Spasojevic, A, Ahadi, P & Elfen, S 2026, 'Design of tailings embankments over soft and very soft subgrades', in AB Fourie, M Horta, M Oliveira & S Wilson (eds), Paste 2026: Proceedings of the 28th International Conference on Paste, Thickened and Filtered Tailings, Australian Centre for Geomechanics, Perth, pp. 1-11, https://doi.org/10.36487/ACG_repo/2655_06
(https://papers.acg.uwa.edu.au/p/2655_06_Spasojevic/)
Abstract: Over the last 5 years, Ausenco’s tailings team has worked on several tailings projects involving designing and constructing tailings facilities over the soft and very soft subgrades. The type and the size of the facilities varied. Still, most involved the construction of 50–70 m high embankments/structural zones overlying soft and very soft soil deposits. The construction of such big facilities over soft subgrades warrants special care when it comes to the modelling of embankment-induced pore pressures and the non-linear effect of effective stresses on the angle of shear resistance.
The current practice of the design of tailings embankment slopes on top of soft subgrades is based on the ‘undrained strength analysis’, which combines the ‘B-bar’ methodology for the assessment of embankmentinduced pore pressures with SHANSEP (stress history and normalized soil engineering properties) methodology for the undrained shear strength and total analysis slope stability. Embankment construction over a soft foundation creates a temporary, transient condition of excess pore pressure within the soft subgrade. Due to low permeability, the excess pore pressure within the clay cannot readily dissipate. This produces a non-steady state situation with elevated pore pressures and lower safety factors, and it could lead to slope failure. 
In this paper, an alternative, effective-stress-based methodology of slope stability analysis that combines the finite element method (FEM) and limit equilibrium method (LEM) is proposed. The FEM portion of the methodology is used for modelling the embankment excess pore pressures, while the LEM is used for the effective-stress-based stability calculations. The FEM considers the dilatancy law of the soft subgrade material, while the LEM accounts for the pressure dependency of the angle of shear resistance. In this way, the methodology covers the full spectrum of effective stresses, from the low values (where the angle of shear resistance typically attains higher values) to very high values corresponding to the ultimate state of the embankment.

Keywords: slope stability, staged construction, soft ground behaviour, effective stress analysis of undrained processes