The use of unbound structural pavement layers is fairly common in many countries. A number of mechanistic-empirical design models have been developed for these layers relating the plastic deformation response of the unbound material to the ratio between imposed shear stress and the shear strength of the material. In the case of South Africa, the shear strengh of the material used to be characterised by the Mohr-coulomb shear strength parameters. The shear strength of unbound material os, however, highly dependent on the relative density and degree of saturation of the material. This paper presents a model that characterises the shear strength of unbound materials including the effect of relative density and degree of saturation. The model is calibrated with very high accuracy for two examples consisting of a crushed stone and natural gravel material