Shotcrete under Squeezing Rock Conditions during the Construction of the Multifunctional Station Faido of the Gotthard Base Tunnel

The Gotthard Base tunnel (2 x 57 km) is the world's longest tunnel under construction with a total length of 153 km of tunnels, including all the additional structures of the project. All tunnels are supported with shotcrete either from the Back Up of the four TBM's or with spraying mobiles during excavation by drilling and blasting. The focus of the paper is rather the Know How transfer of detailed shotcrete technology and shotcrete experience, than description of the general tunnel project. The paper presents the point of view of the contractor at the Gotthard Base Tunnel, Faido section, the Joint Venture TAT. The following items are discussed in detail: (1) The general behaviour of shotcrete under squeezing rock conditions and consequences and proposals for future projects; (2) Limits of tunnelling support with shotcrete, with steel fibre reinforced shotcrete and methods used under the enormous squeezing rock conditions; (3) Prequalification and specifications for permanent shotcrete quality with alkali-free accelerators; (4) Adaptation of the obtained experience from the prequalification to the real site conditions at the beginning of the project; (5) Quality control and statistics of the site test results; and (6) Detailed analysis of the adhesive properties between the individual shotcrete layers. To investigate and analyse the adhesive properties in detail, various parameters were taken into account: age of the substrate shotcrete, zones under squeezing rock conditions in comparison to zones with stable hard rock conditions, and different methods of cleaning the substrate before spraying the next layer.