Seismic site response and the amplification of ground motions are significantly affected by the combined effect of the dynamic stiffness of the soil and the depth of the soil. Current design practice, however, either uses an oversimplified approach to soil classification (e.g., "soil" vs. "rock"), or ignores the effect of depth by accounting only for the 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 1989 Loma Prieta, California, earthquakes provides an opportunity to assess and improve empirically based predictions of seismic site response.
This report presents a geotechnically based site classification system that includes a measure of the dynamic stiffness of the site and a measure of the depth of the deposit as primary parameters. The measurement of a site’s shear wave velocity profile is not essential for the proposed classification system. This site classification system is used to analyze the ground motion data from the Northridge and Loma Prieta earthquakes. Period-dependent and intensity-dependent spectral amplification factors for site conditions are presented.
The proposed classification system results in a reduction in standard deviation when compared with a simpler "rock vs. soil" classification system. Moreover, results show that sites previously grouped as "rock" can be subdivided as rock sites and weathered, soft rock/shallow stiff soil sites resulting in an improved site categorization system for defining site-dependent ground motions. The standard deviations resulting from the proposed classification system are comparable with the standard deviations obtained using a more elaborate (and costly) average shear wave velocity classification system.
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