Characterisation of unsaturated geotechnical properties of filtered magnetite tailings in a dry stack facility

Abstract

Filtered tailings and dry stacking are gaining attention due to problems with slurry-deposited tailings facilities and consequent catastrophic failures around the world. Factors such as recent technological advancements in filtration, lack of water, regulatory pressures and societal demands are shifting focus towards dry stacking. With the growing attention to filtered tailings, a major challenge is the knowledge gap on unsaturated soil properties required to understand the performance of unsaturated tailings in the dry stack facility, especially the hydraulic and shear strength behaviours. This paper presents details of investigations to characterise unsaturated properties of filtered tailings material in a large-scale dry stack facility located in a highly net positive evaporation area in Western Australia. Field and laboratory tests were conducted to understand (i) hydraulic and water retention/storage functions, and (ii) shear strength responses to relevant stress state variables. Historical cone penetration tests with pore pressure measurements (CPTus) conducted annually between 2013 and 2017 consistently indicate negative pore pressures within the dry stack up to 25 m depth. Soil water characteristic curves (SWCCs) developed from laboratory water retention tests show that the tailings material has a low air entry value and desaturates quickly under low matric suctions below 18 kPa and enters residual zones around 70 kPa, reaching residual suctions at 100 kPa. The SWCCs suggest that the unsaturated tailings potentially maintain large matric suction and low hydraulic conductivity which may require a substantial amount of surface water flux over a period to dissipate it, and complement the CPTu results. The magnitude and duration of surface water flux on site may not be sufficient to dissipate the large matric suctions due to short, low and infrequent rainfall and high evaporation rates over a long dry summer. Triaxial and direct shear test results also indicate the significant difference between shear strength responses of saturated and unsaturated tailings to stress state variables (net normal stress and matric suction). For the saturated tests, the tailings shear stress response to the relevant stress state variable (net normal stress) is fairly linear. However, for the unsaturated tailings, plots of shear stress versus matric suction show non-linearity (curvilinear shape) in the failure envelope and higher shear strength results from matric suction compared to net normal stress. The investigations have provided insights into the water storage characteristics and shear strength responses of the filtered tailings to stress state variables and sheds light on why the dry stack maintains unsaturated conditions. Steep stack angles up to 44° observed on site are due to matric suction. For filtered tailings with a low air entry value and located in low rainfall areas with high evaporation, permanency of matric suction may develop to govern dry stack hydraulic and shear strength performance.