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January 22, 2013

ACEC Best in State: Gold award
Complexity

HNTB

Project: U-Link tunnels under Interstate 5

Client: Sound Transit



Photo courtesy of HNTB [enlarge]
HNTB designed four concrete box structures to support retaining walls along Interstate 5 in preparation for tunneling under the roadway.

When Sound Transit needed to ensure that the tunnels for its new $1.9 billion University Link light rail under Interstate 5 in Seattle would be safely and efficiently bored, they tapped Northlink Transit Partners, a joint venture of HNTB, Aecom and Jacobs Associates, to make sure it happened.

Getting the 21-foot-in-diameter twin tunnels under I-5 was consistently regarded by Sound Transit as one of the highest risk portions along the alignment, both for its technical difficulty and for the risk of delaying tunnel boring if this preparatory work was not completed on time.

The challenge of mining two tunnels under a complex and heavily traveled highway is very risky, especially at a location adjacent to a steep, urban landscape. Specifically, what can be constructed underground to support massive retaining walls while creating windows through the walls?

HNTB met this difficult challenge with a simple but highly effective design to excavate four pits and construct four concrete box structures to hold the I-5 retaining walls in place so the tunnel-boring machines, or TBMs, could pass through the walls. Pits adjacent to the walls were excavated, and portions of the walls that were in conflict with the tunnel were demolished then backfilled with material mineable by the TBMs.

The opening in the retaining walls and the width of the box structures allowed for a clearance of up to 36 inches horizontally and 30 inches vertically for the TBMs.

The acceptable movement for the northbound elevated lanes of I-5 during boring would be just half an inch, while the allowed movement for the retaining walls would be one inch. Actual movement of the roadway was later determined to be just a quarter of an inch, while the retaining walls moved just nine-tenths of an inch.

Sound Transit’s goals were to construct twin light rail tunnels without harming I-5, disrupting the traveling public or damaging private property on Capitol Hill. HNTB provided the engineering expertise that assured these goals were met.


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