A massive precast concrete tunnel replaced two decaying bridges in Middlebury, Vermont, as part of a $70 million bridge and rail project. Launched by the Vermont Agency of Transportation (VTrans) the project was designed to improve safety and increase freight train capacity.
The two bridges are 300 feet apart and span a single set of depressed Vermont Railway tracks. Located in downtown Middlebury, with one bridge on Main Street and the other on Merchants Row, the two 100-year-old spans had severe cracking, delamination and spalling and concrete had periodically fallen on the railroad tracks below.
In addition to replacing the two deteriorating bridges, the 360-foot tunnel corrects the problem of insufficient clearance for double-stack rail cars. This project lowers the vertical rail bed for more than 3,000 linear feet, increasing the vertical clearance beneath the bridges to 21 feet without impacting the grade of the streets and sidewalks above.
These changes benefit the Vermont Railway which is a vital component of the state’s economy. The Vermont Rail System (VRS) hauls over 25,000 freight cars each year which lowers truck traffic on state routes, as well as reducing wear and tear and congestion on state roads. One double-stack rail car can carry the equivalent load of eight trucks.
According to VTrans, the project necessitated intricate planning and design to balance community needs, regional transportation, and constructability. For most of the project’s duration, the rail line had to be kept active. In order to avoid rail closures and accelerate the schedule, precast concrete became the best choice for a project of this size.
Precast concrete U-walls allowed VTrans to transition from the track’s current grade down as it enters the tunnel and then transition back up to the existing grade of the railroad, and to support the slopes on either side of the tracks.
Precast’s quick installation time and its simple clamshell design allowed pieces to be joined on site very quickly and bolted to the ground as they were set on site. All precast pieces were set in July of 2020 within the 10 week schedule and the entire project is to be completed in 2021. During those summer months the railway was temporarily detoured around Middlebury while the tracks are lowered and the precast concrete tunnel was installed.
The design and construction team monitored the walls and foundations of historic buildings for construction vibrations and reduced the threat of flooding from Otter Creek (the state’s longest river).
The new concrete tunnel has 18-inch walls and outside-to-outside dimensions of approximately 22-feet-wide by 29-feet-tall. In addition, new bridge railing, sidewalks, and street approaches were part of the project improvements.
The tunnel was constructed with 422 precast concrete U-Walls that are joined by bolted connections that are sealed with grout, butyl rubber and elastomeric bearing pads, which are designed to keep water out of the tunnel. In addition to the tunnel, precast U-walls were placed at either end of the tunnel as the slope comes up to grade.
The precast U-walls were fabricated by The Fort Miller Co., Inc., Schuylerville, NY and were delivered to Middlbury via specialized flatbed trailers. The units are being set by heavy cranes at the site. The heaviest U-Wall piece weighs approximately 80,000 lbs and the average weight is 65 to 75,000 lbs. according to Joshua French, Fort Milers’ VP of Sales and Marketing.
Before installing the tunnel U-Walls, geotextile fabric and crushed stone formed the sub-base for the precast concrete pieces. The concrete bridge deck on top of the tunnel was the last phase of the project.
To improve reliability and the town’s aesthetic appeal, some utilities were buried underground. Another benefit is that Amtrak will come to town in 2021, connecting Middlebury College students to New York City.
The end result is 9,000 SF of landscaped area covering the tunnel, reuniting the town’s Triangle Park with its Village Green after a 170-year hiatus. Triangle Park had been part of the Green from the 1790’s until 1849 when the Rutland Railroad divided the two by building a depressed rail line right through downtown.
Of importance to VTrans was the long-term durability aspect of precast concrete with the culvert having a 100-year design life and that precast concrete is easy to maintain.
The new precast concrete tunnel and U-shaped approach supports were installed on a new alignment and lower profile to improve safety and clearance. While it impacted the community during construction, the benefits of this project provide a safer rail line and safer roadways, a revitalized downtown streetscape, improved stormwater management, and Amtrak service to New York City.