The paper summarizes the main phenomenological aspects of swelling in tunnels excavated in sulphate claystones and formulates a series of theoretical consideration for their coupled hydro-mechanical-chemical analysis. The effects of tunnelling on rock damage, groundwater drainage and ventilation are discussed and the mechanism of long-term chemical swelling in tunnels due to gypsum crystal growth is described. A coupled hydro-mechanical-chemical analysis of the chemical swelling is presented that takes into account the dissolution of anhydrite and precipitation of gypsum under environmental conditions often occurring in tunnels, as well as their associated hydraulic and mechanical effects. Analyses presented in the paper are based on long-term observations in an instrumented tunnel excavated through sulphate claystones from the Tertiary Lower Ebro Basin (Cataluña, España).