GEOLOGY AND LANDSLIDE MAPPING ALONG HIGHWAY CORRIDORS IN CALIFORNIA

Geologists with the California Geological Survey have prepared geologic and landslide maps along highway corridors throughout California as part of a cooperative project with the California Department of Transportation (Caltrans). For each highway corridor, we prepare a geologic map and a landslide inventory map, then map those landslides most likely to affect the highway and describe of the types of movement that could be expected. We compile the geologic maps and add to them, focusing on the Quaternary geology and landslides. The landslide map classifies each landslide by type, recency of activity, and confidence of interpretation. Our mapping shows that the number, activity, and type of landslides are related to the underlying geology, climate and slope steepness.

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 California’s highways, and thereby help Caltrans plan for and mitigate the hazards.