The boring process
From the top of the Damrak all the way down to Scheldeplein, the route of the Noord/Zuidlijn will be excavated by two tunnel-boring machines. The machines will bore both tunnels with a time difference of about three months between the first and the second.

In the route chosen for the Noord/Zuidlijn, the tunnel-boring machines can follow the existing street pattern. That means that almost no buildings need to be demolished in the process.

The tunnels
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 pass through.

The tunnel-boring machine
Each tunnel-boring machine has a diameter of 7 metres and is approximately 60 metres long. Both machines will be assembled in the launching shaft – a construction pit some 20 metres deep in the water of the Damrak.

Just past Scheldestraat, the bored tunnel will end in the reception shaft – a construction pit about 15 metres deep, where the boring machine will be dismantled and removed.

The head of the tunnel-boring machine consists of a hollow steel cylinder: the shield. The shield is approximately 8 metres long. This is where the tunnel builders work. The shield protects them from both the ground and the groundwater that surround the tunnel.

In front of the shield is the cutting wheel, and behind the shield is where the segments that will make up the tunnel are put into position. The shield is open at the rear, where the completed tunnel sections appear. Each day, the tunnel-boring machine creates about eight metres of metro tunnel.

The cutting wheel at the front of the machine turns slowly, scraping through the ground layer for layer. The sand, peat or clay that the cutting wheel scrapes loose is mixed with water and immediately carried off by conveyor belt.

At the same time it scrapes away the ground, the tunnel-boring machine is also edged forward. It uses jacks to push itself off against the section of tunnel that it has already completed. As soon as the giant machine has made sufficient progress, it creates a new section of the tunnel by putting together a number of ‘tunnel segments’.

Once enough space has been bored open, the digging is stopped temporarily. A special device puts the new tunnel segments precisely into position. Each time, a total of seven segments are necessary to complete a section of tunnel wall.

Once the new tunnel segments have been pushed together tightly, the tunnel has grown a little longer. Immediately after each new section of the tunnel is finished, rails are laid that can carry a train loaded with workers and materials to that point.

There needs to be sufficient space for inserting the tunnel segments. The hole that is bored is therefore one centimetre broader than the tunnel itself. Once a new segment of tunnel has been put into place, the leftover space is filled with a mixture of sand, cement and water that will harden over time.

All of this takes place at a depth of 20 to 30 metres. No one at street level will be able to notice what is going on so deep below the surface.

Reinforcing foundations
Most of the route of the Noord/Zuidlijn runs beneath streets and canals, where there are no foundations. Wherever the bored tunnels come near the foundations of buildings, extra care is required.

The Beurs van Berlage building on Damrak and the ‘Industria’ building on Rokin are two buildings whose foundations lie close to the route of the new metro. In each case, the soil around their foundations is being reinforced with a mixture of water and cement. This will fortify the soil permanently. Along the entire route, foundations that were in a poor state were already reinforced during the preparatory phase.

Dutch know-how
Earlier experience with drilling tunnels through the soft Dutch soil over the past ten years was of great help during the preparation phase.

Since the Tweede Heinenoord Tunnel was bored under the Nieuwe Maas River in 1995, a variety of other traffic tunnels in the Netherlands have been built using this technique, including three for the so-called Betuweroute (the Botlek Tunnel, the Sophiaspoor Tunnel and the tunnel under the Pannerdensch Kanaal) and the Westerschelde Tunnel under the Westerschelde River.