The objective of this paper is to review the slope stability analysis conducted during the initial design stages of a new landfill cell that forms part of a landfill site expansion in Gauteng. The SLOPE/W slope stability software, from the Geostudio integrated software suite, was used to carry out the analysis. Initial trials revealed a significant waste slope instability, which, when further investigated, was attributed to the weak sliding interface between the smooth geomembrane and clay, the 8 m deep basin, the 46 m estimated waste height leaning on a small embankment on the northern side of the cell and the cell basin sloped in the direction of the waste sliding mass. Satisfactory results and eventually slope stability was achieved through a comprehensive optimization process which entailed modelling and running iterations of various trials of a combination of stabilizing berms, geogrids and cutting back the waste slope.