Victory in Vermont: Three Hard Rock Crossings Completed On Line, On Grade and On Time

Vermont’s Middlebury Rail and Bridge Project is no small task — requiring bridge demolition and construction of a new rail tunnel, a new drainage system, and roads for temporary access. And to top it off, construction is being done in the heart of downtown Middlebury, a historic New England college town of 8,700 people.

To compress the project schedule, multiple contract packages are being let out over three years. Local contractor Engineers Construction, Inc. (ECI) was subcontracted to bore three drainage tunnels as part of the Early Work Package 3 (EWP3), a contract that included the drainage system. These tunnels, excavated below the downtown historical district in solid rock, were successfully bored under strict line and grade requirements using a motorized small boring machine, known as an SBU-M.

Re-Establishing the Rail Corridor

Middlebury, on the western side of Vermont, is a historic mill town home to Otter Creek (a 628-sq-mile drainage area), the convergence of three major state highways, and Vermont’s western rail corridor, all within the downtown area. The new project is arguably one of the biggest projects owner VTrans (Vermont Agency of Transportation) has undertaken because of its scope: rail, drainage tunnels, and two bridges, all through a downtown area that is on the National Register of Historic Places. But the benefits of the project outweigh its impacts: “In the 1840s, it was desirable to build the rail through the center of town. 175 years later, this project will allow riders to get on a train in downtown Middlebury and ride all the way down to New York City’s Penn Station on Amtrak’s Ethan Allen service,” said Jim Gish, community liaison for the Town of Middlebury. In addition to the state-of-the-art rail tunnel and rebuilt downtown rail corridor, Middlebury will benefit from new infrastructure for water, sewer, and stormwater drainage lines.

The three-year project, set to finish in 2021, evolved from a specific set of circumstances. In the center of town, there are two roadways that cross over the rail corridor using two bridges originally built in the 1920s. “It is the State of Vermont’s responsibility to maintain all crossings of the western corridor rail line. The bridges were shown to be deteriorating, so these were then demolished and temporary bridges were put in place in 2017. Because the two bridges cross the rail line, federal regulations about the required height between top of the rail and the bridge came into play. “We realized the tracks had to be lowered to accommodate these federal standards, and so the project grew to include replacement of the downtown rail corridor as well,” said Gish.

The Town of Middlebury and VTrans discussed the options, ultimately deciding on a precast concrete tunnel, constructed by cut & cover methods, that would replace the two bridges. A 3500-ft-long section of the rail line downtown would also be rebuilt and lowered by about 4 ft total, thereby significantly increasing the drainage requirements. Over the top of the construction area a newly landscaped park would then be constructed between the location of the former bridges.

Early Work packages for the project included demolition of the existing bridges and construction of temporary bridges (EWP2) as well as the drainage system construction (EWP3). In 2018, general contractor Kubricky subcontracted ECI for the series of three drainage tunnels. The tunnels would be bored upgrade north and south along the rail corridor while a third tunnel would be bored downgrade and terminate at nearby Otter Creek as an outlet.

Choosing the Tunneling Machine: The Motorized Small Boring Unit (SBU-M)

ECI, a Vermont-based, 54-year-old heavy civil construction contractor, had experience with a variety of trenchless equipment. The line and grade requirements, combined with the predicted geology of 20,000 psi Unconfined Compressive Strength (UCS) marble, necessitated hard rock tunnel boring machines.

ECI had used Small Boring Units (SBUs) manufactured by The Robbins Co. on several projects over the years. The equipment ranges from hard rock boring attachments, that work in conjunction with Auger Boring Machines (ABMs), to self-propelled tunnel boring machines, all of it under 78 in. in diameter. The SBU-M, or Motorized Small Boring Unit, is a motorized, manned-entry rock and mixed ground boring attachment meant to bore in geology such as dry alluvium or rock with UCS of 4,000 psi to over 25,000 psi. It is made for use with any Auger Boring Machine from 48 to 78 in. or any standard pipe jacking unit. The SBU-M uses a hydraulic or electric motor to generate torque while the circular cutterhead, mounted with disc cutters or mixed ground tooling, excavates the ground.

