Mechanical modeling of MX-80–Development of constitutive laws

In a previous paper [Åkesson, M., Hökmark, H., 2007. Mechanical model for unsaturated MX-80. In: Schanz, T. (Ed.), Theoretical and Numerical Unsaturated Soil Mechanics, Springer Proceeding in Physics, vol. 113, pp. 3–10] a new elastoplastic model for unsaturated swelling clay was presented. The model was formulated as two differential equations, for elastic and plastic strains, respectively. Each equation described a relation between the (air-filled) macro void ratio and the two independent variables stress and (water-filled) micro void ratio. The established concept of swelling pressure and its void ratio dependence were incorporated in the model. To do this, the grain-to-grain contact stress and the contact area had to be considered. In its original form the model only handled one-dimensional problems, e.g. compression tests with uniaxial strain and water uptake test at constant volume with assumed isotropic conditions.This paper describes a first attempt to generalize the model to address multidimensional problems with the intention to integrate it into the framework of Code_Bright. This was made through definition of an expression for the retention properties and through variable substitution. In this way, four functions for the kappa moduli were identified: two elastic and two plastic. These functions provide unique values for each given state. In the used framework of the Barcelona Basic Model (BBM), all strains are treated as elastic, and there is thus no explicit treatment of any deviatoric yield condition. The only remaining parameter outside the new model is the Poisson’s ratio.