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Check-in time at JFK A new light rail link to New York's John F Kennedy Airport has to pass under two taxiways before it can reach the central terminal area.

USA FOCUS

In mid-May contractors were relaying one of the main taxiways at John F Kennedy Airport, marking the end of eight months of intensive work to complete one half of a tunnel for a new light rail link with airport.

For most of the route, the railway will run in elevated section along the central reservation of the Van Wyck Expressway on 10.5m high supports, but as it approaches the central terminal area it must drop to ground level and then below ground to negotiate its way past two busy taxiways.

The opening of the new AirRail service should mark the end of years of frustration for passengers trying to reach JFK Airport from New York City. Until now the only way to reach the airport has been on the Van Wyck Expressway, which is regularly jammed with traffic, leading to two hour delays. A rail link has been mooted for some time and is now going ahead as part of owner/operator the Port Authority of New York and New Jersey's £4.4bn revitalisation programme for JFK. This includes new and improved terminals, roads and communications across the airport.

The AirRail Transit Consortium, a joint venture of Skanska USA, Perini Corporation and Bombardier Transit Corporation, is building the driverless system under a £581M design, build and operate contract paid for by the Port Authority and from passenger revenue. The 14km line includes a 3.2km loop serving the airport's nine terminals and a 5.3km route to Howard Beach subway station, via a maintenance area and long-term and employee parking areas. These parts are expected to open in spring 2002. A further 5km extension north to Jamaica station in the New York borough of Queens will open the following year, cutting travelling times to Manhattan to under 45 minutes.

Construction of the 1.6km section including the twin tracked tunnel is worth £55M. Because one of the taxiways must be open all the time, the tunnel is being built in two parts. George Tamaro of New York-based geotechnical consultant Mueser Rutledge Consulting Engineers explains that as a result, work had to be carried out within a very tight timeframe.

Contractor for this section, EE Cruz & Co, began work on the first half of the tunnel under taxiway B in September 1998 and had to be finished by mid-May this year, whether or not the tunnel had been completed. Work on the second half of the tunnel will begin in September, when taxiway A will be closed, so outstanding work would have to wait until the May 2000 completion date, with obvious cost implications.

EE Cruz division vice president Joseph Malandro says that the company had to order massive temporary steel beams and precast concrete deck segments that could be laid if work did not finish on time. Fortunately for the firm, work is right on schedule.

JFK airport sits on thick layers of hydraulic fill, organic clay and silt and glacial sand, with gneiss and schist bedrock at depths of up to 240m. Groundwater is a mere 1.5m below ground level in places, so excavations had to be water- tight. These were formed by Frodingham steel sheet piles, chosen, says Tamaro, because of their tight interlock. These were driven in sections 'to maintain the lock', using a 7.5m high and 20m long frame by EE Cruz.

Tamaro says extensive dewatering could not be used because a fuel farm near the site was known to have associated contamination. 'We didn't want to increase the drawdown and spread contamination,' he says. Some pumps are used to dewater within the tunnel.

The base of the 300m long, 30.5m wide and 7.6m deep excavation is sealed with a 1.5m thick sodium silica grout blanket which comes to the toe of the sheet pile wall. This was introduced by geotechnical contractor Trevi/ Icos from ground level using five port tube-a-manchettes arranged on a 1.5m triangular grid.

The 22m wide concrete rectangular sectioned tunnel has to be able to carry the loads from the heavy planes passing along the taxiways above, so it has a 1.7m thick roof slab and a base slab between 2m and 3.3m thick.

As well as the railway, it also has to accomodate an 11.6m wide, two lane airport service road that will replace existing road links to the Van Wyck Expressway.

A pumping station is also included. This will keep the tunnel dry when it is in operation and is the deepest part of the scheme at 13.4m. Construction of this presented problems when the sheet piles were being installed.

'We couldn't even get them to the base of the excavation,' says Tamaro. So grouting had to be used here to allow excavation to final depth. An 11.6m thick jet grout layer was formed up and around the sheet piles.

Tamaro adds that the grouting was also used instead of compaction to ensure the ground around and below the final tunnel did not loosen and to counteract the risk of seismic liquefaction. This was checked using cone penetrometer tests before and after grouting.

Work has to be carried out in the shadow of the daily activites of the airport, with the wing tips of Jumbo Jets just clearing the top of the safety fencing. But it has not stopped the frantic pace needed to finish on time. 'Our fastest concrete pour was 1,200 cubic yards [915m3] of concrete in a ten hour shift,' says Malandro.

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