A subway for Second Avenue - 100 years late
Constructing a new subway line in a major city such as New York throws up a wealth of challenges aside from engineering ingenuity.
It has been almost 100 years in the making, but the citizens of New York's Upper East Side will soon, at long last, have access to a subway. Phase one of the Second Avenue Subway is five years into its eight-year programme and is not just a huge engineering challenge, but a perplexing social programme.
As well as being one of the oldest public transportation systems in the world, the New York Subway is the most extensive, with 468 stations and 842 miles of track. In 2011 over 1.64 billion trips were made on the system, on average 5.3 million every weekday. The east side of the peninsula, though, remained resolutely unconnected to this sprawling network until work (re)started on the new section in April 2007. It's a long story.
Working in such a densely populated environment adds to the complexity of the operation. The area beneath which the construction is taking place has over 100,000 residents per square mile. "It is an extremely difficult project, and I am going to put aside for now the engineering part of it," Dr Michael Horodniceanu, president of MTA Capital Construction, and the man responsible for delivering the project, says. "The biggest challenge from the community point of view is to construct and operate in the area for what will be a period of nine years building in front of people's living rooms and I really mean that. Sometimes we are as close with our construction sites as ten feet from buildings.
"At the low level these buildings contain retail outlets that we are heavily impacting. Dealing with that turned out to be as hard as any engineering challenge that we have faced on this project. In the United States we have only three other census tracts that have more than 100,000 people per square mile: one in Los Angeles, one in Chicago and one in Boston; these are all prisons."
As in most large cities a 'NIMBY' culture exists in New York. "It is very hard, and New Yorkers, like residents from all large cities, would like to see a Second Avenue Subway that is being built on Third Avenue," Dr Horodniceanu adds. "[But this] is a truly fabulous engineering project, and as I am an engineer I can tell you I am overjoyed to be able to be part of this."
The Second Avenue Subway is one of four projects on the table of Dr Horodniceanu, the other three being the extension of the number Seven line, the creation of a new terminal downtown – a multi terminal where seven subway lines come together – and an $8.3bn project to build a new terminal 160ft beneath Grand Central station. These are all major undertakings in their own right.
"We are building another subway – a railway station under the Grand Central, to bring trains into the city underground from Long Island into the heart of Manhattan," Dr Horodniceanu says. "At the moment I have about $18bn of work in my portfolio so it's exciting times. The Second Avenue Subway is exciting because it's the job everyone was dreaming of for years."
A bit of old New York
That's 94 years to be precise. Back in 1919 engineer David L Turner produced a report 'Proposed Comprehensive Rapid Transport System' for the New York Public Service Commission. Amongst his multitude of proposals was one for a six-track subway along Second Avenue with links to the Bronx, Brooklyn and Queens.
Ten years later there were cautious plans for expansion of the network, including the Second Avenue line, but they were first delayed by the Great Depression and then shelved with the outbreak of the Second World War. Various options were mooted in the post-war era but nothing materialised until the late 1960s when the US Congress passed the Urban Mass Transportation Act. New York was granted $600m and its priority was a Second Avenue line stretching from 34th Street in the south right up into the Bronx. Construction eventually began at 103rd Street in 1972. However, once again a financial crisis put paid to the bold plan: construction was halted with only three of the northern tunnels completed.
The shoots for the current plan first sprouted in the mid-1990s, and progressed with the sign off of the final environment impact statement in 2004. The current scheme is a two-line track running from 125th Street in Harlem, through the Upper East Side and Mid-Town right down to the tip of Manhattan in the Financial District.
Unlike earlier subways it was decided that the Second Avenue subway would primarily adopt a tunnel boring strategy. The initial excavation was a 'cut and cover' project at 123rd street that enabled the tunnel-boring machine to be positioned, but from there the tunnel system was bored. There were two overriding factors that prompted this decision: limiting disruption to the public and avoiding the network of utilities cables and pipes that criss-crossed the route, largely unmapped.
"The maze of utilities under the streets of Manhattan is so large that it would have been a folly to try and do this any other way other than tunnel boring," Dr Horodniceanu adds. "However we first needed to create a launching box, and we elected to do that at the northern station where the ground was in a transitionary state between soft ground and rock. From that point on we went down about 85 feet to bore underground all the way south to 63rd Street. This was about 30 blocks, about a mile and a half."
For tunnelling operation soil conditions are key to the success. "We were aware of the soil conditions before we began the project as we had taken boring samples along the route so the design accounted for that," Dr Horodniceanu says. "However the soil is not always exactly how you imagined it would be because you are not always hitting where you need to hit. In some place we hit a lot of rock fragmentation that we did not expect. This seemed to happen at the very beginning of the project then for whatever reason we got lucky and the rock was pretty uniform."
The bedrock underlying much of Manhattan is mica schist known as Manhattan Schist. It is a strong, competent metamorphic rock created when the Pangaea supercontinent formed during the late Paleozoic and early Mesozoic eras. It is well suited for the foundations of tall buildings and the two large concentrations of skyscrapers on the island occur in locations where the formation is close to the surface. In Central Park, outcrops of Manhattan Schist occur and Rat Rock is one rather large example.
