Keywords: coupled-problems, poroelasticity, finite element, chemo-osmotic effect.

Today environmental concerns are a major driving force behind current nuclear decommissioning and its associated waste disposal research and development. Minimising harm to the environment can be achieved in some part by careful design of disposal sites, so that any discharge of undesirable contaminants into the biosphere has the minimum possible environmental impact. Fear of hideous and unexpected radioactive pollution from nuclear waste repository has resulted in severe restrictions in choosing the most geologically stable formation for their storage in many parts of the world. One of the key features in these geological selection criteria is the natural underlying of impervious rocks, e.g. clay-rich formation or zeolitized tuff, which is prone to seismic activities and at the same time, hydrologically stable. This is important so that any possible leakage of the contaminant can be safely contained by the impervious geological layers and thus preventing it from escaping into the water cycle. However, most of these impervious and low permeability formations, whose original constituents are mud, clay and organic material such as algae, are prone to swelling when in contact with groundwater due to the repulsive hydration and electrical forces in operation between the silicate crystals. The electrical forces are usually negligible when compared to the hydration forces in the context of the downward compressive force of the overburden. Added to these complexities is the presence of dissolved chemical/organic compounds in the groundwater. In such a situation, an enhanced pore pressure evolution due to the osmotic membrane effects can develop. Some studies have revealed that these combined effects of hydration swelling and osmotic flow cause an irreversible strain in the clayey soils and if swelling is prevented, substantial swelling pressure can develop, weakening the strength of the material.

In this paper, we report a coupled chemo-physico formulation within the framework of Biot's poroelasticity to describe the phenomenon of hydration swelling and chemo-osmotic effects. The effective stress equation is modified by adding an additional swelling term due to the chemical gradient, while the Darcian flux is altered to allow for the membrane effect. The governing equations are discretised using the finite element method using the mixed Q8Q4 element to enhance the stability of the spatial solution. Numerical experiments based on the apparatus setup by Heidug and Wong [1] are conducted to investigate the various coupling effects due to the presence of chemical solute in the porous media. The osmotic coefficient is responsible for the gradient of the chemical potential, and thereby affects the dissipation of the pore pressure and swelling. The Fickian diffusion coefficient controls the rate at which the solute diffuses through the sample, and in doing so, affects the rate at which the initial pore pressure and displacement change, indirectly. The coupled model developed herein is in fact a weakly coupled model. This is because the rate of change of porosity in the solute equation is ignored and porosity change is assumed to be constant. Future research will relax this restriction and to investigate the effect of permeability change as the porous solid deforms.

1

W.K. Heidug and S.W. Wong, "Hydration swelling of water-absorbing rocks: a constitutive model", Int. Jour. Numer. Analy. Meth. Geomech., 20: 403-430, 1996. doi:10.1002/(SICI)1096-9853(199606)20:6<403::AID-NAG832>3.0.CO;2-7

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