The paper presents comparison of 2D and 3D analyses of an exploratory adit of a NATM tunnel in stiff clays using two constitutive models (hypoplastic model for clays and Mohr-Coulomb model). The stress-release factor β of 2D simulation has been optimised to ensure that the 2D and 3D analyses give the same displecement field. The value of the β factor is, however, not unique to the given adit geometry, among other it dependends on the chosen constitutive model. The constitutive model parameters were subsequently optimised using monitoring data from the adit. Parameters of the hypoplastic model did not change substantially during the optimisation process. This model can therefore be used for predictions based on parameters calibrated solely on laboratory experimental data. The Mohr-Coulomb model gives correct predictions only for unrealistic model parameters and low K0 values.