Engineering geological and geotechnical experiences obtained from a major NATM-tunnel (BBT Brenner Base Tunnel, exploratory tunnels) and a TBM-tunnel (Stanzertal hydropower plant, headrace tunnel) in phyllitic rocks in Tyrol (Austria) are described herein, focusing on driving method-related challenges and technical solutions. Both tunnels are situated in similar rock masses consisting of thinly laminated quartz-phyllites with brittle fracture and fault zones. The reaction of the rock mass on the tunnel headings have been comparable in the sense of the prevailing failure mechanisms, but had different impacts on the tunnel headings depending on the tunneling method (conventional or mechanized heading). As a centralized résumé, it can be stated out, that the conventional tunnel drives offered great advantages at fault zone crossings, whereas the TBM-headings performed very well within good rock mass conditions attaining advance rates of about 50 m per 24h.
In the Tyrolean Alps (Austria), phyllitic rocks are widely spread and several tunnels for infrastructure and hydropower plants were performed in these kind of rocks, especially in the last 50–60 years. Most of them were built by conventional excavation methods (drill & blast resp. NATM), mechanized driven tunnels have been exceptional cases. However, especially for elongated tunnels the discussion on the most suitable tunneling method - in technical and economic reasons - always arises during planning phase weighing the pros and cons of each method. Decision-making is mostly accompanied with the assumption that the chosen method is indeed fitting for most parts of the geological and geotechnical setting along the tunnel alignment but not for all sections. By describing the experiences from conventionally driven tunnels and a TBM-heading (obtained from geological documentation and geotechnical supervision on site), this paper highlights some advantages and disadvantages of the different underground construction methods. Both above mentioned tunnels (BBT, Stanzertal HPP) have been planned applying the guidelines of Austrian Society for Geomechanics (ÖGG 2008, ÖGG 2013).
