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Cover Story

The Big Dig
NFPA 502 drives safety of world's biggest tunnel system
by Ed Comeau and Bill Flynn

NFPA Journal®, May/June 2003

When the Boston, Massachusetts, Central Artery/Tunnel project—more commonly known as the Big Dig—is finally finished in late 2005, it should prove a godsend to the thousands of commuters using it every day. And the fire protection systems installed in its tunnels, as well as the preparation of the city's emergency services, will make their journey not only faster, but also safer.

The Big Dig, with its 7.5 miles (12 kilometers) of roadway, loop ramps, connectors, and three tunnels, is the largest civil engineering project the United States has undertaken since the building of the Panama Canal in 1914, and providing fire protection for such a formidable project involves a great deal of engineering proficiency and practical knowledge.

"The Big Dig should stand as a model for life safety, or even renovations of major urban tunnels. What we have here is quite extraordinary," says William Connell, the project's manager of mechanical and electrical systems design for Parsons Brinckerhoff.

When the elevated Central Artery (Interstate 93), half of which still passes through the heart of Boston, was built in 1959, it carried about 75,000 vehicles a day. Until March 2003, this same six-lane roadway carried 200,000 vehicles a day, an increase that regularly resulted in massive traffic jams, and an accident rate estimated to be four times the national average.

The project, overseen by the Massachusetts Turnpike Authority, replaces this traffic travesty with two major components. The first replaces the elevated Central Artery with an 8- to 10-lane underground expressway, known as the Liberty Tunnel. This tunnel connects to the 10-lane, cable-stayed Leonard P. Zakim Bunker Hill Bridge over the Charles River and to Interstate 93 north of Boston. The northbound connection opened in March. The southbound connection is expected to open in November.

The lowest point of the Liberty Tunnel is 120 feet (36 meters) below Dewey Square at Atlantic Avenue and Summer Street in downtown Boston, where four northbound lanes of the highway cross below the Red Line subway tunnel.

The second phase of the project, which was completed in January 2003, involved extending Interstate 90 (the Massachusetts Turnpike), from south of downtown Boston through the new Fort Point Channel crossing, a nine-lane tunnel beneath the channel and South Boston to the 1.6-mile (2.5-kilometer) Ted Williams Tunnel. The Ted Williams Tunnel leads under Boston Harbor for 0.75-miles (1.2 kilometers) to Logan International Airport in East Boston. It opened to limited traffic in 1995 and to all traffic in January 2003.

Construction fire safety
Any tunnel project presents the local fire department with a challenge, while it's being built and once it's in operation.

During construction of the CA/T, safety has been a priority, and injuries have been well below the national average for tunnel projects. Nor have there been any significant fires.

Guiding much of the life safety and fire protection is NFPA 502, Road Tunnels, Bridges, and Other Limited Access Highways.

"The standard addresses two particular areas," says Art Bendelius, senior vice president with Parsons Brinckerhoff, which is one of the two project managers, and chair of the NFPA 502 technical committee. "The first is an obvious tunnel (that's dug underground) and the second is an air rights structure that is built over a roadway that creates a tunnel. The standard deals with the fire protection issues relating to these structures and some criteria on when the standard applies in relation to detection, suppression, and ventilation."

The preliminary design of the Big Dig began in July 1986, and the final design began in January 1989. Construction on the project began in February 1991.

"When it first started, NFPA 502 was a recommended practice, but now it's a standard. We tried to incorporate it into the design as much as we could," says Deputy Chief John Kenney of the Boston Fire Department. (BFD), who is also a member of the NFPA 502 committee. There are a number of components that meet the requirements outlined in NFPA 502. These include items such as standpipes and their fill points, egress, ventilation, emergency response, and water supply.

"NFPA 502 is internationally recognized," says James Lake, the NFPA staff liaison for the document, "and it covers the entire spectrum of what you would see in terms of fire safety in a road tunnel."

NFPA is working with staff from the United Nations to familiarize them with the provisions of the document to help gain greater acceptance around the world. "There are some countries that have federal guidelines on tunnels, but there really aren't any other standards out there," Lake says.

