Geotechnical Engineering Award of Excellence: 181 Fremont, San Francisco

Kirk Ellison
Senior Engineer
Arup, San Francisco

Soft Soil and Close Neighbors: Geotechnical Engineering for a San Francisco Tower

181 Fremont has an externally-braced superstructure and is supported by a five-story concrete basement with 42 1.5- and 1.8-meter-diameter drilled shafts socketed into bedrock, over 75 meters below grade. The geotechnical design of the tower was challenging, due to the high seismicity and a complex geologic environment. In addition, it is directly adjacent to the 20-meter-deep Salesforce Transit Center “train-box”, which was simultaneously under construction, and with which it shared a shoring wall. The strict resiliency requirements for the tower and its neighbors added further complexity. The subsurface stratigraphy consisted of several units, including liquefiable fill, compressible Young Bay Mud, and the highly variable Franciscan Complex Bedrock, located over 60 meters below grade.

The deep shafts helped to mitigate several concerns, including excessive settlement of a deep clay layer, which has been known to cause problems for other buildings in the neighborhood; overloading of the foundations during seismic shaking; and impact on the seismic performance of the adjacent transit center. As an additional innovation, the external shafts were tied together via a pile-cap ring beam, which enabled redistribution of loads away from the corner piles to avoid plunging during a seismic event.

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