The report of Huang and Kao in the State Mining and Geological Board’ s Field Trip Guide to the Golden Gate Bridge (2006) details the retrofit completely for those who wish to see it all. It also details the historic and possibly future seismic activity and how these sensors will prevent damage from escaping notice and point to needs for repair. In this way, it is hoped that any damage done by wind or earth movement can be repaired before it leads to the collapse of the entire structure. What is truly amazing is that these retrofits were all completed without ever closing the bridge to traffic. The earliest known geological survey of the San Francisco area was done by William Phipps Blake in (1858).
He described the huge variation in the geological structures and deposits, Blake identified most of the geological structures as sandstone and shale, with eruptions of trappean and serpentine rock. He said that all of these were of very recent geological age. Point Reyes and the Farallon Islands seem to be mostly granite, as are the surrounding Santa Cruz Mountains.
The tight sandstone shows eruptions of lime and quartz in many places, though never together, from possible intrusions through steam vents along the sides of racks and fissures. The sandstone was quarried from several locations for construction in the city. Blake noted that “ the extensive metamorphism, and the uplifted condition of all the strata, indicate the proximity of igneous rock. ” He noted that these were not exposed. Blake also noted fine alluvial clay in the marshes south of the city and adobe underlying much of the area of San Francisco.
Many artesian wells pierce this alluvial layer and water is found at 50 to 150 feet. Blake marveled at the presence of formations and rock from various ages covering eons of time. All of this should have indicated the long history of tectonic movement, and the wide variety of different ages of formation creates a less than settled foundation for any construction, especially a bridge. It is probable that only a suspension bridge can survive these conditions over time.
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