Precipitation of gypsum crystals explains the severe swelling occurred in some tunnels crossing sulphate formations. The presence of groundwater and discontinuities, and the proportions of anhydrite and gypsum within the claystone rock mass play an important role for the development of swelling in tunnels. Evaporation of sulphated solutions and dissolution of anhydrite are two different mechanisms leading to supersaturation in gypsum, and therefore to gypsum crystal growth. The second mechanism has a major effect on the development of swelling in the rock. The two mechanisms should be taken into account for the modelling of expansions due to gypsum crystal growth in tunnels. The paper describes the case of Lila tunnel which required a substantial reconstruction after experiencing severe heave problems. Long term records of swelling pressures and stresses in the lining are also given.