Shani Wallis, TunnelTalk

Works have mobilised towards completing the 3.15-mile (5km) U-Link extension of Seattle's light rail transit system. To the project, Sound Transit brings its experience of the 14 mile (22.5km) Central Link system that opened to the public last July. Lessons learned on that project will be applied to avoid on U-Link - problems encountered on Central Link and its 4,300ft (1.3km) underground section at Beacon Hill.

Fig 1. Route of Seattle's U-Link LRT system extension

An eagle eye on TBM excavated volumes, plus separate contracts for tunnel and station construction, and completion of several advanced works contracts, are steps taken by Sound Transit to avoid on the University Link (U-Link) LRT extension the delays experienced on the Central Link project and the voids that developed behind the TBM drives for the Beacon Hill running tunnels.

"Belt weighers are specified on the U-Link TBMs, as is tailskin annular grouting and conditioning for cohesionless soils, so there will be no excuses," said Dick Sage, Construction Managing Engineer of the Beacon Hill and U-Link projects for Sound Link when TunnelTalk visited the Sound Transit offices in June. "We are going to know the 'how' and 'what' of systems implemented by the contractors to prevent over excavation. Back analysis of the backfill operation illustrated that it took between 91-106% of the excavated volume above theoretical to fill the Beacon Hill tunnel voids. It is clear that TBM over excavation created the voids. Belt weighers were on the Beacon Hill TBM but we are going to pay close attention to excavated volumes on the U-Link TBMs."

Fortunately, the stratified glacial geology of the area includes a hard till cap above the tunnel horizon that prevented more serious surface subsidence or settlement above the deep tunnel alignment. After the six voids were discovered and filled with between 200yd³ to 400yd³ of CDF (controlled density fill) per void, there has been no further cause for concern.

Capitol Hill station box site looking south

Abrasive wear is another condition. Problems experienced by two machines working on the Brightwater project in Seattle's glacial deposit geology to the north of the U-Link project highlights the concern. Abrasive geology in combination with a deep alignment and a high watertable that required up to 7 bar compressed air pressures for man-entry interventions on the two Herrenknecht Mixshield drives have caused severe delays at Brightwater and initiated emergency contractual measures to complete one of the troubled drives. Sound Transit will pay attention to measures applied by U-Link TBM manufacturers to monitor machine wear.

Three EPBMs will be used to complete the 3.15 miles (5km) U-Link running tunnels from the Pine Street stub tunnel (PSST) of the downtown LRT/electric bus tunnels north through an underground station at Capital Hill and on to a U-Link terminus station adjacent to Husky Stadium at the University of Washington campus (Fig 1).

Traylor Bros/Frontier-Kemper has ordered two Herrenknecht EPBMs for its twin 2-mile (3km) tunnels for Contract U220 working south from the University of Washington (UW) station access shaft to Capitol Hill station while the Jay Dee/Coluccio/Michels JV on Contract U230 has ordered one Hitachi Zosen EPBM to drive both 3,800ft (1.1km) running tunnels south from a working shaft in the Capitol Hill station to a reception shaft adjacent to the PSST.

As the first on the main working sites, the tunnel contractors are installing the retaining walls around the perimeters of the two open cut stations and will excavate the TBM working shafts within their footprint. The Jay Dee/Coluccio/Michels JV is also excavating the core of the Capitol Station box to base slab level and others will be engaged to build the interior structures and install the M&E works. At UW station, the Nicholson/Condon Johnson JV, under subcontract, is installing slurry walls around the full station box and Traylor/Frontier Kemper will excavate the TBM working/access shaft at the south end. A General Contracting/Construction Management (GC/CM) contract has been awarded to Hoffman for complete excavation of the station box and for construction of its interior structures and M&E installations.

"This is different to the one combined tunnel and station contract awarded for Beacon Hill," explained Joe Gildner, Project Director for Sound Transit. "With the deep mined station dividing the twin tunnels into four separate drives, one contract was the least complicated way of progressing the Beacon Hill work but it did cost us time. We were 11 months late on substantial completion on that contract, in addition to 70 days of agreed time extensions. The U250 GC/CM contract for UW station better suits the scope and complexity of the site. Five were short listed, two presented bids and the contract was awarded to Hoffman. Under the terms of the contract Hoffman can self-perform 30% of the construction works and the rest must be subcontracted."

Progress at the UW station box site

To further control time on U-Link, Sound Transit undertook several advanced works contracts and secured many of the necessary permits. Advanced works contracts included utility diversions and demolition contracts at the station sites and permits were procured for noise and vibration variances for night work of the TBMs and at the stations, and for muck haulage trucks through the city.

A particularly complex advanced works contract was undertaken by Condon Johnson Assoc to clear passage of the TBM tunnels through the large diameter piled retailing walls that support the I-5 interstate highway just north of the PSST. Working according to a design by HNTB. The JV excavated two 60ft deep by 40ft wide (18m x 12m) pits either side of highway access ramps to cut out the tunnel eyes in the exposed 10ft diameter piles that support the double deck highway against a steep slope. The access ramps had to be closed for the year in which work was programmed. When completed, one pit was backfilled leaving the other one available for reception and recovery of the U230 TBM after its two drives. "All went very smoothly and the contract was handed over in May 2010, ahead of the June 5 deadline," said Gildner. "It was a job well done."

