In early planning for the North-South line is was thought that bored tunnel could continue the route into North Amsterdam, an outlying part of the city across the main river around the city, the Ij. But that meant passing with TBMs under the historic late nineteenth century Amsterdam Central station, a centerpiece of the city known to millions of tourists.

"At that stage we still had no bored tunnels in the Netherlands and it was thought obtaining permits for the work would be extremely difficult," says Frank Kaalberg at the project’s lead consultant Witteween+Bos.

Abandoning the link and ending the route at the Central Station would have been impossible; providing a connection to a part of the city that often feels cut off and abandoned was politically crucial. The suburb has a free ferry foot passenger and bicycle service across the river, granted some decades ago by the city ‘in perpetuity’, but is otherwise difficult to reach except by a long road route around. New development and housing there makes the connection now even more important.

The Dutch however are very skilled at immersed tube technology and so it was decided to make the crossing this way. "It could also be shallower with this method" says Kaalberg "because a bored tunnel would have to pass at least 6m below the piles of the Central Station."

The station
The station still had to be passed and the method for doing so has been one of the most complex and difficult parts of the whole project. It has been done with an extension of the immersed river tube effectively, a special element floated into a water filled cavity built beneath the station and then backfilled. The float-in operation was completed at the end of 2011.

Creating the space for the tunnel element using a vast underpinning operation and then building a huge table-like structure beneath the station with jet grouting piles and vertical microtunnelling, has been a gigantic operation in itself. Staggering ground engineering challenges have been met, not least in supporting a listed historic structure without damage, and removing thousands of ancient timber piles to make a space for the new tunnel box.

Since that element was installed, work has proceeded quickly on complex in situ concrete work site in a riverside cofferdam work just outside the station. This will make up a station concourse and passenger links to the main station, and will provide the southern abutment for the immersed tube.

The crossing
It comprises a three element connection from the Central station to the north river bank. Here there is an abutment with an in situ concrete tunnel section, currently under construction, which lifts the line up to surface level. The line runs on at grade once it is into the northern district. The immersed tube section, being carried out this year, is relatively straightforward compared to many other large and deep immersions worldwide, says Maurice Reijm, senior project engineer for contractor Strukton. The specialist marine works firm is working on the immersion in joint venture with dredging specialist Van Oord. Together they are both subcontracted to Zink which has the overall river connection works. Zink is a consortium of Heijmans and Strukton.

"Dredging for the tunnel channel and preparing the site for the immersion operation already began in February" says Reijm. The tunnel is due to floated into position in autumn this year, one element each week from 25 September.

The long coffer dammed construction pit for the in situ work on the north side was built initially as a construction dock for the immersion elements. These were concreted and completed some time back between 2005 and 2007 and have been stored ever since in a part of Amsterdam’s harbor around 12km away. Once monthly inspections ensure that they remain in condition and that interior humidity does not cause any problems for the tanks and pipework inside.

The roughly 450m long construction pit was created from an old yacht marina on the north bank of the river and was used to make the elements two at a time. The contractor Zink built the under station element there as well as the three units required for the river crossing.

The station installation was done by a different consortium but it made obvious sense to use the dock facility for all four elements, especially as Strukton is involved in both jobs.

Tunnel elements are roughly the same size, 8m deep, 140m long and 12m wide to carry twin lines for the railway. The station element is wider because it will be part of the Central Station with connections to the main line railways. It has side platforms for the passengers and walkways to the concourse areas at either end.

But the other units are also unique, each with its own geometry because of a curve on the cross river link.

"The connection is also rising as it comes across the river and so each element changes in both plan and vertical alignment" says Reijm.

Setting up and checking the formwork for concreting these ‘bananas’ was one of the challenges at that stage he says.

It also created some extra issues when the units were floated and then moved to the holding mooring 12km away at Suez Haven. Because of the strange shape he says, the units have a tendency to deviate slightly in the water and need constant correction by the tugboats.

This will be part of the challenge in the autumn when they are brought from the Suez. It means ensuring a five tug configuration with four on pulling ropes to adjust the course and one pusher tug.

