Prediction of fault zones based on geological and geotechnical observations during tunnel construction

During the construction of the Semmering Base Tunnel, Lot SBT1.1, the drives have already encountered several fault zones in the Greywacke Zone. Because of the high overburden, the exact position of these fault zones is unknown at tunnel level; a common problem for all tunnelling projects in mountainous regions. Simple exploration drilling techniques such as percussion drillings, where only cuttings and not cores are won, do not always provide enough information to precisely specify the position of the fault zones or their nature ahead of the face. This is reason enough to examine other possibilities for the short-term prediction of fault zones with differing characteristics ahead of the face. Usually displacement data evaluation provides the basis for a short-term prediction of the system behaviour. However, experiences from Lot SBT1.1 show that applying this approach solely does not always yield satisfying results. A further systematic analysis of selected geological data can improve the short-term prediction. In particular, changes of discontinuity and rock mass characteristics mapped at the tunnel face are analysed to spot significant trends indicating fault zones ahead of the face. These trends are then related to and verified by the results of displacement data evaluation. This combination of rock mass characteristics mapped at the face and state-of-the-art evaluation of displacement data has helped to improve the reliability of short-term predictions during the tunnel excavation.