In this paper a new constitutive model based on the Disturbed State Concept (DSC) is proposed for a lignosulfonate treated highly erodible silty sand. In this model, an elasto-plastic Relative Intact (RI) re-sponse and the lignosulfonate bond strength were used as the two reference states to define the disturbance function. In order to determine the model parameters and to validate the proposed DSC model, a series of labo-ratory direct shear tests were conducted under drained condition. It was observed from the laboratory tests that the lignosulfonate treatment increased the shear strength of soil irrespective of the effective normal stress. Also, the volumetric responses showed dilative behaviour with the lignosulfonate treatment. The comparisons of the model predictions with experimental data clearly showed that the proposed model can accurately cap-ture the stress-strain and volume change behaviour of the silty sand similar to the laboratory experiments.