Most of the rocks around the Golden Gate are graywacke.
Graywacke is a smart name for a complex mixture of sandstone, feldspar, mica and mud that formed in submarine landslides as neighboring land rose to form mountains. These rocks form over 90 percent of the outcrops along California's coastal ranges. Carbonaceous siltstone is a generic name for fine-grained shale that may contain lenses of peat and lignite as well crude oil and gas. These rocks source oil in the San Joaquin Basin Province where they are called Fruitvale shale. Graywacke and carbonaceous siltstone sediments inter-finger in western California's Great Valley sedimentary sequence.
Engineering Geology
Geotechnical properties are mechanical, or engineering, properties of soils and rocks. In earthquake zones such as California, an investigation of these properties at building sites is crucial for the stability and safety of structures. Such a study includes an understanding of the local geology, seismicity and water courses.
Weathering
Weathering has a significant effect on the properties of graywacke because it contains fewer quartz grains than most other sandstones but more feldspars, a silicate mineral, and volcanic fragments. Weathering fractures the feldspar, weakens bonds between crystals and the rock can break down to a clay mixture. Carbonaceous siltstone weathers down to a mass of tar and carbon.
Groundwater
Groundwater always impacts on the mechanical properties of rocks and soils. Clay in graywacke expands when saturated by water and can damage foundations in buildings. Carbonaceous siltstones do not expand to such an extent when in contact with water. Local faulting in California means that groundwater depths and distributions vary within a few feet of sampling. In San Mateo County, groundwater is runoff from higher elevations, rather than water from an underground aquifer.
Liquefaction
Earthquake shaking can reduce the strength of a soil to the point where the soil behaves like a liquid. Clean, saturated sand, silt and gravel are the most susceptible to liquefaction. Generally, these soils do not occur in the greywacke areas or those areas where graywacke inter-fingers with carbonaceous siltstones.
Compressive Strength
The compressive strength and bearing capacity of graywacke depends on its erosion and weathering. The density and bearing capacity of graywacke increases with depth and distance from the weathered zones, and makes it an adequate foundation material. Carbonaceous siltstones are not as common in outcrop and are not used as foundation materials.
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