Geotechnical Aspects of the Boston Central Artery/Tunnel Project, "The Big Dig"

The largest, most complex urban infrastructure project in United States history is the Boston Central Artery/Tunnel Project in Massachusetts. Replacing an approximately 40-year-old elevated central artery (I-93), the project, valued at over $14 billion, consists of a modern underground expressway and existing Massachusetts Turnpike (I-90) extension to Logan International Airport through a tunnel system that is state-of-the-art and includes jacked tunnels, steel and concrete immersed tunnel tubes, and cut-and-cover tunnels. To allow the project to be built, geotechnical innovation was critical due to tight restrictions requiring that while the new tunnel was constructed below the elevated section, the existing structure remain in service. Engineers were challenged with design and construction detail development to maintain the busy downtown corridor's traffic and minimize historic structure, commerce, and tourist trade impacts while allowing safe construction of the tunnel at depths reaching up to 120 feet. One of the largest soil stabilization program had to be undertaken because of poor soil conditions and deep excavations; in the Fort Point Channel area, 690,000 cubic meters of deep soil mixing and jet grouting were completed. The project also used such ground improvement techniques as ground freezing, tunnel jacking, and lightweight fill embankments constructed of lightweight foamed concrete and EPS geofoam. As part of this project, several bridge structures were constructed in addition to the tunnel work, including the world's widest cable-stayed bridge. Surface restoration and landscaping, which will result in a linear park through downtown Boston being created where the elevated highway structure once prevailed, is currently underway, and the 7.5 mile corridor (161 lane miles) is now complete.