Comportement mécanique des milieux granulaires sous sollicitations cycliques

The following work deals with the characterisation of the behaviour of road bases granular material, made of tunnel excavation material and loaded by passing vehicle axles. This granular media, don't correspond generally to the standards demanded for the construction of road pavements. The thesis is a part of the project COST 337 ("Unbound granular materials for road pavements") whose aim is to develop a guideline for the design of road pavements in Europe (material production, laboratory tests, modelling of mechanical behaviour). At the beginning, several loadings of road pavements are described (mechanical, thermal, hydric), concentrating on mechanical loading. Full-scale tests and finite element simulations led to the knowledge of the order of magnitude of stresses and deformations as well as to the understanding of the rotation of the principal stresses, due to the passing of a vehicle axle. This theoretical part is followed by laboratory cyclic tests with sand to estimate the influence of the principal stress rotation on the deformability of granular media and the importance to take this phenomenon into account for the modelling. Further laboratory tests were carried out on high diameter samples (160 and 250 mm) made of tunnel excavation material. Those tests were made by performing a large number of cycles (80'000 to 250'000 cycles) and various stress paths. The aim of the tests was to measure the evolution of elastic (stiffening of material) and plastic (accumulation of permanent deformations) material characteristics during the cycles. Tools have been developed in the framework of image analysis to assess the eventual change of morphology of this non-standard granular media. Finally the modelling of the accumulation of plastic deformations is discussed in accordance to the theory of elasto-plasticity and visco-plasticity. To remove the limits of elasto-plasticity to simulate a large number of cycles, a visco-plastic approach is developed. The elegant approach, replacing the time by the number of cycles, allows to approximate the permanent deformations of a road pavement in dependence on the stress path. It provides a tool to estimate the rutting in road pavements.