Applications of diaphragm wall technology for deep foundations and permanent underground structures for the Transbay Transit Center and the Transbay developments-Paper

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The Transbay Transit Center project in San Francisco combines a five-story multimodal transport hub, including two underground levels, with bus ramps and nearby high-rise development. Its cable-stayed bus bridge required deep foundations for Pylon 9: two 21-by-5-foot reinforced-concrete barrettes extending more than 180 feet below grade and 10 feet into bedrock. Barrettes were selected to accommodate high loads and substantial depth within a limited footprint.<br /><br />Construction faced challenging ground conditions—fill, dense Colma formation, stiff old bay clay, and San Franciscan bedrock—as well as severe site and logistics constraints. Nicholson used hydraulic grabs to begin excavation and hydromills to continue through overburden and rock. To improve concrete placement, the reinforcement cage was redesigned to create greater spacing for tremie pipes and concrete flow. Extensive testing produced a stable, workable concrete mix suitable for uninterrupted pours exceeding 700 cubic yards and lasting about 12 hours.<br /><br />Limited space required each approximately 60-ton cage to be fabricated in sections, transported across Howard Street on a custom trailer, and spliced together as it was lowered into the excavation. Detailed lifting analysis helped protect crane stability and cage integrity. Both barrettes were completed successfully and passed cross-sonic testing.<br /><br />The project demonstrates that diaphragm-wall barrettes are a viable alternative to drilled shafts, particularly for very deep, heavily loaded foundations. While both systems are technically suitable, barrette geometry can also help address asymmetric loads. The authors recommend including barrettes among the standard foundation options considered by structural and geotechnical engineers.

Keywords

Transbay Transit Center

San Francisco

barrettes

deep foundations

cable-stayed bus bridge

diaphragm walls

hydromill excavation

reinforced concrete

bedrock

geotechnical engineering