To the casual observer, Fulton Street in downtown Manhattan is little more than a run down shopping area whose main point of interest is the Ground Zero site at its western end. Here and there narrow subway entrances are squeezed between shop fronts, with even the largest far from obvious.
But underneath Fulton Street is one of the subway's busiest interchange stations. Around 300,000 passengers a day pass through the station's small and confusingly signed underground corridors. The A and C lines run the length of Fulton Street, crossed at three points by the JM and Z lines, Lines 2 and 3 and Lines 4,5 and 6.
This year New York's Transit Authority, advised by consultant Arup, is to start a major $750M overhaul of the station, making it more accessible to commuters and tourists alike.
The centrepiece of this will be a spectacular glass and steel 'occulus' building designed by the UK's Grimshaw Architects to serve as the new main entrance.
The structure comprises a lattice framed, truncated cone which will rise eight storeys above ground and funnel natural light into the underground concourses. It will provide a visual focus for a growing influx of tourists expected to visit the area with the redevelopment of the World Trade Center Ground Zero site, following its destruction in the 9/11 attacks.
But it is the work below ground which is likely to tax the engineers most.
Under Broadway, which cuts across Fulton Street, a new underpass must be excavated to increase underground capacity and ease passenger flows from the main entrance.
Most of this will be done in an open cut, but one section has to be dug below the two platforms serving Lines Four and Five, which share track at this point.
To do this, contractors will need to insert piles and support beams to carry the live subway while excavation work takes place below.
'We will need a fairly dense arrangement of underpinning piles through the platforms and trackbed slabs, ' says Arup structural engineer Tom Rice. Three temporary piles will be installed from street level to support the roof to the side of and between the tunnels, with a line of soldier piles and lagging from platform level to the top of the tunnels.
Under Fulton Street itself, a mezzanine passageway sandwiched beneath the road and above the two platforms serving the A and C Lines will be enlarged, presenting serious challenges. The passageway structure comprises a steel frame with concrete cover.
The mezzanine level is the base of a cut and cover box that sits in the ground independent of the bored tunnels housing the A and C Lines. Vertical loads are transferred to foundations via closely spaced steel columns that pass through the platform and on to a base slab between the running tunnels, which sits directly on the underlying glacial sands and silts.
'There is no scope for inserting new foundations because of the running tunnels, ' says Rice. 'So we have created a new structure with a new floor beam above the existing floor, and cantilevered it out above the tunnels but supported it on the platform columns.
Building the new structure will involve opening up the 10m deep, 8m wide box to street level, and creating a new one just over 11m wide - as wide as Fulton Street.
Construction will start with installation of instrumentation in the cast-iron lined tunnels and buildings running down either side of Fulton Street.
Most have basements and sit on shallow foundations and will be underpinned. Where basements encroach on the new passageway, they will be removed. Two sewers running along the outer walls of the mezzanine box also need relocating as will other services just below street level.
After protective shields have been installed in the roof of both tunnels, secant pile walls made up of 900mm diameter piles will be installed along the road either side of the passageway and propped to resist lateral loads during excavation and demolition of the original structure. Steel columns installed inside the excavation will support a temporary road above during excavations.
Groups of four tie-down anchors will be installed down to competent bedrock at 9m centres along the platform and pretensioned to resist uplift during excavation and construction of the new passageway.
Before the excavation reaches groundwater level (between 9 and 10.7m below ground level), the space between the base of the secant walls and the tunnels will be sealed to provide a cut-off.
The secant walls will be shotcreted with a waterproofing membrane laid over. Finally, the walls will be cut down a minimum of 1.8m to allow construction of the new road.