"With a few exceptions the quality was good. They say the nightmare for a tunneller is to see the daylight while they are tunnelling. At one point we were a bit afraid, but in general we had good rock cover."
One of the major engineering challenges was removing the waste material from the boring and mining operations. The majority of the bored waste was transported back to the northern extremes of the tunnel by an intricate network of conveyors and trucked out of the city from there. But when it came to the mining excavations for the stations another methodology was required.
"The muck removal in an area that is full of all these luxury high-rise buildings is not the most coveted activity by the residents," Dr Horodniceanu explains. "We learnt from our operation to remove the bored waste that if we were to do the same thing in the caverns we would have a real problem with the residents.
"What we ended up doing is building muck houses that were 24ft wide, 45ft tall and 200ft long – they covered an entire city block. All the activities occurred inside. There was noise insulation for the blasting: the area also absorbed all the dust and prevented it escaping to the outside environment. All the mucking was handled in here. The trucks would go in and take away containers of muck that was brought from below using electrical cranes rather than diesel. It was all removed with a minimum impact to the surrounding neighbourhood."
As Dr Horodniceanu implies, there is more to managing projects in major urban environments than engineering. "Someone asked me today about my role on one of the elected official's tours and I said that I was half engineer and half public relations guy," he explains. "It was important to communicate and we have undertaken some real community management.
"There was a need, and this was very important to us, to establish trust with the surrounding residents. I had in effect to put my face on the project: I became the frontline representative. I started by walking the street, going into shops and stores, talking to them about what I could do to make their lives better.
"We have put in good neighbourhood initiatives: workshops every quarter where we invite people from along the corridor and sit them at tables together with our designers, contractors and construction managers. We ask them to identify problems, but also to suggest what solutions they believe we can adopt. Stopping the work and just bitching and moaning is not something that was going to be tolerated, but we ask them to come along with constructive ideas and suggestions. Between us we have done wonders to alleviate some of the issues."
The project is moving on quickly and on schedule to open in 2016, after which the future is again in the hands of politicians as approval is sought to continue with phases two, three and four. But whatever the outcome of the political machinations Dr Horodniceanu and his team will have left a lasting legacy for the citizens of Manhattan.
Boring in New York
The Robbins Main Beam tunnel-boring machine (TBM) is complex in design, yet simple in concept. The front of the TBM is a rotating cutterhead that matches the diameter of the tunnel. The cutterhead holds disc cutters ranging from 11in to 20in diameter, which are positioned for optimal boring of the given rock type.
As the cutterhead turns, hydraulic propel cylinders push the cutters into the rock. The transfer of this high thrust through the rolling disc cutters creates fractures in the rock causing chips to break away from the tunnel face. A unique floating gripper system pushes on the side walls and is locked in place while the propel cylinders extend, allowing the main beam to advance the TBM.
Buckets in the rotating cutterhead scoop up and deposit the muck on to a belt conveyor inside the main beam. The muck is then transferred to the rear of the machine for removal from the tunnel.
At the end of a stroke the rear legs of the machine are lowered, and grippers and propel cylinders retracted. The retraction of the propel cylinders repositions the gripper assembly for the next boring cycle. The grippers are extended, the rear legs lifted, and boring begins again.
The open design allows quick access directly behind the cutterhead for the installation of rock support, making it ideal for unlined tunnels.
Engineering the New York daily commute
Three main techniques are being used for tunnel and station construction depending on the geological conditions and specific facilities that could be needed at street level, such as station entrances and vent shafts. The three construction methods are tunnel boring, cut-and-cover and mining.
Much of the Second Avenue Subway has been built using tunnel-boring technology, in which a powerful circular cutting machine drills a tunnel in rock or soil. The excavated material is then taken to street level and removed by truck. Tunnelling has been done through bedrock wherever possible, which is quicker and more cost effective than mining or cut-and-cover.
There are two types of boring machines: the Tunnel Boring Machine (TBM), used in rock, and the Earth Pressure Balance Machine (EPBM), typically used in soil. A TBM is being used on phase 1, where the bedrock is close to the surface. An EPBM may be used in soil south of 6th Street, because the bedrock is relatively deep.
Cut and Cover
Most of New York City's subway system was built using the cut-and-cover technique. With this method, a trench is cut in the street, the soil is supported by vertical walls and a frame is built to support concrete or metal street decking. The decking allows the street to remain open to traffic and pedestrians while excavation and construction continue in the tunnel below.
Cut-and-cover was used at the original TBM insertion point at 96th Street. Through the cut in the street, excavated material can be removed and equipment can enter or exit the tunnel.
The primary method of mining in rock is drill and blast. This procedure involves drilling many small holes within a rock area and then placing small amounts of explosives in each hole. Under carefully controlled and monitored conditions, explosives are then detonated sequentially for short intervals, breaking the rock while spreading the release of energy from the explosives over a longer period, lessening potential ground vibration at nearby structures. When mining is done in soil, the drill and blast method is not required.
Mining will be used on portions of the tunnel too short to make tunnel boring cost effective, and on curved portions of the tunnel that are too constrained for the TBM. Some stations will be excavated from below street level, and the street will be penetrated to build station entrances and venting.
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