SBT – Tunnel excavation in erosion‐prone rock mass with high permeability and water pressure: Mechanisms, risks and countermeasures

Abstract The excavation of the Semmering Base Tunnel (SBT) through fault zones under high overburden presents significant challenges, especially when these zones transition directly into a highly water‐bearing formation with groundwater pressures of up to 10 bar. Excavation under these conditions demands advanced tunnelling and support measures. To permanently reduce groundwater inflow, extensive pre‐excavation grouting was implemented to enhance the rock mass properties within the fault zone and reduce the permeability of the adjacent carbonate rock. Systematic exploratory drilling revealed a complex sequence of geological units at the transition zone from fault zone into the water‐bearing carbonate formation. As a result, grouting umbrellas with borehole lengths of up to 100 m were installed. Large‐scale face instabilities within the fault zone required further adaptations of the tunnelling and grouting measures. Among other measures, geotechnical monitoring was significantly expanded, as the tunnel passes beneath a motorway and an anchored retaining wall located approximately 160 m above the tunnel crown.