How do you dig a tunnel in marshy ground…under a densely populated city…that is built on an unstable foundation of old wooden piles?
The Challenges of Building Amsterdam’s New Metro — the North-South Line
What kept the builders of the North-South Line awake at night?
The pretend scenario like the one depicted in the photo above was probably at the top of the list.
Other reasons for the builders to miss some sleep: political fighting, environmental concerns, cost overruns, sinking houses, and angry residents facing years of noise and disarray at their doorsteps. At one time or another, each of these challenges seemed insurmountable.
A brief history of the metro
For years now, Amsterdam has looked like one big construction site.
In 1968–after a decade of discussions–the Amsterdam City Council finally decided to build a metro system. They started with the East Line, running from the city center to the east.
Work began in 1971. The project was plagued by cost overruns and delays. Even worse, riots broke out protesting the demolition of houses and buildings in the Nieuwmarkt neighborhood. The East Line was eventually completed, but as a result of the protests, plans for the North/South Line were shelved.
Planning the North-South Line
Meanwhile, the City Council gained experience with two other large projects: the Piet Hein tunnel–at the time, the longest car tunnel in the Netherlands–and the Ring Road A10-West. This gave Amsterdam confidence that it could manage another big infrastructure project.
More importantly, new tunnel boring techniques had been developed, making it possible to dig deep underground in soft soil without disturbing the ground above. Houses wouldn’t need to be demolished.
The North/South Line got the green light, and construction started in late 2002.
A route was chosen that mostly follows the streets. The new metro line is nearly 10 km long, of which 7 km lies underground. Within that, two giant tunnel boring machines have been used to excavate a 3 km tunnel, 20 to 30 meters below the surface–right under your feet. Where the streets were too narrow for two sets of tracks to lie side by side, two tunnels were bored lying on top of each other.
Besides logistics, another big challenge was Holland’s centuries-old nemesis: water.
Amsterdam’s ground water
The water table under Amsterdam is high. If you were to dig a hole anywhere in Amsterdam, it will fill up with water pretty quickly. The soil itself is simply too soft to support a building, let alone transportation infrastructure. Even the first small, wooden houses built along the Amstel kept sinking into the boggy ground.
In this city, you need to dig down 400 meters to hit rock. On the way down, there are alternating layers of clay, peat and sand. As any engineer will tell you, clay and peat cannot support the weight of a building.
The solution? Wooden piles (beams of timber driven into the earth). The foundations of most buildings in Amsterdam are supported on piles 12 meters deep that are anchored in the first layer of sand. The piles of larger buildings are even deeper, reaching to the second layer of sand at 20 meters or to the third at 50 meters deep. Altogether, there are more than a million piles under Amsterdam.
Incidents at the Vijzelgracht
Extra care was taken wherever the tunnels ran near the foundations of buildings. In preparation, some of the wooden piles of Central Station were replaced with an underwater concrete construction and steel piles. The soil around the foundation of the Beurs van Berlage was reinforced by a mixture of water and cement. All along the route, foundations that were in a poor state were repaired and reinforced.
Thousands of cameras were fixed on buildings along the route to monitor any movement in the foundations. Despite all these precautions, two serious incidents did occur.
On 19 June 2008, during the construction of the underground station on the Vijzelgracht, a leak in the concrete wall caused water and sand to flow into the pit. This caused tremors in the wooden piles under the houses nearby, making some houses sink 15 cm into the ground. Residents had to be evacuated. The work was stopped immediately and repairs made. Then, on 9 September 2008, construction started again. The next day, more houses sank, this time 20 cm within the hour. Cracks started to appear in the walls and windows, and doors wouldn’t budge. The row of 300-year-old houses, known as the Weavers Houses, became suddenly uninhabitable.
Construction was stopped, this time for nearly a year. A commission was appointed to determine the cause of the leakage. The City Council chose to finish the project based on the recommendations made by the commission.
Today
Today the construction is nearly finished. The last rail was laid in July 2015 and the stations are being completed. Escalators, lifts, cameras, and systems are currently being installed. Testing for safety will take place soon. The first train is expected to run in 2017, and the entire ride will take only 16 minutes–just enough time to finish your coffee.
All in all, with sinking houses and irate neighbors, the new metro line remains an amazing feat of engineering and human perseverance.
Source:
Gemeente Amsterdam North/South Line–Technical
Written by Barbara Austin.
Categorized as Amsterdam.
Tagged with amsterdam, Infrastructure, North-South line.
Originally published at tedx.amsterdam on September 17, 2015.
