The San Francisco Public Utilities Commission’s (SFPUC) combined sewer system is designed to collect and treat wastewater (sewage and stormwater). The SFPUC’s commitment to protecting the community and environment includes actively upgrading and modernizing this system through the Sewer System Improvement Program, which includes projects aimed at reducing flood risks.
The Lower Alemany Area Stormwater Improvements Project’s focus is to improve stormwater management and reduce flood risks near the Lower Alemany Farmer’s Market, the Interstate 280/US 101 Highway interchange, and the surrounding neighborhood. The project involves a 10-foot-diameter underground sewer tunnel known as the Alemany Auxiliary Sewer (AAS). The AAS will extend from the new upstream diversion structure in Alemany Boulevard between Congdon and Stoneybrook Avenue to Industrial Street via Alemany Boulevard, Gaven Street, and Boutwell Street. Dry weather flows will continue in the existing sewer. When the area is hit with a rainstorm, the water levels in the existing sewer will rise, topping a weir, allowing the combined sewer flows to split, thereby relieving the existing sewer downstream. The existing sewer will have additional capacity, which in turn will alleviate local flooding of the area of the 5-year, 3-hour storm. Construction will involve three shafts to construct the tunnel and other sewer connections and related improvements in the area.
The main construction site will be adjacent to Bayshore Boulevard within Caltrans’ right-of-way. The site will host a 40-foot-diameter construction shaft built half within alluvium and half within rock (Franciscan Formation). The shaft will be the starting point for two tunnel drives—the North and West Drives.
The 836-foot-long North Drive will be mined entirely within rock with one significant 1,000-foot-radius horizontal curve. The North Drive will curve onto Boutwell Street and cross under the northbound and southbound lanes of Interstate 280 before exiting at the Boutwell Shaft. The Boutwell Shaft is the transition point between the 10-foot ID circular tunnel and the 990-foot-long, 13-foot-wide by 7-foot-high cut and cover box sewer. The 5,470-foot-long West Drive extends west from the Bayshore Shaft under the northbound and southbound lanes of US 101 onto Gaven Street, then onto Alemany Boulevard before exiting into the Alemany drop shaft. The West Drive will be mined through alluvium under US 101 for about 1,100 feet before transitioning into rock for the rest of the drive. The West Drive includes several 1,600-foot-radius curves to remain within the limits of the City right-of-way.
The tunnel portion of the contract has been set up to allow the 10-foot ID tunnel to be constructed using a tunnel boring machine (TBM) with one or a combination of lining systems. The contractor will be allowed to use a precast bolted concrete segmental lining erected in the tail shield of the TBM or to use a concrete jacking pipe installed and jacked from the Bayshore Shaft. The 10-foot ID is a size that could be considered too small for segmental linings and too large for jacking pipe. With the long West Drive, it is possible the tunnel contractor could start out jacking pipe before transitioning to a segmental lining to finish the drive.
The environmental documentation for the project will be finalized in the first quarter of 2025. A design submittal will be made to Caltrans to obtain their acceptance of the tunnel crossings under US 101 and Interstate 280 before finalizing the design. The current schedule is to go to bid in Q4 of 2025 after obtaining Caltrans encroachment permits, with construction starting in 2026.
Delve Underground is the prime consultant for the SFPUC, providing tunnel engineering and design services to construct the new sewer tunnel, which will connect the existing sewer to the existing junction structure at Industrial Street and Barneveld Avenue.