Infrastructure in the United States is in critical condition. Without adequate maintenance and proactive financial planning, much of our water and wastewater infrastructure risks operating beyond its useful life. Infrastructure designers today must account for several competing priorities: replacing aging infrastructure, adapting to more stringent regulations, increasing system reliability, and responding to climate-change-related risks to make our systems 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 address these concerns. SVCW launched the Regional Environmental Sewer Conveyance Upgrade (RESCU) program to effectively modernize their system and build resilient and durable infrastructure for their communities. The $253M Gravity Pipeline is a key project in the program being delivered using progressive design build, a first for the tunnel industry.
The SVCW RESCU program consists of a series of projects that will 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 an existing, aging wastewater force main with a 3.3-mile-long gravity pipeline. Arup is providing multidisciplinary services including geotechnical, tunneling, hydraulics, civil, electrical, mechanical, structural, and seismic engineering and materials analysis.
3.3mi long pipeline
The new gravity pipeline will serve many needs, primarily replacing an existing force main that has corrosion and leakage issues with a new tunnel and pipeline that are designed for a 100-year lifespan and will resist microbial-induced corrosion.
The pipeline has been designed to account for variations in flow related to usage fluctuations and climate change and will provide significant storage capacity during storm events. This operational approach allows stormwater to be stored and then pumped to the treatment facility in a controlled and equalized manner, extending the lifespan of pumps and saving energy costs by pumping during off-peak hours. The pipeline has also been designed to maintain conveyance through major seismic events, an important consideration given the proximity of the San Andreas Fault, just 4 miles away. The influent is treated at SVCW’s existing facility before being discharged so that it doesn’t adversely affect the ecology of the receiving water body, the San Francisco Bay.
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 naming competition for the TBM. The winning entry provided by a community member was “Salus,” named for the Roman goddess of health and wellbeing.
The tunnel has been excavated in two tunnel drives from a 60-foot diameter, 60-foot-deep diaphragm wall launch shaft. Two 40-foot-long, 20-foot-span sequential excavation method tunnels were excavated to facilitate launching 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 at a 36-foot diameter retrieval shaft at the front of the existing 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 maximizes innovation and design efficiency while minimizing construction schedule 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 collaboratively 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 program success factors were embedded into 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
Material selection and designing for resilience
Designing sewer pipelines is challenging, especially when designing for durability. In the case of SVCW’s new pipeline, the approach to resiliency 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 the pipeline materials tend to not last so long when exposed to sewage. Several corrosion-resistant lining alternatives were considered, but ultimately a robust fiberglass reinforced plastic mortar (FRPM) pipe was selected to ensure the long-life durability requirements are achieved. When complete, this is 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 to accurately convey the design intent.
Tunneling was completed in June 2021 and the project is slated to be online in the middle of 2022.
Arup presented a convincing team of tunneling 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 tunneling 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