Sound Transit’s University Link Light Rail Extension

Seattle, Washington


Twin-bore tunnels connect downtown Seattle with the University of Washington


• Bored through massive retaining walls under I-5
• No significant settlements, no impact to I-5 traffic
• Delivered early and under budget

By threading its University Link light rail extension through 3 miles of twin-bore tunnels, Sound Transit circumvented surface traffic on one of the Pacific Northwest’s most congested corridors. HNTB, part of the integrated joint venture team of Jacobs Associates and AECOM, was contract manager for design of both the Interstate 5 undercrossing and the Capitol Hill to Pine Street tunneling, which concluded by boring into an active transit tunnel.

To support massive retaining walls under I-5, HNTB designed four structural concrete boxes that held the partially demolished retaining walls in place, while the tunnel boring machine mined through the boxes to create the light rail tunnels. Traffic on I-5 flowed the entire time the TBM was active.

At the time tunneling was complete, there were no significant settlements along the alignment. The U-Link extension finished early and significantly below budget.