Caracteristiques mecaniques d'un gravier de fondation routiere dans l'etat non sature

Major civil engineering works carrying out large volumes of granular fills such as highways, railways, dams and dykes, require a better understanding of these materials. This will permit to develop a more efficient design and execution method that will lead to better risk management.;Saturated soil parameters such as shear strength, Young modulus and strain parameters are often used for design and stability analysis of civil engineering works. Pore water pressures on soil layers above the water table are negative but the effect of suction is generally neglected. This assumption can be questioned, is it reasonable when most of the service life of the major parts of these engineering works are above the water table? This thesis entitled "Determination of the mechanical characteristics of an unsaturated granular soil" is intended to study the behaviour of a granular material used for civil engineering structures such as roads.;Laboratory tests permit to simulate different types of loading undergone by engineering works. To carry out this study, recent developments in experimental procedures were used (triaxial shearing apparatus, electro-mechanic controllers for volume and pressure along with the axis translation technique to induce suction and the unsaturated state). This research highlights some aspects of unsaturated granular soil behaviour and the effects of suction on Young's modulus, shear strength and volumetric stain.;Consolidated Drained shear (CD) and Constant Water Content shear (CW) tests were performed. Test results indicate a general improvement of tested material stiffness with the increase of suction.;Test results also indicate increases of shear stress peak with suction. General volume change response of samples is first to contract, followed by dilatation as axial strain increases during shearing. In other words, void ratio slightly decreases before reaching peak stress and is followed by an increase when further sheared. Such a behaviour is typical of so called "dense granular soils" indicating that the void ratio before shearing is below the critical void ratio. CD and CW triaxial test results also indicate that there is a non-linear increase of apparent cohesion with applied suction. This study shows a nonlinear decrease of friction angle with respect of increasing matric suction. Young's modulus calculated from tested samples data shows that an increase in suction induces a proportional effect on Young's modulus. Results of the CW triaxial test show that matric suction decreases as the shearing process develops. These findings are consistent with previous published research.;CD and CW triaxial tests demonstrate that presence of suction improves mechanical characteristics of the tested granular sample during shearing. This sufficiently proves the need to take into account the unsaturated state of granular materials in the stages of design, execution and service life of civil engineering works.;Key words: Unsaturated granular soils, triaxial shearing, suction, shear strength, Young's modulus.