Report - PEER - University of California, Berkeley

ABSTRACTSeismic site response and the amplification of ground motions are significantlyaffected by the combined effect of the dynamic stiffness of the soil and the depth of thesoil. Current design practice, however, either uses an oversimplified approach to soilclassification (e.g., "soil" vs. "rock"), or ignores the effect of depth by accounting only forthe average shear wave velocity over the upper 100 feet of a site profile (e.g., 1997 UBC).The significant quantity of ground motion data recorded in the 1994 Northridge and 1989Loma Prieta, California, earthquakes provides an opportunity to assess and improveempirically based predictions of seismic site response.This report presents a geotechnically based site classification system that includesa measure of the dynamic stiffness of the site and a measure of the depth of the deposit asprimary parameters. The measurement of a site’s shear wave velocity profile is notessential for the proposed classification system. This site classification system is used toanalyze the ground motion data from the Northridge and Loma Prieta earthquakes.Period-dependent and intensity-dependent spectral amplification factors for siteconditions are presented.The proposed classification system results in a reduction in standard deviationwhen compared with a simpler "rock vs. soil" classification system. Moreover, resultsshow that sites previously grouped as "rock" can be subdivided as rock sites andweathered, soft rock/shallow stiff soil sites resulting in an improved site categorizationsystem for defining site-dependent ground motions. The standard deviations resultingfrom the proposed classification system are comparable with the standard deviationsobtained using a more elaborate (and costly) average shear wave velocity classificationsystem.iii