The existing south access road to San Francisco’s Golden Gate Bridge, known as Doyle Drive or Route 101, was structurally and seismically unsafe and required replacement. This project, located within the Presidio, is part of the primary north-south highway and provides key transit linkage through San Francisco.
The foundations for the new South Viaduct Bridge were extremely challenging due to the requirement for 200-foot deep, 12-foot diameter drilled shafts installed adjacent to historic landmarks. The viaduct bents are founded on 12-foot and 8.5-foot diameter drilled shafts, while abutments and retaining walls were installed on smaller diameter drilled foundations. The bridge design required a single drilled shaft below each column. The 12-foot diameter drilled shafts required permanent steel casing to be installed up to 160 feet deep under extremely strict vibration limitations.
Malcolm employed the world’s largest Oscillator to advance the casing without inducing ground vibrations, while a spherical grab excavated overburden soils within the casing. A BG40 rotary drilling rig was used to bore an 11.5-foot diameter rock socket which was drilled below the permanent casing tip. The seismic loads resulted in a very dense reinforcement configuration with individual pile cages weighing up to 150 tons. A custom designed tipping frame and unique suspension system was required to handle and splice these exceptionally long and heavy rebar cages.