The naturally deposited soil usually does not consist of pure coarse or fine-grained soil but of a mixture of both. The mechanical behaviour of a saturated fine sand mixed with varying amounts of low-plastic fines was evaluated by monotonic as well as high-cyclic triaxial tests. The test results were used to conclude on the effect of fines content on the critical state, phase transformation line, secant Young's modulus, the residual strain accumulation as well as strain amplitude during drained cycles of the mixtures in relation to the global void ratio as well as to the equivalent void ratio. It was found that while the choice of void ratio definition is important for the uniqueness of the critical void ratio, both approaches can be used as state variables for the phase transformation line. However, some seemingly contradictive results are found from the drained high-cyclic tests. Eventhough, an increase of the residual strain accumulation with decreasing fines content compared at the same initial equivalent void ratio is rendered by the laboratory data, a unique and on fines content independent relationship between eacc could be established only with respect to the initial global void ratio.