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Channel crossing One of the key projects of the Central Artery is the immersed tube tunnel crossing of the narrow Fort Point Channel in south Boston.

USA FOCUS

The Fort Point Channel crossing will link downtown Boston with the rapidly developing area around the city's new convention centre and beyond, via the Ted Williams Tunnel to Logan Airport.

Not only does it play a key role in the whole scheme, it is also the first time that a concrete immersed tube tunnel has been built on this scale in the US.

The narrow Fort Point Channel, an extension of Boston harbour, lies just to the east of the new I-90/I-93 South Bay Interchange. Via cut and cover tunnels the immersed tunnel will eventually link into the jacked tunnels and carry the I-90 on to the South Boston Interchange.

The rectangular sectioned concrete tunnel will carry nine lanes of traffic, four eastbound and five westbound, including a high occupancy vehicle lane.

Although nowhere near the scale of recent immersed tube tunnels such as the 3.5km 0resund Tunnel in Denmark, the 335m long tunnel is complicated by the unusual and different shapes and sizes of its units, the close proximity of sensitive buildings such as the US post office, a tight working area and an historic and active subway tunnel. And two of the units will provide support for a massive ventilation building and a new road bridge.

Main contractor for the $324M contract is US firm Modern Continental Construction. The immersed tube tunnel was designed by Acer Engineering, subconsultant to Gannet Fleming.

One of the most unusual aspects of the crossing is the foundations needed to support the sections as the tunnel passes just 2m above the Red Line subway running down the centre of the channel. Instead of sitting on a prepared bed in a dredged trench, four of the units will sit on large diameter drilled shafts, explains Bechtel/Parsons Brinckerhoff resident engineer John Bales.

The twin tubed brick tunnels were built in 1915 and while they are still in good condition, there were concerns that any long term settlement of the new units above might damage them. Some 110, 2.2m diameter and up to 48m long drilled shafts have been installed from barges in the channel and into the underlying bedrock at depth. The steel casings are vibrated through the fill, alluvial clays, a thin layer of till and the soft Boston Blue Clay in the channel and are finished off by using a polymer drilling fluid for the rock sockets up to 12m deep. In effect, the tunnel acts as a bridge over the subway.

On the underside of the four units, downstands have been cast that will join directly on to the top of the piles, 'like giant Lego blocks'. Two different types of fixing are used. The two units supporting the vent building, WB1 and EB1, are joined by 1m diameter shear locks fitted to the top of the shafts that will be grouted in when the units are in place. The WB2 and EB2 units will also be grouted inside rubber gaskets between the units and the shafts.

The last two units, WB3 and EB3, will not be above the subway, so they will be founded on grout bags laid between the units and the glacial till.

All the units will sit in a 3m deep trench dredged in the centre of the channel. The bored piles will be cut off just above the base of the excavation and the grout bags placed in it for the eastern units.

But as a safety measure during construction, bulkheads have been fitted in the Red Line tunnels between South Station and Broadway Station. These can be used to seal off the tunnel if there is a breach, preventing flooding of Boston's entire subway system, according to Bechtel/Parsons Brinckerhoff's Al Franzen.

In the huge casting basin on the eastern edge of the channel, four of the six tunnel units are nearly complete. This 305m long, 91m wide and 15m deep excavation sits neatly in a site next to razor blade manufacturer Gillette's US headquarters.

It was formed using 730m of diaphragm walls supported by 1025 multi strand anchors at four levels. The 'gate' of the basin is formed by a cofferdam made up of six, 20m diameter interlocking circular cells each built from seven connecting 'peanut' cells of 21m long sheet piles and filled with gravel ballast. It is kept dry by 10 deep dewatering wells inside and 12 recharge wells outside pump water back into the ground, to prevent settlement of neighbouring structures.

The site was originally home to an oil tank and was then used as a dump for razor blades. Franzen explains that careful excavation was needed, not only to prevent the spread of contamination but also because of Gillette's concern that razor blades, well preserved in the oily ground, would be stolen.

The four units include the two most complicated sections (EB1 and WB1) incorporating the foundations for the ventilation building and also the column supports for the new Dorchester Avenue bridge. The unusual shape of these units means that floating them is more difficult than normal and careful control of water ballasting is needed as they are placed.

Tunnel units are all around 9m high and are between 34m and 53m wide and 100m and 130m long. The largest units weigh 50,000t. As well as internal temporary ballast tanks, each unit is fitted with four 6m high stabilisation cylinders at each corner, used to immerse the units into their final positions.

When GE visited site, temporary bulkheads were being fitted to the units and ballast tanks built inside. These will be filled with water before part of the cofferdam is removed to flood the basin, allowing it to become tidal. Units will then be checked for leaks and if necessary the basin sealed, drained and repairs made. If all is well, the units will then be floated out and placed (see box).

By summer 2000, all the units should be in position. The casting basin, which will then become part of the cut and cover route east, will be joined to the two eastern units and the western end of the tunnel sealed off against sheet pile walls. The Gina gaskets will be strapped together and the tunnel dewatered simultaneously from both ends, to prevent a high build up of differential water pressure at the end of each unit that could cause them to be pushed out of position.

The contract, due to finish in 2002, will be completed by cut and cover tunnels to the jacked tunnels of the South Bay Interchange on the western side and to adjacent contracts to the east.

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