Engineering Marvels

The Central Artery project has enough engineering marvels to fill a textbook. The designers and builders of the project faced difficult soil conditions, tight working spaces, proximity to construction of huge glass and steel office towers and fragile old brick buildings, the need to hold up an elevated highway while tunneling directly beneath it, and - most important - keeping Boston open for business throughout 14 years of construction.

The categories below show how the project turned these challenges into spectacular engineering solutions.

construction of I-93 Northbound to I-90 Eastbound ramp

Slurry Walls

The Central Artery/Tunnel Project included the largest use of slurry walls anywhere in North America. What's a slurry wall? It was the single most important construction technique on this gigantic project, the primary tool of the designers to fulfill the most important promise to the people of Boston: Keeping the city open for business and traffic moving during more than a decade of construction. 

A slurry wall is a concrete wall that runs from the surface of the ground down to bedrock. It defined the area to be excavated for the underground highway and eventually formed the actual walls of the new Central Artery. "Slurry" refers to a clay-water mixture that is pumped into the excavation for the wall to keep the sides intact until concrete is poured. (More)

Supporting the Elevated Highway

Boston's Central Artery/Tunnel Project promised to keep the city open for business during more than a decade of construction. That meant traffic must flow, and in terms of the number of drivers affected, the biggest piece of that promise was keeping the elevated Central Artery in service and carrying its full load of more than 190,000 vehicles a day while the underground expressway was dug directly underneath.

The old elevated was six lanes wide. The new underground expressway is eight to ten lanes wide. Every footing that supports the columns holding up the original Artery sat directly in the path of the new highway tunnel. Every one of those footings was removed, but without taking the existing highway out of service. The entire weight of the elevated highway -- it contained enough steel to build five Tobin Bridges -- was shifted onto new supports resting on the walls of the new tunnel. This load shifting process was called "underpinning."  (More)

Tunneling Under the Red Line

The lowest point of the underground Central Artery is 120 feet 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 highway runs under Atlantic Avenue, perpendicular to the subway under Summer Street. The Massachusetts Bay Transportation Authority is also building a new service at Dewey Square, an electric bus line that will run in a tunnel from South Station to South Boston. Originally called the Transitway, now called the Silverline, the new service will connect the South Boston Seaport district to the rest of the T's system for the first time.

Because of the Silverline and the need for a lobby above the two transit services, there was no place for underground highway but underneath it all. That presented two major challenges. First, the rest of the Central Artery tunnel was built using slurry walls, the concrete panels dug from the surface down to bedrock. How do you dig a slurry wall when there's a subway tunnel in the way?

The second challenge was the fact that the subway tunnel would settle if a four-lane tunnel were dug beneath it. That would damage the rail line. How to put a tunnel under the subway without shutting down the trains?

The answer was a process called "underpinning." A gigantic, reinforced concrete table or bridge resting on bedrock was dug out of the ground beneath the subway tunnel to support the subway while providing a secure place for the highway.

The Casting Basin

The difficult problem of crossing Boston's Fort Point Channel with a highway tunnel was solved in a very big way - in a place called "the casting basin."

As part of the Central Artery/Tunnel Project, the Massachusetts Turnpike (I-90) extends from its current eastern end, through a tunnel beneath South Boston to the Ted Williams Tunnel, Logan Airport, and East Boston. Fort Point Channel, a narrow extension of Boston Harbor into South Boston, lies just east of the I-90/I-93 (South Bay) interchange. To cross the channel, Central Artery engineers decided to use tunnel sections lowered into a trench, much like the technique used to build the Ted Williams Tunnel under the harbor. (More)

Tunnel Jacking

The Big Dig has extended I-90 (the Massachusetts Turnpike) under Boston's Fort Point Channel into South Boston, where it meets up with the Ted Williams Tunnel. I-90 goes underground approximately where it crosses I-93 (the Southeast Expressway) at the South Bay Interchange. But before it reaches the channel, I-90 also passes beneath nine active railroad tracks carrying commuter and Amtrak trains into South Station, Boston's busiest rail terminal. (More)

The Operations Control Center

The Massachusetts Turnpike Authority maintains a proactive Operations Control Center (OCC) that is staffed 24 hours a day to monitor traffic and provide quick incident response.  The OCC monitors the I-90/I-93 system of tunnels, ramps, bridge, and surface roadways in the Metropolitan Boston area, including the Sumner, Callahan, Prudential, and City Square tunnels, and I-90 West to the New York border. (More)