The existing south access road to San Francisco’s Golden Gate Bridge, known as Doyle Drive or Route 101, is structurally and seismically unsafe, and requires replacement. Built in 1936, Doyle Drive has reached the end of its serviceable life. The project is located within the Presidio of San Francisco, part of the Golden Gate Recreation Area, and is part of the primary north-south highway and transit linkage through San Francisco. The foundations for the new South Viaduct Bridge presented quite a challenge to Malcolm Drilling Co. due to the requirement for deep, large-diameter drilled shafts installed adjacent to historic landmarks. Drilled shafts with 12-foot diameters were constructed to depth of approximately 200 feet.
The viaduct bents are founded on 12-foot and 8.5-foot-diameter drilled shafts, while abutments and retaining walls are 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-foot depth 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 systems were required to handle and splice these exceptionally long and heavy rebar cages.
Conditions at the site comprised of alluvial soils, extending to depths up to 160 feet, overlying bedrock of the Franciscan Complex. All shafts have been designed to develop their load-bearing capacity both in end-bearing and side-friction in the bedrock only. The Franciscan Complex is composed of sandstone, shale, limestone, chert, serpentine and greywacke – mixed in a seemingly chaotic manner. Engineering properties of the Franciscan vary significantly within limited vertical and horizontal intervals. Ground water was encountered between 20 feet and 60 feet below working grade.
The shaft integrity was tested using Gamma-Gamma and CSL test methods, while visual inspection by the Mini SID camera ensured a clean base that was required for end-bearing capacity. Malcolm Drilling employed synthetic slurry to stabilize the open rock socket to ensure compliance with the specifications through carefully controlled slurry exchange procedures. The concrete mix design was developed with a special emphasis on workability, which was needed for an extended period for the tremie concrete placement operation. The mix had the characteristics of self-consolidating concrete (SCC), with 50 percent of the Portland cement replaced by slag and fly ash.
From very humble beginnings, Malcolm Drilling has experienced growth over the past 4.5 decades to its present position as a preeminent specialty foundation contractor in the world.