Case Study on Design and Installation of Displacement Auger Cast Piles into Franciscan Bedrock Formation

The project consists of design and construction of a six-story multi-family and senior housing development along the waterfront of the City of San Francisco. The project site was once located at the fringe of the San Francisco Bay prior to the construction of the seawall in the 1860s. Subsequently, man-made fill was placed behind the seawall to create the current waterfront. San Francisco is in one of the most seismically active areas in the world with various active faults within 30 miles in proximity of the San Francisco Bay Region. The North San Andreas and Hayward-Rodgers Creek faults are within 10 mi of the City of San Francisco both capable of generating earthquakes of moment magnitude (M) greater than 7.0. The general subsurface conditions of the site consist of a 20- to 40 feet-thick layer of highly variable artificial fill, which is susceptible to liquefaction based on the geotechnical investigations; the fill also contains considerable amounts of construction debris. Underneath the liquefiable fill, a highly compressible soft clay deposit locally known as young Bay mud (YBM) varies in thicknesses from 5 to 50 ft. Within portions of the site, alluvial deposit was encountered in soil borings underneath the YBM layer. YBM and/or alluvial deposits are underlain by Franciscan bedrock formation at depths ranging from 50 to 80 ft below the ground surface. The Franciscan formation is a complex metamorphosed and deformed bedrock within the subduction zone of western coast of North America primarily consisting of greywacke sandstone, shale, and metashale rocks. The top of the bedrock slopes steeply from one end of the jobsite to another within a horizontal distance of less than 200 ft. This paper will provide a case study of the project summarizing the design and construction of displacement auger cast piles extending into the complex Franciscan bedrock. The challenges include steeply sloped bedrock, soil with very low capacities, high downdrag loads, high static and seismic structural loading demand, sensitive adjacent historic buildings, and site logistics and access. The early involvement of a design-build foundation contractor to select the appropriate foundation type with close collaboration with the owners’ engineers and General Contractor was the key to the success and on-time delivery of the project.