For a tunnel to fall under the requirements of NFPA 502, it must be 300 feet (90 meters) or greater in length. Between 300 feet and 800 feet (240 meters), some portions of the standard will apply. If a tunnel exceeds 1,000 feet (300 meters), all provisions in the standard would apply to the tunnel design and operation.

BFD requires that there be a standpipe and fire alarm system in the unfinished southbound Liberty Tunnel, as outlined in NFPA 502, says Connell, who's also a member of the NFPA 502 committee.

"The contractors are required to provide an interim or temporary standpipe system within the area where they are working," he says.

"They also maintain a fire alarm system within the tunnel. This hardwired system provides for a general alarm in the tunnel. The evacuation paths are clearly marked and the workers are trained in them. There are multiple muster points throughout the area of the project."

Knowing who is working in the tunnel could be critically important if an incident should occur so that everyone can be accounted for. When they enter the tunnel, everyone must pass through an electronic accountability system.

History of tunnel fires
There have been a series of tunnel fires around the world in the past seven years that include the English Channel Tunnel railway in 1996 (PDF, 479 KB), the Mont Blanc Tunnel, connecting France and Italy, in 1999 that killed 39 people and Switzerland's Gotthard Tunnel in 2001 that claimed the lives of 11 people.

The design of the Mont Blanc and Gotthard tunnels differs from that of the Big Dig in that they are "single-bore" tunnels, and traffic travels in both directions in one tube. In the Liberty Tunnel, the traffic flows are separated into two different, but adjacent, tubes. There were other differences in these tunnels, too, where they didn't meet the requirements outlined in NFPA 502.

For example, "some didn't have emergency egress, only safety niches," says Lake. When the Mont Blanc tunnel was rebuilt following the fire, the safety niches were connected to galleries through which the tunnel occupants can move if there's an emergency.

Detection in tunnels
The technology exists to put different types of fire detection into a tunnel. However, one of the best methods of detecting a fire is one that does not rely on smoke or heat detectors.

"A fire generally starts in the vehicle, and the amount of heat released into the tunnel is limited," says Bendelius. "You don't see any flames and only light smoke until the fire department is on the scene, and when they break out the window you suddenly see flames."

For this reason, fixed detection may not always work in a timely manner. A fire occurred in the four-lane Ted Williams Tunnel in May 2002, when bus carrying members of the Seattle Mariners baseball team ignited and stopped. The first alarm to the Operations Control Center (OCC) came from the carbon monoxide detectors in the ceiling. The linear heat detectors didn't activate until after the fire department was on the scene.

As a result, tunnels are starting to rely on both video monitors and traffic sensors to indicate when there's a problem. If a fire should occur, the traffic is going to generally slow or stop downstream of the incident. This alerts the people staffing the control room who can then observe what is occurring on video cameras and react appropriately.

"The CA/T is equipped with manual fire alarm pull stations within the tunnel, but there are no automatic detectors," says Connell. "Instead, we have 24/7 monitoring of the tunnel by the operators in the control center by various means, such as closed circuit television, traffic sensors, and carbon monoxide detectors. The CO detectors are there primarily for air quality measurements, but they also double as an indicator that there is a fire."

Suppression in tunnels
There have been many debates about the merits of installing an automatic suppression system in the tunnels. Australia and Japan mandate the installation of sprinklers in tunnels. There is limited support for the idea of installing sprinklers in roadway tunnels in the United States.

Sprinklers may actually be counter-productive when it comes to tunnel fires. Since the heat and smoke are going to move horizontally until the ventilation fans can be reconfigured, it is best to allow the heat and smoke to rise to the ceiling, creating a cooler, clearer area at roadway level where the people are escaping.

"If you dump water on a fire, you are going to cool the fire and the smoke and destroy any stratification that helps people get out," says Bendelius.

There are very few tunnels in the United States with suppression systems installed in them. One is in Boston, but the system has never been commissioned, according to officials. Two others are located in Seattle, Washington. According to fire officials, the reason for requiring the systems was so that hazardous cargoes could use the tunnels without restriction. The systems have never been used to suppress a fire.

