Mission Bay is a reclaimed area of San Francisco that was previously a shallow bay. The site was reclaimed in the late 1880s and early 1900s using a variety of soil materials and debris, including debris from the 1906 San Francisco earthquake. The fill was not properly placed and is typically potentially liquefiable. Underneath this fill is weak, compressible marine clay known as Bay Mud.
The Exchange project is different because we used an alternative foundation type that had never been used in Mission Bay. Strong foundations are particularly important in this neighborhood because of its poor soil conditions.
With these poor soil conditions, most buildings in Mission Bay are being supported on deep foundations that gain capacity in either a dense sand layer, which is present in part of the area, or in bedrock.
Driven piles have typically been the most economical foundation type to support the buildings in Mission Bay. However, driven piles could not be used for The Exchange project because of concerns of noise and vibrations during installation that would impact the nearby UCSF Medical Center.
Langan worked closely with the owner, structural engineer, and general contractor to evaluate alternative pile types. The team asked us to participate in foundation subcontractor interviews so that we could provide input on the different proposals and ask pertinent questions.
Malcolm Drilling proposed a foundation type that had never been used in Mission Bay: rock socketed drilled shafts. With shallow groundwater, poor fill, and weak clay, drilled shafts were not typically feasible in this neighborhood; however, The Exchange site is near the shoreline of the original bay, and bedrock becomes shallower and Bay Mud becomes thinner toward the bay margin.
To evaluate the constructability of the drilled shafts and determine their capacity, Langan drilled the test location, including 30 feet of rock coring. With information we gathered from the test location, the team asked Malcolm to design a test pile, including Osterberg load cells for load testing.
The drilled shaft was successfully installed and the load test provided data for design of the foundation — which resulted in cost savings to the owner. The key to moving forward with this foundation type was good communication, strong relationships, and productive meetings among the design and construction teams.
Answer provided by Lori Simpson, PE, GE, Principal
Lori Simpson has over 26 years of experience in providing geotechnical investigation, design, consultation, and construction observation services for a variety of geotechnical engineering projects, especially sites with reclaimed land. Her projects include commercial and residential buildings, high-rises, infrastructure improvement for new developments, sports facilities, and investigations for seismic upgrades.