The new motorway link making up the Nordtangente, the north section ring road, is only 3.18km long but may be one of the country’s most expensive stretches of road. Work began in the early 1990s with an overbridge and tunnelling began in earnest in the mid-1990s. The project will continue until 2006.
Most of the road – 2.8km – is in cut-and-cover tunnel, although there is also a double-deck crossing of the Rhine and a short surface section near the border. Complex works include routing under the main station and below old and occupied multi-storey apartments.
Several 200m-long sections of the cut-and-cover tunnel are under way in the St Johann area of the city, and others are in design, according to project director Hans-Joerg Schlegel.
The object of the SFr1.3bn ($0.73bn) link is to relieve the city streets of 40% of the traffic. Much of this enters the country from the nearby French border, where the French A35 was completed some years ago. On the other side of the Rhine river a motorway link from Germany enters the city on the east side and passes on a ring-road link. The city has a Rhine port for barges and carries a large part of the Swiss export business into France and Germany.
The project comprises four big sections, each with its own tunnelling problems.
In the first, Horburg, sector a 250m-long stretch of the tunnel had to be built below apartments. A system of adjacent 2.8m-wide mini-tunnels was bored underneath the buildings, each formed from an umbrella of mini-piles, then concreted as hollow polygonal box tunnels. These were linked with lateral beams, supported on 30m-deep piles at the side of the apartments. Once this platform was in place, including a central row of piles, the excavation for the double motorway box beneath was possible, using a top-down method of construction. Two 2-lane tunnels were formed with a dividing wall. Maximum settlements were 10mm.
Top-down below station
A section on the other side of the river used a different sequence of top-down construction beneath the main St Johann station. A steel deck underneath the rail lines was supported at the sides. In the position of the central wall a row of steel columns was embedded in piles (see diagram) to support the steel plates above. The excavation and concreting proceeded below. Cover for the tunnel here, and along much of the route, is about 4.5m.
Construction of other lengths, many running along street lines, is typically within steel-driven piles and lagging (the so-called Berlin wall), or within secant piles anchored back at two levels. Most of the work has been in 200m tranches.
A concreted box for the motorway is formed within the initial excavation, with walls between 400mm and 800mm thick.
For waterproofing, the ‘white box’ method of concreting has been used, which involves induced crack injection sealing. Swiss-based firm Rascor carried out work on the most recent section in St Johann, using a patented crack inducer at 10m intervals. The plastic inducer, which fits at the end of a pour before the next is made, is moulded to form a guide path for subsequent acrylic injection.
Rascor explains that no liner is needed with the system, and leaks are more easily detected.
Related Files
New motorway tunnels
Twin motorway tunnels
