Paula Wallis, TunnelTalk

Stiff competition is anticipated for the first and largest tunneling contract on the Central Subway project in San Francisco. Bids for the 1.7 mile (2.7km) TBM twin bored running tunnels will be opened next, Wednesday June 8, 2011. The estimated $225 million contract is the first of seven construction bid packages to be let for realization of the $1.6 billion extension of the Third Street light rail line from I-80 to North Beach. The subway extension includes three underground stations and one on the surface (Fig 1). The San Francisco Municipal Transportation Agency (SFMTA) is expecting very competitive bids from leading national and international tunneling contractors.

New subway alignment

"The Agency hosted two conferences that have been well attended by prime contractors from all over the world," said Nathaniel Ford Sr, SFMTA Executive Director/CEO. "This first tunnel construction bid package will employ multiple suppliers and hundreds of construction workers throughout the city and the region."

The contract includes procurement of two TBMs by the contractor; construction of the TBM launch box and retrieval shaft; the Fourth Street portal; Moscone Station and Union Square/Market Street Station end walls, tunnel excavation and installation of a one-pass precast segmental lining; and cross passages and sump pump wells . The TBMs will advance the twin bores concurrently from the Fourth Street portal where the line, running in the middle of Third Street, goes underground at Interstate Highway 80.

Tunneling will reach a maximum 100ft (30.5m) deep at the point where the alignment extends under the existing BART and MUNI Metro tunnels just before the Union Square/Market Street Station. EPBMs are anticipated for the drives, most of which lie below the groundwater table and through poorly consolidated silty and sandy deposits of the Colma Formation.

There is also a section of competent to highly fractured Franciscan Bedrock and a small section of soft clay (Fig 2). Muck removal will be from the Fourth Street Portal.

The estimated $168 million Union Square/Market Street Station (UMS) will be advertised later this year. Construction works will include a 225ft (68.5m) center platform with mezzanine and concourse plus a connection to the BART Powell Street Station and modifications to the Powell Street Station Apple Store entrance and to the Union Square parking garage.

Geological profile

The Chinatown Station, with a cost estimate of $144 million, will be advertised in the first quarter of 2011. This station, located in one of the most densely populated neighborhoods in the US, will be a mined operation, excavated using NATM/SEM. The contract includes platform and crossover caverns, a crosscut cavern and an offsite headhouse. Protection of adjacent buildings is also part of the contract scope and value.

The estimated $87 million Moscone Station is scheduled to be advertised in the second quarter of 2012. The contract includes the station cavern, a 200ft center platform,a mezzanine and concourse plus building protection and an offsite headhouse.

Rendering of the Chinatown Station

The final contract, worth about $95 million, for the trackwork, systems and surface station at Bryant, will be advertised in the third quarter of 2012.

Design of the project is divided into three contract packages. The first, valued at $5.7 million, was awarded to the Parsons Brinckerhoff and Telamon JV for utility relocation and tunnel design. The second $39 million design package, went to the Parsons Brinckerhoff, Michael Willis Architects, and Kwan Henmi Architects JV for station design. The Dr Sauer Group, as specialist sub-consultant, is designing the NATM/SEM works of the Chinatown Station and the special tunnel connections.

The final design package, worth $32 million for system integration, is held by the HNTB - B&C Transit JV.

Central Subway Partners-AECOM & EPC Consultants JV, holds the $147.4 million program and construction management contract. Key elements of the project include minimizing surface disruptions and managing advanced ground improvement and construction methods, such as deep concrete diaphragm slurry walls, secant pile walls, and compensation grouting as part of the building protection requirements.

When complete, the Central Subway will extend light rail service to the Financial District and Chinatown, as well as provide direct connections to BART and Caltrain, which are the Bay Area’s two largest regional commuter rail services. The Central Subway is scheduled to open to the public in 2018.

Table 1. Construction procurement contracts up for bid

Table 2. Design and management contracts awarded

References

SFMTA Central Subway

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