Although a great amount of practical experience has been gained in the last decades, tunnel design in swelling rock is still a very challenging task, as the recent examples of the Engelbergtunnel in southern Germany and the Chienbergtunnel in Switzerland demonstrate. Reliable prediction of swelling pressures and swelling deformations especially in anhydritic rock is extremely difficult due to the heterogeneity of the material and the complexity of the involved transport mechanisms. However, modern design codes and engineering practice demand capacity checks for tunnel linings, which usually can only be provided by numerical analysis with an appropriate constitutive model. Such a constitutive swelling model, which adds swelling strains in dependence on the stress level and accounts for the time dependent evolution of swelling, has been implemented for Plaxis. This article compares the results of a numerical back analysis with this model to in-situ measurements, using swelling parameters derived from laboratory swelling tests.