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Growth factor

Transport links to the expanding Port of Antwerp have reached saturation point. The port’s right bank has good transport links but no room to expand. The left bank, separated by the River Schelde, has room to develop but rail routes are constrained by a bottleneck further south.

Port growth

So to enable the port’s growth, a new rail link must be built beneath the port to connect the two sides.

However, it is difficult ground to tunnel in and expensive to build. As a result Belgium’s railway maintenance manager Infrabel has joined forces with consortium LocoRail to finance and maintain the link, while construction will be managed by another consortium LocoBouw (see box for details).

Currently, all road and rail traffic passes through the Kennedy Tunnel to the south of the port. When the Liefkenshoek link opens in 2014 it will just be used for freight trains allowing direct access to the port. This will remove the bottleneck at the Kennedy Tunnel, freeing up passenger rail services between Amsterdam and Brussels.

The first stage of the 16.2km link is an open cutting which connects to the existing − and now renovated − 1.2km cut and cover Beveren Tunnel. This was built in the 1970s prior to construction of the Waasland Canal in anticipation of a transport link that never came to fruition.

Then there is a second, smaller section of open cutting before the next, and biggest part of the project, which is the two single-track 6km bored tunnels underneath the River Schelde and a canal dock (B1/B2). It cuts through challenging ground conditions with a very small margins for error. The open cuttings will need diaphragm walls, which are down to a maximum 40m depth.

Deep impact

Tunnellers will be passing underneath two major watercourses in a sandy layer, but just above a clay layer. “Depth of the tunnel is very important − too high and the tunnel will float, too low and it will pass through the clay,” says LocoBouw director of works Alex Vandemeulebroecke.

Engineers decided to use an 8.39m diameter Herrenknecht pressure balanced tunnel boring machine (TBM). Once completed the internal tunnel diameter will be 7.30m. With only 600m of the 6km tunnel in clay, Vinci Construction Grand Projets designed the tunnel for the fine sand that covered the rest of operation.

“It’s not that easy.You must be careful, if there’s too much pressure then there would be a blow-out”

LocoBouw director of works Alex Vandemeulebroecke

To begin boring, tunnellers inject bentonite at the front of the cutterhead, which makes a watertight membrane. The pressure is controlled by the amount of bentonite pumped in. “It’s not that easy,” adds Vandemeulebroecke. “You must be careful, if there’s too much pressure then there would be a blow-out.”

The first TBM, called Fanchon, began its operation in February 2010, with the second, Bobette, starting its journey two months later. Tunnelling moves along at an average of 16m per day, with the Fanchon TBM slower to allow the cross passages to be completed. Over 800,000m³ of spoil will pass through the two machines, which is treated on site and then used to help create sea defences close to the port.

As the TBM pushes through the earth, a seven segment concrete ring is pushed into position. A total of 53,000 pre-fabricated concrete segments will be used in both tunnels, which are being delivered to site at a rate of 1,200 per week by train from Germany.

Piling on the pressure

Tunnelling through the sand has been relatively straightforward, but underneath the River Schelde, the TBM comes within 10m of the riverbed. The bottom of the river is disturbed ground, which means it is not as dense as naturally formed ground. Furthermore, as it is a tidal river, the hydrostatic pressure changes by up to 4bar of pressure around the tunnel. “Water pressure was a major issue. Every hour we made checks,” adds Vandemeulebroecke.

Engineers needed to stabilise the soil underneath the canal dock so that the TBM could pass through and so in May 2010, 25,000m³ of slow-setting mortar over a four-day period was poured into the canal basin. The operation involved four concrete mixing plants, four pumps and a total of 240 people.

In July 2010, a further 20,000m³ of rapidly settling concrete was poured in the same conditions. This concrete serves as a counterweight during the boring operations and will protect the tunnels from any ship impact.

“The tunnels pass just a few metres below the reconstructed canal bottom,” says LocoBouw tunnel director Lionel Suquet.

Between the bores are 13 shafts and eight cross passages. These have been constructed, in all but one case, by freezing the ground beneath the tunnel.

“It takes about 250 days from start to finish to construct a cross passage,” adds Suquet. Pipes are inserted through the earth to the other tunnel bore filled with brine. This freezes the ground, enabling engineers to excavate between the two bores.

Lattice effect

Once the engineers have broken through to the other bore, a steel lattice arch is placed in the hole, and concrete pumped in. During the next stage, the passage is lined with a waterproof membrane, after which a steel rebar cage is fitted and poured with concrete to form the platform.

The cross passages are placed every 250m underneath the river, and every 300m along the rest of the tunnel. “They will only be used by the train driver because it’s a freight only tunnel,” adds Suquet.

Once the tunnel is completed, LocoBouw will install a state of the art active fire suppression system along both tunnels, which spray foam in the event of a fire. Completion of the second tunnel is expected in July, with construction finished in mid-2013. Infrabel will begin installing the railway infrastructure, such as the tracks, signalling and overhead lines, from 2012 until its opening in 2014.

All being well the Liefkenshoek rail link will open in mid-2014 and LocoRail will be responsible for the maintenance of infrastructure until 2051.

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