GEOLOGIC AND GEOTECHNICAL CHARACTERIZATION OF SEDIMENT FOR LIQUEFACTION-INDUCED DEFORMATION HAZARD MAPS IN THE NORTHERN SANTA CLARA VALLEY, CA

We have collected and reviewed 668 geotechnical boring logs in the northern Santa Clara Valley at the south end of San Francisco Bay in Central California to (a) characterize geotechnical and geologic properties within and between geologic map units and (b) develop a map estimating the depth to the top of the Pleistocene. Qualitative geologic information and quantitative geotechnical boring log information are linked together; each layer in each boring is assigned a geologic map unit designation thereby allowing us to use quantifiable geotechnical characteristics in the characterization of each geologic unit. This geologic and geotechnical characterization is then used in a feasibility study that combines new models for predicting liquefaction-induced strain to develop regional (1:24,000-scale) hazard maps based on predicted surface deformation resulting from liquefaction.

In the northern Santa Clara Valley, study of sediment layers described in geotechnical boring logs reveals that: 1) less than half of the sediment layers described are coarse enough to liquefy, whether analyzed by the total number of layers or the total boring length assigned to each map unit, 2) the median (N1)60,cs value for sediment with liquefiable textures in most cases is less than 15, a value previous researchers have considered an upper bound for sediment likely to experience large-scale liquefaction related deformation, 3) the method of Youd et al. (2001) results in median (N1)60,cs values that tend to be 2 to 3 blows/ft higher than the values calculated using the method of Seed et al. (2003), 4) the median Factor of Safety value for most geologic map units is much less than 1, indicating that the layers in this area with potentially liquefiable textures are prone to liquefaction when shaken at the 10%-exceedence-in-50-years levels, although a high percentage of the sediment in the study area consists of fine-grained materials that are likely too fine to liquefy, and 5) LPI values for younger deposits tend to be lower than the LPI values for older deposits. The results of the feasibility investigation reveal that late Holocene deposits are likely to experience the greatest liquefaction-induced strain, while older deposits are likely to experience significantly less horizontal and vertical strain in future earthquakes.