Development and outcomes of a tailings slope stability comparative design exercise

Tailings storage facilities (TSFs) have undergone an unacceptably high failure rate for decades, leading to an increased focus on improving the safety of these structures. One aspect that has not received sufficient attention is examining how reliably and consistently engineers analyse the stability of TSFs. An understanding of the current state of practice is needed as there are a range of analytical methods available to engineers, while the high failure rate of these structures strongly points to some deficiencies in practice. To examine some of these issues, a tailings-focussed slope stability comparative design exercise was organised to compare the methods and results used by tailings engineers to analyse the same slope within which the phreatic surface was to rise under a specified pattern. Twenty-eight practitioners participated in the exercise to predict at what phreatic surface level the slope would fail. A wide range of predictions were made, ranging from assuming the slope would liquefy and fail instantly (i.e. before any rise in phreatic surface), to a range of techniques based on (i) yield strength ratios, (ii) stress path methods, and (iii) numerical analyses, each predicting various failure levels, and finally some predictors who applied drained strengths, resulting in predictions that the slope would not fail.