The potential use of bentonite buffers and sealings in engineered barriers (EB) for high-level radioactive wastes in deep repositories has motivated many experimental studies and numerical developments to characterize and reproduce the main thermo-hydro-mechanical (THM) responses of such materials when subjected to changes in moisture content and temperature. Hydration performed at room temperature and at high-temperature values usually leads to changes in stresses inside the barrier and, consequently, to the evolution of the porous structure and to a homogenization process of initially heterogeneous bentonite buffers. These complex transient processes are mainly dependent on the water content and the dry density inside the barrier. A constitutive model for expansive geomaterials has been developed with the explicit consideration of two overlapped pore levels coupled through a strain mechanism and a local water mass transfer between both structural levels. This mathematical model has been formulated in terms of both classical and generalized plasticity approaches and implemented in a finite element code (CODE_BRIGHT). The model capability in reproducing the main THM behavior expected to take place in a confined EB has been evaluated by the simulation of hydration tests carried out in the CIEMAT laboratory (Madrid, Spain). Two groups of experimental tests were considered: the hydration at room temperature of heterogeneous 10cm-height columns made of compacted and pelletized FEBEX bentonite and the hydration of a heated 50cm-height column of MX-80 pellets. In both cases, the numerical modelling with the current double-structure formulation reproduced the changes in the clay fabric (homogenization process) and the evolution of the water content and the swelling pressure due to re-saturation as well as the thermal volumetric expansion (for the non-isothermal case) induced by the thermal gradient inside the bentonite column.