Miracle on (and Under) Second Avenue

A New York City off-again, and now on-again, public infrastructure project depends upon intense collaboration between architects and engineers

Sara Hart

With derisive taglines like "the most famous thing that's never been built" and "the line time forgot," news this spring that New York City's Second Avenue Subway (SAS) was about to start construction was met with nearly unanimous approval tempered by cautious optimism, despite the guarantee of years of unceasing construction affecting hundreds of thousands of Manhattan residents, institutions, and businesses. Public consensus acknowledged that the long-term advantages outweigh the short-term disruptions and inconveniences.

Predicted to cost $16.8 billion (in 2004 dollars), the mega-infrastructure project is the largest public-works undertaking in New York in 50 years. To be built in four phases, over an estimated 16 years, the new subway-also known as the T Line-will serve approximately 500,000 riders daily at 16 new stations along 8.5 miles of new track. It will relieve the overburdened Lexington Avenue Line, the only subway serving the East Side, which reportedly operates at 120 percent capacity.

Almost from the moment of conception in 1929, the project became the perennial victim of every 20th-century economic crisis and vagary of war. During the Great Depression, the project was scaled back and then postponed indefinitely in 1939. A new master plan was considered, then suspended in 1941 along with other nonessential public works, when the U.S. entered World War II. Another postwar scheme was shelved due to huge inflation experienced during the Korean War. Multiple iterations later, construction began in 1972, only to be abruptly halted as New York City teetered on the brink of bankruptcy.

Another 20 years went by, and finally the Metropolitan Transit Authority (MTA) and New York City Transit (NYCT) began the Manhattan East Side Alternatives Study. The goal was to recommend a course of action to reduce crowding and delays on the Lexington Avenue Line and create mass-transit accessibility for the far East Side of Manhattan. The study team compiled a list of more than 20 alternatives to resolve existing and future transportation issues, including the resurrection of the master plan for a new Second Avenue Subway.

The Second Avenue Subway will be built in four phases over an estimated 16 years. Image: Courtesy DMJM Harris/Arup Joint Venture

In July 2004, the Federal Transit Administration (FTA) certified that the requirements of the National Environmental Policy Act had been satisfied for the Second Avenue Subway project. By December, preliminary engineering was completed for all four phases. Two years later, in April 2006, Extended and Final Preliminary Engineering was done. The FTA then authorized the MTA to begin the Final Design of Phase I. On April 12, 2007, the governor, other state and city officials, and several transit administrators broke ground for the fourth time since 1929 and declared the project under way. And they mean it this time.

It takes a big village to build a subway

Public infrastructure projects are by definition enormously complex undertakings, involving dozens of local, state, and federal agencies; community boards; arts commissions; and teams of architecture, engineering and construction consultants. Officially, the MTA and the NYCT are the clients. In that role, the MTA assembled a team to plan, design and engineer, schedule, and oversee construction of the phased project. This team, a DMJM Harris/Arup joint venture, is a complex, expansive architecture and engineering partnership. The principal players of the joint venture, representing all the design and engineering specialties, work under one roof under the authority of the NYCT's Second Avenue Subway Project, an entity of the MTA Capital Construction Company.

"There are many stakeholders involved in every phase of planning and design," says Anil Parikh, SAS program manager. "We conduct working groups with technical advisers, the design team, and user representatives to discuss constructibility, operations, and maintenance, and then investigate options, debating the pros and cons of each," he explains.

1. The Second Avenue Subway Stations, whether vaulted or rectangular in profile, are column-free to the extent possible. This structural strategy will improve passenger flow, visibility, and security. In order to achieve the column-free structure without excessive cost or large beam depths, economical structural spans and spacing modules are incorporated into station design. 2. The underplatform and overtrack exhaust systems are central to the air-tempering and ventilation strategies. Exhaust ducts and fans will remove air from areas closest to the two largest sources of heat in the station-the brakes and car air conditioners. In the winter, by turning these exhaust systems off, enough heat will be retained within the station to render mechanical heating unnecessary. 3. During a fire, the mechanical system operates in an emergency mode and extracts smoke from the tunnel and underground stations. Simultaneously, the system provides fresh air to passenger egress areas, to create zones of higher air pressure and to minimize migration of smoke. 4. The lighting design for the Second Avenue Subway will improve the passenger experience with brightly lit walls, enhanced lighting at transition points, and the use of daylight, especially at entrances. Energy-efficient technologies, in concert with a stationwide lighting control system, will be incorporated throughout. 5. The new subway line's signage will employ both fixed and variable messaging elements. The centrally controlled variable-messaging system will display updated train information and emergency information. Fixed and variable signage will be integrated into the architectural features to increase visual clarity and enhance navigation. 6. Through geometry and the strategic use of sound-absorptive materials, the stations will provide comfortable aural environments with a clear and intelligible public address system.

