U-Bahn-Tunnel Baulos C 23

The tunnel section C-North completes Hanover's urban railway network through linking up the suburbs located in the north with the existing urban railway lines. For the 474 m long construction lot C23, at the tendering stage, the tunnel was to be completed through a shield drive under compressed air with cast segmental lining with the remaining part to be achieved through cut-and-cover. A special proposal for completing the tunnel by the shotcreting method, partially under compressed air and ground freezing was undertaken. In this connection, the inner shell was created in two stages using watertight concrete. The major portion of the 311 and 317 m long running tunnels were driven from the open excavation. The drive without compressed air was accomplished from the construction trench for the emergency exit, with the upper track being created under the protection of ground freezing - and an approx. 165 m long counter-drive in the direction of the starting shaft. Below a filling layer of up to 4 m thick, sands and gravels are to be found in a 12 to 14 m thick deposit. A part-section between loose soils and stable bedrock is disturbed locally to varying degrees through rubble stone comprising clay and silt with gravel/sand intrusions. In this zone, tunnelling operations were carried out using compressed air and ground freezing. The groundwater was located approx. 4 m beneath the land surface and could not be lowered. The excavation walls were created from overcut bored piles in the cut-and- cover section. The following sections were completed by shotcreting under atmospheric conditions: the lower tube (track 9) from the emergency exit in the direction of Lange Laube, the upper tube (track 10) in the direction of Lange Laube with the roof protected by an iced zone, as sands and gravels are present at roof height here. A 2000 collector was also undercut by this drive. The lower tube (track 9) from emergency exit in the direction of the starting shaft as counter-drive with a total length of 162 m. The remaining tunnel sections were completed from the starting shaft under compressed air. The surrounding groundwater called for air overpressure of 1.2 bar. In the starting zone, an iced layer some 1.20 m thick was created here per tube, as the existing sand and gravel layer first moved upwards as the drive progressed. Dry-spraying was applied for all drives. In addition, to the measurements normally carried out for the shotcreting methods (convergences, aligning the drive, surface levelling) tunnelling influences on the soil were observed by means of extensometers in 5 measurement cross-sections. These cross-sections are concentrated in the vicinity of the high-riser belonging to the Continental AG. The settlements from both tubes together were not permitted to exceed 2.5 cm in the roof at the upper surface. A brine unit with an output of 650 kW was used to produce the ice. The cooled brine was pumped into the icing pipes at approx. 40 °C. The frozen zone for a single iced section was created by means of 10 or 11 individual pipes with a standard length of 42 m. 37 m of this total was used for driving purposes. At the end of the tunnelled section, an enlargement in the form of a cavern was created. It provided room for the jumbo required to install the freezing pipes for the next section and for distributing the brine. As both tubes are located very close to each other, the caverns have been staggered.

Country: Germany
Region: Niedersachsen
Tunnel utilization: Traffic
Type of utilization: Underground, urban and rapid transit
Client: Landeshauptstadt Hannover
Main construction method: Trenchless
Type of excavation: Drill-and-blast
Lining: Shotcrete (partial compressed air, ground freezing)
No. of tubes: 2
Tunnel total length: 475 m
Cross-section: 2 x 37.4 m²
Contract Volume: 49.2 Mio. DM
Construction start/end: 1988-1991
Opening: 1991