What is the Gravity Pipeline Project?
Silicon Valley Clean Water’s long-anticipated gravity pipeline project continues to progress with site work and shaft construction activities in anticipation of delivery next July of a tunnel boring machine (TBM) from Germany. The $18.2 million TBM being built by Herrenknecht Tunneling Systems of Schwanau will be disassembled and shipped to Redwood City through one of four ports in Northern California and then reassembled on site.
Tunnel launch shaft sitework began in September northeast of the intersection of Shoreway Road and Redwood Shores Parkway/Holly Street intersection. Site work involves grading and general preparation for the shaft and tunneling construction activities that are planned to start in November 2019 and continue through the spring of 2022.
The gravity pipeline, which the TBM will install from the north edge of Inner Bair Island to the new facilities at the front of the treatment plant at 1400 Radio Road, is a major component of SVCW’s Regional Environmental Sewer Conveyance Upgrade (RESCU) program. RESCU improvements will enable the safe and reliable transport of wastewater from four-member agencies’ collection systems to the treatment plant.
The first tunnel section to be constructed will be located between the Shoreway Road/Redwood Shores Parkway TBM Launch Shaft and the Inner Bair Island Inlet Shaft. This tunnel section will be located under Redwood Shores Parkway (Holly Street), Skyway Road, the San Carlos Airport property (adjacent to, not under, the runways and taxiways), and Pulgas Creek, where it will connect to the recently completed 48-inch force main pipe on Inner Bair Island. This tunnel section will be approximately 5,200 feet in length (slightly less than a mile) and will be installed 20 to 35 feet below ground.
The second tunnel section to be constructed will be located between the TBM Launch Shaft and the TBM Retrieval Shaft located at the SVCW treatment plant, with most of the tunnel located beneath the Redwood Shores Parkway public right-of-way. This tunnel section will be approximately 12,400 feet in length (about 2.4 miles) and will be installed roughly 35 to 60 feet below ground level (to the top of the tunnel).
Tunneling is used as an alternative to open cut methods. Tunneling using TBMs has the advantages of limiting the disturbance to the surrounding ground and allowing long distances between shafts. This significantly reduces the negative impact of construction to the public and cost of construction and is suitable to use in heavily urbanized areas.
Wanting to collaborate with a contractor and designer from the project’s earliest stages, SVCW joined in a Progressive Design-Build agreement with Barnard Bessac Joint Venture, along with design engineer Arup, to design and construct the gravity pipeline. Barnard Bessac JV and Arup are working with SVCW to take the design of the gravity pipeline project from 10 percent to 100 percent.
To ensure contractor involvement from the design phase continuing through construction, SVCW divided the contract into two stages, where Stage 1 takes the design to 60 percent, and Stage 2 brings the design to 100 percent and includes all required gravity pipeline tunnel and shaft construction.