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Tunnel breakthrough for Seattle’s light rail extension project

The Northgate Link light rail extension project in Seattle, Washington has reached a tunnelling breakthrough.

The tunnel boring machine (TBM), known as Brenda, has completed the third segment of the twin 3.5 mile tunnels that will serve light rail trains from Northgate to the University District.

The TBM has excavated 200,000t of material during its journey to reach the U District station site.

The machine will now undergo maintenance at the site before continuing south and connecting with the University of Washington Station.

A second TBM, named Pamela, was launched last November and is expected to reach the U District site early next year.

Each machine weighs 600t and is more than 91m long, including the trailing gear. The cutterheads are 6.5m in diameter.

The TBMs are expected to complete tunnelling by the third quarter of 2016. Tunnel finishes and cross-passages, which connect the train tunnels and provide emergency exits, are scheduled to be complete by the first quarter of 2018.

The $1.9bn (£1.2bn) light rail extension is expected to be operational in 2021.

Watch the TBM breakthrough:

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