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Digging Boston out of a jam Boston's Central Artery is one of the most significant urban transport projects in the world. The logistics of building a major highway through the city's downtown area are

USA FOCUS

Boston's I-93 highway is in trouble. Designed to carry 75,000 vehicles a day when it opened in 1959, the steel elevated road through downtown is now overloaded with 190,000 cars, lorries and buses crammed daily into its six lanes. Jams are not confined to the normal rush hours. In true US style, everything is bigger, with traffic hold-ups lasting seven or eight hours a day.

The road is one of the most congested in the United States, with an accident rate four times the national average. Delays are estimated to cost motorists $500M every year.

'The problem is that we can't divert the traffic, fix the problem and bring the traffic back in; we can't expand the existing road; can't build a new one over the waterfront; and can't use a TBM at depth,' explains Terry Brown of the Bechtel/Parsons Brinckerhoff project management team.

The solution had to be environmentally friendly, allow traffic to flow at the same levels during construction, and had to solve the road's capacity problem for a number of years to come. 'The unique challenge is to keep Boston working,' says Brown.

There are also some formidable geotechnical challenges. Much of the natural harbour, originally a series of small inlets, was backfilled to allow Boston to grow, leading to a highly variable mix of fill, sands, silts and peat overlying the Boston Blue Clay. There is also a high water table.

Work started on the Central Artery/Tunnel project (CA/T), known locally as the 'Big Dig', in 1991. The new 12km long road system is being built by the Massachusetts Turnpike Authority at a cost of $10.8bn (70% paid for by federal government and 30% by the Commonwealth of Massachusetts). There are 109 separate contracts in the project, which has two major parts.

The first deals with the deteriorating elevated section of the I-93. This is being replaced by an underground expressway from eight to ten lanes wide, running along the same alignment. It is being built top-down between diaphragm walls beneath the overpass. Special low headroom rigs are being used to form nearly 8000m of diaphragm wall in 3m long wall panels up to 1m thick and 35m deep. Steel I-beams will be inserted in the top of the walls, and, along with the tunnel roof slab, will provide temporary support for the elevated section. This will allow the existing foundations to be removed, clearing the way for excavation of the underpass. When the tunnels are completed, the elevated section will be demolished and replaced with open spaces and low impact development.

At the northern end of the route, two new crossings will be built over the Charles River. At ten lanes wide, the larger of the two is one of the widest asymmetrical cable-stayed bridge ever built.

The second part of CA/T is the extension of the I-90 Massachusetts Turnpike which now ends south of downtown. This will run in tunnel beneath south Boston and the harbour and on to Logan Airport east of the city. The first part of this section, the Ted Williams Tunnel, was finished in December 1995, It relieves southbound traffic flows from the airport by carrying 25,000 commercial vehicles a day.

The project includes four major highway interchanges to connect the new roads to the existing regional system. At Logan Airport, a junction will link I-90 with Route 1A and the airport road system. In south Boston the I-90 will be joined underground to the rapidly developing waterfront and convention centre area.

At the southern end, one of the most complex parts of the project and the most challenging geotechnically is the South Bay Interchange. Linking the I-90 and I-93, the junction will be on six levels, two of them underground. This involves large tunnel jacking with ground freezing to get under mainline railway tracks; and immersed tube tunnelling and cut and cover techniques to cross the Fort Point Channel and existing subway lines. Many of the techniques have not been carried out on this scale in the US before.

Now at its construction peak, project spend is $3M a day. Vast sums are being spent on the elevated section just to move traffic on to temporary routes to allow parts of the overpass, especially on and off ramps, to be demolished, making way for new build. There are 50 formal traffic changes a week including lane shifting, night-time closures and temporary ramp construction. 'Most of the lane shifting is finished now,' says Brown. 'Some 75% of the diaphragm walls are in and the deck slab should be complete by 2000.'

Some £2bn is being spent to keep the city moving during construction, and close consultation with environment agencies and the local community over this and other mitigation measures was essential. These include reducing noise levels, restoring wetlands to the north of Boston, and using spoil to cap a landfill on Spectacle Island and to build an artificial reef.

Other benefits are an estimated 12% reduction in city-wide carbon monoxide levels, and creation of 60ha of open land, including 11ha along the existing route and 16ha of downtown park land.

The new system is due to be fully open by 2004. When GE visited site at the beginning of May, the project was almost 52% complete. The next milestone is the bridge across the Charles River, due to be finished later this year. The I-90 extension to the Ted Williams Tunnel and Logan Airport will open in 2001.

For the I-93, the northbound underground lanes will be finished first, in 2002; the southbound will switch to the northbound elevated section and the southbound elevated section demolished to allow breakthrough of the Dewey Square Tunnel. Southbound traffic will run underground from 2003.

By 2010, Brown says, the central artery is expected to comfortably carry 245,000 vehicles daily, with the Ted Williams Tunnel carrying 94,000 vehicles a day. The rush hour period will be slashed to just a couple of hours in the morning and the afternoon. The road is expected to be able to carry traffic levels comfortably for 15 years or so after it is finished.

But the state is also spending $2bn on public transport, explains Brown. 'It hopes to get people on public transport and keep them on it.' This, he says, should increase the life of the new road.

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