Across the United States, fundamental infrastructure in is critical condition - especially when it comes to water and wastewater. Without adequate maintenance and proactive financial planning, much of this essential framework risks operating beyond its useful life.

Infrastructure designers today need to address multiple priorities: replacing aging infrastructure, adapting to more stringent regulations, increasing system reliability, and responding to climate-change-related risks to ensure systems are resilient to natural and man-made hazards.

Silicon Valley Clean Water (SVCW), a joint powers authority operating the connected sewer systems and treatment plant for Redwood City, San Carlos, Belmont and the West Bay Sanitary District in California, are working with Arup to meet these priorities head on. SVCW launched the Regional Environmental Sewer Conveyance Upgrade (RESCU) programme to modernise their system and build a resilient and durable water treatment infrastructure for their communities. Central to the programme is the $253M Gravity Pipeline, a key project being delivered using progressive design build - a first for the tunnel industry.

The SVCW RESCU programme consists of multiple projects to improve the reliability of SVCW’s wastewater conveyance and treatment system. In collaboration with Montana-based heavy civil construction experts Barnard Construction and the French tunnel boring specialists Bessac, Arup has co-located with SVCW to design the replacement of the aging wastewater force main with a 3.3-mile-long gravity pipeline. To realise this project we are providing multidisciplinary services including geotechnical, tunnelling, hydraulics, civil, electrical, mechanical, structural, and seismic engineering, along with materials analysis. 

A gravity pipeline with a 100-year lifespan  

The new gravity pipeline will serve multiple needs, replacing an existing force main with corrosion and leakage issues with a new tunnel and microbial corrosion resistant pipeline designed for a 100-year lifespan. 

The pipeline has been designed to deal with variations in flow related to usage fluctuations and climate change effects. It will also provide significant storage capacity during storm events, with stormwater stored then pumped to the treatment facility in a controlled and equalized manner to extend the lifespan of pumps and save energy costs by pumping during off-peak hours. The pipeline has been designed to function through major seismic events, an important consideration with the San Andreas Fault just 4 miles away. Influent is treated at SVCW’s existing facility to make sure any discharge doesn’t adversely affect the ecology of the San Francisco Bay. 

Pipeline perameters

The existing force main pipeline will be replaced with a 3.3-mile-long, 13.5-foot internal diameter precast concrete-lined tunnel housing an 11-foot and 10-foot diameter fiberglass reinforced polymer mortar pipeline. The 15.5-foot outside diameter tunnel has been constructed using an earth pressure balance tunnel boring machine (TBM). In July 2019, SVCW and Redwood City held a competition to name the TBM, with the winning entry “Salus” named after the Roman goddess of health and wellbeing.

The tunnel has been excavated in two tunnel drives from a 60-foot wide and 60-foot-deep diaphragm wall launch shaft. Two 40-foot, 20-foot-span sequential excavation method tunnels were dug to launch the TBM. The first tunnel drive was excavated approximately 1 mile to the south, with the majority of the tunnel excavated adjacent to an active regional airport. The tunnel then passed under a tidal estuary prior to entering a 25ft by 60ft TBM retrieval shaft. A permanent vortex drop shaft will serve as the connection between SVCW’s existing Menlo Park force main and the new gravity pipeline. A pipe jacked adit midway along the first tunnel drive will provide the permanent connection to the existing Belmont and San Carlos systems, which combine flow within a new hydraulic baffle drop structure.

The second tunnel drive terminates with a 36-foot wide retrieval shaft in front of the SVCW wastewater treatment plant. The shaft will be used as a surge and flow splitter for the new headworks facility. 

A progressive procurement approach

SVCW procured the Gravity Pipeline using the Progressive Design Build delivery method, which maximises innovation and design efficiency while minimising construction scheduling and whole life costs. This is a new procurement approach for the North American tunnel industry, which allows for greater collaboration between the owner, designer, and contractor to workshop potential designs and arrive at a preferred solution. 

Arup co-located for two-and-a-half-years with SVCW, the Barnard-Bessac Joint Venture, and the design build team to effectively manage key technical challenges, develop cost-effective solutions, and ensure SVCW’s RESCU programme success factors were embedded in the design. The process included:

  • Partnering workshops to enhance project transparency and team communication
  • Big Room workshops to facilitate critical decision-making, such as the approach to corrosion-resistant tunnel linings, energy dissipation structures, and TBM launch configuration
  • Pull planning with all parties early on to identify where the project schedule could be accelerated. The process accelerated design items critical for achieving project milestones, allowing for the early procurement of the TBM and segment moulds, and resulting in an approximately 6-month reduction in project schedule

Arup presented a convincing team of tunnelling project experts who brought with them a strong collaborative approach to project delivery. There were many decisions to make in the design phase of this project and Arup provided the Owner with necessary information throughout this stage to allow the project to develop within SVCW’s delineated schedule and available budget. I gladly and wholeheartedly recommend Arup’s tunnelling project team; they have served SVCW well throughout the design and construction portions of our Gravity Pipeline project.  

Teresa Herrera

Manager, Silicon Valley Clean Water

Material selection and designing for resilience

Designing sewer pipelines is challenging, especially when designing for durability. For SVCW’s new pipeline, our approach considered the exposure conditions presented by the ground and groundwater to the outside of the tunnel and the unique warm and corrosive conditions experienced by the inside of the pipeline. It is unusual to design sewer systems for a 100-year design life because pipeline materials tend to not last long when exposed to sewage. Several corrosion-resistant lining alternatives were considered, but ultimately a fiberglass reinforced plastic mortar (FRPM) pipe was selected to ensure durability requirements are achieved. When complete, this will be the largest installation of FRPM pipe in North America. 

Collaboratively focused design

When the project was awarded, the solutions for the drop structures, connecting pipework, and temporary diversions were not resolved. Arup's hydraulic and civil engineers led collaborative workshops with SVCW’s operations and maintenance team and the Barnard-Bessac Joint Venture to identify economical solutions that were compatible with the project’s design criteria. Arup also worked closely with SVCW’s and the joint venture’s preferred suppliers to develop shop drawings for the unique drop shafts that accurately convey the design intent.