This paper presents the results of an experimental investigation of the mechanical behaviour of silty sand regarding instability phenomena using conventional triaxial tests under drained and undrained conditions. First, a series of monotonic triaxial tests was carried out on reconstituted clean sand samples with emphasis placed on the effect of basic parameters such as relative density and confining pressure. To highlight the effect of fines content on the initiation of instability and on the critical state, a second series of monotonic triaxial tests was conducted on sand/silt mixture samples reconstituted with different percentages of fines. In this series, one group of samples was prepared with the same overall void ratio, while in the second one, all the specimens had the same intergranular void ratio. The effect of fines on the evolution of the maximum and the minimum void ratios was also investigated. The results obtained show that fines content has a significant influence both on resistance at the onset of instability and on the position of the critical state line of silty sands. In addition, they also revealed that the critical fine content for this soil is about 20%. We guess that, beyond this value, the effect of fines content on the mechanical response of the soil is reversed as reported by several authors in the literature.