Regenerating wastewater infrastructure in the US

Much of the water and wastewater infrastructure in the US is in a critical condition. Without adequate maintenance and proactive financial planning, the infrastructure risks operating beyond its useful life.

The vital sewer upgrade in Redwood City, California, capable of withstanding earthquakes and extreme weather, addresses multiple priorities, including replacing the ageing infrastructure, adapting to more stringent regulations, increasing system reliability and responding to the risks posed by climate change.

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, worked with engineering and sustainable development provider Arup to meet these priorities head on.

SVCW launched the Regional Environmental Sewer Conveyance Upgrade (RESCU) programme to modernise the system and build resilient and durable water treatment infrastructure for its communities. Central to the programme was a $253 million gravity pipeline, a key project delivered using the Progressive Design Build (PDB) project delivery method, and a first for the tunnel industry.

The RESCU programme consisted 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 co-located with SVCW to design the replacement of the ageing wastewater force main with a 3.3-mile gravity pipeline. Multidisciplinary services were employed to realise the project, including geotechnical, tunnelling, hydraulics, civil, electrical, mechanical, structural and seismic engineering, along with materials analysis.

“Designing sewer pipelines is challenging, especially when designing for durability,” says a spokesperson for Arup. “Our approach considered the exposure conditions presented by the ground and groundwater outside the tunnel, and the unique warm and corrosive conditions inside the pipeline. It is unusual to design sewer systems for a 100-year lifespan, because materials tend not to last long when exposed to sewage. Several corrosion-resistant lining alternatives were considered, but ultimately a fibreglass reinforced plastic mortar (FRPM) pipe was selected to ensure durability.”

Gravity pipeline

The existing force main pipeline has been replaced with a 3.3-mile, 13.5ft internal diameter precast concrete-lined tunnel, housing an 11ft and 10ft diameter FRPM pipeline. The 15.5ft outside diameter tunnel was constructed using an earth pressure balance tunnel boring machine (TBM). In July 2019, SVCW and Redwood City held a competition to name the TBM, which was given the moniker ‘Salus’ after the Roman goddess of health and wellbeing.

The tunnel was excavated in two tunnel drives from a 60ft by 60ft diaphragm wall launch shaft. Two 40ft, 20ft-span sequential excavation method tunnels were dug to launch the TBM. The first tunnel drive was excavated approximately a 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, provides a permanent connection to the existing Belmont and San Carlos systems, which combine flow within a new hydraulic baffle drop structure.

Progressive procurement

SVCW procured the gravity pipeline using PDB. This is a new 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, Barnard-Bessac Joint Venture (BBJV) and the design build team to manage 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.

“Arup presented a convincing team of tunnelling project experts who brought with them a strong collaborative approach to project delivery,” says Teresa Herrera, manager at SVCW. “There were many decisions to make in the design phase of this project and Arup provided the necessary information throughout this stage to allow the project to develop within SVCW’s delineated schedule and available budget.”