Fire department operations
BFD is responsible for responding to incidents in the Interstate 93 tunnel, along with the fire departments from neighboring Cambridge, Somerville, and Logan Airport. Because of the size of the tunnel and the limited access, it's important to coordinate the operations with a number of different agencies and departments.

Whenever there's a fire department response for a fire in the tunnel, units will respond to four locations.

The first is to have units respond with the flow of traffic. These companies attempt to reach the scene of the incident by following the normal flow of traffic.

However, this may be a problem because the fire in the tunnel will probably stop the traffic. Furthermore, "the first companies (responding with the flow) may have to come from some distance and there is a chance they may not actually make it to the fire scene," says Deputy Chief Kenney. For this reason additional companies respond and stage outside of the tunnel where the traffic is coming out. "Once we know that traffic is stopped and we get the confirmation from the tunnel control center, then we send companies into the tunnel against the flow."

A third company is sent to the standpipe fill station. "Everything is covered by a standpipe," says Kenney, even if it isn't inside the tunnel itself. "Any place we can't get ready access, there is a standpipe system. For example, when you come out of the tunnel, it is 40 feet (12 meters) down from grade and you cannot access it easily, so the standpipe is continued in that area."

All of the standpipes are dry systems with two remote fill points, as called for in NFPA 502. The tunnel has been broken into smaller response zones, with each standpipe having two fill points.

"We tried to group as many of the fill points in one location as possible," says Kenney. "The standard on the fill times from NFPA 502 is 10 minutes, but for most of them they are less than 5 minutes."

A fourth company is sent to the opposite fill point for the standpipe which is usually at an emergency exit stairs as outlined in NFPA 502.

"We surround the incident," says Kenney. "It is a heavy response, but we have to commit a lot of resources because of the life safety and limited egress points. Also, it takes a few minutes for the ventilation system to take over if there is a fire." By placing the second fill point in an emergency egress stair tower and having a company respond to that location, the company can serve double duty.

To help coordinate all of the responding units, as well as with other agencies, the tunnel is equipped with radio repeaters that allow for communications within the tunnel.

"We have all four (fire) channels down in the tunnel along with Boston police, EMS, and Massachusetts State Police," among others, says Kenney.

Ventilation system
Ventilation is an essential factor in any tunnel fire, and the CA/T is no exception. When the project is completed, vehicles will travel much of the route underground, since roughly half of the 7.5 miles is in a tunnel. The vehicle exhaust will then be collected and dispersed high into the atmosphere away from pedestrians, residents, and motorists through seven new ventilation structures.

Carbon monoxide emissions are diluted to meet ambient air quality standards and are then released through the stacks in the vent buildings. This ventilation system, along with fire detection, life-safety, and traffic-control systems (known as the Integrated Project Control System or IPCS), are run from the OCC in South Boston.

The IPCS is fully redundant, with a backup system running in another building less than one-quarter mile away. One late addition to the IPCS following the September 11, 2001, attacks was the installation of motion detectors outside the vent buildings and at other high-security locations.

To provide the necessary empirical data needed to design the ventilation system properly, the Massachusetts Highway Department, along with its project manager, a partnership of the Bechtel Corporation and Parsons Brinckerhoff, entered into an agreement with the Federal Highway Administration in 1994 to conduct tunnel fire tests in an abandoned West Virginia mountain tunnel.

Called the Memorial Tunnel Fire Ventilation Test Program it became the largest on-site research project focused on tunnel ventilation in the world and provided important data on smoke and heat handling for various types of ventilation systems.

Nearly 100 fires were set in the tunnel. Engineers installed instruments to monitor and record air temperature and velocity, and gas concentrations. Not only did the fire testing allow Bechtel Parsons to design and build a ventilation system suitable for a project of this magnitude, but it provided the Federal Highway Administration with significant data on jet fan ventilation systems.

"Those tests accomplished a lot in developing useful life safety data about large ventilation systems in tunnels," says Connell.

The project's ventilation is a two-way full transverse system in which fresh air is blown through ducts under the road or in a tunnel wall and circulated through the tunnels by fans in the vent buildings. Simultaneously, vehicle exhaust is extracted through openings in the ceiling to rooftop exhaust stacks in the vent buildings and then dispersed into the atmosphere. The highway tunnels' three fan/shaft ventilation systems use about 14 miles (22.5 kilometers) of duct work, 139 double-width centrifugal fans that are 10 feet (3 meters) in diameter, 35 jet fans, and 8 axial fans. Eleven of the project's ramp tunnels will use a jet-fan-based longitudinal/semi-transverse ventilation system.

