Switzerland

Biel east bypass link: TBM tunnelling and the Längholz Tunnel

The Autobahn A5 Biel bypass in the Canton of Bern closes one of the last remaining gaps in Switzerland‘s national road system, and is subdivided into a west and an east link, both around 5 km in length; the east link includes the 1.2 km long Büttenberg Tunnel, in the north-east, and the Längholz Tunnel (2.33 km) to the south-west, each of them twin-bore, 11 m internal diameter tunnels featuring segmental ring support (30 cm; 6+1), a concrete interior lining and a total of eleven transverse galleries. The four bores are being driven in sequence along two axes, using a single tunnel-boring machine (S-452; manufacturer: Herrenknecht AG), with a 12.56 m cutter head diameter.

After completion of driving of the Büttenberg Tunnel in early 2010 and subsequent surface relocation of the entire TBM and its back-up train (dismantling and reassembly would have been not only more time-consuming but also more expensive) across a distance of 630 m near Orpund, involving the negotiation of a 500 m radius bend, an upward gradient of 2.5 % and a downhill section of 3.7 %, excavation work on the first axis of the challenging Längholz Tunnel has now also been completed, with the breakthrough occurring on October 4, 2010, after tunnelling in EPB mode with the use of foam (up to 38 m/td) through more than 1000 m of non-cohesive rock in the groundwater, and the negotiation of multiple intersections with large supply conduits carrying (for example) gas and water, under an overburden varying between less than 6 m and a maximum of 40 m.

The TBM will then be partially dismantled and returned to Bözingenfeld to be reassembled for heading of the second tunnel axis. Work here is to begin in the spring of 2011, with the concreting work for the invert arch floor base and inner lining starting simultaneously in the two bores already completed.

Responsible for performance of these works is the ATUBO Arbeitsgemeinschaft Tunnel Umfahrung Biel Ostast consortium, consisting of Walo Bertschinger AG, Bern/Zurich, PORR Schweiz AG, Altdorf, and Specogna Bau AG, Kloten. ⇥G.B.

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