Big Dig
The Big Dig is the official term for a massive project to replace the existing elevated Central Artery (Interstate 93) through the heart of Boston with an underground highway. The design includes a new tunnel to Logan Airport. Due for completion in 2004, it is the most expensive highway project in American history (roughly $15 billion, or more than $1 billion per mile). The Zakim Bunker Hill Bridge at the project's northern end has already become a new symbol of Boston.
Reworking such a busy corridor without seriously restricting traffic flow required a number of state-of-the-art construction techniques. Because the old elevated highway (which remained in operation throughout the construction process) rested on pylons located throughout the designated dig area, engineers first utilized slurry-wall techniques to create 120 ft. deep concrete walls upon which the highway could rest. These concrete walls stabilized the sides of the site, preventing cave-ins during the excavation process.
Other challenges included an existing subway tunnel crossing the path of the underground highway. In order to build slurry walls past this tunnel, it was necessary to undermine the tunnel and build an underground concrete bridge to support the tunnel's weight.
The Zakim Bunker Hill Bridge, designed by Swiss designer Christian Menn, represents the terminus of the project, connecting the underground highway with I-93 and Route 1. A distinctive suspension bridge, the Bridge is supported by two forked towers, which are connected to the span by cables and girders.
On January 17, 2003, a 1.3-mile tunnel section of the Dig opened, connecting the Massachusetts Turnpike to Logan International Airport. The tunnel is expected to reduce the trip from downtown Boston from 45 minutes in traffic to 8 minutes. The next phase, taking the elevated Interstate 93 and putting it underground, is scheduled for completion by early 2004.