Despite the potential excellent performance of TBMs in favourable ground conditions, the presence of fault zones or heavily jointed rocks represent important geological hazards encountered during tunnel excavation. The effects of these challenging environments on the final tunnel construction time and costs can be investigated through a specific computer code: the Decisions Aid for Tunnelling (DAT). In this framework the DAT simulates the tunnel excavation in several geological profiles, where changing ground scenarios are described in terms of different “fault zone” classes (from highly fractured rocks, to faulted and crushed material). For each class a certain reduction of the TBM advance rate is specified based on real data analyses. Although the great uncertainty, the results give a reliable estimation of the effect of degrading rock mass conditions on the tunnelling performance. Finally, a real case-study has been simulated by DAT in order to validate the use of the “fault zone” classes (and the relative advance rate reductions) in the estimation of the final time of tunnel construction. The predicted time values prove to be very close to the ones recorded on the field, confirming the importance of a more detailed and comprehensive characterisation of difficult ground conditions such as fault and highly fractured zones.