Customer Stories

Tunnel Beneath the Bay

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/ Project Surveyor Sean Fitzpatrick, PLS, operates a Trimble S6 in the Bay Tunnel. Remote control and automated pointing enables him to quickly capture precise measurements to tunnel control points. the City of San Francisco, and one of the largest water infrastructure programs in the nation. With more than 10 years of work already completed, WSIP work continues on replacement, repair and seismic mitigation to pipelines, dams, tunnels and other facilities. One of the biggest needs is to replace aging pipelines near the south end of San Francisco Bay. When water from the Tuolomne reaches the Bay Area, it goes into four pipelines that transport it around and over the southern reaches of San Francisco Bay to the nal destination at the Crystal Springs Reservoir. Two of the pipelines cross the bay, resting on the bay oor or supported on antiquated trestles crossing environmentally sensitive areas. To increase pipeline capacity and reliability while protecting the area's wetlands, SFPUC decided to replace the pipes with an underground pipeline. The pipeline project is overseen by SFPUC's Project Manager Johanna Wong, PE, MS. The new pipe will be installed in a dedicated tunnel constructed roughly 100 feet below the bay oor. Known simply as the "Bay Tunnel," the new conduit is 15 feet in diameter and more than ve miles long. The digging is done by an earth pressure balance tunnel-boring machine (TBM), a type of tunneling system well suited to the dense clays that make up much of the bay oor. To launch the TBM, the project's tunnel contractor, Michels/Jay Dee/Coluccio Joint Venture (MJC) excavated a shaft 58 feet in diameter and 124 feet deep in East Menlo Park on the west side of San Francisco Bay. According to MJC Project Engineer Ed Whitman, the TBM and launch shaft are normal parts of a major tunneling project. But the Bay Tunnel contains an important distinction from Whitman's past projects. In other TBM-built tunnels, several vertical access shafts (Whitman calls them "manholes") are built at intervals along the tunnel. In a similar sized-tunnel that Whitman worked on in Ohio and California, manholes were spaced roughly 1,000 to 1,500 feet apart. Among other functions, the shafts enable project surveyors to connect geodetic control points on the surface to the control points in the tunnel, making adjustments and corrections as the TBM moves ahead. Because the Bay Tunnel is under a body of water, manholes aren't possible. As a result, all the survey control (essential to steering the TBM) is tied to one end of the tunnel. "It's like taking a ve- mile shot off a 50-foot backsight," says Whitman. "From the survey perspective, it's not for the faint of heart." MJC selected Towill, Inc., of Concord, Calif., to provide surveying services for the Bay Tunnel project. Whitman said that Towill--which specializes in surveying and mapping--had the experience and condence needed to control a 27,000-foot run without intermediate manholes. In turn, Towill assigned Sean Fitzpatrick, PLS, to serve as project surveyor. Fitzpatrick has worked on several major tunnel projects and has gained extensive experience with guidance systems for TBMs. Assisted by Party Chiefs Eric Jones

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