This paper proposes a general formulation which allows to adapt existing constitutive models developed for dry or saturated soils to take into account suction effects. The main advantages of such a conceptualisation is to reuse reliable models presenting good capacities to closely simulate soil behaviour under complex loading paths. These loading paths may lead to sophisticated behaviour (contractancy followed by dilatancy in the course of loading etc.) well simulated by such models. The choice of a particular effective stress, combined with suction, is made. The definition of this effective stress is based on the formulation of an equivalent pore pressure. Following the approach presented, an existing elastoplastic model (CJS model), is extended to cover unsaturated states. A classic geotechnical in situ investigation method (pressuremeter) is then simulated. This simulation allows to determine the effects of suction on the mechanical behavior of the soil.