SUMMARYThis paper focuses attention on the development of a numerical model of the hydro/thermo/mechanical behaviour of unsaturated clay and its consequent verification and validation. The work presented describes on-going collaboration between the Cardiff School of Engineering and Atomic Energy of Canada. The model development, which was carried out at Cardiff, can be described as being based on a mechanistic approach to coupled heat, moisture and air flow. This is then linked to a deformation analysis of the material within a 'consolidation' type of model. The whole is solved via the finite element method to yield a computer software code named COMPASS (COde for Modelling PArtly Saturated Soil). Some aspects of verification and validation of the model have been addressed in-house. However, the purpose of current AECL work is to provide an independent, rigorous, structured programme of validation and the paper will also explore the further validation of COMPASS within this context.

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The main thermodynamic forces active in the near field of a nuclear fuel waste multiple barrier system during the unsaturated stage are the gradients of temperature, fluid pressure, and chemical potentials. The processes that have a direct impact on the transport mechanism of heat and moisture within the clay-based compacted buffer material are coupled. The major problems in describing coupled heat and moisture flow (CHMF) in unsaturated swelling clay barriers are the lack of experimental data and the lack of a method to estimate CHMF diffusivity parameters (CHMF-DIPAR). This paper presents the experimental procedures, the analysis of experimental data, and a method of determining CHMF-DIPAR. Several series of one-dimensional tests were performed to examine the transient temperature and moisture flow in buffer material compacted at a dry density of 1.67 Mg/m3 and optimum volumetric moisture content (VMC) of 0.28. CHMF-DIPAR were calculated using the measured temperature and moisture distributions combined with an analytical solution of CHMF equation and a square-root time (SQRT) technique.

Several series of one dimensional tests were performed to examine the moisture and soil water potential distributions in the buffer material compacted to a dry density of 1.67 Mg/m3 and volumetric water content of 0.28. Diffusivity parameters were calculated using the measured moisture profiles combined with the finite difference method. Powell’s optimization was used to determine the material parameters.It is experimentally demonstrated that the moisture distribution is highly dependent on the test boundary conditions. When volume change was allowed to take place during infiltration process, the buffer material adsorbs more water than in the case of no volume change. This is attributed to an increase in hydraulic conductivity by one order of magnitude. As the volumetric water content increases soil water diffusivity increases and soil water potential decreases.