Different European Laboratories (Administration, Universities) were awarded a laboratory twining project between 1990 and 1993, under the "Science" programme, which closure has taken place with Euroflex '93 Symposium (Lisbon, 1993). This research aims to harmonise and develop rational procedures to take into account the mechanical characteristics of soils and unbound granular materials, to apply to the design of flexible pavement structures. A round-robin test programme for unbound granular materials carried out by different laboratories enables to establish reliable test procedures for characterisation of elastic and plastic behaviours of these materials. The procedure for elastic behaviour is summarised in this paper. Concerning the modelling of flexible pavement structures, the behaviour of unbound granular materials was described by a nonlinear elastic constitutive law (Boyce), with which a classic plasticity criterion was added (Drucker-Prager) in order to give a stress field in the material compatible with its strength. This model was implemented in FEM codes: CESAR LCPC-2D and 3D, and Noel 8-2D of the University of Nantes. The results of FEM-2D (single wheel) and FEM-3D (dual wheel) calculations realised for most representative flexible pavement structures are presented.