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The crews building the Silver Line through Fairfax County soon will reach a major milestone — the completion of the large aerial guideways that will carry the new Metro line.

Only a few of the 2,700 cast concrete segments are left to be put in place, pending the scheduling of road closures.

“It’s a significant milestone,” said Marcia McAllister, communications manager for the Dulles Corridor Metrorail Project. “The aerial work was considered to be some of the most difficult work. It was also the most visible to the community.”

About six miles of the 11.7-mile first phase of the Silver Line required aerial structures. The first phase connects to Metro’s Orange Line tracks in Falls Church and extends through Tysons Corner, including four stations there, and down the middle of the Dulles Toll Road and Access Road, terminating at Wiehle Avenue in Reston.

All of the aerial guideway construction is expected to be completed by the middle of this month, McAllister said. Then, subcontractor Rizzani de Eccher will disassemble the huge, yellow horizontal crane, called a truss, that has become a familiar sight above the streets of Tysons Corner.

At one point, there were three of the 366-ton trusses working on the project simultaneously.

“It was the only place in the world something like that was happening,” McAllister said.

Aerial guideway construction represented about $170 million of the $2.6 billion costs for the first construction phase.

Each of the 25- to 40-ton segments was fabricated at Dulles International Airport and delivered by truck to the construction site. Although they look similar, each segment is different and had to be assembled like the pieces of a puzzle.

The end of the aerial work won’t mean an end to road closures for the rail construction, although it was that work that was responsible for many nighttime closures when the truss had to work over a roadway, including the Capital Beltway.

“There will definitely be less disruption at the Dulles Toll Road and [Va.] 7,” McAllister said.

Overall, the first construction phase is about 77 percent complete. Construction is expected to wrap up next summer, followed by a testing period before rail service begins near the end of 2013.

Once the aerial guidways are completed, crews will be continuing to work on the five stations, as well as continuing to install tracks.

One of the next visible changes to the landscape around the rail project will be the construction of pedestrian access structures and pedestrian bridges over the roadways.

Crews currently are working on foundations and supports for the pedestrian bridges, and the pre-fabricated steel bridges will be delivered this fall.