Peter Dejong/associated press


A quarter of a million travellers pass through Amsterdam’s Centraal Station every day and few realize the construction of a subway system is going on beneath their feet.


Seventeenth-century masons built Amsterdam on a foundation of wooden poles planted in soggy, sandy ground, leaving behind a beautiful architectural museum — but one with walls prone to sinking or crumbling without warning. So, how do you dig a subway under it? Very carefully.

Construction of a new “North-South” line for this city of canals and rivers began in 2003, and is presenting Dutch engineers, famed for their ingenuity in keeping this waterlogged country dry, with devilish challenges.

“The politicians told us: ‘We want a subway, we’re prepared to pay for it and accept some disruption, but the one thing we absolutely don’t want is any damage to the city,’“ said Johan Bosch, the project manager.

The solution: 7,000 mirrors hung in clusters of three on buildings along the four kilometres of the route that’s underground. Measuring devices shine infrared beams onto each mirror once an hour, measure the reflection, and feed data into a central computer.

After triangulating, the computer raises the alarm if buildings shift more than 0.5 millimetres in any direction.

The system, unique on such a large scale, has told townspeople something they may have guessed: theirs is a city in motion.

“We now know that whole segments of the city move by themselves, a number of millimetres over the course of a season,” Bosch said.

Scheduled for completion in 2013, the $2.4-billion US project stretches 9.5 kilometres and will transport an estimated 200,000 people daily.

The biggest technical challenge is building a subway stop directly beneath the city’s main train station — a landmark 19th-century building — while it remains in use.

A quarter of a million travellers pass through Centraal Station every day and few of them realize construction is happening beneath their feet.

“It’s probably better that people don’t see this,” said Bert van de Zande of Strukton, the contractor responsible for this part of the project, pointing to I-beams supporting the building’s main columns.

Strukton workers eventually will dig an 18-metre-deep trench under the station, and connect it to the Ij River, which flows directly behind.

When all is ready, a segment of concrete tunnel 140 metres long will be buoyed with air like a submarine, floated into the trench, and then allowed to settle into place.

And they expect that to work? “We haven’t missed a deadline yet,” Van de Zande says.