Learning From the Big Dig

Public Roads, July, 2001 by Daniel C. Wood

It may not look much like a school, but Boston's Central Artery/Tunnel (CA/T) Project -- the Big Dig -- is providing plenty of lessons for transportation planners and engineers from all over the world.

The sheer scope of the project, of course, is enough to attract international attention. Designed to replace 12 kilometers (7.5 miles) of aging urban highway through the tangled heart of Boston, the Big Dig has been compared in its extent and complexity to such landmark engineering projects as the Panama Canal and the Chunnel.

Within the United States, nothing like the CA/T Project has been attempted before. It involves replacing the elevated Central Artery highway (I-93) with an eight-to-10-lane expressway, building a 10-lane cable-stayed bridge across the Charles River, extending the Massachusetts Turnpike (I-90) to Logan Airport, and constructing four major highway interchanges. And all this must be done while keeping traffic and commerce moving through one of the nation's oldest and most historic cities.

Because of the innovative and unprecedented nature of the Big Dig, there has been broad interest in nearly every aspect of the project. To help facilitate the transfer of CA/T Project technology, the Federal Highway Administration (FHWA) has established the Innovations and Advancements Program, designed to share knowledge gained from the Big Dig with the national and international transportation communities. The program focuses on specific categories of topics: items that represent a cost or time savings, topics that exemplify superior quality, new and/or innovative technology, and items that would prevent others from "reinventing the wheel."

Discussed below are some of the areas that are attracting the most interest among transportation professionals who are eager to learn the lessons being taught by Boston's Big Dig.

Tunnel Jacking

Problem: Construct an underground roadway without disrupting traffic on nine active railroad tracks -- including commuter tracks that carry 150,000 people into and out of Boston every workday -- right above the roadway.

Solution: Construct the tunnel adjacent to where you want it to go and shove it into place using a technique known as tunnel jacking.

The CA/T Project extends the Massachusetts Turnpike under the Fort Point Channel into South Boston, where it meets the Ted Williams Tunnel. The turnpike (I-90) goes underground where it crosses the Southeast Expressway (I-93) at the South Bay interchange and passes beneath the tracks carrying Amtrak and commuter trains into South Station, Boston's busiest rail station.

To carry out the tunnel-jacking operation, three concrete jacking pits were dug alongside I-90 just east of I-93. Tunnel boxes 24 meters (80 feet) wide and 12 meters (40 feet) high were built inside the pits. The plan was to break the head ends of the concrete pits and push the tunnel boxes into place with massive hydraulic jacks.

But the Big Dig tunnel-jacking operation, the largest such operation ever attempted, faced a special problem -- the poor quality of the soil. Pushing the tunnel boxes into place without stabilizing the soil could cause the railroad tracks to settle, threatening train service. The solution was to freeze the soil ahead of the tunnel boxes, using hundreds of steel pipes that were driven into the ground between the tracks. A brine mixture that stayed liquid below 0 degrees Celsius (32 degrees Fahrenheit) was pumped into plastic pipes within the steel pipes by a freezing plant located near the railroad tracks. The brine was circulated back to the freezing plant and returned to the pipes again; the circulated brine over a period of several weeks froze the ground outward from the pipes.

The freezing allowed the ground to be excavated without settling. (It also caused the ground to expand, but allowances had already been made for this movement, and the track operations were unaffected.) The frozen soil ahead of the tunnel box was excavated by a machine called a road header. The soil was chewed up by the machine's rotating grinder, removed out of the back of the tunnel box, and carried to the surface by a crane. The tunnel boxes were then pushed into place by two sets of hydraulic jacks.

Slurry Walls

Planners of the Big Dig promised the people of Boston that the mammoth construction project could be accomplished without bringing the life of the city to a halt. Traffic would continue to flow, they vowed, and business would go on with little or no disruption. Slurry walls have helped the builders of the Big Dig keep that promise, and in many ways, these walls are the foundation of the CA/T Project. In fact, the Big Dig represents the largest single use of the slurry-wall technique in North America.

Slurry walls, which are similar to drilled shafts, are concrete walls that run from the surface of the ground down to bedrock, defining the area to be excavated for underground highway construction. Their immediate purpose is to keep construction trenches from collapsing while the soil is being removed. They are also used on the CA/T Project to support temporary traffic decking above the excavation. In the final stage, the walls are incorporated into the permanent tunnel structure.