A major flood at the southern end of the 3.5km rail tunnel in 1997 forced the contractor to scrap its original construction method. The contracting consortium of Philipp Holzmann, Hochtief, Bilfinger & Berger and Dickerhoff & Widmann is now nearing completion of the last of four of the main tunnel sections after relocating machines to the north of the project.
When the £1.5bn four-bore tunnel is complete it will carry high speed and regional trains 574m from Gleisdreieck, north to a new station built in open cut under Potsdamer Platz. On the other side of the station four more drives each 705m long will take track to the insitu cast tunnel sections built inside the casting box.
Initially it was planned to start the four southern drives using two Herrenknecht tunnel boring machines. The idea was to use the Potsdamer Platz station box as a turning area for the two earth pressure balance TBMs so they could complete the two parallel drives. The TBMs would then have been moved to the longer 705m drives to the north.
But the access shaft at Gleisdreieck flooded in 1997 just as tunnelling began. The shaft flooded within hours and the TBMs were rapidly redeployed to the Reichstag site to begin work on the northern tunnels instead. These then tunnelled south and were turned around at Potzdamer Platz before heading north again. Three of the four bores are now complete and construction of the fourth is in its last 100m.
Work to dewater the shaft at Gleisdreieck has just started. It was left in limbo while insurers for DB and the contractor wrangled over who was responsible.
However, when the TBMs are moved to this shaft to start again on the southern tunnels, they will no longer be able to use the new Potsdamer Platz as a turning area because work on the new station is now too far advanced. Instead DB expects the contractors to drag the TBMs back through the first two completed bores before starting on the remaining two.