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 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. Pushing the tunnel boxes into place without stabilizing the soil could cause the railroad tracks to settle, threatening train service. The plan was to break the head ends of the concrete pits and push the tunnel boxes into place with massive hydraulic jacks.īut the Big Dig tunnel-jacking operation, the largest such operation ever attempted, faced a special problem - the poor quality of the soil. Tunnel boxes 24 meters (80 feet) wide and 12 meters (40 feet) high were built inside the pits.
To carry out the tunnel-jacking operation, three concrete jacking pits were dug alongside I-90 just east of I-93. 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. The CA/T Project extends the Massachusetts Turnpike under the Fort Point Channel into South Boston, where it meets the Ted Williams Tunnel. Solution: Construct the tunnel adjacent to where you want it to go and shove it into place using a technique known as 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. 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."ĭiscussed 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. 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. And all this must be done while keeping traffic and commerce moving through one of the nation's oldest and most historic cities.īecause of the innovative and unprecedented nature of the Big Dig, there has been broad interest in nearly every aspect of the project. 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. Within the United States, nothing like the CA/T Project has been attempted before.
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. The sheer scope of the project, of course, is enough to attract international attention.
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.
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