During the early stage of the engineering, it is typically accepted that having a pseudostatic FOS below target is not acceptable and mitigation measures are required. While a pseudo-static FOS below target does not necessarily mean that the embankment will fail, it suggests that permanent deformation should be expected. Therefore, quantifying the load as a function of the targeted performance criteria (also called the acceptable permanent deformation level) that is specific to the project could demonstrate the safety of the embankment.
SRK used a performance-based design approach to assess the pseudo-static safety of an embankment built on Eastern Canadian soft and sensitive Barlow-Ojibway clays. The assessment allowed an acceptable permanent deformation level for the embankment and associated loads to be established, which better determine the safety of the embankment.
