The Semmering Base Tunnel (SBT), one of the major infrastructure projects in Europe is part of the Baltic-Adriatic railway corridor and connects the two federal states of Lower Austria and Styria. Due to its total length of 27.3 km and complex rock mass conditions, the project is divided into 3 construction lots. The eastern construction lot SBT 1.1 Gloggnitz Tunnel crosses major fault zones adjacent to water-bearing carbonatic rocks (Grassberg), with a potentially high water inflow in combination with high water pressure. Crossing the fault system north of the Grassberg is one of the biggest challenges of lot SBT 1.1, due to the poor rockmass quality within the fault system and 10 bar water pressure within the Grassberg. Systematic probe drillings indicated a complex sequencing of geological units at the transition from the fault zone into the water-bearing Grassberg unit and required a modification of the original design approach.
The easternmost part of the Semmering Base Tunnel (SBT), lot SBT 1.1 Gloggnitz Tunnel, comprises the construction of two single-track tunnel tubes being undertaken conventionally with excavator and drill & blast (Gobiet, G.; Wagner, O.K. 2013). Due to the complex rock mass conditions at SBT 1.1, the tunneling work is carried out from the portal at Gloggnitz and the intermediate access Göstritz, with a 1.2 km long access tunnel and two 250 m deep shafts (Hauer, H. et.al. 2022). The total length of the construction lot is approx. 7.4 km (Figure 1).
At chainage 3900 m, starting from the portal Gloggnitz, an approx. 50 m long fault system was predicted, marking the northern border of the Grassberg, an approx. 800 m long carbonatic rock mass, with a groundwater pressure of 10 bar.
The carbonatic rocks of the Grassberg are part of a permomesozoic stratification, which is formed by silicious rocks (schists, phyllites, quartzites) and carbonatic members (limestone, dolomite, rauhwacke, breccia). The whole sequence has undergone a multi-phase tectonic imbrication followed by large displacements along major strike-slip-faults, which caused the embedding of other stratigraphic units (e.g. crystalline rocks of the Semmering) within the formation. Such strike-slip-faults are adjacent to the Grassberg in the north and the south. Therefore, the border areas of the Grassberg are considered a geotechnically challenging zone, where water-bearing, jointed carbonatic rocks occur in the close vicinity of cataclastic mica schists with very low permeability.