In December, MassDOT transferred southbound traffic from the old bridge to the new half. That moved the project into phase four, which includes removing 4,250 tons of steel from the old bridge that carried 70,000 vehicles daily across the Merrimack River, between Newburyport and Amesbury. HNTB led the design to replace the 65-year-old twin-barrel steel-arch truss bridge with a network tied-arch bridge—the only one of its kind in the state—plus several other overpasses along a 3.5-mile corridor of I-95. 

Aside from coordinating dozens of subcontractors and material suppliers “to get everybody firing on all cylinders” at the start, the biggest challenges have been dealing with the highly variable subsurface conditions, including in the river, and the compressed right-of-way, Daigle says.

“Between two of the existing spans near pier two, there’s only one to two feet of water at low tide. So, we can’t position any large, floating equipment there because the rocks would ground out and destroy the vessel,” Daigle says. The team employed a gantry system to address those constraints.

“Some of the largest lifts we made were steel superstructure items and precast-concrete deck elements,” Daigle says. “Those were all done with a crane that had virtually unlimited range of movement over the area where we needed to set the pieces.

“We use more than usual temporary steel on the project—in excess of 600 million pounds of towers and foundation steel—to facilitate construction,” Daigle adds. Walsh also self-performed all the steel erection.

Tidal fluctuations of up to 9 ft in a six-hour period can occur in the Merrimack, making work even more difficult the first two winters. “In cold winter months, the river ices [over] but not completely,” Daigle says, “Partly due to the tide and partly to the salinity of the water, icebergs form and flow in both directions because of the tide. That was a challenge that first winter, when we had a lot of marine equipment on the water as we were working on the piers for the new bridge.”

Working beneath the old bridge in a constrained environment precluded the use of cranes. When crews began building two of the new piers, they had only about 26 ft between high tide’s water elevation and the underside of the old structure.

“In most normal jobs, you would have unlimited headroom because there would be no structure,” says Daigle. “You might have something adjacent, but not overhead. [For those piers] we used compact drilling machinery and telescopic-boom cranes to work below the structure with traffic still on it.”

To combat the daily freeze-and-thaw effects of severe winter weather and the corrosive effects of deicing chemicals, MassDOT hired Claremont, N.H.-based manufacturer Canam to fabricate all the steel for the bridge superstructures and use an innovative zinc-metalizing-spray system as protective paint. “We’ve finished work on the northbound bridge and are working on steel in our shop for the southbound bridge,” says Ken Allgair, senior director of projects at Canam.

The team and Newburyport officials are discussing the possibility of blasting three piers of the old bridge. The team has permits for conventional impact hammering but hopes to pursue underwater blasting, using grapples attached to conventional excavators, to minimize time and disruption to fish migration. However, the team is still awaiting approval from state and federal agencies.

In the final phase, crews will construct a shared-use path—MassDOT’s first of its kind—for bicycles and pedestrians on the interstate bridge.

The project is the largest of five “megaprojects” in MassDOT’s $3-billion accelerated bridge program, which state legislators enacted in 2008. The program includes the fast-track completion of fourteen I-93 bridges in 10 weekends (ENR 11/14/11 p. 17). Another of the mega­projects is the $255-million Longfellow Bridge restoration (ENR 9/29/14 p. 26). The others involve the Fore River Bridge, Burns Bridge, and the Fall River crossing over I-195.