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Extended THM formulation for predicting the long-term behaviour of unsaturated clay barriers

It is envisaged that unsaturated compacted expansive clays will often be the prime component of engineered barriers in the containment of high-level radioactive waste. Such barriers are subjected to a set of complex actions that involve a variety of coupled thermo-hydro-mechanical (THM) phenomena. Conventional coupled THM formulations to tackle this problem [1] involve the simultaneous solution of a series of balance equations (solid mass, water mass, air mass, internal energy and momentum) together with a suite of constitutive equations and equilibrium restrictions that describe the various phenomena and their interactions. Performance of long term tests has revealed that the conventional formulation faces significant difficulties to predict adequately the long term behaviour of engineered barriers.

Consequently, the formulation has been extended to incorporate new features that may be relevant for long term conditions. They are: i) coupled processes such as thermo-osmosis, corresponding to Onsagerian off-diagonal terms, ii) the existence of a threshold gradient in the generalized Darcy’s law, and iii) the consideration of fabric changes of expansive clay during hydration.

The consequences of incorporating each one of those features (as well as some combinations) are explored in relation to the following cases: a) experimental results of long term (10 years) infiltration tests under isothermal and non-isothermal conditions, b) observations of a long term (16 years) large-scale test simulating an engineered barrier subjected to heating and hydration, and c) predictions of long term barrier behaviour in the context of a specific repository design.

[1] Gens, A. (2010). “Soil-environment interactions in geotechnical engineering”. 47th Rankine Lecture. Géotechnique, 60, 3-74.

Author(s):

Antonio Gens    
Technical University of Catalonia (UPC), Barcelona Tech
Spain

Beatrice Pomaro    
University of Padova
Italy

Marcelo Sanchez    
Texas A&M University
United States