Rutting, due to permanent deformations of unbound granular layers, is the principal mode of degradation of flexible pavements (bituminous layer thickness less than 12 cm). The large number of load cycles necessary to describe the long term behaviour of pavements is the main difficulty when plasticity based incremental formulations are used. This difficulty can be overcome using the shakedown theory. The elastoplastic model presented in this paper is based on the simplified method of Zarka. This method is usually used to describe the cyclic behaviour of materials with kinematic hardening like steels. This method has been adapted for unbound granular materials by introducing a yiel surfac e taking into account the influence of the mean pressure on the mechanical behaviour of the granular media. The model is validated by comparing its predictions with results of repeated load triaxial test carried out on Missilac sand. Finally, the results of finite lements modelling of the long-term behaviour of a flexible pavement with the simplified model are presented