SBT – Refurbishment of a collapse in erosion‐prone rock mass

The Auebach Valley is the morphological expression of one of the main fault systems in the Semmering area. The varying lithological composition of erosion‐prone fault rock and groundwater‐bearing carbonates poses challenges for design and construction. In difficult conditions, overlapping exploratory drillings in front of tunnel drive are essential to improve the geological‐hydrogeological model and to assess geotechnical risks. However, under the prevailing geological‐hydrogeological conditions, the drilling methods reached their limits. Despite the ongoing exploration and concluding risk assessment, a very large‐scale water and ground inflow with subsequent tunnel face collapse and development of a sinkhole on the surface could not be prevented. The article outlines the reasons for the occurrence and the formulation of methods to deal with the issue. I t addresses geological‐geotechnical analyses of the incident and explores the limitations of geotechnical surveillance in complex heterogeneous rock masses. Finally, an evaluation of the costs associated with managing the incident is conducted.