Granular soils subjected to seepage flow may suffer suffusion, i.e. a selective internal erosion. Extending the classical approach of poromechanics, we deduce a new form of the Clausius-Duhem inequality accounting for dissipation due to suffusion and we deduce restrictions on the constitutive laws of the soil. We suggest (i) a possible coupling between the seepage forces and the suffusion kinetics (ii) an extension of an existing elastoplastic model for the skeleton mechanical behaviour. Numerical integrations of the elastoplastic model are carried out under drained axisymmetric triaxial and oedometric conditions. As a result, we prove that the extended model is able to qualitatively reproduce the suffusion induced strains as well as the strength reduction experimentally observed. Predictions on the oedometric behaviour of suffusive soils are also provided.