EFFECTS OF SOIL CROSS-SECTIONAL MODELING ON THE SITE DESIGN SPECTRA AND FUNDAMENTAL PERIOD

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2004-11-23

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The most important tools in earthquake engineering are the site design spectra and fundamental period. Design spectra allow the determination of the expected ground acceleration as a function of the structure's period, while a site fundamental period indicates the resonance period of the site. The National Earthquake Hazard Reduction Program (NEHRP) Provisions allow the determination of a site design spectrum by a general procedure, which applies to most of the locations within the U.S.A. and its territories, or by a site specific method for some locations that have a certain class of weak soils. This dissertation addresses the general procedure that uses site classification to determine the site design spectra. A site classification can be determined from a soil profile modeling by one of three methods; the site weighted average standard penetration test uncorrected blow counts, the site weighted average shear wave velocity, and the site weighted undrained shear strength. The shear wave velocity profile for a certain location can be determined by invasive tests such as cross-hole tests, or by the more economical non-invasive tests such as Controlled Source Spectral Analysis of Surface Waves method, which is also referred to as the CXW test. In addition, NEHRP Provisions require modeling of only the top 30 m of the soil cross-section.

     The dissertation has four main objectives: to examine the effects of the various soil profile modeling methods on the site design response spectra; to examine the effects of the various soil profile modeling methods on the site fundamental period; to examine the effects of increasing the bedrock depth beyond the NEHRP depth of 30 m on the site fundamental period; and to examine the effects of the presence of embedded soft soil layers within the soil profile on the site fundamental period.

     To achieve these objectives, a seismically active site was chosen for field testing, as well as a five-station seismic array was installed and utilized.  Different tests were performed and seismic activities were recorded at different locations within the site, then dynamic analyses were performed that were then used to accomplish the dissertation's objectives.

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