The reasons for the San Francisco Municipal Transportation Agency’s (MuniMetro) Central Subway project can be viewed from several aspects, but all in common with the need to provide a better public transport to the densely developed Downtown Financial District and Chinatown areas of the city. San Francisco has had a long but discontinuous history of public transport, especially featuring its famous cable cars on its hilly streets, a few routes of which are preserved. The ever-present hills and increase in road traffic make a more efficient system vital.
The Central Subway is designated as Phase 2 of the T Third Light Rail Project. Phase 1, or the 6.9 mile (11km) long Initial Segment has been running since April 2007. It goes on the surface from the southern suburbs of the city to link into the Caltrain Terminal and then hugs the shoreline to the famous Emabarcadero waterfront and bay ferries where it goes underground before taking a 90 degree left turn to run along Market Street, one of the city’s main thoroughfares, and the original MuniMetro. This results in a big loop in the route, which will be short-cut by the Phase 2 Central Subway.
The 1.7-mile (2.7km) Central Subway route leaves Phase 1 at the Caltrain Terminal station at Fourth and Brannan Streets on the surface to plunge beneath the Interstate highway route (to the Bay Bridge) at a portal and continue along Fourth Street to Market Street/Union Square where it interchanges with the MuniMetro and the Bay Area Rapid Transit (BART) regional route. After this point it then continues along Stockton Street to the new Chinatown Station at Jackson Street. There are also outline plans for an underground extension of the route to link with the North Beach suburb, but not as part of this project (figure 1).
There is currently no rail route servicing central San Francisco in a generally north-south direction. Not only do private vehicles congest the area, but bus services are also overcrowded. One of the major expected benefits of the Central Subway is a reduction in travel time between the Caltrain Terminal and Chinatown from 20 minutes by bus to 8 to10 minutes by the metro.
In addition to this central function, the Central Subway will provide regional connections via Caltrain, BART and the T Third Phase 1 from the outlying communities of Mission Bay, Bayview and Hunters Point.
Apart from convenience, the Central Subway should also offer the environmental benefits about which California is famously keen including reduced air and noise pollution. Forecast ridership is expected to reach 76 000 by the year 2030.
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Phase 2 or Central Subway project funding will be drawn from federal state and local resources, currently amounting to USD1578.3M. Although the federal funding, 48 per cent of the total, has been planned, it has not actually be allocated formally yet. Nevertheless the program is proceeding on the assumption that funding will be made. Of the rest, 29 percent is due to come from city/local sources and 23 per cent from the state of California. Full funding agreements are expected at the end of 2011.
Construction tasks
Although comparatively short, the Central Subway route is mostly underground and faces many construction challenges including the famous San Francisco topography, the need to maintain support of many adjacent surface structures and the need for seismic engineering.
Separate contracts are being or have been let for site investigation, preparatory works, the tunnels, and four stations.
The twin-bore running tunnels, each 1.6 miles (2.6km) long, are to be driven by two earth-pressure balance (EPB) TBMs simultaneously from a portal site now being prepared on Fourth Street, located between a new surface station platform at Fourth and Brannan, and the Interstate I-80 flyover. The tunnelling contract bid invitation, with an estimated cost of USD200M, will be advertised in Q4 this year with a scheduled first notice to proceed in March 2011. The work is expected to last 40 months and will be the first major contract to be bid.
Luis Zurinaga of the project management joint venture, Zurinaga-Cordoba, working for the San Francisco MTA, explained some of the scheduling difficulties to T&T North America. The first notice to proceed with tunnelling, once funding guarantees have been obtained, is essentially clearance to procure the TBMs in advance by the successful contractor to allow for manufacturing lead times. “We don’t want to wait until full funding has been received,” he says, “or that will mean project delays. So the first notice will be to acquire the TBMs, the second will be for preparation of the TBM launch shaft, and the third to launch the TBMs. The successful bidder will also have the option to use only one TBM if it shows it can meet the schedule. In order to keep things going we’ve already let one contract to move utilities at the launch pit and Moscone Station, and there will be a second utility diversion contract let soon for Union Square Station.”
The TBMs will install a one-pass precast concrete segmental lining, pasing through the staton areas before they are constructed. Between Market Street and Chinatown the tunnels make a steep incline to follow the surface topography and pass adjacent to part of the Stockton street tunnel, which is nearly 100 years old. In this area of high ground the geology is mainly fractured rock.
As with the rest of the San Francisco Bay area, the geology is a real mixture necessitating extensive site investigation. The tunneling horizon is mainly in sands and superficial deposits together with weak rock of the Franciscan Formation. Ground control is particularly important, especially where the water table is relatively high. Here no dewatering is allowable, as this would create further instability in the overlying marine clays. Consequently extensive monitoring for possible subsidence along the route, as well as adjacent to stations, is needed, with over 250 buildings being fitted with instrumentation.
