A series of monotonic and cyclic triaxial tests were performed on a sand with fines sourced from Christchurch, New Zealand. The sand was sieved and then mixed to give three soils with different fines contents. The undrained tests were used to examine the effects of fines on the strain softening behaviour under monotonic loading and liquefaction resistance in cyclic loading. Two reference states were used as a basis for evaluation of the effects of fines: the relative density and the steady state line within the state-concept framework for sand characterization. The addition of fines to the sand base caused downward movement of the steady state line in the Dr-p' plane (e-p' plane), and this effectively increased the potential for strain softening or flow deformation. Samples prepared at an identical relative density showed decreasing cyclic strength with increasing fines content. Conversely, samples at an identical initial state relative to the steady state line showed increasing cyclic strength with the fines content.