Key Decisions Made to Launch SVCW’s Long-Anticipated Gravity Pipeline Project

Silicon Valley Clean Water’s (SVCW) long-anticipated gravity pipeline project was authorized to begin one of its key site preparations in a series of actions by its four-member Commission.

The gravity pipeline project, which is to be constructed by tunneling from the north edge of Inner Bair Island to the new front of plant facilities at the treatment plant at 1400 Radio Road, is a major component of SVCW’s Regional Environmental Sewer Conveyance Upgrade (RESCU) Program. RESCU features conveyance system improvements to transport wastewater from four-member agencies’ collection systems to the wastewater treatment plant.

The Commission authorized:

  • The tunnel launch shaft sitework to begin in September northeast of the intersection of Shoreway Road and Redwood Shores Parkway/Holly Street intersection. Site work involves grading and general preparation for shaft and tunneling construction activities that are planned to start in November 2018.
  • The design, manufacture, and purchase of a tunnel boring machine (TBM) from Herrenknecht Tunneling Systems of Schwanau (Germany). The $18.2 million TBM will take 11 months to build at the factory in Schwanau after which it will be disassembled and shipped to Redwood City in July of 2019.
  • Purchasing a $3.5 million crane to be used at the TBM launch site. The crane is a highly specialized equipment and will be built specifically for this project that will begin in the fall of 2019 and continue until late 2021.

Once the shaft construction is complete, the shaft will be prepared for assembly and launch of the TBM next summer. Tunnel support activities will be conducted in the area adjacent to the launch shaft, including storage and handling of tunnel support materials, soil removed during the tunneling activity, and the pipe that will carry wastewater once the tunnel is placed into service in early 2022.

The use of the Shoreway Road/Redwood Shores Parkway site for tunneling support activities allows the construction traffic to move directly between US101 and the TBM launching area, keeping the majority of the construction traffic away from the Redwood Shores residential area. Once the new pipeline is in service, the launch shaft will be filled, and the site restored and returned to the County’s San Carlos Airport.

Tunneling is used as an alternative to drilling and open cut methods. Tunneling using TBMs have the advantages of limiting the disturbance to the surrounding ground and allowing long distances between shafts. This significantly reduces the cost of construction and is suitable to use in heavily urbanized areas.

The gravity pipeline will replace the remaining 48-inch and the entire 54-inch reinforced concrete pipe segments of the existing force main. A portion of the 54-inch will be repurposed by relining that section between the Belmont Pump Station and the San Carlos inlet to the new gravity pipeline near the existing San Carlos Pump Station.

The top of the tunnel will be situated roughly 20 to 60 feet below the ground surface between the northern end of Inner Bair Island and treatment plant. A new, 11-foot inside diameter fiberglass reinforced pipe (FRP) will be installed within a 13.5 -foot inside diameter concrete tunnel (16-foot outside diameter) in two separate sections of tunnel, constructed from the TBM Launch Shaft located just north of the Shoreway Road and Redwood Shores Parkway/Holly Street intersection. The wastewater in the pipeline will flow by gravity (unlike the current under-pressure force main).

While the tunnel and carrier pipe are continuous from the influent end on Inner Bair Island to the treatment plant, the tunnel will be constructed in two separate sections. The first tunnel section 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 and will be installed at a depth of 20 to 35 feet below ground surface.

The second tunnel section 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 right-of-way. This tunnel section will be approximately 12,400 feet in length and will be installed at a depth of roughly 35 to 60 feet below ground surface (to the top of the tunnel).

During the early 1970s, the existing 48-inch and 54-inch diameter force mains were constructed of reinforced concrete pipe (RCP) with bell-and-spigot joints spaced at 12-foot intervals. These sections of pipeline were initially built to function as gravity sewers and subsequently repurposed to be pressurized force mains. Now the pressure in the force mains is carefully managed to minimize leaks through complex operational procedures.

The existing force main is located in “young bay mud,” which is an unstable soil stratum, and since the joints are spaced every 12 feet, they move in the unstable bay mud and consequently leak, requiring repairs along streets, in backyards, and within biologically sensitive environments. In addition to periodic leaking joints, corrosive soils outside the pipe and corrosion from sewer gases inside the pipe have weakened the pipe. When surges in flows such as during a power outage occur, the resulting pressure and suction surge conditions can break the weakened pipeline resulting in major spills of sewage.

A priority for SVCW is to complete the new gravity pipeline in a timely and cost efficient manner to minimize the risk of spills from the existing pipeline.

See a short video of tunneling using a TBM:

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