One of the most important and most commonly encountered problems in geotechnical earthquake engineering is the evaluation of a site response during an earthquake. The estimation of site response during an earthquake has been performed by using analytic and numerical site response analyses (SHAKE Analysis). 1 The natural frequency (resonant period) of a site is the most influential parameter in the estimation of site response during an earthquake. 2 The natural frequency of a site is usually estimated by numerical analysis or a simplified wave propagation equation. Engineering soil parameters used for the site response analysis (numerical analysis and simplified wave propagation equation) are acquired by in situ and laboratory tests; however, uncertainties in using the above parameters always exist. The uncertainties can be classified as follows: (1) disturbance of the soil specimen during the sampling procedure; (2) inherent experimental errors; and (3) obstacles in the in situ tests. Due to these uncertainties, if the natural frequency of the site can be estimated directly from in situ tests, it would be more reliable than numerical site response analysis results. However, over the past decades, surprisingly few attempts have been made to artificially represent an earthquake's motion by using such methods as a rotating mass or explosion. 3 '4 Thus, in this article, a spectral analysis method for direct estimation of the natural frequency of a site is executed at the experimental site by forced vibration tests. It uses the combination of a hydraulic hammer and dynamic compaction. Detailed site characterizations were also performed by using such in situ tests as a standard penetration test (SPT), a cone penetration test (CPT), and tests for shear wave velocity measurements. Soil particle motions, which were measured in a downhole receiver array as well as on the ground surface during the forced vibration tests, were used for estimating the natural frequencies of the experimental site, and they were compared with the results estimated by the analytic and the numerical analyses.
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