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Fairytale of New York - Second Avenue Subway takes shape

Londoners may think Crossrail has been a long time in the planning, but New Yorkers are finally seeing the Second Avenue Subway project come to fruition after an epic 80 year wait. Alexandra Wynne reports from the Big Apple on a scheme that will take a big bite out of its core.

In over 50 years New York City has not seen a subway project on a scale to rival the one now underway beneath a long stretch of the east side of Manhattan.

This is the Second Avenue Subway, and it is a mega project at that. Costs were put at $16bn (£10.58bn) in 2004 and the scheme will involve a medley of construction methods as well as all the associated challenges of tunnelling for 13.7km beneath the entire length of densely populated Manhattan.

These challenges might sound familiar to those embarking on London’s Crossrail, but in New York they have been a reality for the construction team since its work started in April 2007. Now, despite several major interruptions, the neighbourhood along Second Avenue east of Central Park is witnessing evidence of construction in progress. Work is focused on building the first of four phases.

At present this is the only phase to have secured funding – to the tune of £2.85bn – and it covers a section running south from 96th Street to 63rd Street. Eventually the scheme will provide a two-track line linking the Metro-North suburbs and Harlem at 125th Street to the financial district at the southern tip of lower Manhattan.

Aecom Transportation, formerly DMJM Harris, is lead partner in a joint venture with Arup, which is consultant for the entire scheme. Aecom Transportation holds a two thirds stake in the joint venture. Its scale has not escaped the attention of those involved with its delivery. “It’s a once in a lifetime project,” says Aecom Transportation vice president and project manager for the scheme Christopher Bennett. “I had always dreamt of working on a project like Second Avenue Subway and now, after 80 years’ planning, I’m the one who’s responsible for getting it done. It’s history in the making.”

New York’s Metropolitan Transportation Authority (MTA) appointed the joint venture in December 2001 for conceptual design and preliminary engineering work. The joint venture began its final designs in April 2006. Bennett says that 90% of the project will be a tunnel boring machine (TBM) job. The rest is made up of cut and cover and mined drill and blast sections – typically around the 16 stations – each an average of 275m in length, which are planned for the whole scheme.

Aside from tunnelling the railway, the first phase also includes building a cut and cover station at 96th Street, which is one of the shallowest at about 15m deep. In addition, stations at 86th and 72nd streets will be mined, a challenge given the number of high value, high rise properties close to these sites.

The two mined stations are at depths of between 25.9m and 27.4m in rock and the construction method for these deep rock caverns is one that Bennett says helps to ease any concern for what lies above. “[Mined stations] are less disruptive. There are some very picky communities and expensive real estate with some 30-storey to 40-storey high rises there. This way we only needed two shafts for excavation work,” says Bennett.

Much of the current focus is on cut and cover sections between 92nd and 95th Streets. The MTA awarded the first construction contract worth £226M in March 2007 to S3 Tunnel Constructors, a joint venture comprising of Skanska USA Civil, Schiavone Construction and JF Shea Construction. This is for the portion of Phase 1 work which involves tunnelling between 63rd and 92nd Streets and building a 248m long by 23m wide TBM launch box, which will eventually become part of the 15m deep 96th Street station.

The contract also includes two access shafts for the 72nd Street station. Slurry or diaphragm walls, 1.1m wide and 6.1m long and around 35m deep, flank the sections between 93rd and 95th Streets. Where rock is shallower between 91st and 93rd Streets, 1.1m diameter secant piles do the same work at reduced depths.

Once the walls are installed the team excavates the earth between them and builds the box structure using the bottom up construction method. Temporary decking forms the top of the box and performs two vital functions. It braces the excavation – supporting the installed walls – while carrying Second Avenue road traffic as excavation work continues below.

Along the whole of the Second Avenue route tunnelling and stations will likely be up to about 30m below Manhattan. In the same way that buildings have swelled to cover so much of the island above, below ground is not exactly uncharted territory. Arup director of construction David Caiden says: “It’s a spaghetti of tunnels, utilities, pipes and cables – I’ve never seen anything like it.” Bennett adds: “We pretty much have the same thing to deal with as London [for Crossrail] weaving above and below underground obstructions.”

In addition to utilities, within the four phases tunnels and stations will have to go over and under various Subway lines, Amtrak railway lines and a road tunnel linking Manhattan to the borough of Queens to the east.

Tunnellers will have to face geological anomalies along the way. The geology of Manhattan varies along the Second Avenue Subway’s length as the line passes through rock and soft ground. Ground conditions comprise sands, silts and clays overlying Manhattan Schist but Bennett says there is more to contend with: “We have got faults and shear zones – there’s some difficult rock with some fractured rock there too.” Hard rock 6.7m diameter TBMs will do the work on this first contract. “Once the TBM gets going I predict it will be straightforward,” says Caiden. He expects progress to be at a rate of about 20m per day.

The first TBM is a hybrid machine – meaning that Robbins manufactured it but that it is in the process of being rebuilt at Schiavone’s yard in New Jersey with additional parts from Herrenknecht. The machine is expected to make its debut on site this summer.

But apart from the technical challenges, there is another problem – how to find enough tunnelling staff to do the job. “There’s a lot of other tunnelling work going on in New York right now and there’s a lot of competition for experienced labour,” says Bennett. To illustrate the point, the S3 consortium is also carrying out tunnelling work to extend Line 7 on the city’s Subway system. And while some of the Second Avenue contractors are involved with Line 7, as well as work on the Fulton Street Transit Center, Bennett says it is a challenge ensuring their resources are focused on the Second Avenue work.

Construction of the entire Phase 1 portion is scheduled to be complete by 2015. Before completion of the entire new Second Avenue line, the first phase will act as an extension to the Subway’s Q line and will carry 190,000 passengers per day. New Yorkers will no doubt be keeping their fingers crossed that confidence in the project remains strong throughout the current economic crisis so that the entire scheme will finally become a reality – a century after it was first conceived.

Click here for diagram and images of the Manhattan transfer

Second Avenue Subway: the line that nearly never was
If Londoners have felt frustrated by the on-again-off-again nature of Crossrail, residents of New York have lived through an epic wait for the Second Avenue Subway since it was first conceived in the 1920s.

It suffered its first setback with the Great Depression in the 1930s. Then in the 1942 the Second Avenue elevated railway was torn down to make for property developments along the route. Its departure seemed to pave the way for an underground equivalent to take off, but America’s involvement in the Second World War meant attention was diverted elsewhere.

The Third Avenue elevated railway met the same fate as that on Second Avenue in the mid 1950s and since then the Lexington Avenue Subway line has suffered overwhelming pressure and overcrowding as the only line taking passengers north and south along Manhattan’s Upper East Side while the Upper West Side luxuriates in access to three subway lines.

The Second Avenue Subway’s fortunes revived temporarily in the early 1970s, and there was a groundbreaking ceremony in 1972. But the city hit a financial crisis and work halted in 1975. Now the project is back on track and hopes are alive that the new Subway line will be finished.

Who’s who

  • Designer
    Aecom Transportation/Arup joint venture

  • Client
    Metropolitan Transportation Authority (MTA)/New York City Transit

  • Client project manager
    MTA Capital Construction Company

  • Contractor
    for first section in Phase One S3 Tunnel Constructors joint venture comprising Skanska USA Civil, Schiavone Construction and JF Shea Construction

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