GEOLOGY AND LANDSLIDE MAPPING ALONG HIGHWAY CORRIDORS IN CALIFORNIA
Within the varied geologic environments and terrain of California, we find that several factors influence the type, numbers and activity of landslides. The physical properties of the geologic units are one of the most important among these factors. On the most basic level, soft, intensely sheared bedrock of the Franciscan complex melange tends to fail as earthflows while some weakly-cemented late Tertiary nonmarine sandstones tend to fail in debris flows. Geological details can also have important influences on the type and activity levels of slides. On the Big Sur coast, similar-appearing plutonic rock units have very different susceptibility to landslides, especially debris flows, which may be related to the relative biotite content.
Detailed maps of the geology and landslides along highway corridors in California allow Caltrans to consider the geologic hazards and potential landslides in planning for construction projects and contingency planning for landslides. Already corridor maps prepared by the California Geological Survey have been used in evaluation of repair or realignment options for unstable portions of Highway 101 and in the long-term planning for landslide mitigation and repair along Highway 1 along the Big Sur coast. With the completion of maps for additional corridors we hope that we are increasing the knowledge of the geologic environments of landslides along Californias highways, and thereby help Caltrans plan for and mitigate the hazards.