Importance of tailings properties for closure

Managing tailings facilities often includes stability and deformation analyses from both short- and longterm perspectives. Behind these analyses, geotechnical theories are applied on the studied material. Design for closure is often considered to cover a period of 1,000 years, during which time the closed tailings facility should be stable, both physically and environmentally. Since the use of tailings facilities only covers about 100 years back in time, there are still no data on how tailings behave in the long term. In Sweden, correlations to natural formations have been used to predict the long-term stability of tailings dams. Natural formations that have been stable since the latest ice age (approximately 10,000 years ago) are considered good examples of long-term stability. These are mainly formed by glacial till, a natural material that has been subject to erosion, transportation, and sedimentation processes. Tailings material, however, is artificially created and differs in some respects from natural soils. Recent laboratory work on tailings from one mine in Sweden indicates unexpected behavior during shearing. In direct (simple) shear tests, contractant tendencies were initially observed, but these were followed by a remarkable compression after maximum shear stress was reached. Pore-water pressures were measured correspondingly and indicated an increase parallel to the decreased sample volume. The reasons behind the unexpected deformations after reaching maximum shear stress are not fully understood and explained, but they are assumed to be attributable to particle breakage and rearrangement in the soil skeleton. The geotechnical theories normally used for tailings might therefore need an upgrade to cover the phenomena recent laboratory tests indicate. This paper discusses possible effects of the observed change of tailings properties with regard to closure of tailings management facilities.