According to the geological-hydrogeological forecast, it was assumed that occurrences of cellular dolomite at tunnel level was only to be expected in subsection 2.1.
In subsection 2.2, which includes the highly fissured Hochstegen zone (Hochstegen marble), and in section 3, divided by two shear zones of the Olperer fault system, distinctive groundwater flow systems and thus an increased risk of water leakages in the tunnel resulting in an increased risk for water resources on the surface were expected. Therefore all tunnel excavations in the subsections 2.1 and 2.2 as well as in the Olperer Shear Zone must be excavated under the protection of sealing injections against 85 bar ground water pressure.
During the planning phase, it was assumed that the cellular dolomite could not be excavated with a tunnel boring machine and would have to be treated beforehand. For this purpose, the zone was first crossed by the exploratory tunnel. After that, a secondary attack from the exploration tunnel to the main tunnel level was carried out, consisting of a ramp and a crosscut. Subsequently, from this crosscut, the western tube and later the eastern tube were excavated cyclically through the cellular dolomite. After the successful advancements of the subsections 2.1 and 2.2 of the exploratory tunnel and the successful advancement of the subsection 2.1 of the main tunnels the construction lot H52 ended in December 2023. From January 2024 on the subsequent lot will complete the remaining sections towards the state border.