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Belgians go Dutch with Amsterdam tunnel job

IT IS only natural that a Belgian consortium which beats four Dutch consortia to a 100 million tunnel job in Amsterdam should feel very pleased with itself.

But wasnt it going a bit far to decide to build the immersed tube tunnel segments in Belgium and then transport them 300 km by sea to The Netherlands?

This is exactly what the winning consortium, Combinatie Piet Hein Tunnel (CPHT), chose to do and, to be fair, the factors which inuenced the decision were not nationalistic. They were those two great motivators: time and money.

The 1,900 m long Piet Hein Tunnel is the largest immersed tube tunnel in The Netherlands 1,265 m is under water. It will provide a route under the Amsterdam Rheincanal from the city centre to the A10 orbital motorway.

The tunnel will carry a dual carriageway road in two directions and has a third chamber which will take a railway, to be installed later.

The original tender documents suggested casting the eight tunnel segments in an old dock in Amsterdam, not far from the tunnel site. But this did not appear to be an ideal solution, says CPHT project manager Lode Franken: When we looked at the dock in Amsterdam it was too small we would have had to make the elements in two steps, he says. Also, the costs, due to local environmental standards that had to be met, were too severe.

So CPHT looked to Antwerp, where it found the perfect dock, which had been used for casting segments of Belgiums Liefkenshoek Tunnel.

Mr Franken explains that the proposal to build in Antwerp was kept going throughout the three-month tender period while the consortium investigated whether it was feasible.

There were four main questions which CPHT had to answer: Was the dock in Antwerp available? Would the tunnel segments be able to pass through the various waterways on the journey to Amsterdam? Could the segments survive the sea conditions? And would anyone insure it?

Luckily, the answer to all these questions was yes, but the contractor had to make some

design modifications first.

The biggest challenge was building segments to withstand the increased bending moments from waves on the North Sea. To do this, CPHT increased the amount of prestressing by a factor of five.

Mr Franken explains that, while pre-stressing is usually a temporary measure to cope with bending during sinking and placing, they were able to convince the client that the pre-stressing cables could act as permanent reinforcement.

The London Salvage Association insured the operation but made certain stipulations before giving the go-ahead. The transportations had to take place between May 1 and September 1 so that the wave height did not exceed 2 m (the ultimate design height was 3.5 m). And additional steel bulkheads were added behind the usual concrete ones at either end of each box segment as a back-up in case the first set failed.

At the time of tender, CPHT put in a price for the original proposal and for the Antwerp proposal, which was 10 million guilders (4 million) cheaper.

The client, the Municipality of Amsterdam, accepted the cheaper option in August 1992 and detailed design began.

Between June 1993 and October 1994, CPHT built the eight 40,000-tonne elements in the dock at Antwerp. Four of the elements are straight, four are curved but each has the same approximate dimensions: 160 m long, 32 m wide and 8 m high.

The curved sections are needed because the plan shape of the tunnel is a shallow S.

By March 1995, the elements had been fitted out ready for their sea journey and the dyke was removed from the mouth of the dock to oat the elements. From then on, it was a case of watching the weather forecasts so that the contractor could catch the right tides and make sure that the waves in the North Sea were rising no more than 2 m. The route from the dock in Antwerp to Amsterdam is tortuous: it passes through the Kallo lock in Antwerp into the narrow Western Scheldt, up the North Sea coast, through the Ijumuiden lock and into Amsterdam.

It was nerve-wracking for the project team as the first element left Antwerp May 5 last year.

From the moment we went through the first lock, there was no way of returning, says Mr Franken.

But all eight sections made it through safely and the weather forecasts proved reliable on all but the last trip, when a localised depression whipped up 2 m waves. But the segment survived.

The contractor moored the segments near the tunnel site before sinking them in the 36 m-wide dredged trench. The trench is not level the east end of the tunnel is lower to allow 8 m of water for ship canal traffic to and from Germany to pass over.

CPHT sank the first five sections, working from west to east, in just a month. At this point, the contractor was forced to take a two-week break not because of a hitch but because an annual boating event in Amsterdam was taking place. This was no bad thing, says Mr Franken: The break was very good for our physical well-being. Although techn- ically we could have carried straight on, we all needed the rest.

After this interlude, the two eastern sections were sunk, leaving one section to be slotted into place.

The final section was 1 m shorter than the gap, but this was intentional, and designed to enable the last section to be manoeuvred into place.

Finally, the two ends of the tunnel were stitched together. By September 18, it was possible to walk through the tunnel.

The next step is to tidy up inside the tubes. The contractor has removed the bulkheads and is now concreting the joints. Installation of the electrical and mechanical services lighting, signalling, ventilation is about to begin. This stage accounts for 10 per cent of the whole project cost.

The first car should drive through the tunnel in April 1997.

TUBE TRAVEL: Contractor CPHT cast eight segments for the Piet Hein immersed tube tunnel in a dry dock in Antwerp (top left) before ooding the dock (top right) and oating them. Next came the 300 km journey up the coast (bottom left) to their final resting place in Amsterdam (bottom right)

From the moment we went through the first lock, there was no returning

Lode Franken, CPHT