Amulti-section concreting method is being used for the construction of tunnel elements to be used on the two 2.5km long immersed tube crossings on the new high-speed rail line from Amsterdam to Antwerp. The method has been designed to help cope with possible sinkage of the tunnel into the Netherland’s notoriously soft ground.
The two virtually identical tunnels take the 300km/hour track under two branches of the Rhine-Maas delta, the Oude Maas just south of Rotterdam and the Dordtsche Kil further towards Belgium. Each has twin tubes, one for each direction, of five 134m long tube elements and two central 150m long elements. Each element is 18m wide and 8m high. At each approach is 750m of cut and cover tunnel.
Most of the 100km long route has been piled, including at-grade track, to cope with the soft ground of the sea-reclaimed polders. Ground comprises typically a 12m-15m thick layer of peaty silts overlying medium dense sand for another 12m-20m. The water table reaches ground level.
But piling would have been far too difficult in the heavily shipped river channels, explained Peter Hoogen, tunnel project manger for government client the HSL-Zuid. To cope with an allowed 300mm sinkage over the length of the central immersed sections of the tunnels, the elements will be cast in 25m long sections, with “zero-millimetre” joints between them. A small amount of flexing will be possible, though vertical movement in the joint will limited by male-female shear keys. Between any single section, only 1mm-2mm vertical movement is allowed.
For the float-out even this much flexing is not needed and units will be temporarily pre-stressed. Strand will be heated to relax it, once the units are submerged on site, and then cut.
Construction of all immersed tube segments take place in a dyked-off yard off the Oude Maas river.
Contractor, the consortium Bouwcombinatie HSL Drechtse Steden, plans its first float-out immediately after the initial flooding of the construction yard, due in August 2003 when concreting finishes.
The Dutch consortium, made up of Ballast Nedam, Van Hattum & Blankevoort, Strukton Betonbouw, Van Oord ACZ, Waterbouw, HBG Civiel, and Maasdiep VOF, has the overall US$500M design-build contract for the construction of the tunnels, a 2km long bridge and 8.9km of piled slab track.
