Geotechnical characteristics of soft rocks of the Inneralpine Molasse – Brenner Base Tunnel access route, Unterangerberg, Tyrol, Austria

The engineering geological investigation for the environmental impact assessment of the more than 20 km long route section Schaftenau–Radfeld in the Lower Inn Valley (Tyrol) was conducted in 2017/18. As a central structure, a nearly 3 km long tunnel will be built through the Unterangerberg. The bedrock of the Unterangerberg consists of turbiditic, marine deposits of the Inneralpine Molasse with interbedded layers of marl, claystone and sandstone. The proximity to the Inntal shearzone and exogeneous effects like several glaciations have softened the surface of the inherently weak bedrock and led to heterogeneous mechanical properties. The monotonous lithology is therefore accompanied by an inhomogeneous rock mass characterization. This paper describes experiences of the geotechnical investigation of this weak rock. It was observed that only a quick transport of samples from the drilling site to the laboratory can guarantee for test results in accordance with the standards. The results, lying in the transitional zone between soft rock and hard soil, are in good accordance with data from other studies.