The paper addresses the issue of modelling the hydromechanical behaviour of swelling clays that exhibit evidence of multi-porosity. A double structure constitutive model developed for such materials is presented fi rst followed by a double porosity formulation intended for the performance of coupled hydromechanical numerical analysis. Hydraulic equilibrium between porosity levels is not assumed; instead a water exchange term is postulated. Constitutive model and formulation are then applied to the simulation of a number of experimental tests involving the hydration of a mixture of pellets and powder; a heterogeneous material in which the presence of two structural levels in especially marked. It is shown that the complex behaviour exhibited by this material is satisfactorily reproduced. Insights into the basic processes that underlie the observed macroscopic behaviour are also obtained.