page_title: Leipzig City Tunnel
section_title: Construction Technique
hierarchical_section_title: Leipzig City Tunnel / Construction Technique
caption_attribution_description: Deutsch: City-Tunnel Leipzig, Haltepunkt Markt, Blick von oben English: city tunnel Leipzig, station "Markt", top view
context_page_description: The City Tunnel is a twin-bore railway tunnel for the city-centre S-Bahn in Leipzig. It links Leipzig Hauptbahnhof with the central Markt station, Wilhelm-Leuschner-Platz station and Bayerischer Bahnhof.
  Construction began in July 2003. The first bore was structurally completed in March 2008, the second in October 2008. The tunnel and new tracks linking it with the rest of the network were opened for commercial service on 15 December 2013, the date of the timetable change in December 2013.
context_section_description: The two 1438-m-long tunnels were largely dug using a 65-m Tunnel Boring Machines. The start of the tunnel boring machine work was in the pit at Bayerischen Bahnhof, from where they worked towards the Central Station. There it was dismantled and re-built at Bayerischen Bahnhof, after which it started a second time. At Bayerischen Bahnhof and at all other stops between, three slurry walls were constructed. The slurry was mixed with cement and hardened in the slot. This mass then formed a waterproof wall through which the tunnel boring machines could run without the surrounding water and rock penetrating.
  The tunnel has an excavated diameter of nine metres and was driven by a fluid-based plate. This generates a positive pressure that prevented the ingress of water and rock into the tunnel. Using this approach, however, there is a danger that the pressure is lost due to old water wells with the collapse of the ground in front of the tunnel boring machine. The coverage varies from eight to sixteen metres. The shortest distance between the building foundation and tunnel roof is under the Petersbogen underground carpark at 2.20 metres. The tunnel was in the ground water at a pressure altitude of approximately 16 m to the bottom of the tunnel. The lining of the tunnel was carried out with seven re-enforced concrete segments, which form a tunnel ring, which are 40 cm thick and a keystone. For the two tunnels approximately 13,000 segments were required.
  When tunnelling in soft ground it is expected that the tunnel will sag due to soil loss (sagging of the soil after tunnelling or mining operations) which cannot be avoided completely. The lower tray is about 60 to 80 m wide. The depth depends (among other things) on the type of coverage, the shear strength of the soil and the overlay. A maximal deviation of 20 to 30 mm is anticipated after the second tunnel bore. All buildings that are in the tunnel route and affected, have been examined in advance. (Drilling, geotechnical investigation, Bauakteneinsicht)
  The crossing under the main railway station is not done by the tunnel boring machine, but by manual digging. There were extensive measures to secure the main railway station against the builting work. The plan was to freeze the pit edge and drive through this frozen ground under the station. The plans include two tunnel excavations with a diameter of 2.40 m using compressed air and a cover plate. From this side vertical tubes were inserted to freeze the ground were built and the ground was frozen into a waterproof layer.
caption: conscripts and braced excavation of the Markt station
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