In the present work, the hypoplastic model for sands by Wolffersdorff, (1996) coupled with Intergranular Strain Anisotropy (ISA) by Fuentes et al., (2020) is extended to account for the influence of fines content on the undrained cyclic response and liquefaction resistance of the material. A set of undrained cyclic triaxial tests on clean Ottawa C778 sand and on the same sand with additions of bentonite or laponite were simulated to evaluate the model’s capabilities. The comparison between experiments and element test simulation results suggest that the proposed extended model is able to accurately describe the behavior of clean Ottawa C778 sand and the significant influence of plastic fines in the response of the mixtures, such as the reduction in the accumulation rates of strains and pore water pressure under cyclic loading, and therefore, the improvement in the liquefaction resistance with increasing fines content.