The use of open tunnel boring machines in squeezing rock in the Gotthard Base Tunnel / Einsatz von offenen Tunnelbohrmaschinen bei druckhaftem Gebirge im Gotthard Basistunnel

In the Faido section of the Gotthard Base Tunnel (GBT), two open gripper tunnel boring machines (TBM) with a bored diameter of 9.4 m are each driving about 11 km of single-track tunnel. No extensive fault zones were expected in the entire section except for the Piora-Mulde. A special tunnel was driven to investigate the Piora-Mulde, from which investigation boreholes were drilled down to tunnel level. Because of the finding of sugar-grain dolomite, the tunnelling work was tendered by drilling and blasting as well as mechanically, but the tenders led to the award being based on TBM tunnelling.
In the course of construction, a stretch some hundreds of metres long unexpectedly had to be bored through squeezing rock, where the temporary support was destroyed and the West TBM and backup almost became jammed. This article describes the experience from the mechanical tunnelling of the Faido section and then gives details about the intended support, the design of the TBM, the difficulties that arose and the experiences gained.