Thermomechanical model of hydration swelling in smectitic clays: I Two-scale mixture-theory approach

A thermomechanical theory of hydration swelling in smectitic clays is propo sed. The clay is treated as a three-scale swelling system wherein macroscop ic governing equations are derived by upscaling the microstructure. At the microscale the model has two phases, the disjoint clay platelets and adsorb ed water (water between the platelets). At the intermediate (meso) scale (t he homogenized microscale) the model consists of clay particles (adsorbed w ater plus clay platelets) and bulk water. At the macroscale the medium is t reated as an homogenized swelling mixture of clay particles and bulk-phase water with thermodynamic properties defined everywhere within the macroscop ic body. In Part I, the mesoscopic model governing the swelling of the clay particles is derived using a mixture-theoretic approach and the Coleman an d Noll method of exploitation of the entropy inequality. Application of thi s procedure leads to two-scale governing equations which generalize the cla ssical thermoelastic consolidation model of non-swelling media, as they exh ibit additional physico-chemical and viscous-type terms accounting for hydr ation stresses between the adsorbed fluid and the clay minerals. In Part II the two-scale model is applied to a bentonitic clay used for engineered ba rrier of nuclear waste repository. The clay buffer is assumed to have monom odal character with most of the water essentially adsorbed. Further, partia l results toward a three-scale thermomechanical macroscopic model including the bulk phase next to the swelling particles are derived by homogenizing the two-scale model with the bulk water. A notable consequence of this thre e-scale approach is that it provides a rational basis for the appearance of a generalized inter-phase mass transfer between adsorbed and bulk water. C opyright (C) 1999 John Wiley & Sons, Ltd.