- The porosity of soils is considered to be a directional measure and its distribution is characterized by a functional form. This form has been used to extend the critical state soil mechanics framework to include the effects of structure in soils. A new internal plastic energy dissipation formulation has been proposed to account for fabric arrangement. New expressions for the yield locus, and the plastic stress-strain response of structural soils have been derived. The applicability of the concepts to model the plastic stress-strain behaviour of a number of soils is illustrated. The advantage of the new model is very well indentified in modelling the stress-strain behaviour of K0 consolidated and natural clays.