A massive precast concrete tunnel is replacing two decaying bridges in Middlebury, Vermont, as part of a $70 million bridge and rail project. Launched recently by the Vermont Agency of Transportation (VTrans), in collaboration with the Town of Middlebury, the project is designed to improve pedestrian and vehicular safety and increase freight train capacity on the state’s western rail corridor.
Built between 1920 and 1921, the two bridges are about 300 feet apart and span a single set of depressed Vermont Railway tracks. Located in downtown Middlebury, with one bridge on Main Street/VT 30 and the other on Merchants Row, the two spans were marked by severe concrete cracking, delamination and spalling, even visibly rusting rebars, and concrete has periodically fallen on the railroad tracks below.
In March 2017, Transportation Secretary Joe Flynn issued an emergency order for VTrans to install two temporary bridges to replace the 100-year-old structures while construction of the bridge and rail project is underway. Subsequently, the Main Street and Merchants Row Bridges were demolished in mid-summer 2017, with vehicular traffic proceeding on the temporary bridges. While this work was let under a separate contract, it is considered part of the overall bridge and rail project. The steel, truss-panel Mabey bridges will remain in service for the project duration.
The Middlebury bridge and rail project is being managed by VTrans, which has selected consultants VHB as the lead engineering designer, and Kubricky Construction Corporation as construction manager/general contractor. In addition, the Town has a liaison, Jim Gish, who works full time with the design/construction team and maintains an information blog as part of a public outreach program.
Tunnel Benefits Town and Railroad
Construction activities in 2018 include tree cutting along the rail line, building temporary access roads and installing drainage and underground utilities, while VHB completes final design documents.
The 360-foot tunnel will not only replace two deteriorating bridges but also corrects several problems for the railroad. For example, the 19th century bridges did not have enough vertical clearance for today’s double-stack rail cars. This project lowers the vertical rail bed for more than 3,000 linear feet, allowing vertical clearance beneath the bridges to be increased from less than 18 feet to 21 feet – enough to allow passage of double-stack freight trains without impacting the grade of the streets and sidewalks above. Furthermore, the project will change the rail alignment, thus reducing the existing sharp curve and resulting in better horizontal clearance for trains. In addition, improving drainage and placing trains in a tunnel will alleviate water ponding and the risk of icing problems in winter.
These changes benefit the Vermont Railway, a vital component of the state’s economy. Operating on much of the defunct Rutland Railway, the Vermont Railway is the main part of the Vermont Rail System (VRS), which also owns the Green Mountain Railroad. Trackage is owned by VTrans except in New York, where Vermont Railway operates a line owned by the Boston and Maine Corporation. The VRS hauls over 25,000 freight cars each year, with nearly 90 percent of traffic serving Vermont businesses. What’s more, the company maintains a number of U.S. and Canadian freight interchanges.
Transportation officials point out that moving freight on Vermont’s western rail corridor lowers truck traffic on Routes 7, 22A, and other state routes, as well as reducing wear and tear and congestion on these roads. One double-stack rail car can carry the equivalent load of eight trucks.
Flood Prevention is Part of Plan
On project completion, there will be about 9,000 square feet 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.
According to VTrans, the bridge and rail project has called for intricate planning and design in order to balance community needs, local and regional transportation, and construction feasibility. For most of the project’s duration, the rail line has to be kept active since a number of communities along the corridor depend on trains to deliver essential goods and commodities. In line with this, VTrans negotiated with VRS to obtain feasible work windows that enable construction to proceed without interrupting rail service.
Other considerations that had to be addressed by the design/construction team included:
The latter consideration was especially important. The name Otter Creek is a misnomer that belies its potential flows. It is not a creek but actually a river, in fact the state’s longest river, flowing about 112 miles south to north in a meandering fashion. Otter Creek drains 936 square miles, receiving water from several other rivers en route to its terminus in Lake Champlain. Near the town of Middlebury, extremely high flood crests have been recorded at the local USGS station. The highest one occurred more than a century ago, in March 1913, when the station reported Otter Creek’s highestpeak flow and highest crest on record – a town-devastating 17 feet above normal. High flows in Middlebury have been a continuing reality over the past century. More recently, following Tropical Storm Irene in August 2011, the USGS gauge at Otter Creek Falls recorded the flood crest as the 8th highest recorded level – 7.31 feet above normal.
Stormwater Infrastructure Requires HDD
The $70 million bridge and rail project is expected to take about three and a half years between 2018 and 2021. Final design efforts began in 2017 together with some preparatory work including demolition of the two existing bridges and installation of two temporary Mabey bridges.
Other work in advance of main project construction includes relocating some utilities underground, installing stormwater infrastructure using horizontal directional drilling, and building two temporary construction access roads. This work is intended to be completed during the first year of construction, along with the completion of final construction documents.
The second and third years are slated for lowering the rail track and building the tunnel. This entails installing such excavation support as soldier pile and lagging, sheet piling and gravity block walls, for track lowering and allowing excavation to proceed safely near surrounding buildings, wing walls, retaining walls, streets and other structures.
Massive Precast Tunnel
With supports in place, existing tracks will be removed and excavation for the new lower profile for approximately 3,550 linear feet of track will proceed. The precast concrete tunnel and permanent U-shaped approach supports for about 1,400 feet of track will be installed on a new alignment and lower profile. The new concrete tunnel will have 18-inch walls and outside-to-outside dimensions of approximately 22-feet-wide by 29-feet-tall, according to preliminary design documents. In addition, new bridge railing, sidewalks, and street approaches will be built during the third year of the project.
In the final construction season, the contractor will finish track work; complete necessary street reconstruction including paving, line striping, crosswalk work, and signage; and conclude landscaping of the reunited Triangle Park/Village Green Area.
Town Fully Engaged
Reflecting on the incipient four years of construction, Middlebury Liaison Jim Gish commented that it would be a significant challenge for many in the community, but pointed out that there were also advantages to be gained:
“The benefits of this project include a safer rail line and safer roadways, a revitalized downtown streetscape, improved stormwater management, and Amtrak service to New York City,” he said.
Gish indicated the town is committed to the $70 million project.
“This community is fully engaged in the effort to support its local businesses and preserve the qualities of life that make Middlebury unique.”
Photos courtesy of Jim Gish, Middlebury Liaison.
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