The elevated Central Artery had six lanes. The new underground expressway has eight to ten lanes.
In total, the project built 161 lane miles of highway, about half in tunnels, and four major highway interchanges in a 7.5 mile corridor.
The old road had 27 on- and off-ramps; the new one has 14. With improved streets, local traffic can exit the highway to surface roads while through traffic continues under the city.
The project excavated a total of 16 million cubic yards of dirt, enough to fill a stadium to the rim 16 times.
About two-thirds of the dirt went to landfills and other sites. Moving all that dirt took more than 541,000 truckloads. If all those trucks lined up end to end, they'd back up 4,612 miles. That's all the way to Brasilia, capital of Brazil, as the crow flies, or to the Panama Canal over land.
The rest filled more than 4,400 barge loads and went to Spectacle Island in Boston Harbor, and capped an old dump to make way for a new park.
3 million cubic yards of clay went to New England cities and towns to cap landfills that had reached capacity.
Concrete and steel
The project placed 3.8 million cubic yards of concrete. That's enough to build a sidewalk three feet wide and four inches thick from Boston to San Francisco and back three times.
The project installed more than 26,000 linear feet of steel-reinforced concrete slurry walls. These formed the walls of the underground highway as well as the supports for the elevated highway during construction. That's five miles of slurry walls, the largest application of this technique in North America. This is all resting on bedrock up to 120 feet below the streets of the city.
Reinforcing steel used in the project would make a one-inch steel bar long enough to wrap around the earth at the equator.
Elevated Central Artery demolition removed enough structural steel to make five Tobin Bridges.
First, most, biggest
Building the South Boston connection between the underwater part of the Ted Williams Tunnel and the land-based approach took great effort. It happened in the widest and deepest circular cofferdam in North America. A cofferdam being a watertight structure from which you pump water so that construction can take place inside. A ventilation building was also built inside the cofferdam.
The Ted Williams Tunnel interface in East Boston between the land-based approach and the underwater section is 90 feet below the surface of Boston Harbor, the deepest such connection in North America.
The project's seven-building ventilation system is one of the largest highway tunnel ventilation systems in the world.
Traffic using the Metropolitan Highway System (including the underground Central Artery, the Ted Williams Tunnel, and the Mass Turnpike out to Route 128) is monitored by an advanced traffic management and incident response system. Radio and cellular phone signals are re-broadcast into the tunnels.
The Leonard P. Zakim Bunker Hill Bridge is the widest cable-stayed bridge in the world and the first hybrid and asymmetrical design in the United States, using both steel (in the main span) and concrete (in the back spans).
The project required the largest use of segmental bridge construction and the largest application of steel box girders in the United States.
Crossing the Fort Point Channel included the most extensive use of concrete immersed tube tunnels in the United States, the first installation of jacked vehicle tunnels in North America (and one of the largest in the world), and the second use of soil mix construction on the East Coast (the first use came in East Boston on the Ted Williams Tunnel).
The project included the largest geotechnical investigation, testing and monitoring program in North America. The purpose was to identify conditions in the path of tunneling work, and help prevent buildings from settling during the digging.
Parks and open space
The project created more than 300 acres of new parks and open space. This includes 27 acres where the existing elevated highway stood, 105 acres at Spectacle Island, 40 acres along the Charles River, and 7 acres as part of an expanded Memorial Stadium Park in East Boston.
Three quarters of the 27 downtown acres will remain open. The rest is set aside for modest development, including retail, commercial, and housing uses in low-rise buildings.
More than 2,400 trees and 26,000 shrubs were planted at Spectacle Island. Another 2,400 trees and more than 7,000 shrubs were planted downtown.
Underground utility relocation
The project's underground utility relocation program moved 29 miles of utility lines. These lines are maintained by 31 separate companies. All told, about 5,000 miles of fiber optic cable and 200,000 miles of copper telephone cable were installed.
The Central Artery/Tunnel Project used the latest applications for preventing damage to Boston's vital subsurface infrastructure. Damage prevention efforts were continually updated.
Odds and ends
Because of the new highway system, Boston's carbon monoxide levels dropped 12 percent citywide. How can a road carrying more cars reduce pollution? Because it will keep traffic moving, so that emissions will reduce significantly.
The cable-stayed bridge across the Charles River used 1,820 miles of steel wire to form the seven-wire strands. These strands are in turn bound together to form the support cables, the largest being a foot in diameter.
About 150 cranes were in use project-wide.
The first 25 percent of construction took five years to complete; the second 25 percent took about two years to complete (the half-way point was reached in the spring of 1999). By the summer of 2004, construction was 94 percent complete.
During the peak of construction (1999 through 2002), workers did about $3 million of work each day.
At the peak of construction activity about 5,000 construction workers were on the job.
The deepest point of the underground highway is 120. That's beneath the Red Line subway tunnel at Dewey Square (Atlantic Avenue and Summer Street). The highest point is at State Street, where the highway passes over the Blue Line subway tunnel and the roof of the highway is the street above.
The project included 118 separate construction contracts, with 26 geotechnical drilling contracts.