Nonlinear Soil Properties Estimated from Downhole Array Recordings at Kashiwazaki-Kariwa Nuclear Power Plant in the Niigata-Ken Chuetsu-Oki Earthquakes

Abstract eng:
Nonlinear soil properties are back-calculated using strong motion downhole array recordings at the Service Hall of the Kashiwazaki-Kariwa nuclear power plant during the 2007 Niigata-ken Chuetsu-oki earthquakes. A technique used for this inversion is genetic algorithm combined with a one dimensional equivalent linear response analysis in which soil damping ratios vary with Fourier amplitude of shear strain in the frequency domain. The Holocene and Pleistocene dune sands with Vs = 200-350 m/s at depths smaller than about 70 m showed strong nonlinear behavior with shear modulus ratios of about 0.01-0.8 and damping ratios of about 20-35 % at maximum shear stains of about 2x10-3 - 3x10-2 during the main event. The reduction in shear modulus ratio is consistent with previous laboratory test results for sand consolidated under similar confining pressures but the back-calculated damping ratio seems to be higher than that of the previous studies. The mudstone with Vs =500 m/s located at depths greater than about 70 m did not show any strong nonlinearity during the main shock, with shear modulus ratios of about 1.0 and damping ratios of up to 5 % at maximum shear strains up to about 5x10-4 - 2x10-3.

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