Marking the occasion with a low-key March 27 ceremony muted by the war in Iraq, officials at Boston's $14.6-billion Central Artery/Tunnel project began a phased opening of the critical $4-billion Interstate 93 northbound segment, which runs through the heart of the downtown area.

It is the second major CA/T opening this year, following the $6.5 billion I-90 opening in late January (ENR 1/20 p. 16).

From March 26 through March 29, CA/T workers closed and dismantled some ramps on the existing six-lane viaduct and constructed tie-ins to the new tunnel and bridge replacement. The new segment, which officially opened to the public March 29, stretches 3.5 miles from south of the city across the Charles River and connects with the Tobin Bridge in Charlestown to the north. It contains 1.5 miles of tunnel and the 1,800-ft-long cable-stayed Leonard P. Zakim Bunker Hill Bridge.

Work on the project started in 1993 with a major utility relocation program. That was followed by tunnel slurry wall construction, viaduct underpinning, tunnel floor and roof segment pours and construction of three ventilation buildings, two of which are underground structures. "Vent building #4, near Haymarket Square, was particularly challenging because of groundwater infiltration," says Keith S. Sibley, CA/T's I-93 milestone manager. "The four-story building is completely underground–the deepest hole on the project–and we had to run three different grouting programs to tighten up the foundation because of the unusually fractured rock. We also installed post-tensioned rock anchors."

OPEN Ceremony (above) marks completion of a crucial I-93 segment in Boston. (Photos above and top courtesy of Bigdig.com)

Almost five miles of slurry walls were poured in the decade-long effort, making the CA/T one of the largest concentrations of the technique in U.S. construction history. "Slurry walls are the unheralded success story of this project because without them, we could not have dug the tunnel and maintained existing traffic on the viaduct and surface streets," says Michael P. Lewis, CA/T project director. "Slurry walls kept Boston open for business and made the project possible."

Three major ramps from the viaduct will be demolished this spring, with the whole structure being demolished next year after the I-93 southbound opening. One unplanned challenge for the remaining demolition work and new tunnel construction will be the Democratic National Convention, scheduled for July 2004 in Boston. "The viaduct demolition will be well under way by then and it actually frees up space for the operational needs of the DNC and the city," says Matthew J. Amorello, chairman of the Massachusetts Turnpike Authority, which oversees the CA/T.

The four-lane northbound tunnel and the new ramps will help alleviate some existing heavy traffic congestion, but will also require extensive driver education and advisory work.

Still greater construction challenges remain. "This opening is important to the public, but northbound plus two weeks is when we relocate inbound traffic to open new work zones for construction of the last 800 ft of tunnel box," says Sibley. "In addition, we need to refurbish the existing Dewey Square tunnel. To do that, we have to relocate the tunnel center wall and lower the floor slab in order to achieve the initial southbound opening. It's a huge amount of work."

No heavy lifting will be needed to name the tunnels. Gov. Mitt Romney (R) led the effort to name the I-93 tunnel the Liberty Tunnel in honor of Massachusetts veterans. "The war in Iraq serves as a fitting reminder of the sacrifices made in the name of liberty," says Romney. He reached the naming deal with legislative leaders on March 24.

The I-90 tunnel was named the Thomas P. "Tip" O'Neill Jr. Tunnel after the former speaker of the U.S. House of Representatives who was instrumental in getting funding for CA/T. The green space above the I-93 tunnels was quietly named the Rose Kennedy Greenway months ago in honor of the Kennedy clan matriarch.