Peter Kenyon, TunnelTalk

Bids are open for the single contract package of station construction, surface, track and systems on the US$1.6 billion San Francisco Central Subway Project. The timing coincides with arrival in San Francisco Port of the first of two EPBMs that will excavate the 2.7km twin running tunnel extension of the Third Street light rail line.

Table 1. Bids for stations, surface, track and systems Company/ JV Evaluated bid (US$) Tutor Perini 852,067,000 Amorso/ FCC/ Southland JV 879,777,000 Skanska/ Shimmick/ Stacy & Witbeck JV 955,881,600

Tutor Perini has come in as the lowest bidder of three, with a spread of nearly US$104 million (Table 1). The lowest bid is approximately US$100 million higher than had been estimated. "The bids, opened last Thursday, (April 18, 2013) are under review and not yet awarded. We will work with our policy board and funding partners to move forward with this major project," said Sarah Wilson of Jacobs Associates and Resident Engineer for the project's Central Subway Partnership program management/construction management team.

The original intention had been to let out each of the four station construction packages (three underground and one at grade) separately, but in October last year (2012) it was decided to group all the station construction packages, along with track and train control systems, into a single package.

Chinatown Subway Station to be constructed using SEM

"We advertised the combined contract in October 2012. Combining the contracts allows the Central Subway Program to capitalize on cost and schedule efficiencies and reduce constraints involved in coordinating and integrating four separate construction packages," said Wilson.

Full contract scope comprises:
• Chinatown Station (underground) with entrance at
   Stockton St and Washington St;
• Union Square/Market Street Station (underground)
   with entrances at Stockton St and Geary St and
   Stockton St and Ellis St;
• A concourse connecting the Union Square/Market
   St Station to the existing Powell St Muni/BART
   Station;
• The Yerba Buena/Moscone Station (underground)
   with an entrance at 4^th Street and Clementina St;
• The 4^th and Brannan Station (surface-level) on 4^th
   Street between Brannan St and Bryant St;
• Surface tracks on 4^th Street between 4^th and King St and the subway tunnel portal on 4^th Street between Bryant
   St and Harrison St;
• Trackbed, tracks, switches and train control systems within the Central Subway tunnel.

SEM is selected as the excavation method for Chinatown Station, with cut-and-cover between slurry walls as the method for Union Square/Market St and Yerba Buena/Moscone Stations.

EPBM for Central Subway

Bids are opened for the station packages as construction of the 2.7km twin-running tunnels advances towards the tunneling phase. The first of two EPBMs has arrived in San Francisco from the Robbins factory in China and is currently being transported to the reception shaft for the Barnard/Impregilo/SA Healy JV. The JV won the tunneling contract with a bid of US$233.5 million in June 2011.

"The first TBM is in the port of San Francisco and will be delivered to the site this week. The second TBM will arrive in early June," said Wilson.

"Two 6.27m diameter EPBMs procured by the contractor, and furnished by Robbins, will be used to complete the drives. Contract specification also comprises five cross passages, a launch box, a retrieval shaft and four station headwalls. Lining will be with gasketed precast concrete segments with an i.d. of 5.44m," she added.

2.7km Central Subway alignment

Construction of the launch box on 4^th Street between Harrison and Bryant is complete and is being prepared for TBM assembly. "Assembly of the first machine will begin this week and launch is planned for early June, at which time assembly of the second machine will commence with launch planned for mid-August," said Wilson.

The extraction shaft, to be excavated on private property on Powell St in North Beach, is not yet built.

Wilson added: "There is no contract milestone associated with tunnel completion, but Crosspassages 1 and 2 are required to be completed by June 2014."

TBM challenges
"The particular challenges of this project include the dense urban environment and curved tunnel alignment. Use of the TBM will help us to minimize impact to the community, and the design of the TBMs will help us to negotiate curves using active articulation," said Alessandro Tricamo, Chief Engineer for the Barnard/Impregilo/Healy JV. Both machines have been designed to enable smooth excavation around tight turns down to 140m in radius.

The two EPBs have been additionally engineered with mixed ground cutterheads using knife edge bits and carbide tools. Shaft-type screw conveyors will aid in excavation through poorly consolidated silt and sand deposits of the Colma formation below the ground water table.

Robbins horizontal belt conveyors will travel behind both machines for efficient muck removal during tunneling. "We estimated that a continuous conveyor would help us improve the production rate of the TBMs on steep slopes (up to 7%), where a traditional mucking system using a locomotive would have been a challenge," said Tricamo.

References

EPBs for San Francisco as station bid opens - TunnelTalk, March 2012
Central Subway tunnels contract awarded - TunnelTalk, June 2011
Tight race for Central Subway running tunnels - TunnelTalk, June 2011
Bidding signals start of San Francisco Subway - TunnelTalk, June 2011

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