Strong ground motions from the Mw 6.6 2007 Niigata-ken Chuetsu-oki earthquake were recorded by a free-field downhole array installed near the Service Hall at the Kashiwazaki-Kariwa nuclear power plant. Site conditions at the array consist of about 70 m of medium-dense sands overlying clayey bedrock, with ground water located at 45 m. Ground shaking at the bedrock level had geometric mean peak accelerations of 0.55 g, which reduced to 0.4 g at the ground surface, indicating nonlinear site response. Ground settlements of approximately 15±5 cm occurred at the site. A site investigation was performed to develop relevant soil properties for ground response and seismic compression analysis, including shear wave velocities, shear strength, relative density, and modulus reduction and damping curves.
Ground response analysis of relatively weak motion aftershock data provides good matches of the observed resonant site frequencies and amplification
levels, suggesting that the velocity profile and assumption of one-dimensional response may be reasonable over the frequency range of interest. Initial analysis of the aftershock data was performed using equivalent-linear and nonlinear ground response procedures in which soil backbone curves were matched to test data for modest strain levels < ∼0.5%. These analyses over-predicted high-frequency ground response, suggesting that the laboratory-based small strain damping level is too low for field conditions. Increasing the small strain damping to about 2-5% in the sand layers provided improved results. Subsequent ground res ponse analysis of strong-motion data produces a strain localization phenomenon where large strains developed at a depth having a velocity contrast. Accordingly, we introduced a procedure to modify the large-strain portion of backbone curves to asymptotically approach the shear strength. When implemented in nonlinear and equivalent linear ground response, the strain lo calization phenomenon is removed, strains in the upper portion of the soil profile are increased, and predicted ground motions match observations reasonably well.
Measured relative densities ( DR) at the site ranged from approximately 40% (from tube samples) to 65% (from SPT correlations). A material specific volumetric strain material model for that range of DR was developed, which when coupled with the shear strain demands from the ground response analysis, predicts ground settlements ranging from 8 to 12 cm for horizontal ground shaking and 12 to 18 cm when vertical ground motions are considered. Those settlement estimates are reasonably consistent with the observations from the site.