3D finite element simulations are conducted to better understand the soil-structure interaction of rectangular foundations resting on a sand embankment over soft mine tailings. To account for large strains, an updated Lagrangian formulation is used in which the stiffness matrix is updated with the non-linear change of the soil geometry. The sand is modelled using a modified Mohr-Coulomb (MMC) soil model where the friction and dilation angles are varied with deviatoric plastic strains. It is shown that failure over a moderate slope angle of 22° and normalised sand embankment height (H/B) of 0.26 (H is the embankment height and B is the footing width) involves a deep seated circular slip plane, involving the sand and clay layers, where the foundation was close to the crest of the embankment. The failure pattern turns gradually to typical punch-through with an inclined sand shear plane as the distance of the footing edge exceeds 1.13B from the crest. The results of this study agree well with those from model centrifuge experiments, and can be utilised in developing better analytical models and equipment capable of live monitoring of slope deformation.