Assessment of Soil Amplification of Earthquake Ground Motion Using the "CARES" Code Version 1.2 (NUREG-1750)
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Manuscript Completed: September 2001
Date Published: September 2001
Division of Engineering
Office of Nuclear Reactor Regulation
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
This report describes the results of two studies to assess the soil amplification of earthquake ground motion by varying three parameters pertinent to the soil and rock supporting nuclear power plant structures. Both studies used version 1.2 of the computer code CARES, Version 1.0 of which was developed by Brookhaven National Laboratory for the U.S. Nuclear Regulatory Commission. The model is a single layer of soil overlying a rock halfspace. In the first study, the input earthquake ground motion (corresponding to the Regulatory Guidel.60 spectrum) was specified at the rock level. The parameters varied in the first study were: the shear wave velocity of the soil stratum, the thickness of the soil stratum, and the shear wave velocity of the rock half-space underlying the soil stratum. The study analyzed the effects of varying each parameter on the motion at the ground surface and at 40 feet (ft) below the ground surface. Three values of soil layer thickness were considered: 50 ft, 100 ft, and 200 ft. Three values of the shear wave velocity of the soil stratum were considered, 1000 ft per second (fps), 2000 fps, and 3000 fps. The three shear wave velocities of the rock were 5000 fps, 7500 fps, and 10000 fps. The results of the first parametric study show significant shifts in the frequency at which the maximum spectral acceleration occurs when the soil thickness is varied from 50 ft to 200 ft and when the shear wave velocity of the soil stratum is varied from 1000 fps to 3000 fps.
No frequency shifts are seen when the shear wave velocity of the rock medium is varied from 5000 fps to 10000 fps, but significant changes in the maximum spectral amplitude of acceleration are observed for the 100 ft and 200 ft thick soil layer. The second parametric study used the seismic ground motion specified by a rock sitespecific spectra input at the rock outcrop, varying only the thickness of the soil layer and the shear wave velocity of the soil layer. A comparison of the results of the two parametric studies shows that the shapes of the spectral responses for both types of ground motion input are quite similar to the corresponding cases of soil thickness and soil shear wave velocity. However, the spectral amplitudes obtained for the sitespecific rock spectra input are lower than the corresponding values for the Regulatory Guide (R.G)1.60 input because the amplitudes of the input spectral accelerations of the site-specific rock spectra are lower than those of the R.G. 1.60 spectra at all frequencies larger than approximately 0.18 Hz, even though the peak ground acceleration (PGA) of the sitespecific rock spectra was set at 0.3g by scaling the actual PGA of 0.24g by a factor of 1.25 to match the 0.3g PGA of the R.G. 1.60 spectra.