Empirical relationships are developed for estimating undrained critical shear strength based on experimental triaxial tests under monotonic loadings. The effect of fines content on undrained shear strength is analyzed for different combinations of density states. The parametric study indi-cates that in terms of soil void ratio, fine content properties, undrained critical shear strength may increase, or decrease as the amount of fines content increases, consequently showing the vulnera-bility to liquefaction influenced by the fines content percentage. A series of monotonic undrained triaxial tests were undertaken on reconstituted saturated sand-silt mixtures specimen. Beyond 30 % of fines content, it is shown that a fraction of silt participates in the soil skeleton chain force. In this context, the concept of the equivalent intergranular void ratio may be an appropriate parame-ter to express the critical shear strength of the soil under investigation. This parameter is able to control the undrained shear strength of non plastic silt and sand mixtures for different density states.