Abstract Degradation and swelling of clayey rocks bearing calcium sulphate produces considerable damage to civil engineering works. A recent case was the Lilla tunnel pertaining to the AVE Madrid–Barcelona high speed railway line (Spain), were the floor slab heaved up to 80cm in the term of a year and a half. The present work is based on field and laboratory observations from that case. Under no confinement, degradation of the Lilla rock starts immediately upon contact with water. Degradation develops along a time span of several months and includes development of a crack pattern, significant swelling and the extensive growth of gypsum needles within the degraded rock mass. A simple numerical model is developed for this process. The purpose of the model is to reproduce the swelling process observed in the Lilla tunnel from which records of heaves and swelling pressures are available and to check the likeliness of several possible driving mechanisms for the swelling phenomenon. The main conclusion of this analysis is that the dissolution of anhydrite and precipitation of gypsum is the most likely mechanism among the three considered. The determinant factor behind this conclusion is the physical fact that gypsum has a relatively low solubility, which means that the mass of calcium sulphate transported and precipitated as solute is always small.