Should Seattle become home to another professional sports franchise anytime soon, one might suggest they adopt a TBM mascot. A number of machines have been busily mining beneath the city streets, and the mammoth Alaskan Way Viaduct is expecting its 17.5m Hitachi Zosen EPBM to be fully assembled this year.

With TBMs completing their work on the University Link, or U-Link, tunnels, Sound Transit is ready to bid the twin bore tunnel construction contract for its next extension, to Northgate. At the far northern end of Seattle, this urban center has one of the region’s shopping malls, multi-family housing and a community college. It’s also adjacent to the Interstate Five corridor.

"It’s a very busy area of Seattle and it sees a lot of transit ridership," says Don Davis, executive project director with Sound Transit for the North Link. "This is probably the best ridership segment in our current program, and we’re expecting about 62,000 daily boardings on this link."

The Northgate Link, or North Link, component of Sound Transit’s 23-mile (37km) north-south regional mass transit system is a 4.3-mile (7km) light rail extension (Figure 1). Connecting the University of Washington to Northgate, via the densely-populated Roosevelt neighborhood, the alignment is mainly in tunnels through softground with the last mile elevated as it reaches Northgate’s shopping mall (see Figure 2).

This corridor is currently served by an extensive express and local bus system, but it’s already very congested, explains Davis. This primary link to downtown Seattle will help in many ways. "It’s one of the higher growth areas, and it’s just the general need for the capacity that rail transit brings instead of bus transit."

From the University of Washington Station (part of U-Link), the North Link alignment continues north, underground to the U-District Station (previously called Brooklyn), then to the Roosevelt Station, also underground. The tunnel will end at a portal and the remaining mile or so will be elevated to the third station, Northgate.

With a number of similarities between the two projects, Davis and colleagues are optimistic about this next step. There are, however, differences in ground conditions, and the U-Link experience has shed some light on what’s to be expected up north.

Extended family
Sound Transit has already bid an advance demolition contract for station sites to take down existing structures. The tunnel construction contract will be advertised in March and includes all of the TBM tunnelling, the portal at the north end of the alignment, cut and cover excavations for the two underground stations and cross passages. The current estimate for contract is in the range of USD 500M.

Two separate contracts will finish each underground station. Another contract will construct the remaining elevated mile of light rail. Several other contracts include utility location and systems work, among others. Jacobs Associates is responsible for all of the tunnel design and design for support of excavation for the stations.

Shannon & Wilson is involved on the project for geotechnical work, and Parsons Brinckerhoff is the lead designer for the aerial guideway and the aerial station at Northgate.

As an extension, the twin bore tunnels will be the same dimension as the U-Link and Central Link projects. Finished tunnel diameter will be 18ft, 10in (5.7m), and at minimum the TBM diameter will need to be around 6.5m.

"As far as the tunnelling itself goes it’s very similar," says Davis. "We have basically the same tunnel section in the Northgate segment as we do in the University segment. The difference, is of course is the geology."

The two stations will be between 80ft and 90ft (24.4m and 27.4m) deep, and the deepest part of the alignment is underneath the University of Washington’s campus, with an approximately 150ft (45.7m) invert. Near the portal, the alignment has an invert of approximately 40ft (12m), the least amount of cover in the whole drive.

"There are the same geologic conditions prevalent right through the U-Link project and the Northgate Link in that we have a variable sequence of interlayered glacial, interglacial and non-glacial soils as we did on U-Link and also on Central Link," says Isabelle Lamb, design manager for Jacob Associates. She and Davis have worked on both projects.

On the North Link alignment there are numerous contacts between hard cohesive soils and then variable permeability non-cohesive soils, such as sands and gravels, as well as boulders and shears. "So it’s similar in the environment [of] the University Link. But the big difference here on Northgate Link is we have a lot more of your sands, the non-cohesive soils," she explains.

"On U-Link a majority of the U-220 contact, which went from University of Washington station to Capitol Hill, was predominantly the cohesive silts and clay — the really hard silty clay. And through that material we had channels of sand cut through. On Northgate Link, we have a predominate of the more granular, non-cohesive sandy soils and fills. So it’s just a real mix of soil."

Driving apart
The total TBM drive will be approximately 18,391ft (5,605.5m), with a 4,773ft (1,454.8m) reach between the University of Washington station and the U District station, 5,530ft (1,685.5m) between U District and Roosevelt station, and from there, another 8,088ft (2,465m) to the portal.

The cut and cover U District station and Roosevelt station are going to be approximately 400ft (121.9m) and 490ft (149.4m) long, respectively.

TBMs will be launched from the Roosevelt station, and Sound Transit anticipates a total of three machines will be used on this contract. Two EPBMs will complete the drives south to the U-Link’s University of Washington station, past U District station, approximately 10,303ft (3,140m) each. A third machine, either EPBM or slurry, will do both drives between Roosevelt station and the portal.

"The potential for use of a slurry machine is because of the granular, non cohesive soils that we anticipate for that northern end," explains Lamb.

A retrieval shaft will be built at the north end of University of Washington station. The TBMs will drive into the shaft and be extracted from the station.

Shannon & Wilson has been helping with groundwater modelling for the station excavations, looking at discharge flows and interpretations of the ground conditions. For the TBM launch and retrieval at Roosevelt station the contract will call for pre-excavation jet grouting. While for the U District station, because of the proximity of structures, the design team is looking at dewatering rather than pre-excavation grouting.

"We also have pre excavation grouting proposed for some of the cross passages where we believe the groundwater conditions would make straight SEM excavation a little difficult," explains Lamb.

One of the project’s biggest challenges is its 23 cross passages, however the ground conditions will be different from the U-Link project and the groundwater pressure head is lower further north.

At the alignment’s deepest point beneath the university there is a maximum pressure head of 2.9 bars, explains Lamb. "On University Link, in comparison, the pressure head was approximately 5.2 bars. As we go to the north the pressure head gets much less, and it’s less than 2 bars."

Few differences can be distinguished between the projects beyond the ground conditions. "We’re passing under the University of Washington’s central campus and that has requirements with respect to noise and vibration, and long term vibration impacts from the operating system," Davis explains.

He later adds, "We had some rather significant challenges on University Link passing under a ship canal and passing under the I-5 freeway with very minimal overburden on the tunnel. We had only 15ft (4.6m) of cover when we passed under the freeways. We don’t have those same challenges on Northgate Link.

"It’s different challenges, different geology and different kinds of development. But it’s very similar. It gave us the opportunity to look at the University Link and look at any of the issues they ran into on University Link and take into account as we progress on Northgate Link."

For example, on U-Link’s Capitol Hill station, when the soils were not as expected, the project team re-did the dewatering design, explains Lamb. "It’s a bit of a learning experience. We are now seeing how the soil behaves so we are very optimistic about how the ground will behave on North Link"