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.

In contract SBT1.1 of the Semmering Base Tunnel, the single‐track main tunnels are being driven from the portal in Gloggnitz and from a temporary intermediate starting point in Göstritz. The temporary intermediate starting point in Göstritz is being constructed in order to reduce the risk to the tunnel construction time for the Graßberg‐Schlagl fault system. During the construction phase, the contractor developed optimisations of the planned logistics concept for the intermediate access point in Göstritz, which led to a redesign of the shaft head cavern system. Shortly before the excavation of the shaft head cavern, an unexpected and geotechnically relevant fault zone was encountered in the access tunnel. In order to investigate the geological situation in the area of the shaft head, extensive additional investigation measures were carried out. Due to the updated geological forecast, the layout of the shaft head cavern and the support concept had to be changed. Thanks to the measures taken, the shaft head cavern system cold be successfully excavate without any damage to the support.

Following a rockslide in a former quarry near the town of Dürn-stein/Wachau, extensive remedial measures were necessary to restore the damaged rail infrastructure and ensure long term safety. To provide for safe working conditions during the phase of construction of additional protection measures, a geotechnical risk management system based on the observational method per EC 7 was developed and implemented. Geotechnical monitoring was a key safety management element. The paper discusses the specific challenges, practical approaches and general experiences associated with the use of the observational method for the monitoring of cliff faces.

The article describes the fundamental geotechnical model assumptions in the design phase of the Semmering Base Tunnel. Two selected case studies show the verification of the geotechnical model during tunnel construction. The verification process is essentially based on geotechnical monitoring in combination with specific back analyses. It is shown that particularly in complex geotechnical conditions, such as deep tunnels in weak rock mass, the designer can only assess a range of expected behaviour. Most information about system behaviour and thus about the geotechnical model conceptions can only be gained during construction. An improved understanding of the geotechnical model provides the potential to identify and minimize geotechnical risks earlier and to adapt excavation and support measures to the actual conditions.

The Semmering Base Tunnel with a total length of 27.3 km is being excavated from the Gloggnitz portal as well as from three intermediate faces at Göstritz, Fröschnitzgraben and Grautschenhof. The geological conditions of the project are characterized by tectonically extremely complex rock formations with difficult geotechnical conditions. The excavation passes through numerous fault zones with deep overburden. During the excavation works, some large‐volume collapses occurred. In this article, the collapse events are reconstructed from the available geological and geotechnical information. The reconstruction of the observed system behaviour is based on analytical and numerical calculations. The aim is to identify the respective failure mechanisms.