Systems comprising geosynthetics constitute a preferential slip line when they are laid down on the slopes of civil engineering structures and their stability must be checked with care. With this objective in mind, as well as that of improving the characterisation of friction at geosynthetic interfaces and therefore the calculation of the dimensions of such systems on slopes, an apparatus of the "inclined plane" type was designed. Two full-scale experiments validated the many results obtained with this apparatus and, in particular, highlighted the influence of wetting of the materials on friction angles and confirmed the influence of saturation of the protective soil cover on the stability of geosynthetic systems. In parallel with these experiments, using existing calculation methods and integrating the hypotheses concerning the application of the various hydraulic conditions validated by our experiments, a geosynthetic system slope stability calculation software programme was developed.