Water infiltration into a large-scale in-situ experiment in an underground research laboratory

This paper describes the re-saturation behaviour of a large-scale in-situ experiment carried out at Atomic Energy of Canada's (AECL) underground research laboratory. The experiment, known as the isothermal test, examines water inflow, from the surrounding rock, into highly compacted, unsaturated buffer material. Comparisons of physical measurements and a finite element numerical simulation are presented of water uptake and distribution during the 7-year life of the test. The numerical simulations were performed using an approach developed to model the thermo/hydraulic/mechanical behaviour of the system. A comprehensive data set of the physical behaviour was available from AECL. The simulation of the experiment, using a 'conventional' hydraulic conductivity variation, revealed that neither the duration nor the pattern of moisture influx could be modelled accurately. Further study suggested that the expansion of the microstructure of the bentonite, as the material saturated, would tend to reduce the void spaces in the macrostructure, as the material, overall, was constrained from swelling. This in turn was likely to reduce the material's hydraulic conductivity. Incorporation of these ideas within the approach yielded significant results. Both the pattern and the rate of water uptake were now modelled with much greater accuracy. The approach adopted is different from the 'conventional' method, in the sense that the material's 'effective' hydraulic conductivity decreases as moisture content increases, as opposed to increasing, as is the 'normal' case. The repercussions may be significant in terms of total time for re-saturation of the buffer. The process is significantly delayed as a result of this phenomenon. Also, the pattern of moisture distribution during water influx is quite different. These general conclusions, in turn, may be significant when considering the overall performance of a disposal repository, as the total time for re-saturation would be increased. This in turn may be of interest in performance assessment considerations.