Both the technical boundary conditions and the usual advance rates achieved during hard rock TBM shield drives severely limit the ability of the on-site-personnel to document the geological conditions and the system behaviour at appropriate intervals. A systematic and continuous short-term investigation of the rock mass conditions is performed on the construction lot KAT2 of the Koralm Tunnel (obtained by impact drillings and geophysical methods). The difference in scales and the fact that no continuous inspection of the ground conditions can be performed lead to a gap between the prognosis and the observed behaviour and a direct comparison is seldom possible. Based on the daily comparison between the observed behaviour and the analysed machine data, a state-of-the-art interpretation method has been developed. This method allows reliable conclusions regarding the primary aspects of the system behaviour and thus enables deductions on the geological/geotechnical conditions which have been encountered. The identification of the relevant parameters and suitable analysis methods has been performed in a "trial and error" manner: machine parameters deemed relevant in theory have been continuously compared with the observations on site and depending on the results, found usable or discarded. Reliable knowledge about the face stability, qualitative degree of fracturing of the rock mass, occurrence of over excavation in the cutter head area, the state of the annular gap and the blockiness of the rock mass can be deduced by applying this method. The results are used on daily basis by the on-site geologist and geotechnical engineer in order to verify the prognosis and recommend additional/auxiliary construction measures.