SBT – Shaft excavation in erodible rock mass

During construction of the Semmering base tunnel, two shafts with a depth of approx. 250 m were excavated from the intermediate access Göstritz in lot SBT 1.1. During shaft sinking in poorly consolidated brecchiae, regressive erosion occurred along existing fractures behind the shotcrete lining, causing water and material ingress into the shaft. An extensive grouting campaign was executed, finally allowing for a significant reduction of water ingress and erosion phenomena. Subsequently, the modification of the groundwater management turned out as a key factor: The original external water collection system with ring drains outside the shotcrete lining was abandoned in favor of systematically draining the groundwater into the shaft. This modification significantly minimized new water pathways behind the shotcrete lining, thereby reducing potential erosion channels. For similar projects, it is recommended to catch groundwater in non‐erodible layers, avoid external drainage systems, grout spots with murky water ingress immediately, and minimize drilling through the lining.