One of Europe's biggest civil engineering projects is in full swing in the shadow of Berlin's recently refurbished Reichstag building. Just north of Germany's new parliament is a huge open cut, watertight box in which contractors are casting three separate tunnel tubes for a road, a railway line and a subway. The projects have a combined value of £2.3bn.
The 150m long, 110m wide, 20m deep box will house the three tunnels and make it easier for each to cross the River Spree. The 2.4km cut and cover road tunnel, the 3.5km bored railway tunnel and a subway line all converge at the southern edge of the diaphragm wall structure. It allows the complex cluster of structures to be built more easily in the confined space.
Berlin's high water table - 3m below ground - meant that the box needed to be stabilised with extensive ground anchoring in the base slab and the diaphragm walls. The structure has to withstand uplift forces of about 170kN/m2 and lateral ground pressures of about 220kN/m2.
Tunnel alignments are dictated partly by the fact that they have to cross the River Spree to the north. Once over the river, traffic from the tunnels enters the underground section of the vast new Lehrter Bahnhof station, now also well into construction. Railway tunnel gradients of 12% dictate the horizontal tunnel alignment as tracks return to the surface beyond the station.
Getting the tunnels under the Spree is a major engineering challenge. The tunnel box has to thread itself under the river with just 1m clearance between tunnel roof and river bed. A bored crossing for the three routes was ruled out because of the expense and the risk of a flood during construction.
Instead contractors diverted the river to the north, creating a casting pit for tunnel sections. Now the river has been rediverted back to its original position to allow remaining sections to be cast at each end.
North of the box the new £283M multi-level Lehrter Bahnhof rail interchange is beginning to rise out its own 200m wide by 400m long foundation pit. Prefabricated steel support columns for the intermediate floors are sprouting upwards and the insitu cast concrete deck of a new bridge for east-west railway traffic is progressing across the station pit on concrete-filled steel columns.
When complete in 2002, the new steel and glass roofed station will replace the existing one, which carries east-west urban and inter-city trains between Zoologische Garten and east Berlin's main stations at Alexanderplatz and Ostbahnhof. It will provide the city's main junction for rail traffic travelling from north to south and east to west.