Paula Wallis, TunnelTalk

Three new water tunnels progressing concurrently in the San Francisco Bay Area are the backbone of a $4.3 billion system-wide upgrade by the San Francisco Public Utilities Commission (SFPUC). With more than 80 contracts in its Water System Improvement Program (WSIP), the owner is trying to get as many of the jobs to the street in short order to take advantage of lower construction costs.

The Commission's Chief Financial Officer recently reported that nine projects in the WSIP, estimated to cost $289.7 million will instead cost about $220.8 million, due mainly to falling materials costs. The Crystal Springs Tunnel, the shortest of the program's three, is already into construction. The Bay Tunnel, a five-mile undercrossing of the San Francisco Bay, is out to bid, and an invitation to tender the New Irvington Tunnel will hit the streets in the coming weeks (Fig 1).

Fig 1. WSIP program with Crystal Springs Bypass Tunnel and New Irvington Tunnel identified

Shank/Balfour Beatty JV secured construction for Crystal Springs Bypass Tunnel in October 2008 with a bid of $55.7 million. With the shafts completed, launch of the TBM, designed by Shank, is planned for the coming weeks. In early October, the newly assembled TBM will be lowered approximately 155ft into the south shaft to begin excavating the 4,200ft long tunnel.

Crystal Springs Tunnel TBM

Geology along the alignment is identified as highly-variable Franciscan Complex, which is a local bedrock formation ranging from very competent sandstone to weak shales. Arup/Brierley Associates JV designed the 4200ft long x 8ft diameter (1.28km x 2.4m) tunnel with its welded steel final liner. Jacobs Associates is construction manager on the project for the SFPUC.

Designed by Shank and fabricated by Hitachi Zosen in Japan, the TBM has a cutterhead dressed with 23 x 17in disc cutters with a maximum torque of 593,920ft-lbs and an operating thrust capacity of 1,150,000lbs. The trailing gear of five gantries was fabricated by Shank.

As tunneling operations commence on the Crystal Springs contract, the October 29, 2009 deadline for bids on the estimated $380 million Bay Tunnel contact is fast approaching. The single 5-mile x 9ft (8km x 3m) diameter tunnel will be the first bored tunnel under the San Francisco Bay.

With one shaft at either end of the drive, access will be challenging along the alignment that will reach depths ranging from 75ft to 110ft (22-33m). EPB excavation is specified for the drive in mostly sandy and silty clays. A section of Franciscan bedrock near the end of the drive will require outfitting the TBM with disk cutters for the final push to the eastern shaft. Jacobs Associates designed the Bay Tunnel and Jacobs Engineering was awarded the construction management contract.

Original Irvington Tunnel construction

Hatch Mott MacDonald recently secured the construction management contract for the program's estimated $338 million New Irvington Tunnel in Alameda County. The project is currently in the contractor pre-qualification stage with an invitation to tender planned for November 2009.

Designed by URS and Jacobs Associates, the tunnel will be excavated in four different directions simultaneously using a roadheader with sections of drill+blast. Construction is expected to begin in spring 2010 and be completed in 2013.

The tunnel will be located just south of the existing Irvington tunnel, built in the 1930s, and at a depth of 150ft to 700ft below ground. The 3.5 mile long alignment has a horseshoe shape with excavated dimensions of approximately 12ft by 14ft with an 8.5 to 10.5ft i.d.

The three tunneling projects are integral parts of the $4.3 billion Water System Improvement Program, to repair, replace, and upgrade a system of aging pipelines, tunnels, and dams.

References

Bay tunnel (video) - TunnelTalk, March 2008
Crystal Springs Tunnel awarded - TunnelTalk, October 2008
SFPUC