Design-bid-build procurement
The end of 2016 is the target date for opening services on the new U-Link extension and at this early stage, the critical path is excavation of the Capitol Hill station box. "We have the baseline schedules from each of the contractors and they are in compliance with the program," said Gildner. "Further out, the critical path will be installation of the rail. This will be complicated by the fact that there is no portal structure on this extension. Rail will have to be installed in pieces via the station box accesses and welded in situ. This will need precise programming."

The U-Link stations and tunnel drives are designed to 100% by the NTP JV of Jacobs Associates as lead with HNTB, and AECOM. Construction Management for the extension is by the START JV of CH2M HILL, as lead, and Jacobs Engineering. Parsons Brinckerhoff continues to play its role as principle consultant to Sound Transit since the start of the LRT program in the early 1990s.

One of the earliest risk mitigation steps by Sound Transit was to cancel the proposed deep mined station at First Hill. At 220ft deep, the SEM station cavern excavation would have been deeper than the 180ft deep Beacon Hill SEM station on the Central Link and involved considerable time, cost and constructability risk. Excavation of the Beacon Hill station cost the client much of its time overrun on the Central Link and accounted for $33 million or 12% increase of the $297 million bid price to $312 million. "This is below the 15% contingency allocated to the contract" said Dick Sage, who was Construction Managing Engineer for the Beacon Hill project and is now for the U-Link project. It also excludes the result of claims yet to be resolved to close out the Beacon Hill contract. "We hope to have these resolved before the end of the year."

Fig 2. Build out plan of ST2 to 2023

In cancelling the deep mined station at First Hill in a 2008 revision of the project, passengers instead will travel to and from the major hospital facilities in the First Hill area by an extension of Seattle's street car system which is being built by the City of Seattle. Cancellation of the First Hill LRT station also allowed slight realignment and shortening of the U230 running tunnels between Capitol Hill and UW stations.

Services on the 14 mile (22.5km) Central Link with its deep station and tunnels at Beacon Hill opened in July last year. Since then ridership numbers have increased from a starting number of 14,000/day to 20,000/day currently. That is down on initial forecasts but the current economy's drop in employment has had an affect. Despite this Seattle's case for securing public transit funding from the FTA (Federal Transportation Administration) remains among the highest in the country.

In January 2009, Sound Transit secured an $813 million FFGA (full funding grant agreement) from the FTA towards the $1.95 billion construction cost of the U-Link. This is added to a $17.8 billion roads and transit construction budget approved by voters in 2008 financed by an increase in sales tax over 20 years. Local funding already existed for construction of the U-Link project. The new funding will provide the local contribution for construction of extension of the system south beyond the current stop at Sea-Tac Aiport to Redondo/StarLake, east from the downtown Seattle transit tunnels to Bellevue and Overlake, and further north from Northgate to Lynnwood. Construction of this ST2 expansion of the system is programmed over the next 13 years to be in service by 2023.

In all areas, controlling cost is a major objective for Sound Transit. Failure of the first efforts to procure the northern extension to University of Washington as a two separate and then one revised design-build procurement contracts in the late 1990s was a reach too far admitted Gildner. The scope of the project and the risk elements involved were too ambitious to undertake as a design-build projects.

For U-Link, all construction contracts bid to date have been awarded for less than the Engineer's Estimate – 22% less for the U220 contract; and 34% less for the I-5 TBM passage advanced works. "This is a sign of economic times," said Gildner, "with tighter competition and lower commodity prices for steel, concrete, fuel, and lower labour costs. During construction we apply the standard cost control tools. All main contracts are with Sound Transit; almost all civil works are procured on a design-bid-build basis with contractors working to 100% final designs; and particular specifications are in the contracts. We include a GBR; contract bid documents are placed in Eskrow for reference in the event of a dispute; the GBR and the geotechnical data report (GDR) are part of the contract; and we establish a three-member DRB to help resolve claims as they arise. The one thing we don't do, is include the geotechnical interpretive report in the contract. This is based on information gathered by field studies and is rarely updated. It never becomes a final document. It is more for the designer and feeds into the writing of the GBR. It is for the Construction Management team to then evaluate claims of differing site conditions and make recommendations to the owner."

In fact, in last week, (July 23, 1020), Sound Transit was selected by the National Institute of Governmental Purchasing for the Outstanding Agency Accreditation Achievement Award that accedits professional excellence in public purchasing policies and practices.

With a wealth of experience gained on the Beacon Hill project, with adoption of the design-bid-build procurement model, with a favourable economic environment, and a clear mandate of support from the general public, Sound Transit is set to see its network of services expand significantly over the coming years, starting with construction of the current University of Washington extension.

References

Beacon Hill voids investigation - TunnelTalk, July 2009
Seattle embraces new transit system - TunnelTalk, July 2009
Americans approve billions for transit projects - TunnelTalk, Nov 2008
Mobilisation of Seattle's U-Link extension - TunnelTalk, July 2010
Build-out plans for the Seattle LRT - TunnelTalk, July 2010

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