Completion next year of the 2.4km road tunnel between Lehrter Bahnhof to the north and the Landwehrkanal in the south will return part of Berlin's vast Tiergarten park to its original grass and woodland state for the first time since the early 1960s.
The tunnel will replace a road hurriedly built through the Tiergarten when the Berlin Wall isolated the quickest north-south road link in east Berlin. Like most underground structures in Berlin it has had to contend with the city's high water table, just 3m below the surface.
Contracting group Tiergartentunnel, comprising Strabag, Bauer Spezialtiefbau and Berliner Tief & Vehrhersbau, is currently casting insitu tunnel sections inside the road, rail and subway tunnel casting box just to the north of the Reichstag at a rate of one 10m section per week. These will form the last of the 140 sections which make up the tunnel. Inside the casting pit the 10.5m wide sections are cast from the bottom up in 10m lengths.
The main cut and cover section to the south through the Tiergarten is now complete and awaiting the start of road surfacing work. This cut and cover section was built inside 25m deep diaphragm walls sunk into sandy gravel ground and anchored back. Spoil was excavated to leave a section of trench flooded with ground water.
A 1.5m thick base slab was then cast underwater between the diaphragm walls and anchored to withstand uplift forces of 170kN/m2. With the slab in place, water was pumped out and the roof slab inserted.
As the route of the road tunnel follows the line of the surface road through the Tiergarten, Berlin's city government decided to use this corridor as the site for a cut and cover structure. When complete the surface road will be scrapped and the route will become parkland again.
Environmental constraints have led to an extensive ground water monitoring programme to make sure that work on the tunnel does not upset natural ground water systems. The Tiergarten is one of Berlin's major green spaces and there was concern that tunnelling would damage its trees.
Engineers have also had to be careful not to accidentally lower ground water around the Reichstag. 'The whole Reichstag rests on timber piles so we have to maintain ground water levels so as not to expose them,' says Martin Ritterbach, an engineer with the Berlin housing and transport ministry's technical consultant Schusslerplan. If ground water is lowered, there is a danger that the piles will rot, undermining the stability of the high profile government building.
Ritterbach says the new road tunnel will be much safer than the Mont Blanc tunnel, where 41 people died in a fire in March (NCE 1/8 April). The Tiergarten tunnel is relatively shallow and has four emergency exits to the surface as well as an interchange exit at Lennestrasse. The 23m wide honeycomb tunnel structure also has two bores separated by a central wall with fire doors every 300m to 350m.