By then the river trench and two steel sheet pile cofferdams will have long been installed and prepared. The cofferdams at either end help reduce the dredging required by allowing a 19m deep trench to be formed without a long shallow batter either side.

"We will also have placed six foundation pads, two for each unit, where the secondary end of the element will rest on steel pins through the tunnel floor," says Reijm. The precast pads are being made currently and comprise a tapered concrete block with a steel plate on the top.

The tunnel units will be brought into position with the secondary end as the ‘bow’ during transport.

That protects the delicate rubber Gina gasket which is on the primary end of the unit, to seal it against the steel frame of the abutment or the next unit.

"That means they have to be manoeuvred around and the pusher tug moves to the other end" he says. The unit will then be eased as far as it can into the 80m long coffer dam with the tugs at first and then by hauling inwards with a winched cable connection.

Once inside, the element will be connected to two big shear leg floating cranes, with a three point configuration for the cables.

"These will be the main adjustment for the elements as they are then ballasted and sunk" explains Reijm. Unlike most immersion operations he says, side to side and tilt adjustment cannot be done by altering the distribution of the ballast water in the tanks inside the elements.

"The elements are pretty narrow and therefore the ballast tanks have to go right across the width inside." End to end orientation will still be possible however.

The units will also use an innovative survey system from Geocon for the immersion itself, the so-called taut wire measurement developed for the Korean crossing. This replaces the usual survey tower system, which uses a frame mounted on the unit keeping a survey point visible above water for positioning by total station observation.

"Taut wire is exactly as described," says Reijm, "a wire kept at constant tension by a hydraulic roller system. Distance and angles on the wire, in plan and vertically, can be measured to determine the element’s position. "It is very accurate."

Underwater, positioning and connection will be done by traditional means says Reijm, though with an innovation there too. That means use of primary beams on the gasket ends which rest on the abutment or the previous section, where there are catches installed.

Jacks inside the hollow beams will pull the Gina gasket tight against the steel facing plate before the joint area is pumped out and water pressure pushes the seal even tighter.

Unlike normal primary end beams however these will also have vertical jacks installed. It means the beams can be raised and lowered to adjust the element position. "And by using the jacks in the opposite direction on the two beams it is possible to slightly alter the orientation of the element," says Reijm.

The combined system, which eliminates the need for a separate vertical adjustment mechanism, was developed for work on a crossing in Limerick in Ireland, he says.

At the secondary end, meanwhile, the pins will rest on the steel foundation plates leaving the bottom of the element 500mm clear of the trench floor. In the under space a sand filling is pump installed via tube connections to a hollow lip around the concrete base of the units.

Connections are made every 10m along, placing the sand in ‘pancakes’ as they build up underneath and gradually fuse together. Load reduction on the vertical primary beam jacks and the secondary pins, are used to measure the back pressure from the sand to indicate when the foundation has completed.

"Then the secondary pins are withdrawn," says Reijm.

Each unit has a 24 hour window for this placing operation, one a week into October, when the river traffic on the busy waterway will be stopped completely.

For the third and final unit there is additional work.

Like the first it will be winched into a cofferdam area. But it is installed in the opposite direction, with its Gina gasket against the far abutment. That leaves a space between its secondary end and the secondary end of the middle unit. This closure joint, 1.5m long, has to be sealed with steel formwork panels placed by divers.

"The shear legs will lower down formwork panels and then we can pump out the water from the sealed area," explains Reijm.

Operations in the days after each float will continue with a gravel locking fill placed around the element and then a timber and rock armour above to complete the backfill. "That protects against dragging anchors and the like" he says.

Barring unlikely earthquakes, or hurricane winds which are equally unlikely in the quiet early autumn, things should then be complete outside, leaving the internal work to be done inside the tunnel.

An important job is cutting the post-tensioning which keeps the elements stiff during the float-operation. Each element has six segments with concrete construction joints between them which allow some flexibility to accommodate differential settlements.

The temporary concrete bulkheads will also be removed and the immersion equipment and ballast tanks stripped out. Concrete bases will need casting to provide a track bed and adding weight to reduce buoyancy. Then it will be handed over to the track work and signally teams.