Geotechnical Engineering Award of Excellence: 500 Folsom, San Francisco
Senior Associate & Vice President
Langan Engineering, Oakland
Performance-Based Earthquake Responsiveness
As San Francisco’s business hub expands southeast, the mixed-use development at 500 Folsom replaces residual infrastructure left from the Embarcadero Freeway that was damaged by the 1989 Loma Prieta earthquake. With market-rate and affordable housing units, the 42-story tower occupies the entire site, with six levels of underground parking.
For lateral restraint during excavation, shoring was provided by up to five rows of tiebacks and one row of internal cross-lot bracing. This served to cut off groundwater, and support adjacent shallow-foundation structures and a freeway off-ramp. To confirm the tiebacks would not impact the adjacent elevated freeway, a test inclinometer program was installed to check for soil strain at depth during tieback loading.
Due to the close proximity to the San Andreas fault, a site-specific, performance-based earthquake response spectra were developed for critical modes of the building, testing three levels of shaking and pulse-like motions. A ground improvement system mitigated liquefaction and transferred building loads to deeper strata through a closed-cell grid pattern of jet-grouted columns. The focused treatment of the potentially liquefiable soil at depth created significantly less waste than a more conventional approach of soil-cement columns mixed at the ground surface. Extensive analysis was performed to estimate static and dynamic settlements, and to develop subgrade moduli for design of the mat. The analysis was complicated by the variable stiffness and settlement behavior and variable subgrade mat pressures and conditions.