In this study a series of swelling and breakthrough tests were performed on a target S/B mixture in proportion 80/20 in dry mass. Tests were carried out on specimen compacted to the same dry density but changing the osmotic suction of the pore water. The tests aimed at determining the relationship between the pore water composition, the swelling behaviour and the breakthrough pressure. The obtained results showed that an increase of the osmotic suction produces a reduction of the swelling capacity and of the breakthrough pressure. This behaviour is associated with the reduction, or even a total lack, of the capacity of the bentonite to fill up the voids among the sand grains.