The autobahn ring Biel east branch / Autobahnumfahrung Biel Ostast

The Biel autobahn ring closes one of the last gaps in the Swiss autobahn network, and the east branch of this ring requires two twin-bore tunnels in difficult and variable geology. All four tunnel bores are being driven by an earth pressure machine with 12.6 m diameter. Right at the start of the tunnel section, the route passes under a canton road and the main line of the Swiss federal railways SBB. This article describes the contractor's "table bridge" proposal for the section under the canton road and the railway line and the work of the earth pressure machine and the transfer of the TBM between two tunnels is described in more detail.
The term "table bridge" describes an underpinning structure like a table top constructed in advance to enable the construction of the tunnels under the SBB tracks and the canton road without disturbing rail traffic. All loading from rail operation, earth surcharge and dead load is supported by the table construction and transferred into the underlying rock. Both tunnels are being driven using an earth pressure shield despite the changeable geology. The use of various operating modes - Open, Semi-Closed and Closed Mode - has enabled successful boring under difficult geological conditions. In order to be able to convert the TBM between modes quickly and without rebuilding work, the contractor is using a screw conveyor for the removal of excavated material along the entire route. Between the two tunnels, the TBM including backup was moved a distance of 631 m without dismantling work thanks to a newly developed system, which has saved many months of construction time.