Innovative Tunneling Beneath the Emerald City

There’s no fast way to add more than four miles of track, most of it underground, to the Sound Transit Link Light Rail system in the Seattle area. That’s the challenge a joint venture of several tunneling contractors faced when starting its portion of the $1.9 billion Northgate Link Extension in 2013, and the project is still keeping them busy three years later.

Sound Transit dedicated more than a quarter of the project’s estimated expense — $440 million — to general contractor JCM Northlink LLC to handle the tunnel segment. JCM is a joint venture between Jay Dee Contractors of Livonia, Michigan, Frank Coluccio Construction of Seattle and Michels Corporation of Brownsville, Wisconsin.

After digging two large vertical shafts and a portal to be used as access points, JCM began digging the twin-bore tunnels from Northgate to the already-excavated University of Washington station and existing light rail track. Crews used tunnel boring machines (TBMs) to simultaneously dig two parallel tunnels. The tough soil didn’t make the job easy. The TBMs hit harder ground than anticipated, slowing down the machines as cutters and scrapers heads wore down or broke against cobbles and boulders.

As tunneling moved along, crews started digging cross passages between the tunnels. The passages are typically constructed using pneumatic tools and mini-excavators, but JCM recently began using a different approach: Brokk remote-controlled demolition machines. They made the switch to limit worker exposure to harsh tunnel environments and reduce the inevitable fatigue and potential injuries that come with using handheld tools all day. The equipment sped up the excavation portion to less than three weeks, shaving off more than a week of work per cross-passage compared to handheld tools.

Crews typically started the cross passages by pairing a Brokk 400D with a rock drill to drill holes for steel spiles. Then JCM used a Brokk 260 paired with a hydraulic breaker to break through the tunnel wall. Finally, JCM brought back the B400D with a hammer and a road header to complete the passage. Each passage took seven to nine weeks to complete.

Other cross passages did not go as smoothly. Seattle gave JCM much more groundwater than expected, causing complications and prompting project leaders to decide to freeze the soil around 10 of the passages to prevent structural issues and ensure safety. Crews installed freeze pipe then pumped them with a brine solution, chilling the soil to a frozen 10 degrees Fahrenheit and making it an easy target for a Brokk machine’s hydraulic breaker.

After three years of tunneling with the TBMs, JCM completed the final breakthrough in September 2016. When finished in February 2018, the crews will have removed about 725,000 cubic yards of material — enough to fill about 242 Olympic-sized swimming pools. Project completion is scheduled for 2021.