BIG PUSH Opening of the southbound Interstate 93 tunnel box allows the final demolition of the 50-year-old viaduct it replaces. (Photo courtesy of Big Dig/ Matt Poirier)

The third and final tunnel opening this year on Boston’s $14.6-billion Central Artery/Tunnel project went as scheduled Dec 20 when officials and workers activated the Interstate 93 southbound lanes.

Opening the $2.6-billion segment now allows traffic to flow both ways through the I-93 tunnel box and over the 10-lane asymmetrical cable-stayed Leonard P. Zakim/Bunker Hill Bridge spanning the Charles River. It also allows final demolition of the existing 50-year- old viaduct it is replacing.

The opening initially was planned for 12 months after the opening of the $4- billion I-93 northbound segment (ENR 4/7 p. 15). But the CA/T team decided to speed things up by tying the new mile of tunnel running from the Charles River south to Congress Street into the existing east bore of the Dewey Square tunnel that was carrying northbound traffic. "This ties into [yet] another half mile of existing tunnel for the entire southbound trip," says Keith S. Sibley, CA/T director of construction. "This is the culmination of the project. We’re now in the final phase," says Michael P. Lewis, CA/T project executive.

The biggest challenge was finishing the last 1,000 ft of cut-and—cover tunnel box adjacent to the northbound alignment. The work included placing roof girders up to 90 ft long and weighing over 50 tons to span a three-lane tunnel and two-lane collector ramp. Connecting deep air-duct shafts into the tunnel box also was a problem because of the curved plenums. All work took place while the existing southbound viaduct and northbound tunnel were open. "It was a difficult jobsite to get to and congestion was the main issue," says Sibley.

There still is plenty of work left to be done on the project. About $1 billion in construction is under way, including demolishing the viaduct and the double-deck High Bridge, building new ramps to the cantilevered lanes on the Bunker Hill Bridge and completing two major interchanges located at Logan International Airport and the junction of I-90 and I-93. CA/T officials are predicting substantial completion by May 2005. Once the viaduct is removed next year, a surface restoration program will create a new greenway through the heart of downtown Boston. Earlier this year, CA/T also opened the $6.5-billion I-90 segment (ENR 1/20 p. 16).

Because of "size, technological challenge and working conditions, the CA/T was a major challenge–bigger than the Chunnel and Hong Kong Airport," says C. Matt Wiley, program manager for Bechtel/Parsons Brinckerhoff, CA/T project consultant. "It’s the same magnitude project, but its location in downtown Boston made the difference."