Aboveground and underwater
The aboveground part of the Noord/Zuidlijn in Amsterdam-Noord will run along dikes and through viaducts. From Buikslotermeerplein until just before the northern bank of the IJ River, the new metro line will be built inside the central reserve of Nieuwe Leeuwarderweg.

As its path approaches the IJ, the metro line will gradually descend into an excavated tunnel until it reaches the immersed tunnel at the bottom of the river. Up to that point, the builders will be using traditional construction techniques.

The metro tunnel beneath the IJ River will be constructed in the same way that the IJ Tunnel was built for car traffic some 30 years ago. The tunnel elements (caissons) will be built in a dry dock and then towed to the appropriate position in the IJ. There, they will be immersed, one by one, precisely into slots that will have already been dug out in the river bottom. Hence the name ‘immersed tunnel’. Dozens of tunnels have already been built in the Netherlands in this way. The dry dock where the tunnel elements are being built is next to Sixhaven.

Immersion tunnel in a channel
Across the IJ, the Noord/Zuidlijn will need to continue further beneath Central Station. This passage is one of the most difficult aspects of the entire construction project. Since the tunnel under the IJ will lie at a depth of 18 metres, the connecting metro tunnel beneath the station will also need to be relatively deep.

The builders also need to take into account that Amsterdam’s Central Station is a national monument and that its foundation consists of 3000 wooden piles. An important condition is that no harm be done to the monumental station during the construction of the Noord/Zuidlijn.

Metro station
The metro station under Central Station will be 130 metres long. It will be beneath the main (central) pedestrian passage of the station. The metro tunnel will be as narrow as possible to keep the impact on the existing station as low as possible. The total breadth of the tunnel is approximately 21 metres.

Here, the metro tunnel will be constructed using two different techniques: the ‘cut-and-cover’ method and the immersion method. With the ‘cut-and-cover’ method, a channel that opens into the IJ River will be dug beneath the existing station. A pre-fabricated tunnel element will be manoeuvred into the channel from the river and then sunk into position.

A major advantage of this way of working is that there is no need for a dry excavation site. It does require the building of a special table-like construction, however. This construction will need to be strong enough to bear the entire station. And it will need to be built from within and beneath the existing station, without inconveniencing travellers there.

The caisson method
The metro tunnel beneath Central Station will open into the new hall that will be located beneath the existing square in front of the station. From this hall, the metro route will run further underground to the water of the Damrak, where the bored tunnel begins.

This part of the metro tunnel will be built using the caisson method, which was also used to build the Oostlijn (East Line). In this method, caissons or individual sections of tunnel are constructed at street level and then lowered into position in the ground by washing away the soil beneath them.

The bored tunnels
What makes the Noord/Zuidlijn a unique project is the fact that two tunnels are going to be bored beneath the historical centre of Amsterdam: one for each set of metro tracks. From the top of the Damrak all the way to Scheldeplein, two gigantic tunnelling machines will be boring the route of the metro line.

The machines will bore both tunnels with a time difference of about three months between the first and the second. The tunnel shafts will have a diameter of about 7 metres. They will be 3.8 km long – or 3.2 km if you don’t count the length of the platforms in the stations that the boring machines will be passing through.

Both tunnel-boring machines will be assembled in a construction pit deep in the water of the Damrak. This is the shield-launching shaft. The parts of the machines will be floated here on the water. The reception shaft of the bored tunnel will be located in a construction pit at Scheldeplein. There, the boring machines will be disassembled once again and removed.

The underground stations
The construction of the stations is taking place by means of various different techniques. For the stations that will be built aboveground, the building technique will be like that used for normal construction projects. However, for the Rokin, Vijzelgracht and Ceintuurbaan stations, all of which are located deep under the surface, a special building technique will be used.  These three stations are being built in areas where there is a lot going on – areas that will need to remain accessible. That is why these stations are being built for the most part in covered excavation sites and using the ‘cut-and-cover’ method.

The ‘cut-and-cover’ method
With this method, workers first make the walls and then place a roof on top of them. The so-called diaphragm walls consist of panels of reinforced concrete that are usually 1.20 metres thick. These walls reach a depth of nearly 40 metres.

Before the walls can be made, deep trenches first need to be dug. To prevent those trenches from collapsing in on themselves, they are immediately filled with bentonite, a mixture of water and clay. Next, steel reinforcements are lowered into that mixture. Then the trenches are filled with concrete from the bottom up while at the same time the bentonite is pumped out again from the top.

No piles need to be driven in making these walls. The roof that comes to rest on top of them is also made of reinforced concrete. This creates a giant concrete box full of soil several metres below the ground. Now the soil can be removed from the box. Above this box, the situation at street level will quickly be restored to normal, with traffic flowing just as it did before. The station can then be finished underground.

Rokin Station
The construction of the three ‘deep’ stations will take place in phases to allow for the re-routing of traffic. For Rokin Station, for example, the former car park on Rokin was removed to allow for the building of the walls of the first half of the station. Once those were completed, the tram rails and the traffic were rerouted so that the walls and roof of the other half of the station could be built.

When those are finished, the situation on Rokin can return to normal. And the first half of the station can be given a roof as well. After that, the rest of the work can be done aboveground and will only require a much smaller work site.

Building on the street level
The bored tunnel will end just past Scheldestraat in Amsterdam-Zuid. That is where the boring machine will reach the so-called reception shaft: a 15-meter deep construction pit in which the device will be dismantled and removed.

For the final couple of hundred metres of the Noord/Zuidlijn, in front of Amsterdam RAI, the tunnel will be built from the street level in a huge trench that is currently being excavated. Europaplein Station will be located near the roundabout in front of Amsterdam RAI at a depth of about eight metres.

Here, the tunnel will already be less deep, and it will be ascending gradually until it reaches the surface in the central reserve of the A10 ring way just beyond RAI Station. The final stretch of the Noord/Zuidlijn towards Zuid Station will be built aboveground.

Choice of technique
In choosing among the possible building techniques, the goal of keeping the disruption and inconvenience for the surroundings to a minimum played an important role. In three different ways, efforts are being made to ensure that the people of Amsterdam will experience as little inconvenience as possible from the construction of the Noord/Zuidlijn:

• A route was chosen that runs for the most part under canals and streets.
• Nearly 4 km of the metro tunnel will be bored well below the surface. This will not bother anyone aboveground.
• The underground stations in the city centre are being built for the most part in covered construction pits.