U-Bahn Berlin, Lot D 79- Undercutting the Karl-Bonhoeffer-Hospital

The U-Bahn construction Lot D 79 embraces an underground section divided into two separate, single track tunnel tubes with the Stations Karl-Bonhoeffer-Hospital and Rathaus Reinickedorf. Between these two stations, the Underground passes beneath the grounds of the Karl-Bonhoeffer-Hospital, the Reinickes Hof residential area with high-risers of up to 12 storeys and the Nordgraben. An emergency exit was planned for roughly the middle of the 1,080 m long running tunnel. The tunnel tubes have an internal diameter of 5.70 m. The drive mainly passes through glacial marl as well as clay, fine sands, medium sands and gravels. Stone Intrusions must be reckoned with. The groundwater Level is located 2.5 m beneath the ground surface up to 4.5 and 7.0 m above the tunnel roofs. In order to carry out undercutting as gently and settlement-free as possible, shield tunnelling with a hydro-shield was selected for this construction section. The rock guard in front of the feed pump as well as lock at the foot of the pressure wall facilitated the salvaging of any larger boulders that were encountered. In order to remove obstacles, the excavation chamber is accessible through a lock. A monocoque support with watertight reinforced concrete segments was foreseen for the tunnel. The segmental rings are 1.10 m wide and comprise 7 segments and a keystone. Neoprene strips set in continuous grooves seal the joints. Atmospheric pressure was applied during the lining of the tunnel. This signified a considerable easement for the tunnellers working underground and the cessation of impositions on health caused by compressed air. The emergency exit from the tunnel between the two stations had to be submersed as a caisson before the shield drive to avoid any lowering of the groundwater at this point. The space, which was to accommodate the subsequent tunnel, was initially filled with lean concrete, which the shield then drove through. In the zone with the emergency exit openings, steel segments were installed instead of the reinforced concrete cassette segments used elsewhere in the tunnel, which were lined after completion of the two shield drives in order to create the opening for the stairway leading to the emergency exit.

• Country: Germany
• Region: Berlin
• Tunnel utilization: Traffic
• Type of utilization: Underground, urban and rapid transit
• Client: Land Berlin, Der Senator für Bau- und Wohnungswesen, Berlin, Abt. Vll
• Main construction method: Trenchless
• Type of excavation: Shield machine (SM)
• Lining: Reinforced concrete segments
• No. of tubes: 1
• Tunnel total length: 1328 m