Chienberg Tunnel: redeveloped 6 Years after Opening

The 2.3 km long, twin-lane Chienberg Tunnel, which bypasses Sissach, opened shortly before Xmas 2006 after a 6-year construction period. It is used by some 15,000 vehicles per day. The construction costs almost doubled at the time from 180 to 330 million CHF (275 million euros) owing to

a 4,000 m³ cave-in 2 years ­after starting up resulting in an 8-month delay (added costs of 25 million CHF/21 million euros) and

through rapid heaving of the tunnel floor occurring unexpectedly in 2004 of up to 75 cm over a 370 m long section on account of gypsum keuper swelling as a result of ingressing water during tunnelling. The rigid base invert was supplemented by an underlying deformable zone as a counter-measure. This was produced in the form of an evacuation route and cable duct (added costs 100 million CHF/83 million euros). The tunnel possesses an up to 1.10 m thick inner shell with ring closure roughly every 450 m.

So far the measures designed to combat the heaves have proved themselves, however, redevelopment activities have become necessary on the yielding support for the base has risen by up to 76 cm in the course of the last 6 years (removed via suction excavator) on account of a lack of rock anchors in the invert and the tunnel floor (in this case the carriageway surface) becoming slightly curved at one side owing to lateral swelling pressure. A new slip system was devised with the tunnel standing on 2 concrete beams, which rest on novel deformable zones. In order to counter the swelling pressure from below, 700 rock anchors have been drilled 20 m deep into the soil and pretensioned accordingly through the renovated deformation zone. Work was due to finish in late 2012. In the process more than 600 m³ of material had to be removed. It is estimated that intervention of this type will first have to be resorted to again in roughly 6 years.⇥G.B.


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