New York’s Subway is one of the oldest, busiest, and most comprehensive in the world – but can be difficult to negotiate. This is particularly true of Lower Manhattan, where the Capital Construction division of the city’s Mass Transit Authority (MTACC) is currently in the process of constructing the new Fulton Street Transit Center.
The project will consolidate a dozen existing Subway lines within an attractive light-filled space with logistically clear connections, and also provide a new 400ft long, 25ft wide concourse linking to the World Trade Center PATH station.
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A legacy of friction
The transit companies that built the original Subway lines in New York were jealous business rivals, with neither wanting the public to ride the other’s lines. “Friction” was deliberately built into the system says Craig Covil, Principal and Tunnel Engineer with Arup’s New York office, lead consultant for the Fulton Street Transit Project. There were originally no connections at all between the sub-surface cut-and-cover tracks that run under Manhattan, to discourage passengers from changing lines.
As the system grew the friction became more and more of a problem. Although some new connections and pedestrian routes were added after integration of the Subway under the city’s Metropolitan Transit Authority (MTA) in the 1940s, these were mainly add-ons as part of other rail projects. At key interchange points, such as Lower Manhattan, where Wall Street and the World Trade Center site are located, it was necessary to leave one line, walk along the street, and enter another to change routes.
The connections have evolved very much as an afterthought, says Covil, and like the surface entrances to the subway system, are relatively invisible. A major link was formed with the construction of the transverse A/C line station (see figure 1), built as a long subterranean box sitting over the slightly deeper A/C platform tunnels; however, this mezzanine footway has multiple ramps and links and is narrow and cramped.
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By GlobalDataIn this long passageway, and on the old narrow street pattern of lower Manhattan, there is daily confusion. One of Arup’s Senior Tunneling Engineer’s, Michael Deutscher, says he cannot go on site with his official helmet on without being asked for directions by dazed looking visitors and tourists.
Clear connections
It has long been the ambition of the MTA to change all this, and the Fulton Street Transit Center Project aims to do just that.
According to Uday Durg, MTA’s project director, the Center will take in a dozen different lines, restructuring four stations and widening or building new underground walkways between them. Further links will be added to the new World Trade Center and PATH stations.
Several contracts make up the project: One for renovation of the 2/3 station (figure 1); a major new connection under Dey Street that includes a new above ground entrance; and two contracts for the Transit Center itself.
Surface demolition and a 80ft (25m) deep secant pile perimeter wall for the Center’s two-storey deep basement is now complete but the major work is yet to start. Above ground, this building is to be a four-storey glass and steel building, designed by London-based architect Grimshaw & Partners, providing a main entrance to the east-west A/C line station and the 4/5 line station, as well as the new Dey Street connection.
The building will draw a lot of attention with its light airy spaces and the spectacular “occulus” skylight, which sits at an angle at the top of a “dome-shaped” tower (figure 2). The building’s interior lining will actually be a sculpture by James Carpenter, an artist who works with glass and metal, and will consist of a web of transparent light-reflecting panels that captures light from the occulus and throws it all the way down into the deepest levels of the building.
In order to integrate the Fulton complex into the local historic district, whose landmark buildings shaped New York City, the Center will also incorporate an existing building on the site. The historic Corbin building was built in 1888 and was reputed to be tallest building in the city at the time, as well as the first to feature OTIS elevators.
Structuring logistics
Structural design work for the old and new buildings, and the transition structure between, is highly complex and some current wrangling over cost persists in New York. However, below ground work at the site is the real focus at present.
Covil explains: “The central problem is that the connection point with the 4/5 line station is at the end of the A/C line station. Therefore passengers load themselves into the front of the trains in order to be ready for the change to the next train; they have to pass though a limited passageway at the corner of a right angle between the lines to get onto the other platforms.”
With everyone trying to make the change in the same corner at the end of the platforms, queues and congestion prevail. Train “dwell time” in the stations is extended at rush hour, with a knock-on effect to the whole system. The solution is to widen this right-angled changeover.
To do it, the MTA took over a city block on the corner between Fulton Street and the downtown section of Broadway, where the two stations meet. “Strictly speaking it is a half block” says Covil, this is where the Transit Center is located. Within the new building, an open space will drop to platform level, with stairs and escalators connecting to the platforms along half their length.
