The San Andreas Fault has a great impact on California methods of geotechnical investigation.
Geotechnical investigations are the study of the physical and engineering properties of soils and rocks. They include surface exploration such as geological mapping and subsurface exploration such as soil sampling by drilling boreholes and digging trenches to locate faults. These studies are necessary to establish whether any land development or construction may be undertaken at a given site. California methods of geotechnical investigations are governed by the state's earthquake hazard. The results of such investigations can restrict the development of any site.
Fault Hazard
The San Andreas fault system dominates earthquake hazard in California. The state is crisscrossed by other major, regional or minor faults that are either inactive -- those that have not moved for 11,000 years -- active or potentially active. California geotechnical engineers must identify every possible fault location at the site under investigation.
Groundwater
The investigation of groundwater is crucial to geotechnical investigations in urban areas. An influx of groundwater can undermine the stability of slopes and foundations. A sudden decrease of groundwater due to drought or because a fault diverted water flow can cause subsidence and structural failure in buildings. Engineering geologists in the California Department of Water Resources have pioneered a number of geotechnical methods. These include the first computer model of a groundwater basin and an analysis of the penetration of coastal aquifers by seawater.
Liquefaction
Liquefaction is a process when earthquake ground shaking causes soil particles to lose their cohesion. This cohesion supports the soil skeleton. The soil behaves like a fluid when the cohesion disappears. Previously buried pipelines can float above the liquefied soil while building foundations fail and cause structures to sink into the soil. Geotechnical engineers must evaluate which soils are prone to liquefaction.
Lateral Movement
Lateral movements occur in steep slopes such as earth dams and embankments after earthquakes. They are a sliding circular wedge of earth that can involve the whole of the slope and cause failure of earth dams. Soil liquefaction around the arc of the wedge causes the sliding. Such dam failure causes reservoir water behind the dam to flow over the top and cause extensive flooding and damage downstream. This happened in 1971 in California during the San Fernando earthquake with the complete failure of the Lower San Fernando dam and partial failure of the Upper San Fernando dam. California geotechnical engineers must pinpoint this hazard on all sites close to steep slopes.
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