The paper first presents an overview of the issues associated with radioactive waste disposal and follows with a brief description of deep geological disposal for high level radioactive waste. The performance assessment of these underground repositories requires a good understanding of the basic phenomena that affect the succession of barriers separating the waste from the biosphere. A key safety element in the design of these facilities is an engineered barrier, made up of highly expansive clay, that surrounds the canister containing the waste. This barrier is subjected to a complex set of coupled thermo-hydro-mechanical (THM) phenomena that must be correctly modelled and understood. The paper presents a coupled THM formulation that incorporates the most relevant processes occurring in the barrier and their mutual interactions. The formulation is then applied to the modelling of an in situ test that simulates repository conditions at full scale. The tests involves heating and hydration of the engineered barrier. It is shown that the modelling results represent satisfactorily the observations from the test, including those obtained dafter a partial dismantling stage. This exercise demonstrates that the basic phenomena appear to be well understood and that the formulation and associated computer code are useful tools for the coupled analysis of this type of THM processes.