The phenomenon of swelling in tunnels excavated through sulphate-bearing rock is quite frequent in some geological formations. The paper describes the case of the Lilla tunnel excavated in anhydritic-gypsiferous of Eocene age. Very large swelling displacements were observed affecting the first few meters below the tunnel inverts. In the paper, a fully coupled hydro-mechanical and chemical analysis is presented that takes into account the dissolution and precipitation of the anhydrite-gypsum chemical system and their associated hydraulic and mechanical effects. The analysis provide a better understanding of the swelling ground phenomena and offer a firmer foundation for taking engineering decisions concerning mitigating measures.