Abstract : The main technical objective of this PhD thesis deals with the development of a soilbehavior numerical tool. It should be robust, efficient and adapted to model themechanical behavior of geotechnical structures (e.g. embankment dam) under theworst loading scenarii such earthquakes.In the finite element method, the quality results is directly linked to the soilconstitutive model, the integration scheme and the numerical resolution. In this PhDThesis, the ECP elastoplastic soil model is introduced in Code_Aster through animplicit scheme. An implicit scheme ensures to respect the theoretical formulation ofthe model. The ECP constitutive model is one of the models available in theliterature to represent the behaviour of different kinds of soils under cyclic loadingsand it is used since the 80's by hydraulic engineers at EDF. It is expressed in termsof effective stresses and infinitesimal strains. The developments are validated forlaboratory tests in a large scale of loading paths. On the other hand, the study of asand embankment was performed and compared to the results obtained with thefinite element software GEFDyn developed at ECP.The ECP model is based on a non-associated flow rule and it is able to reproduce asoftening behavior. When shear bands occur in the structure, these properties leadto a pathological sensitivity of the results depending on the mesh size. Therefore, aregularization technique has to be used to circumvent this problem and to obtainobjective results with respect to the mesh. The second gradient of dilation model isthus chosen to be coupled to the ECP model and in this way, to ensure a spatiotemporalindependence of results. However, this mesh independence still evolves ina potential domain of solutions, when instability occurs. The simulations of drainedbiaxial tests on laboratory samples show a dependence of shear bands thickness inregard to the initial stress state and material properties.An analytical problem of a dilatant shear band is used to extract the key factors.These theoretical solutions are compared and validated to numerical responses,which are in good accordance. A bearing capacity problem was also solved todemonstrate the potential of the method. The conclusion of this work establishes themain role of initial stress state over the shear band thickness in the context of thesecond gradient of dilation model and the models based on Cam-Clay approach.