A complex project to overhaul a San Francisco wastewater facility has reached a critical foundations phase.
For residents in San Francisco, the “big one” is never far from their thoughts. Aside from the major earthquake which almost destroyed the city in 1906, the last major seismic event to hit was in 1989. That one caused serious structural damage and brought down a section of the double deck Oakland Bay suspension bridge.
The prospect of the next big quake occupies a large part of James Mitchell’s mind. He is project manager for the MWH&Webcore joint venture comprising Colorado wastewater contractor MWH Constructors and San Francisco construction firm Webcor.
It is delivering ($440M) £319M of enabling and foundation works for the £1.2bn South East Plant (SEP), San Francisco’s next generation sewage works.
The SEP’s new Biosolids Digester Facility Project (BDFP) will bring the city’s sewage processing system up to modern standards while producing nutrient-rich organic by-products – known as Class A biosolids – resulting from wastewater treatment.
It will also capture and treat odours more effectively and is designed to accommodate an expected 900mm sea level rise by 2100.
When operational, the SEP will be able to handle up to 80% of San Francisco’s sewage and stormwater flows. According to Mitchell, the new facility will make an immediate impact on San Francisco’s air quality.
“The plant is within the city, and surrounded by residents and other stakeholders who will see benefits for years to come,” he says.
“So, the intention of the project is to improve the neighbourhood and increase the efficiency of the treatment process. [In addition] the current buildings are founded on ageing foundations and it needs to be more seismically resilient.”
The foundations will be heavily reinforced for earthquake resistance
The BDFP is being delivered for the San Francisco Public Utilities Commission (SFPUC) as part of a 20 year multi-billion dollar sewerage system improvement programme. It is being constructed next to an existing facility which has operated beyond its life expectancy.
The SEP’s main anaerobic digestion facility, known as building 610, will include five anaerobic digesters. Neighbouring building 615 will house biosolids dewatering equipment, while building 613 contains waste gas burners to deliver up to 5MW of power for the SEP. Buildings 606 and 607 contain solid odour control systems and solids pre-treatment equipment is in building 600.
Work at the SEP is taking place in several phases. Scope 1 works – demolition and enabling works – were substantially completed in November 2020. Scope 2, which includes the foundations for all 10 facilities plus the construction of the main digester facility, received notice to proceed in July 2020 and is scheduled for completion in April 2025. Further phases for the plant are being developed by SFPUC and the MWH&Webcor JV.
The current buildings are founded on ageing foundations and it needs to be more seismically resilient
“At the main facility housing the biodigester tanks, building 610, we are forming the 30m by 150m wide, 12.2m deep basement with a 914mm thick diaphragm wall,” says Mitchell.
“It extends to a depth of 50m, with panels containing longitudinal reinforcing bars that are called No 18 bars, which are 57mm diameter bars with 152mm spacing – there’s some serious amount of rebar going into this.
“The structure will be tied down with 526, 300mm diameter micropiles and a 1,828mm thick base slab. It all needs to be built to the seismic standards here,” he says. “If we do have a big [earthquake], this wastewater treatment plant will be one of the most resilient facilities of its era.”
Mitchell has experience constructing the shafts for Crossrail’s Whitechapel station. He explains that the foundation phase for building 610 is due for completion in March 2022 and involves installing 1,190 piles of various sizes to support the 10 new main facility structures.
These include 664 piles, which are a combination of continuous flight augur piles and drilled shafts between 910mm and 1,210mm in diameter with a depth range of 15.2m to 27.4m.
I have never seen a sewer so heavily encased in concrete and founded on piles
The piles have been bored through a mix of “young bay” mud – a mix of soft and firm clay – Colma Sand and backfill which includes debris from the 1906 earthquake.
Building 610’s basement includes an additional 526, 304mm diameter micropiles sunk to depths of up to 15.2m. These will ensure the SEP can withstand a magnitude 7.8 earthquake on the San Andreas fault, and a 7.1 earthquake on the Hayward fault and resist buoyant uplift.
Three structures – the solids pre-treatment structure, the biosolids dewatering plant, and a pipe tunnel – are built with 1m thick walls formed using the cutter soil mixing method installed to a depth of approximately 21.3m, supported by internal steel bracing. The pipe tunnel is between the main anaerobic digestion facility and the solids pre-treatment structure.
Three 7.6m deep basements are being built for the solids pre-treatment structure, the biosolids dewatering plant and the pipe tunnel.
Mitchell notes: “In the enabling works scope we had to divert a sewer. I have installed sewers before and I have never seen a sewer so heavily encased in concrete and founded on piles. Every structure, whether it is a main structure or whether it is a housing pipe structure, is all founded on pile caps ranging in depth between 914mm to 1,828mm and typically 914mm to 1,200mm piles.”
Foundation work was also complicated by the need to remove piles which had supported three now demolished warehouses.
Up to 300 piles were drilled out using a Kelly rig and oversized casings and then backfilled with controlled, low strength material.
The foundation work for building 610 has the added complication of including three rows of tiebacks to help transfer structural load into the ground.
There are 209 tiebacks per row as part of the 365m long diaphragm wall which has been built to a depth of 48.7m.
The maximum length of the tiebacks is 35m, and their maximum load capacity of 1,120kN. The tiebacks had to be threaded through piles installed for other already built structures, including digestion cooling towers (building 604), thermal hydrolysis (building 605), one of the odour control buildings (building 606) and the main digester facility.
Mitchell explains that building information modelling (BIM) was used for “a lot of the heavy lifting” at an early stage and throughout to ensure the build stayed on schedule.
He says: “As each pile is installed, we’ve had to upload it to the BIM model almost in real time and with the as-built locations we were able to install the tiebacks between installed piles of the adjacent structures without hitting any.”
After half a century of waiting, San Francisco can finally look towards a fully modern water treatment facility capable of withstanding the biggest earthquake.
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