The 8,341–m–long Blessberg Tunnel is part of Deutsche Bahn's 190-km–long high-speed railway line between Nuremberg and Erfurt. The southern section of the Tunnel is situated in mainly karstified Muschelkalk rock strata over a length of some 430 m, with the overburden from the tunnel crown to the surface ranging from 6 m to 70 m. In the course of the conceptual design, a comprehensive exploratory programme was carried out for this tunnel section, including 7 core drillings as well as geophysical investigations (geoelectrics, refraction seismics and reflection seismics). During excavation of the top heading, large karst cavities with widths of up to ten metres were encountered. The karst cavities were found to be part of an approx. 500–m–long cave. Further huge karst cavities were encountered with an overburden of about 8 m above the tunnel crown.

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