Although many alternatives are identified for every stage, all decisions must answer to a higher power-the assessments of the painstakingly researched environmental-impact statement. Because the SAS project is the beneficiary of federal funding, such an evaluation was required prior to construction in accordance with the National Environmental Policy Act. The client and the FTA recently completed the final environmental impact statement (FEIS). The document notes that although it is final with regard to starting construction, it can be amended as previously unaddressed issues or unanticipated complications arise.

Some of the many factors the FEIS takes into account are the project's potential effects on transit service and roadway congestion, and social and economic conditions. It also considers issues such as air quality, noise and vibration, energy and natural resource use, and contaminated material disposal. Mitigation measures to reduce localized impacts are described in the document. These are requirements, not guidelines. Assessments are based on "reasonable worst-case scenarios," which means that while there may be alternatives for any given process, the FEIS evaluated the one with the greatest potential for disruption. The assumption is that approved alternatives, by definition, fall within the limits set by the FEIS for the worst-case scenario.

Between rock and a hard place

On-site blasting and excavating are the inaugural steps of most construction projects, and ground breaking usually proceeds without drawing much attention. In the case of the SAS, the sheer breadth and depth of the tunneling effort required considerable evaluation and detailed explanation in the FEIS. The document describes three approved tunnel methods-tunnel boring technology, cut-and-cover, and mining.

Transparent canopies will define station entrances, provide protection from the elements, and bring daylight into station mezzanine levels. . Image: Courtesy DMJM Harris/Arup Joint Venture

Some of the tunnels will be excavated using powerful circular boring machines that drill horizontally through the earth. The project will employ two types of boring technologies-a tunnel boring machine (TBM), for cutting through bedrock, and an earth-pressure-balance machine, to bore through soil. Although the street must be excavated in order to insert either type of machine at the desired depth, the process causes little additional surface disruption.

Most of New York City's existing subway system was built using the cut-and-cover method. Although it causes the most surface disruption of the three technologies, it is the easiest method for building a section of tunnel that is relatively close to the surface. This method involves digging trenches from the surface, holding back the surrounding earth with retaining walls, and constructing a frame to support a concrete or metal street deck. Cut-and-cover will be used to excavate more than half of the tunnels and to create entrances at every station location.

The third technique is mining, which will be used on portions of tunnel too short to make tunnel boring cost-effective, or for curved sections of the line where the radius is too tight for a TBM. To create tunnels using this method, contractors drill many small holes within a rock area and then place small amounts of explosives in each hole. Under carefully controlled and monitored conditions, explosives are detonated sequentially for short intervals of time, breaking the rock while dissipating the release of energy, lessening the potential for ground vibration at nearby structures.

The profile of the Second Avenue Subway stations and tunnels will be either vaulted or rectangular in shape, defined by the excavation method employed. Platforms (top left) and mezzanine levels (top right) will be largely column-free to aid passenger navigation, visibility, and security.

When mining is done in soil, this so-called "drill-and-blast" process will not be necessary. Soil and rock can be excavated and removed using backhoes, bulldozers, or a clamshell shovel suspended from a crane.

Regardless of which mining method is used, shafts will be required to remove the excavated rock, soil, and debris. Cranes, small rail cars, and conveyors will be used to bring this spoil to the surface. Most of the excavated material will be clean crushed rock, which can be used to fill abandoned mines, build artificial reefs, reinforce bulkheads, and pave roads.

Architecture goes underground

While the tunnels are the conduits for transit services, they are also permanent volumes that define and confine the limits of the architectural and engineering efforts. Tunnel construction techniques yield two basic profiles for the station volumes-vaulted spaces created by the circular motions of the TBMs, and rectangular ones defined by the cut-and-cover method's soil-supporting slurry walls.

DMJM Harris, the architectural arm of the joint venture, has devised modular, transparent canopies at street level to define entrances, provide shelter, and create an openness to contradict the reality of descending into a cavern. Skylights will penetrate sidewalks, wherever feasible, to invite sunlight as far into mezzanines and platforms as possible. The subterranean experience will be enhanced by other factors, including way-finding strategies, temperature control, lighting, and acoustics.

