First metro element floated under Amsterdam Station

On 8 July a joint venture of rail engineering company Strukton and dredging expert Van Oord successfully floated an immersed tunnel section, called ‘Priscilla’ under Amsterdam’s historic Central Station on the River Ij, and lowered it 7m for securing later. After removing pontoons that had carried it into position, the tunnel section was left ‘suspended’ from rods securing it to the support slab of the Central Station constructed earlier. The work is part of the City’s new North-South metro Line 52, and is just one of many engineering challenges being overcome, mainly associated with the soft, waterlogged ground of sand, silt and clay, and how to ensure support of historic building that currently rely on wooden piles.

The operation is believed to be the first ever operation of this kind. Manipulation of the tunnel section included positioning by floating in a sort of temporary canal between pile walls and under a microtunnelled canopy, requiring considerable delicacy and accuracy in positioning. Following correct location and removal of the pontoons, work could start on backfilling around the precast tunnel element and final securing over a period of around six weeks.

Before the insertion operation could take place 3000 wooden piles had to be removed from under the station and replaced with a concrete slab to support the centre of the old building. The removals were carried out by divers using hydraulic chainsaws and underwater welding and cutting.

The tunnel element measures 136m long, 22m wide and weighs 20 000 tonne. It was cast by a joint venture of Heijmans and Strukton Betonbouw at Suezhaven in the Amsterdam docks and towed-pushed with the assistance of tugs and pontoons along the river and into a specially constructed lock on the side of the river. The element was then rotated through 90 degrees for hauling into the excavated ‘canal’. In its contract the casting consortium has to supply a total of four immersed tube elements totalling 800m in length.

The project consultants on the first phase for Amsterdam City Council are a consortium called North-South Line Consultants (Adviesbureau Noord/Zuidlijn), including Royal Haskoning.

Meanwhile, one of two 6.880m-diameter Herrenknecht Mixshield TBMs, named ‘Molly’, used by the Saturn contractor consortium on Contract 4.2 of the project has been approaching the deepest part of the 3.8km-long twin-bored route, at 30.7m below datum, whilst following under Apollolaan to Cornelius Troost Square another 160m away. After this there is a climb to Ceintuurbaan station. Saturn comprises Zublin and Dura Vermeer. Average boring progress has been 15m (ten rings) per day with a daily record of 23m, beating ‘sister’ TBMs ‘Noor’ and ‘Countess’.

The 9km-long new metro route runs from the A10 ring road and Amsterdam Zuid main rail station to the same ring road to the north of Amsterdam city centre at Buikslotermeerplein on the reclaimed land that was formerly the Zuider Zee across the River Ij. In addition to the immersed tube tunnel in the river, other section will feature surface construction, TBM-bored tunnels through the city centre, and surface construction again at the southern end. Exhibition goers will be interested to know that it will link the RAI congress centre with the city centre and Central Station. It should take 16 minutes to complete journey the whole line.

The project has been many years in the making from plans in 1968, including a period if idleness from September 2008 to August 2009, due mainly to political and public worries about cost over-runs and damage to historic buildings. However, on 1st July 2009 the Amsterdam City Council decided to go ahead following advise from a special committee.

Transporting the floating immersed tube element to the Amsterdam Central Station on the River Ij [Photo: You Dubbelmann/Gemeente Amsterdam On board the pusher tug as immersed tube tunnel element 'Priscilla' is transported along the River Ij to Amsterdam Central Station [Photo: Strukton