This study is set in the context of deep geological waste disposal facilities planned in the Callovo-Oxfordian (COx) claystone. The behavior of the argillite zone located adjacent to the galleries, damaged during their excavation, is the subject of our study. During the post-closure phase of the galleries, this damaged zone progressively resaturates, causing a phenomenon of crack sealing primarily due to swelling by water absorption in the clayeous matrix. The objectives of this work, are to improve the understanding of the self-sealing mechanisms of the claystone, and to develop a model to simulate the response of a representative zone around a gallery element during the post-closure phase. In this model, the hydration-induced swelling taking place at the macroscopic scale is represented by an unrestrained (free) strain controlled by the resaturation process of the claystone. The COx ability to absorb water even at full saturation requires a detailed description of the free strain that characterizes the swelling of the material. Investigating the swelling under constraint when fractured lips come into contact is also required. Inspired by the BExM constitutive model applicable to highly expansive soils, it is possible to account for the free strain that represents swelling as a function of the capillary pressure until the full saturation of the material. Moreover, the use of a non-linear viscoelastic-viscoplastic numerical model allows including the creep phenomenon which affects the self-sealing properties of the claystone. Numerical developments are performed using 3D samples of representative elementary volumes of the fractured claystone, composed of the clay matrix in which are dispersed cracks of defined shape and size. A numerical multiscale technique based on a modified FE² method is used to simulate the response of a macroscopic gallery region, which is discretized into zones with similar crack density parameter estimated from experimental data.