Gold Award – Structural Systems
HNTB – University Link Tunnels Under I-5
Client – Sound Transit
One of the most congested bus routes for Sound Transit is Downtown Seattle’s Pine Street to University of Washington- a five-mile journey that takes 30 minutes to travel. Sound Transit considered all options to improve trip times and reliability, exhausting all surface options until deciding upon underground tunnels. Light rail tunnels were decided upon to improve commute times from downtown to the University. HNTB was tasked to design a solution to enable the boring of the tunnels without movement of I-5 retaining walls or the roadway surface.
The tunnel option took the shortest route, crossing under the I-5 freeway, which presented difficulties given the history and make-up of I-5. The I-5 freeway is 60-years old, a depressed and double-decked freeway flanked by retaining walls. Preventing movement of the freeway infrastructure was a difficult task considering the complex and heavily-traveled highway is surrounded by a steep and highly-urban landscape. HNTB designed a special geostructural concrete box supported by pilings that would hold up the massive retaining walls while active boring was performed under I-5.
The geostructural boxes were built sequentially, from the top down, utilizing the existing cylinder pile retaining walls and 3-foot diameter piles on four sides. The box structure transferred the forces from the existing retaining walls to these new piles that resisted both lateral sliding and rotation. Once the excavation and casting of a structure was complete, the existing wall piles were demolished sequentially from the bottom up, and the boxes were backfilled with controlled density fill. The fill provided soil support and as a mineable medium the Tunnel Boring Machine (TBM) could mine.
The tolerance for highway pavement settlement was one inch, one-half inch for elevated lanes, and one inch of deflection at the top of the retaining walls. HNTB’s geostructure enabled the tunnels to be bored within the tolerances. The pavement moved only .25 of an inch, while the retaining wall movement was less than .9 inches. An impressive feat considering the tunnel is only fifteen feet below I-5.
When the construction equipment was gone, the only signs of the project were fresh pavement, a new coat of sealer on the walls, and a few small survey targets, all of which were planned. The geostructures allowed the TBM to bore twin tunnels under I-5 without any significant problems – the general public was unaware the boring had occurred.
The University Link Light Rail project will greatly enhance the lives of Seattle’s citizens by providing a public transportation option that is highly efficient and economic. The new light rail tunnel is expected to convey commuters from downtown Seattle to the University of Washington in six minutes. In addition, the light rail uses electricity for propulsion, thereby reducing greenhouse gases and reliance on carbon-based fuels. Access to key destinations like the airport, downtown, and university district will be improved. HNTB’s geostructure design successfully addressed the challenges of the project, regarded as one of the highest risk sections of the project.