MIDDLEBURY — Snow and frigid temperatures will soon give way to mud season, spring flowers and then a cacophony of construction work in downtown Middlebury.
Jim Gish, the local liaison to the downtown Middlebury rail bridges project, recently released new details on timing for the most disruptive phase of the $72 million construction project, which will take place this spring: installation of the 360-foot concrete tunnel that will supplant the Main Street and Merchants Row rail bridges.
Workers spent much of the past year laying excavation-support infrastructure along the rail line. This included driving 333 mini piles — long metal pipes — and 350 temporary metal sheets into the ground along the length of the project, which begins roughly 400 feet south of the Merchants Row bridge crossing and ends at the northernmost limits of the Marble Works property.
These supports stabilized the rail bed slopes to allow them to receive the massive concrete tunnel that will be installed in sections — while Main Street and Merchants Row are closed to traffic. Vermont Railway freight trains will divert around Middlebury during the 10-week closure.
Here, according to Gish, are key dates to remember as spring approaches:
•  Monday, May 4, to Thursday, May 21: Merchants Row will be closed to through traffic and the temporary bridge will be removed. Parking spaces will be lost along both the upper and lower stretches of Merchants Row.
 In addition, the Battell Block driveway will be closed. Access to the Battell parking lot will be through a temporary access road running north from Water Street along Otter Creek.
Sidewalk access to all Merchants Row buildings will remain open, but the sidewalk connecting Town Hall Theater to the Battell Block will be closed, requiring a walking detour through the town green.
Gish said there will be no construction during Memorial Day weekend, including during Middlebury’s Memorial Day Parade and Middlebury College graduation.
Construction hours are set for 7 a.m. to 5 p.m., Monday through Friday.
•  Wednesday, May 27, to Wednesday, Aug. 5: Main Street will be closed to traffic — except delivery and emergency services vehicles — from Bakery Lane/Mill Street to Seymour Street. Eleven parking spaces will remain open in front of the Post Office on Main Street.
As rail traffic is also diverted, the Main Street temporary bridge will be removed, as will be the wooden footbridge over the rail corridor, and Printer’s Alley will close to pedestrians.
 This phase of work will proceed 24 hours a day, seven days a week.
Sidewalk access to all Main Street buildings will remain open, but the sidewalk connecting the Post Office to the National Bank of Middlebury will be closed at the rail corridor. This will require a walking detour through the Marble Works or across Cross Street Bridge. An Addison County Transit Resources shuttle will connect the Marble Works to the intersection of Main Street and Merchants Row.
Eventually, the rail corridor will be excavated and rebuilt and the two downtown bridges replaced with a tunnel. Main Street and Merchants Row will reopen and all downtown parking will be restored.
Much of the construction activity will precede the closing of Merchants Row and a lot will follow the reopening of the downtown roadways, but the key construction this year will take place during these three months, according to Gish, who promised much more construction detail in the coming weeks.
Gish will also soon share plans for managing impacts on the downtown. Those plans are being developed by the Neighbors Together group, the Better Middlebury Partnership, the Vermont Agency of Transportation and the town of Middlebury.
Meanwhile, Gish noted progress in work leading up to the tunnel installation.
Last week, the Maine Drilling & Blasting team completed five months of drilling mini-piles and tiebacks to stabilize the rail slopes, according to Gish. They will return later this spring to drill additional mini-piles. A second Maine team this week will be testing the installed tiebacks.
The company ECI continues to install sheet piles in the rail corridor behind the Seymour Street fire station. To date, ECI has installed about 100 feet of a projected 1400 feet of steel sheeting designed to hold the rail abutments in place when the north end of the rail corridor is excavated in June, according to Gish.
Kubricky Construction Corp. workers will spend the next few weeks excavating and installing infrastructure at two sites between the bridges where additional work on the new downtown drainage system was done last year, Gish said.
 
Meanwhile, 75 miles south of Middlebury in Schuylerville, N.Y., the Fort Miller Company has begun making the precast concrete that will form the new tunnel and abutments for the rail corridor, according to Gish.
To date, Fort Miller — a privately held, family-owned company — has produced about 35 of the nearly 400 pieces of precast concrete that will be set in place downtown, Gish said. Those precast concrete pieces will be stored temporarily at the Fifield Farm on Route 30 in Middlebury beginning later this spring.