was a huge technical challenge - HNTB.com


was a huge technical challenge - HNTB.com

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.


U-Link Tunnel Boring

Machine Facts


21-foot outside diameter,

330 feet long including

trailing gear


679,000 pounds

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

Page 01 HNTB DESIGNER Number 98 HNTB DESIGNER Number 98 Page 02

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.”



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.

Temporary Tiebacks

Existing Wall

Pit Wall

Tangent Pile

(Tiebacks not shown)

Elevation View

TBM Direction of Travel

TBM Direction of Travel

Plan View

Existing Wall

Existing Wall


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.”


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 • annjamison@hntb.com

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.

TBM Window


Page 03 HNTB DESIGNER Number 98

HNTB DESIGNER Number 98 Page 04

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