Abstract
Modern building codes including the 2003 International Building Code (IBC2003) and Eurocode 8 include local site effects in design values of ground motion via spectral amplification factors that vary according to the site class. The origins of this approach expressed the amplification factors as a function of the average shear wave velocity in the upper 30 m. This is appropriate because the most important soil parameter affecting earthquake site response is the shear wave velocity. However, IBC2003 and Eurocode 8 also allow the site class to be determined using the standard penetration test (SPT) blow count to facilitate implementation using conventional, non-geophysical tests. The Atlanta, Georgia area is in the Piedmont physiographic province, which consists of highly structured, residual silty sands and sandy silts. Frequently, the depth to partially weathered rock is less than 30 m. At several residual soil sites in and near Atlanta, both in situ seismic and standard penetration tests have been performed to determine site class. The results indicate the SPT tests often produce a site class that is more conservative because the large strains associated with the SPT test obliterate the structure of the residual soil. Because the shear wave velocity is assessed at small-strain levels, the values reflect the structure of the soil and produce an estimate of site class that is more accurate. The non-invasive nature of surface wave tests also allows site class to be determined more easily and accurately at sites with shallow rock. These findings demonstrate the value of using seismic geophysical tests for determining site class in soil conditions like those found in the Piedmont region.
Published Version
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