Every bridge project is affected by timing. Communities need these vital routes to be built or rehabilitated as quickly as possible to minimize traffic disruptions. For the town of Middlebury, Vt., the need for speed was especially critical, as the bridges and railroad in need of upgrades ran right through the middle of town.
The 20-ft-deep rail corridor, which was built in the 19th century, cuts through the center of Middlebury, with two bridges offering the only way for traffic to pass over it. In 2013, when the bridge design phase began, the railroad was plagued with drainage issues, clearance restrictions, and poor track conditions, which had contributed to a train derailment. The two bridges were also deteriorating.
The hazardous situation spurred the city to invest in the $80 million bridge and rail improvement project, which included replacing both bridges, updating the rail corridor, and increasing the overall vertical clearance for the railroad by more than 3 ft. The project would be difficult because of its location and the constant road and rail traffic.
“We knew that working around the railroad schedule was going to be a challenge,” says Aaron Guyette, transportation lead for VHB. “Accelerated bridge construction [ABC] using precast concrete was the perfect solution to keep the project on schedule.”
Even with the ABC approach, the project would still create traffic problems for everyone involved. The railroad ultimately agreed to a 10-week extended track closure, and the city agreed to a 10-week closure of all downtown streets. “The decision to close all downtown streets essentially cut the downtown community in half,” says Craig Rypkema, sales engineer for the Fort Miller Company. However, it was the most efficient solution for all stakeholders involved.
The choice of a precast concrete made the 10-week timeline possible, says Jon Griffin, project manager for the Vermont Agency of Transportation. “Fabricating each component prior to the bridge closure period saved an immense amount of time compared to traditional cast-in-place methods.”
One of the critical elements for the precast concrete producer on this project was ensuring that the pieces for the rail corridor could meet the tight tolerances necessary to make the system watertight. Each joint had to be within a specific range for the joint waterproofing materials to work properly.
The owner, designer, precast concrete producer, and contractor worked collaboratively to find a balance of fabrication tolerances, construction tolerances, costs, and functionality. They ultimately came up with the perfect design. “The constant collaboration helped make this project a success,” Rypkema says.
The team used a precast concrete box structure that would permanently retain surrounding soils and support the existing roadways. The sealed structure was designed to keep groundwater out and convey stormwater from tunnel approaches.
The 345-ft-long concrete box section consists of a series of bottom U-shapes and top U-shapes that join with a keyway to form a clear opening. The tunnel was fabricated with tops and bottoms that could easily be placed together at the jobsite, with U-walls needed to extend upward more than 20 ft.
“It was a challenge to design elements that would be able to be transported to the jobsite and that would easily fit together in the field,” Guyette says. To accomplish this, the design team post-tensioned the precast concrete panels to the U-shaped wall bases at the project site. Individual pieces were then connected longitudinally using galvanized “dog-bone” hardware that could be quickly inserted and tensioned before the next piece was installed. As an added advantage, the box structures could be extended between the two existing bridge locations to create a tunnel and reconnect the historic town green.
“Precast concrete helped us achieve a project that was constructable within the project constraints,” Guyette says. “This means that precast concrete made the schedule achievable, made the project affordable, and helped to achieve the 100-year design life.” And for the first time in nearly 180 years, the town green—now a focal point of the downtown area—has been reconnected and developed into a beautiful community space.