AMSTERDAM, 31 july 2007 - 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 nation 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 dammage to the city'", said Johan Bosch, the project manager. "We need a system so that if things don't go as expected, we don't find out after the damage is irreparable".
The solution: 7,000 mirrors hung in clusters of three on buildings along the 2.4 miles 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 any building shifts more than half a millimeter. A millimeter is the thickness of a paper clip.
The system has already told townspeople something they may have guessed: that theirs is a city in motion.
"We now know that whole segments of the city move by themsleves, a number of millimeters over the course of a season," Bosch said.

Working under pressure
Scheduled for completion in 2013, the $ 2.4 billion project stretches 5.9 miles in all and will transport an estimated 200,000 people daily, adding a new dimension to Amsterdam's traffic of bicycles, trams, cars, taxis, buses and boats.
The biggest technical challenge is building a subway stop beneath the city's main train station, al 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 will dig a 60-foot-deep trench under the station and connect it to the IJ River, which flows directly behind.
"It'll be the world's most expensive covered canal," van de Zande said. When all is ready, a segment of concrete tunnel 450 feet long will be buoyed with air like a submarine, floated into the trench, and then allowed to settle gently into place.
And they expect to work?
"We haven't missed a deadline yet," van de zande said.
This fall, two drills, ech 23 feet in diameter, will tunnel through the city like metallic moles, pulling the contraption that lays concrete tunnel walls.
The initial segment where the drills will descend into the earth is being built using the "caisson" method.
The sealed segment, or caisson, is constructed above the spot where it must go.  Workers operate in a pressurized air pocket directly beneath it, pumping mud upward through a pipe so that the caisson slowly sinks.
Because the streets of Amsterdam are below sea level at high tide, the air pocket is increasingly pressurized as the caisson descends to a final depth of 82 feet. workers must rest in decompression chambers after their shifts in order to avoid getting "caisson disease", the divers' ailment otherwise known as the bends.

Subterranean suprises
The subway stations are being built before the tunneling starts, using an innovative method intended to minimize disruption to street traffic: the walls are built first, then te roof, and the rest of the work can proceed underground.
There are always surprises.
Workers digging space for a wall at the Rokin station were alerted by the mirror monitoring system that buildings on a nearby street began to sway back and forth. 
On further inspection, they found they had run into a layer of horizontal poles at a level where they expected to find sand.
Eventually, Bosch said, workers got through the layer using 36-foot steel tubes with drill bits welded to the end, chewing up the wood and sucking it out as if through a straw.
"It didn't seem prudent to keep on going, so we had to find another way", Bosch said. "I'm sure it won't be the last time we have to improvise".

Source: Chicago Tribune
Author: Toby Sterling