The intensively dissected and highly permeable carbonate rocks of the Grassberg, as well as the tectonically strongly overshaped edge areas of the Grassberg, represent geotechnically particularly challenging zones for tunnelling work. The first exploratory boreholes already revealed water ingress with flow rates of over 100 l/s in combination with significant material discharge. The location of the key zone directly below the S6 expressway required a modification of the original planning approach. Due to the erosion‐prone and unstable rock in the peripheral zone, two large‐scale injection screens with 97 and 75 injection boreholes per shield with lengths of 50–70 m were required for each railway tube. The subsequent excavation was carried out in sections in 7 m long stages, each with 59 pieces and 24 m long cased injection wells. To reduce settlement, an additional 32 pieces of pipe umbrella pipes with a length of 15 m were executed per stage. In the upcoming carbonate, the tunnelling was carried out with injection screens according to the top‐down system. For each 10 m long tunnelling stage, uncased boreholes with a length of between 5 and 20 m were drilled. Depending on the conditions encountered, the injections were carried out using Portland cement, hybrid mortar, ultra‐fine binder, silicate gel and polyurethane.
Bauer et al. (2025) studied this question.