Drilling and Grouting Works for Pressurised Groundwater Conditions of the Semmering Base Tunnel

The Semmering Base Tunnel (SBT), with a total length of 27.3 km, is one of the leading construction projects of the Baltic-Adriatic Railway Network. The tunnel connects the two federal provinces of Lower Austria and Styria and cuts through the eastern part of the Alps. The construction lot SBT 1.1-Tunnel Gloggnitz is characterized by a complex geological- and hydro-geological architecture containing alternating competent geological structures, major fault zones, and corresponding geological transition zones. There are three main water-bearing formations at the construction lot SBT 1.1: (a) Karst-prone blocky limestone with an initial water pressure of 10 bar. The discontinuities form a highly permeable interconnected joint network with a significant storage coefficient of the groundwater table. (b) Massive to blocky dolomite with an initial water pressure of 25 bar. The systematic discontinuities and disturbed zones of subsidiary structures with karst form a permeable, interconnected joint network. (c) The transition zone of the limestone to base structure marks the most challenging area for consolidation- and sealing grouting. Weak fault rocks characterize transition zones with intense fracturing. However, as subjected to high groundwater pressure of 10 bar, these zones are associated with the potential of flowing ground conditions. Based on the overall project requirements, specific drilling- and grouting methods and materials for pre-excavation grouting have been established and successfully implemented in the construction process. The innovations include a Standpipe-Packer substituting a conventional steel standpipe, a specifically cased drilling system with grouting inserts to prevent erosion within the borehole and allow for defined grouting. In addition, this Grouting-Pipe system replaces standard tube-à-manchettes and controls the flushing while drilling with preventers. Finally, a combined cement-polyurethane grout mix (Hybrid Grout) was implemented to stabilize the grout. The implementation of these measures will be discussed in detail, and their benefits to the construction process will be highlighted.