The whole space, enclosed by the secant pile walls, will be much more free flowing, with passenger ways through a series of arches and entrances to the platforms. Escalators will be fitted between unusual inverted arch spread footing foundations, which support the tie-in to the Corbin building.
The lower Transit space will reduce the strain on the existing narrow mezzanine connection over the top of the A/C tunnels. But this will be widened with new ramps and staircases installed, to comply with disabled access standards and fire regulations.
Construction challenges
All this is easier said than done says Deutscher: “The first issue is the water table, which is 33ft (10m) down.” The ground in this part of the city is not the hard schist that made Manhattan so suitable for skyscrapers, but sedimentary deposits “of silt and sand, beneath 6-9ft (2-3m) of soil with utilities.”
The first Subway lines, from about 1905, were built by cut and cover above groundwater level, but by the 1920s the use of compressed air excavation had increased. The A/C line, which cuts across the older lines, comprises twin tubes that sit below the watertable. Above these is the mezzanine, a two level concrete box with ramp and stair connections to the platforms and stations it crosses. The box is an important load on the tubes below, resisting uplift.
To demolish and rebuild it is a complex task. There is no room to establish diaphragm walls between the major buildings either side, or to do anything to the tunnels below. The solution adopted is to complete the enlargements in sections, carefully calculating potential movement below. Essentially, for short sections at a time, temporary internal walls will be built to support the roof, then the old walls demolished, leaving room to widen the mezzanine outwards. That done, new permanent walls can be built and the temporary walls removed.
This means the load on the tunnels below is always present, “but shifts its position,” says Covil. “It creates a slow moving wave of altered stress along the box, but hopefully not suddenly. It will be spread out over time, a year and half of construction in this case.” A complicated 3D and time analysis is made even more complex by the changing cross section of the box as ramps and staircases are encountered.
Analysis of the impact of works on passenger flows has also been carried out. Arup has studied the changes the new ramps and stairs will make during construction using Pedflow software. “We have done this not only in absolute terms to ensure no blockages, but also to establish optimum flows and to determine exactly what ‘level of service’ will be provided,” says Covil, who thinks this magnitude of pedestrian analysis is unique or, at least, unusual for a construction phase.
Current activity
The contract for all this work and the Transit Center above, worth over $1bn, was initially put out for bid early this year. However, the tender has been withdrawn following higher than expected costs. The MTA is currently deciding whether to modify the project, possibly by altering the above ground building. More likely is that the work will be re-packaged into smaller contracts.
Meanwhile, work has been ongoing on the other contracts. The first of three contracts let in December 2004, for renovation of the 2/3 station, new entrances, and upgrading of platforms and stairways to contemporary standards, has been completed.
A second contract at the west side of the project includes work on the R/W line station, which is closest to the WTC site and has been closed since 9/11. “The tracks are in use and trains pass through without stopping,” says Deutscher. Several outages were required for these works, which involved underpinning the tracks with micropiling to allow construction of new underpasses through to the WTC, PATH and Line E stations beyond.
These new connections will be linked to the Transit Center via a pedestrian underpass along Dey Street. The tunnel spans the full 33ft (10m) width of Dey Street, which has historic buildings on both sides including the original AT&T building, where the transatlantic cables from Europe used to be connected.
On the southern side of the street is a 1905 building that houses the famous Century 21 department store, a favourite for tourists. The store insisted on retaining street use for its delivery vehicles throughout construction, this was achieved with a heavy steel temporary deck initially and eventually via a secant pile wall.
The underpass is now nearly complete. Meanwhile, at the end of Dey Street the structure widens – with an additional box formed within secant piles – to become the stair and escalator well for a new glass and steel street level entrance.
The walkway will also connect through into the basement of the Transit Center, currently a large excavation in the ground where a third demolition contract has been completed, secant pile wall built on one side, and the site made ready for the main contract.
The project is now awaiting discussions by the MTA on the main works and the go-ahead for the largest part of the scheme.
Rendering of the future Fulton St Transit Center The historic Corbin building, which will be incorporated into the Fulton St Transit Center Fig 1- Map of the Fulton Transit Center Project components Fig 2 – Section of the Transit Center, showing the central light well and the A/C line Rendering of the new Fulton St Center Rendering of the new Fulton St Center Rendering of the new Fulton St Center
