Innovative Shoring Techniques for Deep Excavation in Downtown San Francisco

The Salesforce Tower is currently the tallest building on the west coast of United States, and the tower foundation construction involves the deepest excavation ever in downtown San Francisco. The foundation excavation utilized five shoring methods in three different construction stages: Cement deep soil mixing (CDSM) soldier pile shoring walls. A strut-wale shoring system with four levels bracing in the Phase I excavation, which allows the excavation down to 20-m below the street level. Twelve (12) reinforced jet grout shafts as excavation shoring extending from the bottom of the Phase I excavation. A 41-m diameter secant pile ring as excavation shoring in Phase II excavation, which extends down from the bottom of the Phase I excavation and provides shoring for excavation to 29-m down from the street level. A three-level tieback system allows excavation in the Plaza area in Phase III of the project. The presentation focuses on the use of innovative shoring techniques in combination with the conventional shoring method to meet significant challenges in the excavation, including very tight shoring deflection limits in the dense urban area, risk of base heaving due to artesian pressures, risk of differential settlement in adjacent building due to the excavation, design of a new shoring system within a braced shoring that were already in place, interfacing with adjacent shoring system and construction at Transbay Transit Center, innovative uses of jet grouting and secant piles as shoring methods in complex soil conditions, and complex construction logistics with multiple construction trades interfacing with shoring and excavation.  The innovative shoring design allows the complex and demanding deep excavation project to be successfully completed without a hitch.