A 10.6m diameter tunnel boring machine (TBM) in south west Germany is being encased in concrete to halt damage to an above ground rail line, caused by a collapse on an under construction tunnel.
The tunnel collapsed on 12 August when the TBM was passing 5m below the Rhine Valley rail line in Rastatt. Project promoter Deutsche Bahn was overseeing work on the new 4.3km long tunnel as part of a major upgrade of its 182km long line. To stabilise the ground beneath the rail line ahead of tunnelling, the soil had been frozen with brine to a temperature of -35ºC.
Deutsche Bahn, which also runs the existing track, said water and soil penetrated part of the new Rastatt tunnel during a driving operation, causing the ground to subside by around 500mm and warping a section of the existing track above. From official drawings and images of the incident around 40m appears to have been damaged. Deutsche Bahn said its immediate focus was on repairing the line, allowing it to reopen to passenger and freight trains.
To do this, it said “extensive” engineering works were taking place to backfill a 160m long section of the tunnel with concrete, it said.
Site workers are installing a 2,000m3 concrete “plug” behind the halted TBM to isolate around 4km of new undamaged tunnel. Around 10,500m3 of concrete – equating to around 1,300 concrete mixer trucks – will fill the collapsed section of tunnel that holds the TBM. Seven holes drilled from ground level through to the tunnel will enable the team to pump the vast quantity of concrete that will fill the machine up to the shield area at its head.
In addition Deutsche Bahn said a 150m length of overhead line would have to be dismantled, involving track, ties and ballast removal, before the repairs could be made. These include the installation of a 120m long, 1m deep concrete slab, which will stabilise the ground, provide structural support for the new cable supports and also serve as the foundation for the new tracks.
Concreting and above ground works are expected to take around three weeks of round-the-clock work, after which the tracks can be rebuilt.
“Deutsche Bahn had envisaged a much shorter timeline, but securing the tunnel and ensuring safe operations on the Rhine Valley railway clearly comes first,” said Deutsche Bahn Netze [a Deutsche Bahn subsidiary] member of the management board responsible for large-scale projects Dirk Rompf. “We are confident that work will be completed on schedule and want passenger and freight service on this heavily travelled section to be restored.”
Tunnel Rastatt consortium spokesperson Klaus Pöllath added: “Considering the complexity of the situation and the how urgent it is to restore service, we are relieved that we have developed this fast but also sound plan together with Deutsche Bahn.”
In total, repairs to the tunnel and line are expected to take until 7 October. However, there is no information available at this stage as to when works to complete the tunnel will resume.
The expansion and reconstruction of the Karlsruhe-Basel line
Until the collapse of the tunnel, works were being carried out to replace the existing, 150 year old, 182km long Karlsruhe-Basel line. Bottle necks, capacity issues and delays have prompted the move.
For the most part, the new higher speed line runs parallel to the existing with sections of tunnel where the above ground expansion has not possible.
Construction on the new line started in 1987 with the Achern station with the Raststatt-Süd-Offenburg section opening in December 2004.
In December 2012, the 17.6km section between Schliengen/Auggen and Eimeldingen was put into operation with the opening of the Katzenberg tunnel.
Station buildings have been completed in Weil am Rhein/Haltingen. Construction of the 4.3km long Rastatt tunnel started in May 2016.