Immersed tunnel construction
To construct the tunnel running below the IJ river and Central Station, the immersion method was used. Four tunnel sections were fabricated in a construction dock near the Sixhaven harbour. One section, to be positioned below the station, was 21 meters wide, 10 meters high and 136 meters long. The three sections making up the tunnel below the IJ river were 12 meters wide, 1 meter high and 141 meters long. A trench was dredged out in the bottom of the waterway to accommodate the tunnel sections.
Creating the immersion trench below the station was many times more complex. While daily train services continued as normal and thousands of passengers were transported to their destination, a partial new foundation was installed below the station using a so-called ‘table structure’. This structure, measuring 130 meters in length and 20 meters wide, supports the station. The table legs are made of two parallel tube pile walls below the platforms and sandwich walls below the station concourse. The tube piles are 1.8 meters in diameter and up to 80 meters in length, and were installed by means of vertical microtunnelling. For the sandwich walls, two rows of tubex piles of up to 60 meters long were screwed into the soil, after which the space between the rows was reinforced using jet grouting. These walls are three meters thick. On top of the table legs, a roof was installed made of concrete piles poured on-site.
Tunnel section immersion
The soil below the station’s newly installed ‘floor’ was excavated, and the old wooden piles were removed. After the trench below the station had been connected to the three tunnel sections immersed in the IJ river, the fourth tunnel section was transported by ship to the trench below the station and immersed. The river’s water level was temporarily lowered for this purpose.
The immersed tunnel sections are connected to the central metro concourse below the square in front of Central Station. Between this concourse and Damrak, the metro tunnel was constructed using the so-called caisson method, which involves constructing a tunnel section (or caisson) on top of a sand bed.The sand is then gradually removed until it is at the required depth.