Keeping a busy Natick, Mass. interchange open while replacing a deteriorating bridge west of Boston is critical for the ongoing Route 27/9 Interchange project that will deliver Massachusetts’s first Diverging Diamond Interchange.

The so-called Natick Big Dig led by the Massachusetts Dept. of Transportation must maintain a fully operational interchange serving some 90,000 vehicles daily while constructing the Route 27/9 Bridge in a tight space, says Christopher Howard, project executive, for GPI, the Bedford, N.H.-based preliminary designer of the project’s base-technical concept design.

The $99 million design-build contract for the Route 27/9 Bridge Replacement and Interchange Improvements project was awarded to McCourt Construction and Jacobs Engineering in June 2023 with plans for modifying the original 2017 partial cloverleaf design.

Crews are relocating 300 ft of the utilities’ overhead wires near the planned bridges’ site to the underground duct bank. They are also relocating 73 utility poles with infrastructure for up to nine utility companies to facilitate the intersection alignment.
Courtesy of Mass DOT

Improving safety and mobility while easing congestion are main priorities for the agency with the interchange witnessing some 100 crashes annually at the on-ramps to Route 9, media reports say. 

Fourteen years in development, the project began construction in December 2024 with substantial completion projected for summer 2030, according to MassDOT.

The project is “a transformative investment in Natick’s future,” said Natick Town Administrator James Errickson in an agency statement. “With improved safety, traffic flow, and connections for pedestrians and cyclists, this redesigned interchange will deliver long-term benefits for our residents, businesses, and the regional economy.”

Howard says one of the most complex parts of the job is eliminating the center bridge pier in the Route 9 median strip to reduce long term maintenance concerns and provide Federal Highway Administration recommended clearance of 16 ft 6 in over Route 9.

“The latter goal requires raising the new bridge approximately five feet higher than the existing bridge, which in turn requires full reconstruction of the interchange ramps and Route 27 approaches to meet this new bridge elevation,” he says.

To ensure feasibility, the team has undertaken “extensive traffic analysis and creative construction phasing” to create the necessary space to build while maintaining access to the existing interchange destinations, Howard says.

Crews finished installing the underground telecommunication and electric duct bank in mid-July before the utilities’ relocation got underway to prepare for building the interchange.
Courtesy of MassDOT

Traffic Calming

Designing the Natick Route 27/9 DDI involved geometry more typically used in roundabout design to reduce vehicle speeds and improve safety while maintaining the operational efficiencies of a DDI.

“This interchange design is quite unique; while there are numerous diverging diamond interchanges (DDI) throughout the United States with many more in construction, most have been built as sprawling interstate interchanges,” Howard says. “Ensuring the interchange design remained context-sensitive to the [compact] location was crucial.”

Howard notes that Route 9 and Route 27, “although vital corridors are not interstate highways,” so integrating compact geometry to ensure low travel speeds was important for safety.

Unlike traditional DDI’s that often include one wide bridge, Natick’s modified DDI design includes three independent bridges—two new single span bridges and a shared-use-path (SUP) bridge in between, Mass DOT says. Having more separation between Rt. 27 roadway barrels on the single spans flanking the SUP bridge provides roadway curvature at both the entry and departure crossover points, requiring drivers to slow down.

“This geometry provides a traffic calming benefit,” with target travel speeds through the interchange of about 20 to 25 mph, the agency says.

Since traffic management is “critical” to keep the interchange operational, the agency and GPI “will closely monitor operations through live camera feeds and quickly work to address any issues that may arise during construction,” Howard says.

Lead designer Jacobs is employing StreetLight mobility analytics platform “to help minimize traffic disruptions during construction and guide the addition of SUPs” with data-driven decision making, a Jacobs statement says.

Source: Panethos 2023

Utility Work

Installation of the underground telecommunication and electric duct bank finished mid-July before eight utility companies began relocating their utilities to prepare for building the interchange, says Tim Pybus, McCourt project manager.

This includes relocation of 300 ft of the utilities’ overhead wires near the planned bridges’ site to the underground duct bank.

“Additionally, 73 utility poles, with up to nine utility companies’ infrastructure items mounted on each pole, require relocation to facilitate the intersection alignment,” says John Goggin, MassDOT spokesman. “The replacement of 2,532 ft of an existing 6-in cast iron gas main to an 8-inch plastic main has also started along Route 9.”

Other major work is scheduled to begin late September with deep drainage installation and retaining walls building to facilitate temporary roadways during the construction staging of the project, says Pybus.

“Crews will begin managing deep drainage using larger 349 Cat excavators to set both the manholes as well as for the deep excavations,” he says. “So far, lead times for most materials have been two to three weeks with no major impacts to material deliveries.”

But he predicts some items would have longer lead times in a few years when bridge work begins.

“We are honored to be one of the first construction companies performing a DDI in Massachusetts,” Pybus says.