The storage of nuclear waste is still an unresolved problem of the nuclear industry, being geological disposal the most favoured option. Most conceptual designs for the deep geological disposal of nuclear waste envisage placing the canisters containing the waste in horizontal drifts or vertical boreholes. The empty space surrounding the canisters is filled by an engineered barrier often made up of compacted swelling clay. A good understanding of thermo-hydro-mechanical (THM) issues is therefore necessary to ensure a correct performance of engineered barriers and seals. The conditions of the bentonite in a typical engineered barrier are being simulated in a mock-up heating test at almost scale. The evolution of the main Thermo-Hydro-Mechanical (THM) variables of this test are analyzed in this paper. Especial emphasis has been placed on the study of the effect of thermo-osmotic flow in the hydration of the clay barrier at advanced staged of the experiment.