Innovative Shoring Techniques for Deep Excavation in Downtown San Francisco-Paper

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The Salesforce Tower foundation required the deepest excavation undertaken in downtown San Francisco, in a dense urban area beside the Transbay Transit Center (TTC). Its shoring design had to control movement near adjacent structures, coordinate with the TTC’s ongoing construction, and address potential base heave from artesian groundwater.<br /><br />The work proceeded in three phases. Phase I used cement deep soil mixing (CDSM) soldier-pile walls with four levels of preloaded internal bracing to excavate to about 66 ft (20 m). Because concrete defects were later found in the tower’s deep load-bearing elements, Phase II required up to 30 ft (9 m) of additional excavation inside the installed shoring. Engineers added a 137 ft (42 m) diameter secant-pile ring and twelve smaller jet-grout rings to support the deeper excavation and allow repairs. A reinforced concrete mat connected these systems to the perimeter shoring. Three-dimensional analyses assessed the interacting shoring systems and excavation stability.<br /><br />Although initial checks indicated a risk of base heave, analyses accounting for the load-bearing elements’ anchoring effect found a factor of safety above 1.5. Monitoring recorded no measurable heave. Maximum lateral shoring deflection remained within the project limit of approximately 1.5 in. (3.8 cm). Phase III used a three-level tieback system to excavate the adjacent plaza.<br /><br />Despite tight working space, schedule pressures, complex construction sequencing, and the need to install new support within existing bracing, the excavation and repairs were completed without reported adverse effects on nearby buildings or roadways. The project demonstrates how coordinated use of braced CDSM walls, secant piles, jet grouting, and tiebacks can enable deep excavation while limiting ground movement.

Keywords

Salesforce Tower

deep excavation

foundation shoring

San Francisco

cement deep soil mixing

secant-pile ring

jet grouting

base heave

transbay transit center

tieback system