There is a wide variety of buildings including historic monuments and conservation areas as well as those of commercial importance. Zurinaga commented, “There needs to be significant pretreatment of the ground, especially near old structures and the BART tunnels which are within five feet (1.5m) of the new tunnels. The work will include compensation grouting. We are prepared to install pipe roofs but hopefully they will not be needed.”
Other geotechnical support measures expected to be considered include jet grouting and chemical grouting, as well as sprayed concrete and NATM procedures for the sequential excavation areas.
Other work included in the tunneling contract will include construction of the TBM launch box and retrieval shaft near North Beach, ‘end walls’ at Moscone and Union Square/Market Street stations, six cross passages, settlement monitoring and the protection of existing utilities, surface structures and the BART tunnels.
Zurinaga explains that the current plan is to abandon the TBM shield(s) at the end of the drives and retreat back with the TBM operational and trailing gear. “The alternative is to construct a retrieval pit, but it may be cheaper for the contractor to abandon the shield,” he says. “It will be up to them.”
The planned contract is of a standard bid and construct type, with the design being given to the contractor, but alternatives are being examined. “Shared risk is being looked at,” reports Zurinaga, “in order to reduce costs. We are absolutely open to contractor cost engineering ideas.”
Stations
The route includes three underground stations that are major construction works in their own right. From the south these are Moscone, Union Square/Market Street interchange and Chinatown, of which Union Square/Market Street is the largest. All have access routes using vertical lifts for use by the disabled as well as stair and escalator access. All also include substantial works for support of adjacent structures, with final designs still being drawn up. Zurinaga says that designs are in the nature of ‘this is the way you could do it,’ with the final decisions on methods left to the project governing board.
At an estimated cost of USD170M, the Union Square/Market Street Station contract will be the first station work to be put out to tender, approximately a year after the running tunnel in Q4 2011. The work, which is expected to last 56 months, includes the installation and commissioning of station services as well as civil work. Commenting on this Zurinaga says, “All parties agree that the Union Square work is the most critical in several ways. The area has ‘high end’ shops and other commercial premises so site housekeeping is very important. There will be restrictions on noise and vibration generation, plus limitations on working hours. Except for the tunnelling itself there will no work at the Christmas shopping time and other holidays, nor at nighttime. These environmental points were important to obtain agreements to go ahead.”
The new station box, 225ft (68.6m) long and at a depth of 100ft (30m), will be constructed by a combination of secant pile walls and cutter soil mixing for stabilization, a process innovative in the US. The box will reach groundwater level under the BART station necessitating water control precautions.
Excavation within the box will be by top-down and sequential excavation methods, mining sideways from the access cut. In addition to the new station box, there will be connections to and modifications of the adjacent BART Powell Street. In the design of the station provision is being made for another possible MuniMetro line connection.
Up the incline at Chinatown the contract, at an estimated cost of USD 150M, is similar but also includes provision for AC and DC traction substations as well as station services. At a depth of 60ft (20m), the basic station box will be 200ft (61.0m) long, but will be extended to 340ft (103.6m) to allow for a crossover. Secant piles or slurry diaphragm walls will form the box walls with sequential method excavation between. The contract for Chinatown station will be put out to tender in Q1 2012 with scheduled notice to proceed the following quarter for a construction period of 52 months.
Moscone Station is the smallest of the three underground stations but has many of the same difficult construction features, especially in view of the proximity of surface structures that must not be disturbed. The estimated work cost is USD90M and will be out to tender at the same time as Chinatown station, with a scheduled notice to proceed during Q3 2012 with a duration of 49 months. The station box, 200ft (61.0m) long and at a depth of 50ft (15.2m), will be constructed using diaphragm slurry walls and top-down excavation. It is on the site of an existing gas station, so there may need to be precautions against leaked hydrocarbons in the ground.
Schedule
In January the MTA received approval from the Federal Transport Administration to enter final design and notice to proceed was issued. Final design work, by Jacobs Associates and others, is under way and due for completion by the end of 2013. The authority also received USD 20M in further funding to support the project design work.
Preliminary construction activity has also started, with a groundbreaking ceremony taking place in February (see Tunnels & Tunnelling InternationalWorld News, 25 February 2010). Utility relocation work to enable construction of the Moscone Station and the portal on Fourth Street between Howard and Brant Streets started in January.
Major construction is due to start from 2013 following full funding agreement.
Fig. 1 – Map of central San Francisco showing the route of the Central Subway and links with other transport routes Site of the launch portal adjacent to the I-80 flyover Luis Zurinaga, project management consulting engineer to the SF MTA Artist’s impression of the largest station and interchange at Union Square/Market Street Chinatown Station will provide much needed extra public transport to this and the North Bay communities