Acoustical integration is arguably the most challenging aspect of transit design. Sound has multiple air- and ground-borne sources-ambient crowd noise, public address announcements, mechanical noise, and track vibrations. Most of the attenuation and mitigation options involve intense architectural and engineering collaboration. "It's a totally integrated process," explains Kenneth Griffin, AIA, DMJM Harris's chief architect for the project. Griffin's team is designing the line's 16 new subway stations and several ancillary structures and is working closely with all the consultants. "We influence each other's strategies," he says.

Ventilation equipment will be housed in street-level, multistoried enclosures (top and bottom left) that will incorporate strategies to reduce noise emission, such as the fan-noise attenuator (top right). To reduce ground-borne noise from the trains, the design team is investigating several options, including a variation on the "lost form" floating track slab. In this configuration, the rail is fixed directly to a concrete tie block that sits in a resilient surround. Existing subway tracks employ either a "direct fix" or "resistant base plate," which provide little or no vibration mitigation.

For example, FEIS criteria calls for "comfortable and controlled aural environments," in which public address announcements are clearly intelligible. Acoustical engineers from Arup collaborated with the architects and NYCT communications engineers to investigate integrated options. Arup measured the acoustical conditions at existing stations to determine how current public-address systems work and assess the effectiveness of sound-absorbing finishes. They analyzed each link in the chain of sound transmission from the announcer's booth through the cabling infrastructure to the speakers and finally to station platforms, determining that the distortion that so often renders announcements unintelligible is cumulative. Back in Arup's in-house sound lab [record, March 2003, page 149], every factor that either improves or degrades the quality of sound is isolated and evaluated independently, so that the system can be designed and specified as a whole rather the sum of its parts.

To help control reverberant noise from the public-address system, Arup is recommending that the mezzanine and platform ceilings include sound-absorbing elements that are merged with the station architecture. "The effectiveness of sound-absorbing materials depends not only on how much sound they absorb, but also on where the materials are located relative to the noise source," explains Arup acoustical engineer Joe Solway.

The SAS public address system, currently in design development, will include a component for delivering verbal instructions in case of emergency, thus eliminating the less-effective alarm system in use at existing stations. With full-scale mock-ups, the architects are now exploring platform-edge and mezzanine-level "service carriers" that would house the system, along with lighting, closed-circuit TV cameras, and cabling, into canopies. These overhead elements will allow contractors to avoid embedding electrical conduit in concrete, thus keeping this critical station infrastructure accessible for maintenance, explains Griffin.

In all aspects of design development, the architects are guided by 12 design principles that consider the interface between architecture and the myriad engineering, life-safety, and urban-design requirements. For instance, one of the design principles calls for a balance between station context and essential "elements of continuity." These are "the physical elements that have a systemwide reference." They are standard planning devices, such as structural grids and modular components, which can be repeated in all the stations to a degree that is practical. Whereas continuity is important to passenger orientation and comfort, it is critical to maintaining structures that are designed and built to perform well for 100 years. Building services, systems, lighting, and graphics must be integrated identically in each station, and the repetition of modular components allows efficient repair and replacement. The office walls of the DMJM Harris/Arup joint venture are plastered with studies showing this integration and design evolution to date.

In addition to the new subway stations, DMJM Harris is designing several ancillary, multistoried structures at street level, which will house exhaust fans and other ventilation equipment serving individual stations and tunnels. The acoustical and mechanical engineers are working to ensure that fans, cooling towers, and handling units are designed with the necessary sound attenuation to reduce noise emission to the exterior and surrounding buildings. These measures become architectural concerns because the mitigation strategies can affect material choice and facade design. As preliminary renderings show, these unoccupied structures will receive the same level of architectural detailing as the subway stations, since their presence will similarly define the surrounding urban fabric.

Sudhir Jambhekar, AIA, principal at New York City−based FXFOWLE Architects, worked on the SAS for three years. Although the firm is no longer involved, the experience led Jambhekar to develop an argument regarding the architect's role in large infrastructure projects, such as the SAS. "Architects assume that there isn't much design opportunity in these building types, but that's just not the case," he insists.

Jambhekar acknowledges that organizational and management hierarchies can be more complicated than those applied to comparable commissions, such as highly specialized institutional projects. Infrastructure projects can take two or three times longer to complete, requiring a commitment of a decade or longer. Architects often work within large teams, which, in addition to the typical consultants, can include a maze of government agencies and regulatory commissions. However, in spite of these challenges, he makes a strong case for architects' involvement: "Infrastructure influences quality-of-life issues more than people think, which is why we should feel obligated to participate."