Construction of the second bore of the Pfaender tunnel / Bau der zweiten Röhre des Pfändertunnels

The Pfaendertunnel bypasses the residential area of the city of Bregenz at the northern part of the A14 Rheintal motorway. The alignment of the 2nd bore of the Pfaendertunnel runs for over 6.5 km under the Pfaender mountain-massif, 70 m west of the existing bore. The route runs through geology described as the "Standing midland molasse", a sequence of beds from the Neogene period (young Tertiary), which today mostly consisting of conglomerate, sandstones, marl-sandstones, marl and clay-marl. The second bore will be excavated continuously by a shielded tunnel boring machine, opening from the north starter-tunnel to the existing bore at the south. The standard cross-section of the tunnel will be circular (bored diameter 11.92 m) with external diameter 11.62 m segmental concrete lining. The paper deals with the main issues related to the design of the alternative TBM tunnelling-tender taking into account the swelling tendency of the rock mass and also deals with the construction-issues of the first road tunnel in Austria being built by the use of a TBM. During initial TBM-drive in the starter-tunnel the machine sagged into loose ground.. The problem of driving a TBM into divided cross sections (bench and invert sections without top-heading) into soft ground and the solution adopted on site is discussed.