ENGINEERING GEOLOGY OF SAN FRANCISCO BAY, CALIFORNIA

摘要

The principal engineering problems that confront the geologist in San Francisco Bay are the strength of foundations, settlement of ground due to imposed load, stability of side slopes, support of piles, and source of borrow sand for fills. Work on the design of foundations for proposed new crossings of San Francisco Bay has shown that each geologic formation has more or less consistent characteristics with respect to these engineering problems. The geologist thus can facilitate the work of the engineer by advising him as to the engineering properties of the rocks and sediments in areas between bore holes or excavations, even though the bore holes may be several hundred feet apart. Bedrock beneath San Francisco Bay consists of the Franciscan formation of Jurassic (?) age. Prior to the deposition of the present sediments in the bay, this formation was severely deformed, broken by faults, and eroded to a surface of considerable relief. The formation thus poses problems in landing caissons for bridges across the bay. In such places knowledge of the relief, firmness, and distribution of weak and strong rocks is essential. The bay sediments consist of five formations of late Quaternary age, which correspond to deposits previously described by Lawson in his folio on the San Francisco Bay region. The oldest sediments belong to the Alameda formation, which ranges from 0 to more than 200 feet in thickness. The formation consists of a series of layers of firm sand, sandy clay, and clay, which everywhere forms a good foundation for engineering structures. The caissons in the east part of the bay on the present San Francisco-Oakland Bay Bridge rest on this formation. In more than 15 years the piers have not settled appreciably. The overlying San Antonio formation, 15 to 120 feet thick, consists of clay and sandy clay, with a little sand. The sediments in most places are moderately firm and give good pile support. The formation settles appreciably under loads of more than 1.5 tons per square foot. The succeeding formation, the Posey, 0 to 50 feet thick, consists of sand and sandy clay. It is firm and, where the underlying San Antonio formation is stiff, forms a good foundation. It also is a source of borrow sand. Subsequent to the deposition of the Posey formation, valleys 150 feet or more in depth and up to 1000 feet in width were formed. These valleys and adjacent flat land were then partially covered with the Merritt sand, 0 to 60 feet thick. This sand in part is wind-blown and in many places is a source of borrow sand. The youngest formation is the Bay mud, which fills the old valleys and blankets the adjacent land. It is soft, affords a poor support for structures and piles, settles under imposed load, and fails readily when covered by significant quantities of fill. The formation causes critical problems in the deep, filled valleys. These valleys lie near the mouths of the present streams, which flow from the mountains. Presumably, the valleys mark the course of drainage when sea level was lower than it is now, perhaps during some glacial epoch.