Finding the lowest-impact wastewater tunnel solution for Silicon Valley Clean Water

Strengthening a critical wastewater system in a complex coastal corridor

To improve the long-term reliability of its wastewater conveyance system, Silicon Valley Clean Water (SVCW) launched the Regional Environmental Sewer Conveyance Upgrade (RESCU) program, a major infrastructure investment designed to reduce the risk of untreated sewage spills, protect public health and the environment, and meet future regulatory requirements.

A key component of the program, the Gravity Pipeline Project, replaced aging force mains between Bair Island and SVCW's treatment plant with a new 3.3 mile gravity conveyance and storage tunnel. This created a more resilient system for transporting wastewater from SVCW's member agencies while minimizing impacts on surrounding communities and sensitive environmental areas.

For communities served by SVCW, the project was about more than replacing aging infrastructure. It was an investment in reliable wastewater service, environmental protection, and public health across the San Francisco Peninsula. Delivering those benefits, however, required navigating one of the region’s most environmentally and operationally complex corridors.

Finding the lowest-impact solution

The route needed to pass through an area shaped by tidal habitat, poor ground conditions, a national wildlife refuge, airport operations, residential neighborhoods, and strict permitting requirements. As part of the Kennedy Jenks team, Mott MacDonald supported SVCW as owner’s advisor, helping identify an underground alignment and construction approach that could balance technical needs with community, environmental, and schedule impacts.

The project team assessed more than 100 alternative concepts, considering alignment options, trenchless installation technologies, permitting needs, property requirements, stakeholder concerns, and construction impacts. This early investment in planning helped define a solution that balanced technical feasibility with the environmental and community constraints surrounding the project corridor.

A trenchless approach was identified as the preferred and most cost-effective option. By avoiding sensitive habitats, airport operations, and residential areas, the approach helped reduce permitting impacts, stakeholder disruption, and potential schedule risks. The alignment was also developed to minimize the need for private property easements, relying primarily on public right-of-way and public property.

Community focused construction approaches

The resulting concept used a 13-foot-inner-diameter conveyance tunnel with an 11-foot-diameter fiberglass reinforced polymer carrier pipe. The tunnel solution provided below-grade in-line storage, avoiding the need for surface-based storage facilities.

Construction planning also considered local impacts. The 16-foot-diameter tunnel boring machine (TBM), known as “Salus”, was used to mine two drives from a central launch location outside the residential area near US 101. This approach avoided additional truck traffic through neighborhoods while supporting efficient underground construction.

Delivering long-term reliability with reduced surface impact

The project was completed in May 2022 using progressive design-build (PDB) – the first TBM project in North America to be completed using this delivery method. We continued supporting SVCW through construction on the Kennedy Jenks owner’s advisor team, reviewing contractor methods and procedures, tunneling and shaft construction estimates, tunnel boring machine data, geotechnical instrumentation, and proposed contractor changes.

By combining alignment planning, trenchless technology evaluation, and constructability insight, the team helped shape a conveyance solution that reduced surface disruption, minimized private property impacts, and supported long-term reliability for a critical wastewater system.