Innovative Shoring Techniques for Deep Excavation in Downtown San Francisco-PPT Presentation

PPT Presentation

Pdf Summary

The Salesforce Tower project involved a 29-metre-deep excavation in downtown San Francisco, alongside a separate excavation for the Transbay Transit Center. The work required careful coordination between adjacent shoring systems, strict control of ground movement in a dense urban area, and measures to ensure excavation-base stability.<br /><br />The main shoring system used cement deep soil mixing (CDSM) soldier-pile walls with four levels of internal bracing. The walls extended into Old Bay Mud to form a “bathtub” that limited water and soil movement. After damage was found in the upper portions of deep load-bearing elements, repairs required further excavation below the concrete slab. Engineers designed a “shoring within shoring” system, including a 34-metre-diameter secant-pile ring and smaller grout-shoring rings.<br /><br />Design analyses assessed base-heave stability and lateral deflection. The project adopted a maximum shoring-deflection limit of 4 centimetres—tighter than conventional guidance—and used stitching piles to help stabilize the excavation base. Step-by-step design checks and continuous field monitoring supported construction beside the neighboring excavation. Automated total-station surveys tracked 100 monuments every two hours, enabling close observation of ground and shoring movements.

Keywords

Salesforce Tower

San Francisco excavation

Transbay Transit Center

cement deep soil mixing

soldier-pile walls

internal bracing

base-heave stability

secant-pile ring

stitching piles

ground movement monitoring