The second Mixshield TBM on the Finnetunnel in Germany was being prepared last month for launch as its sister convertible shield made steady headway with a four month lead on the twin tube rail project.
A Wayss & Freytag-led JV is building the tunnels using a pair of 10.82m diameter Herrenknecht TBMs, and the second machine (S-420) is set to be launched shortly ahead of schedule. Originally, the programme anticipated the launch around December.
The first TBM (S-419) was launched ahead of schedule, in late April – ahead of the planned July start (T&TI, May, p7). By the end of last month the shield had excavated more than 600m, erecting 9.6m i.d. rings (6+1) that are 2m long.
Early geology, driving from the west portal, comprises weathered sandstone with clay/silt in the Finne fault zone, which in addition to high inflows prevents groundwater lowering by closed system dewatering.
The first TBM has advanced in slurry mode, which will be used for the initial approximately 1,500m long stretch of the tube.
However, slurry mode operation would be uneconomical to continue with due to high groundwater head (up to 6 Bar) in new red sandstone. The JV is, therefore, using the sandstone conditions to drawdown the groundwater using 80m deep boreholes. Along these approximately 3,100m long sections the TBMs will be operated in open mode.
When the first TBM switches mode the slurry package is to be installed into the following shield.
The remainder of the Finnetunnel tubes are to be excavated by NATM mostly through dewatered red sandstone. Beyond this 1,350m long stretch, the NATM?method will also be employed in a 850m long dry section. The tubes will be supported by a primary shotcrete lining and secondary (insitu) linings.
The JV – which includes Max Bogl and two units of Porr – was awarded the US$328M contract to build Finnetunnel by DB Netz at the end of 2006.
Finnetunnel is part of the 10.5km long Erfurt to Leipzig/Halle rail line. The 6.8km long twin tubes will have cross passages with airlocks built every 1000m, and the project is to be finished in 2011.
Preparations for the launch to the 2nd Herrenknecht TBM to the Finnetunnel twin tubes, Germany
