With more than 85% of tunnel boring completed on the north-south line for the Cologne metro, the contractor, a Bilfinger Berger JV, is on the home strait with the twin drives using two Herrenknecht Mixshield TBMs. By late May, the 8.39m Ø Mixshield on the 2690m east tunnel had constructed 1664 rings of 1800. In the parallel tube on the South Lot, the sister TBM had erected 1400 rings.

The TBMs set off in June and July last year on the northbound drives from Bonner Wall towards the central rail station through sand, gravel and silt. Cover along the alignment varies between 5m -19m. The Mixshields have 1,100kW drives and are boring below groundwater not far from the Rhine, erecting 400mm thick precast segments (7+1) for the 1.5m long rings of 7.3m i.d.

The large diameters were chosen for the single track rail tunnels to help with later break-out and enlargement, at certain sections, to excavate some underground stations.

A total of six new underground stations are to be built on the new rail link, and due to urban congestion four are to be mined by break-out from the running bores. The contractor plans to use ground freezing and/or compressed air to open up the station excavations. Extensive ground treatment is also being used along the alignment.

A third Herrenknecht TBM, of 6.8m diameter, was working earlier on the project to drive a pair of 260m long tunnels. The runs were completed from July-September last year and then December 2006 to February.

The construction programme for the South Lot is between 2004-2010 and is budgeted at about US$520M. The JV contractor consists of Bilfinger Berger, Wayss & Freytag Engineering and Ed Zublin.

One of the two 8.39m Ø Herrenknecht Mixshields on the project Cologne Mixshield Underground works are well advanced on Cologne’s new metro line Cologne metro works