SOMERVILLE — One of the advantages of extending the Green Line north through Somerville and Medford is that the route follows two existing railroad rights-of-way, reducing the amount of construction work involved in building a new transit line.
Even so, the 4.7-mile project requires moving an enormous volume of dirt just to get the narrow corridors in shape to accept new rail tracks and seven new stations.
“I would say half of this project is just getting a ton of earth-work done,” said John Dalton, the Massachusetts Bay Transportation Authority executive overseeing the project.
The rights-of-way are being widened to make room for Green Line trolleys that will run alongside existing commuter rail tracks. The commuter tracks have been temporarily relocated to make room for the construction but will return to their original location when construction ends.
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Meanwhile, tons of dirt are being removed and trucked to the Somerville-Cambridge border, where it will be used to form an elevated base for a new Green Line vehicle facility.
Nearby, massive concrete structures leap out of the earth to eventually carry viaducts to the new destinations.
Noise barriers, retaining walls, and drainage systems are being installed alongside neighbors’ backyards. And bridges and old buildings are being demolished to make space, marking a huge disruption to Somerville’s road network.
It’s been a lot of work already, but the pace will pick up considerably in coming months as the project closes in on its deadline in late 2021. Between the MBTA and its contractor, GLX Constructors, there are about 390 people working on the project today, and another 200 or so more are expected.
From about $170 million in construction spending now, the project will swell to $400 million in each of the next two years, MBTA general manager Steve Poftak said.
“The next two years are going to be an order of magnitude more work,” he said.
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Detour signs have dotted Somerville since the shutdown of two major roadways in recent weeks. Perhaps the biggest impacts so far are in Ball Square and on Washington Street, just outside of Union Square.
Outside of Ball Square, the Broadway Bridge has been torn down, interrupting through traffic on a major roadway for the next year. And on Washington Street, a commuter rail bridge will be disassembled, one half at a time, over the next two years, closing the street for months at a time.
The bridge on Broadway is a huge project in its own right. The new bridge, scheduled to be built next year, will be 150 feet long, nearly double the length of the old one.
There will be other major disruptions to come. In a little more than a year, the current northern terminus of the Green Line, Lechmere Station, will be shut down and a new one built on the other side of McGrath Highway.
With the bridge on Broadway gone, there’s a sense there’s no going back now for a project that was delayed for years and, in 2015, faced a near-death experience when its budget spiraled out of control.
Still, Dalton noted, the heaviest work is yet to come.
“We have to get a lot done this year, this construction season,” Dalton said.
“It’s going well now, but we’re still in a very early stage of the project, as far as getting the risk exposure behind us.
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“Every shovel of earth you move or every drilled shaft you put down, there’s potential risk.”
Adam Vaccaro can be reached at adam.vaccaro@globe.com. Follow him on Twitter @adamtvaccaro.