For spoil removal, an invert auger enclosed in steel casing runs beneath the bearing housing and drive motor assemblies. To seal off the cutterhead and protect the motor, a steel bulkhead is added to direct the rock chips into the invert auger.

A Launch Shaft in the Middle of Town

For the EWP3, the machine would be launched from a 40 ft deep shaft in the middle of town, requiring not only a fine-tuned construction operation, but also community buy-in. With launch shaft construction sitting squarely between the National Bank of Middlebury and the town’s post office, and right next to the rail line, the project was firmly in the public eye. Receiving pits on to the north and south also had to be excavated to a depth of 30 ft to allow for the 140 to 150 ft runs below ground.

With further measures including reduced boring hours from 7 a.m. to between 3p.m. and 5 p.m. daily, and vibrational monitoring, the community got on board with the construction. Controlled blasting required all activity to shut down for 15 minutes before each of 18 blasts in a coordinated effort. Vibrations were ultimately not enough to be of concern, although minor vibrations from the boring machine were reported by people at the National Bank of Middlebury.

Successful Tunnel Excavation

The 60-in. diameter Robbins SBU-M was launched on Aug. 22, 2018, from the 36-ft deep, 40-ft diameter shaft and took 30 days to bore 140 ft. “We had a Robbins technician on site for the duration of the project, who cross trained our people,” said Tom Loyer, project manager and estimator for ECI’s trenchless division, of the launch and excavation process.

The TBM averaged between 1 and 2 ft an hour cutting through solid rock, and somewhat less when making its way through a seam of mixed dirt and rock. “In drive 1, we hit a small vein of clay that the geotech report did not see, but we worked through it. It got us off grade but we backed up and recut the bottom of the tunnel and we were fine. It wasn’t a major challenge,” said Loyer.

After being transported back to ECI’s shop for inspection, the machine was then launched on a 152 ft long bore that would make the southern leg of the drainage system. The second tunnel was completed in significantly less time than the first stretch of tunnel.

The final 139-ft long tunnel makes up the northern leg of the new drainage system, and was found to include a section of sticky clay, which was verified during construction of the receiving pit. Contractor Kubricky hit the clay just 5 ft down from the surface. The clay was again encountered during excavation. “We hit a 20 ft layer of clay. We had one long 18-hour day, where we lost our ability to steer but were able to push through the heavy clay using water injected through the cutterhead for lubrication,” explained Loyer.

Despite the challenges, the machine made its final breakout on Nov. 13, 2018, about one month ahead of the overall construction schedule. In the course of 12 weeks, the TBM had drilled its way through 442 ft of bedrock, mixed ground including soil and rock, and a challenging final stretch of sticky clay. Excavation rates averaged 1.5 to 2 ft per hour, and, were as high as 20 ft in one day or 3 ft in one hour.

“We finished on line, on grade, and on time. Everyone worked hard and worked together, it was great. The Robbins personnel were great,” said Loyer. The TBM holed through within one inch of line and two inches of grade, and well within the set project limits. Though ECI had extra disc cutters on site, no cutter changes were required over the course of the three tunnels. “At the end of each drive we analyzed the cutters and they looked great throughout.”

Work Still to be Done

With drainage tunneling complete, the main railroad tunnel work is set to begin soon.

“The main project starts this summer,” explained Gish. “The main contract covers the construction of the rail tunnel and the reconstruction of the 3500 ft long downtown rail line. Before they construct the rail line they also need to put in minipiles, tiebacks, and sheeting, to prevent abutments from collapsing as they dig down. This work will wrap up in late 2019. Then in summer 2020, our two downtown roadways will be closed for a period of 10 weeks and rail will be rerouted through the eastern side of the state while we are shut down.”

Vehicle traffic will be rerouted as well around the town center as crews work 24/7 to reconstruct the rail line, remove the temporary bridges, and construct the rail tunnel. Once the construction is done and transportation restored, the focus will turn to the landscaping of new parks in the construction areas and other finishing touches.