The entirely computerized system's also equipped with carbon monoxide sensors tied to the OCC and is programmed to automatically adjust to higher levels of carbon monoxide by pumping more air into the tunnel. At night, when traffic levels are minimal, the ventilation system operates at a lower capacity. As traffic levels increase, the operating levels of the air intake and exhaust fans increase. Full capacity is only needed during an emergency, such as a fire. In that situation, the system's capable of controlling heat and smoke and dilution of carbon monoxide levels.

The West Virginia Memorial Tunnel testing also provided project operators with enough data to conclude that the proper use of the ventilation system during a fire can be of substantial value in controlling smoke and can play a significant role in slowing the growth of a fire until firefighters arrive.

Although OCC operators do have flexibility in the way they use the system, the standard practice is for the air intake pumps to slow down the fire and for the exhaust pumps to speed up to remove smoke from the tunnel.

"In an emergency," says Kenney, "the idea is to place everything on full supply upstream of the fire to supply as much air into the area as possible and push the smoke down the tunnel. Everything downstream of the fire will go into full exhaust, drawing out the smoke."

If a fire should occur, the traffic beyond the location of the fire would normally exit the tunnel. The objective is to provide those people that would be caught in their cars by the stopped traffic with fresh air while the smoke is pushed away from them and exhausted out of the tunnel.

In the event that people need to evacuate the tunnel in an emergency, there are a series of emergency doors located approximately every 1,000 feet (305 meters). These doors lead people either to stairs that they can take to the surface or into an adjacent tunnel through a cross-passage door.

"It's impressive to watch the CCTV monitor to see how well this ventilation system handles a large amount of smoke," says BFD Deputy Chief Joseph Fleming. "During one fire a chief told me he could walk to within 5 feet (1.5 meters) of the source and there was no smoke. He was amazed that he could get that close. The ventilation system is truly state of the art for the biggest tunnel system in the world."

Connell says because the project is so big it needs a ventilation system that has lots of flexibility.

"At one major underground interchange, we're six lanes wide in just one side of the tunnel," Connell says. "It's a situation like that that makes this project different from anywhere else." Connell also cited the ventilation in the Ted Williams Tunnel. He says it has seven zones of transverse ventilation, each with dedicated exhaust and supply fans. "They can all be independently controlled, giving the OCC operator great flexibility in the way the exhaust from a fire is controlled," he says.

Fire department training
BFD has an extensive training program to prepare its personnel for the staged openings of the tunnels.

"A lot of the areas are ready enough that we have a lieutenant and a captain bringing companies down every day, two or three in the morning and then another two or three in the afternoon. They cover the areas that they would normally respond to, the location of the stair towers and we cover the features of the tunnel," says Kenney.

The command staff has also been undergoing familiarization training as well. They have been to a series of seminars that explain how the ventilation systems and fire alarm systems work and then taken them down into the tunnels for more familiarization.

In addition to the familiarization, BFD has been conducting acceptance tests on the systems to ensure that they are operational. This included a live fire using diesel fuel inside of the tunnels so that they can observe where the smoke travels and how the ventilation system copes with the smoke.

Lessons learned
During the project, NFPA 502 went through a renaissance, Connell says.

"When we started this project," he notes, "NFPA 502 was a recommended practice, it wasn't much more than a dozen or so pages, no real specific guides. My involvement in this project and NFPA 502 helped to advance both.

"NFPA 502 grew to what it is today, now a standard…its much more substantive than it was in 1990 or 1991 when we first picked it up…the project, because of its length of design stage and construction, has had the opportunity to borrow the best features from other projects.

"I can't say there were lessons learned, it was more of an evolution. NFPA 502 played an important role. When we sat down in the late '80s to set down the concepts [for the CA/T], it was apparent that there was no guidance…several other tunnels had been commissioned in the US, but not many, yet they all had different features and requirements."