2022 – Design change midstream to mitigate risk of liquefaction for a tailings dam – A case study

Malcolm Barker – GHD – Senior Technical Director – Dams, Alireza Mojami – GHD – Technical Director – Dams and Tailings, Karl Jones – CS Energy – Manager Mining and Ash, Michael Costa – Stantec – Principal Civil Engineer

Tailings dams are dynamic structures with continual changes and complexity owing to the ongoing requirement for increased storage of tailings. There are primarily three construction methods for tailings dams namely the traditional downstream dam, centre line raise and upstream raise where tails are stored above a main starter wall. The most common construction approach used for tailings storage facilities is upstream raise, which is solely responsible for all major static liquefaction events and also contributes to seismic failures. According to the ICOLD Bulletin 121, the failure rate for tailings dams over the last one hundred years is estimated to be 1.2%. This is more than two orders of magnitude higher than the failure rate of conventional water retention dams that is reported by ICOLD to be 0.01%. The Waste Containment Facility (WCF) at the CS Energy Callide power station has been using an upstream raise approach since commissioning in the 1980s. In 2017 CS Energy recognised the increasing potential for liquefaction of the tailings with upstream raise construction and an alternative construction strategy was urgently implemented. This resulted in a composite downstream raise with upstream filter zone. This paper will explore the history of tailings dam failures, identify some of the triggers for liquefaction and discuss the approach used at the Callide facility to mitigate the risk of liquefaction for the WCF.