The recent advances made in modern Unsaturated Soil Mechanics have permitted the development of new analysis methods for the prediction of the behaviour of embankments and other earth structures subject to changes in atmospheric conditions (precipitation, temperature, relative humidity) and under the action of traffic loading. One of the breakthroughs of greatest interest is the possibility to numerically model the processes of volume change in expansive soils when subjected to variations in humidity. In this paper, the analysis employed, based upon the Barcelona Basic Model constitutive model (Alonso et al., 1990), to study the viability of the embankments of the speed railway Montornès del Vallès-La Roca del Vallès (LAV Madrid-Zaragoza-Barcelona-French Frontier), is presented. A coupled flux-deformation analysis has been undertaken to model the water-infiltration process through the embankment, and the associated volume change deformations have been evaluated in terms of rainstorm duration.