SBT ‐ Concurrent grouting during excavation

Abstract The Semmering Base Tunnel (SBT) crosses several geological units that contain significant ground water reserves. These aquifers are under special protection, as stipulated in the Environmental Impact Assessment (EIA) decision. The affected zones are predominantly located within carbonate rock mass blocks, whose properties ‐ particularly regarding permeability and strength ‐ vary considerably between formations. These variations impose stringent requirements on the planning and execution of tunnel excavation. Consequently, customized drilling and grouting concepts were developed for each formation. Particularly challenging for excavation are the intensely fractured and highly permeable carbonate rock mass of the Grassberg, as well as the marginal zones strongly affected by tectonic deformation. During the initial investigative drilling, water inflows exceeding 100 L/s were encountered, accompanied by significant material washout. The paper focuses on the excavation of the Grassberg carbonate unit and describes the technical measures implemented to protect the groundwater. To ensure safe tunnelling, stepwise measures were applied, including top‑down injections and a thicker primary lining with an almost circular profile. In addition, groundwater pressure‑relief drilling, which achieved discharges of up to 215 L/s, reduced the groundwater pressure from approximately 8.7 bar to 0.4 bar, further enhanced the effectiveness of these methods.