Anything but Boring

Future Subway Construction to Use TBM Equipment

by James Murdock

Unlike the originators of the subway system, today's projects will be strewn with complexities of working in and around business, residences, commuters. It will take new methods and new ingenuity.

Modern subway contractors face a huge obstacle that would have been alien to the men who built the original line.

Contractors today must work around existing buildings and infrastructure, taking care to avoid disturbing businesses and residents. A century ago, builders were given free reign to get the job done as quickly as possible, and there was a lot less city to get in their way.

So while New York's first subway was constructed in only four years, it has taken the Metropolitan Transportation Authority almost four years to develop just the environmental impact statement for the Second Avenue Subway. Actual construction could take 16 years or more.

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Though it is incredibly time-consuming, today's detailed planning process is necessary to navigate the increasing complexities of working underground in an urban environment. New York City is one of the most built-up urban areas in the world.

It contains a subterranean network of transit lines, utilities and sewers that, if untangled, could circle the globe many times over. Combined with environmental concerns above ground, this subterranean jungle is the reason that deep tunneling will be the preferred construction method in many of the upcoming subway construction projects.

Tackling Blocky Rock
Tunneling methods have come a long way since 1900. Whereas the drill and blast technique was once the only method of handling Manhattan's schist and gneiss rock, tunnel boring machines are now standard. A TBM doesn't involve explosives,
so it's safer for contractors and creates less risk of destabilizing the ground and buildings above.

Even with today's improved technology, though, working underground is difficult. "Nobody has magic eyes," said Sheldon Roth, director of preconstruction services at Slattery Skanska Inc. "You can't look under the ground and predict what you're going to hit.

"You can sort of predict with borings - you might see some of the soil types and whether there's water or contaminants - but that's like a spotting a needle in a haystack."

And indeed the learning curve in TBM work hasn't been an easy one. Joseph Siano, a senior vice president of MTA Capital Construction Co., said that during the 1970s, contractors working on New York's third water tunnel ran into frustrating and expensive delays caused by "blocky rock" that clogged TBM drill heads.

TBM equipment has improved significantly since 1970s, but to allay contractors' concerns that blocky rock might once again cause delays, the MTA is taking on significant liability risk in its upcoming TBM projects.

"We recognize that the contractor is there to make money, so we need to share the risk, too," said Mysore Nagaraja, president of MTA Capital Construction. He added that the MTA will cover the expense of unforeseen work stoppages.

Siano recently studied TBM jobs worldwide and offered several recommendations for contractors interested in bidding on MTA projects. "You can't skimp on muck trains and conveyors," he said. "You need to make sure they don't break down. Robust equipment is one way to hedge against risk of a work stoppage."

Although it wants contractors to cover their bases, the MTA is still willing to experiment with new TBM equipment. During work on the East Side Access project this summer, it tested a road header machine designed to claw through rock and create underground caverns. Tests were suspended, though, because the road header kept breaking down. Gneiss proved too hard for the machine's claw-like rotating heads.

Everything Old Is New Again
Not all of the upcoming projects will require TBM work. The Second Avenue Subway will require several construction methods.

"Geological conditions from 125th Street to Hanover Square vary tremendously," said Margarita Gagliardi, a senior vice president of STV Inc., which is a subconsultant on the project. "Depending on the geotechnical conditions, there will be hard-ground tunneling, soft-ground tunneling and even some cut and cover."

Today's cut-and-cover method involves different techniques than when it was first used to build the subway in 1900, said Bernard Grand, a design and construction coordinator at Slattery Skanska. Due to concerns over polluted ground water, environmental regulations now prevent dewatering: so whereas tunnel boxes were once built with soldier piles, which required extensive dewatering around the worksite, they are now constructed with slurry walls or jet grout.

Ground freezing, an even newer technique to avoid dewatering, may be used in Second Avenue Subway construction. "It's an innovative way of stabilizing adjacent soil," said Fred Werner, president of DMJM+HARRIS Inc., which is doing the design and engineering for the project in a joint venture with Arup Group Ltd. "It allows you to minimize settlement and potential disturbances."

In addition to starting new projects such as the Second Avenue Subway, the MTA is continuing its station rehabilitation program. Nearly 150 out of 468 stations have been renovated since the MTA began its state-of-good-repair campaign in 1982. As part of its proposed 2005-2009 budget, the MTA has set aside money for renovating roughly 40 more.

Gottlieb Skanska Inc. is no stranger to this work. It has restored tile work and installed new escalators in the rehabs of 15 stations. Arnold Kirsch, company president, said staging is the biggest challenge.

"We have to work around an operating system," he added. "Depending on the station, it can get extremely difficult because the platforms aren't very wide and you can only work when the MTA shuts down service for a night or a weekend."

Fortunately, technology today creates some time savings. Unlike 100 years ago, when each tile had to be set individually, tiles now come in factory-assembled sections.

Behind the scenes, meanwhile, the MTA continues to upgrade signals and other basic infrastructure. It is also poised to begin $591 million worth of security enhancements suggested by a recent vulnerability assessment.

Design consultants on the project include Parsons Brinckerhoff, Washington Group, HNTB and URS.

Though Nagaraja isn't allowed to specify what the security enhancements will entail, he said the MTA hopes to have construction contracts awarded by 2006.