Deep Jet Grouting Grids for Support of a 42-Story Tower Designed Using Performance-Based Seismic Design-Paper

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The 500 Folsom Street development in San Francisco is a 42-story residential tower with a six-level basement, designed using performance-based seismic design. Its deep mat foundation bears on highly variable soils and sloping bedrock. Potentially liquefiable sand near foundation level posed risks of bearing loss and uneven settlement during a major earthquake.<br /><br />The project team selected deep jet-grout columns to mitigate liquefaction and provide more uniform support, allowing the tower to use a conventional mat rather than deep foundations. Installed from street grade before excavation, the columns extended through the liquefiable layers to competent soil or bedrock. Eight-foot-diameter columns, spaced seven feet apart, formed overlapping grids that enclosed the soil in closed cells. Jet grouting was chosen over mechanical soil mixing because it treated only the target zone, reducing construction time, materials, and spoil while avoiding excavation through hardened soil cement.<br /><br />Structural and geotechnical engineers iteratively coordinated nonlinear seismic analyses, foundation loads, settlement estimates, and subgrade moduli to design the mat and ground improvement together. Analyses predicted limited static and earthquake-induced settlement.<br /><br />A test program and ongoing quality control verified installation depth, column geometry, overlap, and strength. Coring showed adequate treatment in sand; where columns narrowed in hard clay, supplemental analyses confirmed that reduced diameters were acceptable. Average unconfined compressive strength exceeded the 350-psi target. Inspection after excavation confirmed the columns were present and overlapped as designed.<br /><br />The project demonstrates how detailed analysis, testing, monitoring, and collaboration among the design and construction teams enabled deep ground improvement to be installed reliably beneath a high-rise tower in a dense, seismically active urban setting.

Keywords

500 Folsom Street

San Francisco

residential tower

performance-based seismic design

liquefaction mitigation

jet-grout columns

deep mat foundation

ground improvement

earthquake settlement

geotechnical engineering