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Uptown underground

Transport - Manhattan's Upper East Side is getting a new Subway line.

Subway commuters on the upper east side of Manhattan are getting a rough ride at the moment. Hundreds of thousands of passengers a day have to squeeze onto the busy Lexington Avenue Line as they battle in to work downtown.

Many have a long walk to get to their nearest station as the Lexington Avenue line runs several blocks east of homes close to the banks of the Hudson.

Things will be different in seven years time, if construction of a new $16bn (£8.4bn) line under Second Avenue gets under way as expected this spring.

The $3.3bn (£1.7bn) phase one of the new line will run parallel to the Lexington Avenue line on the upper east side of Manhattan between 96th and 63rd Street. When it opens in 2012, it will connect directly into the existing Q Line which will carry trains further downtown.

'Phase one will have 200,000 riders on the opening day, ' says MTA's Second Avenue program manager Anil Parikh.

Tunnelling contracts for phase one are expected to be awarded in April. These will be followed three years later by contracts to build stations at 96th Street, 86th Street, 72nd Street and 63rd Street.

Phasing is necessary because of the need to spread the project's vast cost over many years. 'To win federal government financing we had to look at doing the project in phases, ' says MTA Capital Construction president Mysore Nagaraja. 'The first phase of the project is the most beneficial.' It is also relatively easy to build, as the twin bored tunnels will run through hard granite.

The other three phases are more complex as they involve soft ground tunnelling close to buildings, many of which are high rise. Phase four, at the southern tip of Manhattan, will be in reclaimed land, and contractors are expected to encounter buried wharves and even old ships as they work their way towards the terminus at Hanover Square.

Work on phase one will start this year with the excavation of a tunnel boring machine (TBM) launch box at the southern end of the open cut for the 96th Street station. Here soft ground to the north gives way to the hard granite, which characterises much of the phase one route.

MTA, advised by consulting joint venture DMJM Harris Arup, decided to award the tunnelling contracts first because it wanted the TBMs clear of the stations by the time work on them started.

The stations are to be forged underground using drill and blast methods, another reason why completing the tunnelling work first is important.

'The tunnelling contractors will construct shafts to provide access to the stations, ' says DMJM Harris Arup tunnel team leader Peter Chamley. 'It avoids having TBMs coming into a half fi nished station.' MTA and DMJM Arup decided phase one work should be a mixture of design and build and more conventional design, bid and build, where design work is kept separate from construction contracts.

Tunnelling will be design and build, but the more complex and aesthetically demanding station work will be design, bid and build, with MTA consultants carrying out designs for contractors to execute.

'There are so many aesthetic issues with the stations that we did not want to leave the work to a design and build contractor, ' says DMJM Harris vice president Geoffrey Fosbrook.

The 30m wide by 300m long station box at 72nd Street is the most technically challenging part of phase one, says Nagaraja. It will span three tracks, filling a cavern blasted beneath the full width of Second Avenue, so buildings of up to 40 storeys on either side will have to be protected from vibration damage.

'The 30m wide cavern is probably one of the largest in the United States, ' says Fosbrook.

Building the connection between the Second Avenue Line and the Q Line running into the two level 63rd Street station is also technically complex. The twin tunnels have to curve to the west and twist their vertical alignments so they can get under the Second Avenue running tunnels and into the station.

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