*********** Warning from the National Park Service ************
Enter only open areas when visiting the Golden Gate National Recreation Area, do not enter closed areas, because geologic conditions are ever changing. Obey and observe all signs and fencing erected by the park and do not put yourself at risk. The principal engineering problems that confronted the geologists when building the Golden Gate Bridge in San Francisco Bay were the strength of foundations, settlement of ground due to imposed loads, stability of side slopes, and support of piles. *******************************************************
Bedrock beneath most of San Francisco Bay consists of the Franciscan Complex of Mesozoic age. Prior to the deposition of the present sediments in the bay, the Franciscan rocks were severely deformed, broken by faults, and eroded to a surface of considerable relief. Knowledge of the relief, firmness, and distribution of weak and strong rocks was essential in constructing the Golden Gate Bridge.
Over the 600,000 year history of San Francisco Bay, it has fluctuated from being a marine influenced estuary during inter-glacial periods, to being a dry valley crossed by many local streams as well as the large Sacramento/San Joaquin River when glaciers covered the Sierra Mountains. The Bay Mud that covers the present bay floor overlies older bay sediments, and fills valleys cut by streams flowing from the surrounding hills and mountains during low sea level periods. Bay Mud is soft, affords a poor support for structures and piles, settles under imposed load, fails readily when covered by significant quantities of fill, and causes critical construction problems when covering deep, filled valleys.
Knowledge of the geological landscape under the San Francisco Bay was therefore critical for the design and construction of the Golden Gate Bridge. Fortunately, the bay floor is largely swept clean of Bay Mud at the Golden Gate by strong currents. The north tower was built on sturdy chert rock on the shore of the Marin Headlands, but the south tower foundation was built 100 feet underwater on Franciscan bedrock composed of serpentinite rock; a fractured type of rock known to be unstable under heavy loads. The suitability of this serpentinite rock for placement of the south tower was controversial among local geologists and in fact caused a permanent rift between the senior geologists at Berkeley and Stanford University -- Andrew Lawson and Bailey Willis.
In order to construct the south tower, workers had to erect the pier that supports the south tower more than 1100 feet offshore in the middle of the Golden Gate -- the first bridge support ever constructed in the open ocean. To enable the tower foundation to be prepared, divers had to clean a bedrock surface and then pour concrete underwater to create a giant fender that encompassed a football-field-sized area. Water was then pumped out of the fender so that the tower foundation could be built inside. In December 1934, Berkeley geologist Andrew Lawson descended into an inspection well over 100 feet beneath the sea surface and reported that "the rock of the entire area is compact, strong serpentine remarkably free from seams . . . when struck with a hammer, it rings like steel." The fender used to prepare the south tower foundation now serves to protect the pier from stray, fog-bound ships.
In order to receive credit for this Earth geocache, answer the questions below and email answers to me. Although not required it would be wonderful to post a photo of what you see and a description of the weather conditions.
1) Describe what you think the floor of the bay would look like if the water were to be drained
2) Name at least two other types of construction projects for which a geologist would be considered critical.