Different European Laboratories (Administration, Universities)were awarded a laboratory twining project between 1990 and 1993,under the "Science" programme, which closure has takenplace with Euroflex '93 Symposium (Lisbon, 1993). This researchaims to harmonise and develop rational procedures to take intoaccount the mechanical characteristics of soils and unbound granularmaterials, to apply into the design of flexible pavement structures.

A round-robin test programme for unbound granular materialscarried out by different laboratories enable to establish reliabletest procedures for characterisation of elastic and plastic behavioursof these materials. The procedure for elastic behaviour is summarisedin this paper. Concerning the modelling of flexible pavementsstructures, the behaviour of unbound granular materials was describedby a non linear elastic constitutive law (Boyce), with which aclassic plasticity criterion was added (Drucker-Prager) in orderto give a stress field in the material compatible with its strength.

This model was implemented in FEM codes: CESAR LCPC-2D and3D, and Noel 8-2D of University of Nantes. The results of FEM-2D(single wheel) and FEM- 3D (dual wheel) calculations realisedfor most representative flexible pavement structures are presented.