Sound Transit’s U-Link project will connect
downtown Seattle with the University of Washington
via three miles of twin soft-ground tunnels.
The route from Pine Street in downtown Seattle to the
University of Washington is one of the most congested in the
Pacific Northwest. Crushing traffic often stretches the threemile
bus trip into a 30-minute crawl.
Sound Transit considered all options to improve trip times
and reliability along the route, including bus rapid transit lanes,
but costly and scarce downtown real estate made alternatives
nearly impossible. Above ground? Well, no one wanted heavier
traffic on the surface streets or a viaduct that would bifurcate
When Fuller talks of hiccups, he’s referring to the many obstacles
traditional design-bid-build tunnel projects face. In the final 500
feet of the project, the tunnel boring machine (TBM) crossed under
busy Interstate 5 twice into a live light rail transit tunnel.
HNTB, part of the integrated joint venture team of Jacobs
Associates and AECOM, was contract lead for the design of both
the I-5 undercrossing and the Capitol Hill to Pine Street tunneling
that concluded by boring into the active tunnel.
BORING THROUGH MASSIVE RETAINING WALLS
U-Link Tunnel Boring
21-foot outside diameter,
330 feet long including
Advance rate per day:
44 to 50 feet
neighborhoods, obstruct views and add to noise pollution. With
all surface options exhausted, there was only one alternative Seattle’s I-5 is a critical section of highway in Washington. It is Time underground:
left: Go underground.
a depressed, double-decked freeway flanked by retaining walls 14 to 18 months
By threading its University Link Light Rail Transit Project built in the 1960s.
Tunnel depth below
through twin bored tunnels, the agency could circumvent
“The retaining walls consist of up to 10-foot-diameter drilled surface:
surface traffic instead of competing with or adding to it.
concrete shafts with 9-foot-deep encased steel I-beams,” Fuller 100 to 300 feet
Nicknamed U-Link, the extension would provide unparalleled said. “They extend up to 75 feet into the ground. Not your
speed and reliability through Seattle’s densest neighborhoods, normal cantilevered retaining walls.”
shortening travel times to six minutes.
TBMs can’t bore through steel I-beams, which meant crews
The project takes the existing portion of light rail that
would have to extract some portions of the retaining wall drilled
terminates at Pine Street in downtown Seattle and extends it shafts before the TBM could pass through the lower portion of
more than three miles north via the tunnels to the university. the shafts.
The new route is expected to shorten travel times by 24 minutes “We were essentially cutting off the walls at their knees,” said
and add 70,000 boardings a day by 2030.
John Sleavin, Sound Transit’s civil design manager for U-Link.
“The beauty of tunnels is that they are out of sight,” said
To support the walls, HNTB designed four structural
Hugh Fuller, HNTB project manager. “And, they solve your boxes, each adjacent to the portion of the retaining wall the
access problem. Plus, the technology for constructing tunnels TBMs passed through. Crews temporarily tied back the massive
is improving with each decade. This project demonstrates that. walls, excavated the ground adjacent to them and constructed
There were no hiccups at all.”
the structural boxes, then demolished that portion of the drilled
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“HNTB’s structural work has been stellar. They kept major stakeholders informed and thoroughly explained what we were doing
through modeling, different graphic techniques and computer simulation, so it was well understood by everyone involved.”
— JOHN SLEAVIN
CIVIL DESIGN MANAGER, SOUND TRANSIT
Geo-Structural Design Solution for U-Link Tunnels
Under Interstate 5
HNTB engineers designed a special concrete box supported by pilings that
would hold up two massive retaining walls while the tunnel boring machine
actively bored underneath I-5. These illustrations show elements of the
concrete box design and how it was constructed.
(Tiebacks not shown)
TBM Direction of Travel
TBM Direction of Travel
shafts in the path of the TBM. Each structural concrete box
measured approximately 35 feet wide by 55 feet long.
“The structural boxes were a complex geo-structural simple
solution to an extremely challenging problem,” Fuller said.
The Washington Department of Transportation, the owner
of the retaining walls and a major stakeholder in the project,
required that the tops of the retaining walls move no more than
one inch as a result of the construction of the structural boxes.
But, on nearly the first day of construction, as the contractor
placed the first temporary tieback, the top of one of the walls
moved approximately .75 inches. No one expected that.
“The movement was likely triggered by an existing geological
anomaly, probably a slip plane,” said Rich Johnson, HNTB
contract manager and member of the design management
team. “Owners can’t afford to take sub-surface boring samples
every 10 feet of a three-mile-long project. It’s just not practical
or financially feasible. A more practical approach is to take one
boring sample every 300 to 1,000 feet. The gaps between the
borings may ultimately reveal unexpected challenges during
construction. It’s a risk owners take.”
WSDOT was concerned about the lateral movement at the tip
of the retaining wall. If the wall moved that much during drilling
of the tiebacks, how much would they move during excavation?
To diffuse growing concerns, HNTB engaged Sound Transit and
WSDOT in discussions about the basis of the design. HNTB then
modeled the anomaly and, with this new information, reassured
both parties the walls would move no more than 1.5 inches
during construction. WSDOT revised its criteria to reflect the
new calculation. By the time the boxes were in place, the wall
had moved laterally approximately 1.25 inches.
“WSDOT and Sound Transit were happy, and there was no need
for a wholesale change in the original design,” Johnson said.
“HNTB’s structural work has been stellar,” Sleavin said. “They
kept major stakeholders informed and thoroughly explained
what we were doing through modeling, different graphic
techniques and computer simulation, so it was well understood
by everyone involved.”
NO SIGNIFICANT SETTLEMENTS
The tunnels were designed to be about 21 feet in diameter and
a mere one diameter apart, except for the last 500 feet of the
project where they would be separated by two diameters.
Contractors deployed three TBMs to mine the new route.
Two of the machines took off in a sort of staggered drag
race from the University of Washington south to the Capitol
Hill station. From Capitol Hill to Pine Street, a much shorter
distance, one TBM was sufficient.
On its approach to Pine Street, the third TBM had to hit the
east retaining wall structural box spot on, drive through the soil
under I-5 — where the machine’s crown was just 15 feet below
the tires of cars and trucks — intercept the structural box on the
west side of the freeway and conclude by boring into the active
transit tunnel at Pine Street. It then was returned to Capitol Hill
to do it all over again for the second tunnel.
“Traffic on I-5 was moving the entire time the TBM was active,”
Fuller said. “No one even knew it had passed through the
retaining walls, other than the people operating the machine.”
Was the underground crossing a first for the U.S. tunneling
“I certainly know of no other instance where a tunnel has
bored through existing retaining walls,” Johnson said.
The $1.9 billion U-Link project is part of a program called
Sound Transit 2, funded by federal grants, a motor vehicle
excise tax and a quarter-cent sales tax approved by Seattle
voters in November 2008. The plan adds regional express
bus and commuter rail service while building 36 additional
miles of light rail to form a 55-mile regional system.
“All of the tunneling is complete, there were no significant
settlements anywhere along the alignment and most of the
U-Link project is at or ahead of schedule and at or below
budget. You can’t ask for more than that.” Sleavin said.
Construction of the project is slated for completion by
late 2015 with revenue service anticipated in 2016. n
ANN JAMISON, HNTB Client Service Leader, Sound Transit
(425) 450-2506 • firstname.lastname@example.org
Using a tunnel boring machine
under an active interstate highway
was a huge technical challenge.
HNTB’s knowledge of the surface
infrastructure combined with the
firm’s tunneling expertise resulted
in a successful project.
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