Stability assessment for complex structures, particularly the calculation a sliding Factor of Safety (FoS), invariably has numerous simplifying assumptions that typically affect the FoS due to conservative approximations. This was the case during the review of an existing dam with a central arch, gravity abutments, steeply sloped valley profile and complex geological conditions. Empirical calculations encountered limitations due to the complexity of the structure and associated load paths, that inadequately captures the arch behaviour. That is, an arch should effectively be able to be put on “roller” supports and remain stable and within its range of acceptable stress. The quantity of arch thrust being distributed to the foundation and gravity abutments therefore needs to be considered for the gravity abutment sliding FoS calculation. The complexity of this calculation is increased when considering the geometry of the foundation, and arch thrust load which is perpendicular to that of the applied hydrostatic loading. So, how can an appropriate sliding FoS be calculated for these complex structures? The authors have identified a method to assess stability of the dam and estimate the sliding FoS involving incorporation of Mohr-Coulomb non-linear interfaces within a 3D brick Finite Element Analysis (FEA) model. This requires a thorough understanding of FEA, including aspects such as interface parameters and behaviour, convergence requirements and analysis sequencing to name a few. Comprehensive analysis of a structure can allow a refined sliding FoS to be calculated, which may alleviate the need for potentially expensive remedial works.