In the context of nuclear waste geological storage, deep argillaceous formations are likely to be subjected to complex mechanical, hydraulic, and thermal loads. In particular, the argillaceous material can be firstly dried, and then re-wetted. During the latter process, the material experiences swelling and can develop swelling pressure if swelling deformations are constrained. In this contribution, the results of swelling pressure tests on shale performed in totally constrained conditions (isochoric tests) are presented. A constitutive model (ACMEG-S) is used to predict the value of the swelling pressure in such conditions. The model is made of two parts. The mechanical part addresses the stress-strain behaviour of the material, as a result of effective stress variation. An elasto-plastic approach is employed, and Bishop's unsaturated effective stress, which is a function of the degree of saturation, the suction and the externally applied stress, is used as the mechanical stress. The water retention part of the model defines the relation between the degree of saturation and the suction within the material. The results put into light some factors that control the swelling pressure value, in particular the degree of saturation and the plastic behaviour of the material.