Étude du comportement des sols granulaires moyennement denses sous sollicitations monotones et cycliques

In geotechnical engineering, the mechanical behaviour of granular soils under monotonic and cyclic loading continues to be an important research subject because of the devastating human and material damages that a rupture in a foundation or an earth embankment might generate. Although medium-density granular soils are very often found in the foundations, there has been little work carried out on these soils compared to loose soils. An intensive laboratory study of the behaviour of five foundation sands (% of silt varying between 2 and 42%, and D[indice]50 varying between 0,55 and 0,09 mm) was undertaken through a wide experimental program (drained triaxial, undrained triaxial, oedometer, monotonic DSS (direct simple shear), cyclic DSS, resonant column). The monotonic and cyclic behaviour of these sands was related to the state parameter: for the same confining stress and the same density index, soils showing the higher dilatant behaviour were the most resistant. This result was observed under different types of loading. Moreover, the influence of gradation on soil behaviour was analysed. The question of the uniqueness of the ultimate state for soils was raised: the care that should be taken in the determination of this state under drained or undrained triaxial testing conditions was outlined. On the other hand, the effect of static preshearing on the cyclic shear strength was investigated. The importance of the choice of rupture criteria was explained and an interpretation of this rupture as a function of deformation or liquefaction was realised. The criterion expressed in terms of deformation was the most critical in the presence of high preshearing. For the densities ([asymptotically equal to]60%) examined in this thesis, the shear strength generally increased in the presence of preshearing. The resonant column tests confirmed the results of the triaxial and DSS tests as far as the effect of preshearing and the influence of gradation on the shear strength are concerned. Finally, modelling the monotonic and cyclic behaviour of medium-density granular soils was realised in France using the elastoplastic constitutive model of Hujeux. This model derives from the Cam-Clay model, and is utilised by Electricite de France in the GEFDYN Code. This modelling helped in clarifying certain differences existing between the simple shear and the triaxial tests. The differences located mainly at the ultimate state and the friction angle were attributed to the limit conditions imposed by the tests. Good results were obtained by the modelling