Although the majority of soils are unsaturated, their mechanical behaviour is still not well defined. This dissertation presents first an experimental study of an unsaturated silt and then proposes an elasto-plastic constitutive law to model the behaviour of unsaturated soils. The first part of this work deals with the characterisation of the mechanical behaviour of unsaturated soils by means of laboratory experiments on a remoulded silt. Suction is imposed by controlling the excess air pressure in the sample. Several techniques for the measurement of volume changes of samples are developed and compared. The saturated behaviour is studied by means of imposed suction and controlled tests under triaxial and oedometric (uniaxial strain) conditions. Other test performed in Richards cells are used for defining the purely so-called "hydric" behaviour of the soil. The analysis of test results were done in the effective stress plane (σ'=σ-uw,) as well as in the net stress plane (σ*=σ-ua). The hydric tests confirmed the hysteretic nature of the wetting-drying cycles. The hydric and mechanical response changes when the suction raises above the air entry value. Isotropic tests show that the preconsolidation pressure increases with the suction in the log(p')-e plane. However, this increase is rather limited in the net stress diagrams. Deviatoric tests show an increase of stiffness and peak strength with suction at constant mean effective or net pressure. Suction appears to have an effect similar to a mechanical overconsolidation when representing the result in the effective stress plane. Softening appears post peak for the cases of strong suction or weak net mean pressure. The evolution of the critical state is interpreted in terms of effective and net stresses. In the p'-q plane, the critical state line is shown to be independent of the suction (in the studied range of stress and suction); this is not true in the p*-q plane. The yield surfaces F derived from the test results grow with suction and both p'-q and p*-q planes, and the behaviour corresponds to non-associated plasticity. An existing elasto-plastic model originally developed for saturated soils (HISS-δ1) was improved and extended to integrate suction effects. The new proposed model, called δ1-unsat, is described by means of two independent stress state variables: the effective stress σ' and the suction s. Two yield surfaces, coupled through the hardening internal variable, are used in the model; the first one is linked to the mechanical behaviour, while the second one is used for hydric loading. This new model requires five new material parameters: three of them are used for the purely hydric behaviour, one describes the increase of peak strength with the suction and the last one describes the growth of yield surface with suction. The δ1-unsat model is shown to be able to reproduce the principal features of the unsaturated soils: (a) existence on a drying path of a saturated domain with non-zero suction where plastic (irrecoverable) strains appear, (b) elastic behaviour on drying path when the suction is larger then the air-entry value, (c) wetting collapse, (d) influence of mechanical stress state on the hydric behaviour, (e) increase of preconsolidation pressure with suction, (f) increase of peak strength with suction. The quantitative reliability of the δ1-unsat model is good quantitatively. In particular the prediction of the behaviour in the domain of low strains (pre-peak) is well described. The introduction of a damage concept (DSC - disturbed state concept) in the δ1-unsat model allowed the modelling of the post-peak behaviour as well. In conclusion, the mechanical behaviour of an unsaturated silt was characterised. A constitutive elasto-plastic model integrating suction and damage effects was developed and was proved to represent the principal constitutive features of unsaturated soils.