Probabilistic site response analysis for nuclear facilities considering variability of soil properties and its effects on uniform hazard response spectra and ground motion response spectra

Abstract

Because seismic waves are influenced by variability at local soil sites, their effects on the uniform hazard response spectra (UHRS) and ground motion response spectra (GMRS) of nuclear facilities at soil sites are studied by means of a probabilistic site response analysis based on the random vibration theory. Monte Carlo simulations are employed to take into consideration the effects of variability of the layer thickness, low-strain shear-wave velocity, and nonlinear dependence of the shear modulus and hysteretic damping on shear strain at soil sites. A new probabilistic model is proposed to simulate the variability of the layer thickness using an Erlang distribution. Random field models are considered for the nonlinear shear modulus reduction and damping curves. The probabilistic approach is applied to generic soil sites to evaluate their soil-amplification functions. The seismic hazard curves and the UHRS can be obtained using the amplification functions and the hazard curves of outcropping bedrock motions. Then, the seismic risk for structures installed at these types of sites is calculated and the resulting GMRS values for a seismic design are derived. It can be observed from the simulations that the variability in the layer thickness and the low-strain shear-wave velocity influences the soil-amplification functions and the resulting UHRS and GMRS values for the soil sites significantly. The effects increase with the variances of the considered random properties. Because the input motions for probabilistic earthquake response analyses of nuclear facilities are determined based on the UHRS/GMRS values, the effects of random soil sites must be carefully considered to ensure the seismic safety of these facilities.