The nonlinear soil amplification models developed by Walling et al. (2008) are revisited for two main reasons: (a) the simulation database on which the models were developed has been updated and extended, with additional magnitudes and soil profiles, and (b) two alternatives for the input shaking parameter are explored – the Peak Ground Acceleration (PGA) and the spectral acceleration for each period, (Sa(T)). The benefits and limitations of each alternative are discussed.
The model is based on site amplification factors, relative to a VS30 =1170 m/sec site, computed for 53 base soil profiles using the RA SCALS computer program . The base profiles include ten VS30 values (160 m/sec, 190 m/sec, 270 m/sec, 400 m/sec, 560 m/sec, 760 m/sec, 900 m/sec, 1170 m/sec, 2830 m/sec, and 3150 m/sec), up to eight soil depths (25 ft, 50 ft, 120 ft, 250 ft, 500 ft, 1000 ft, 2000 ft, 3000 ft), and four nonlinear soil models (EPRI and Peninsular range models for cohesionless soils and Imperial Valley and Bay Mud m odels for cohesive soils). For each soil profile, the site response is computed for 11 levels of input rock motion: PGA 1170 = 0.01g, 0.05g, 0.1g, 0.2g, 0.3g, 0.4g, 0.5g, 0.75g, 1.0, 1.25g, 1.5g, produced by three point-source magnitudes: M = 5.0, 6.0, 7.0.
For each base soil profile, the shear-wave velocities, layer thickness and the nonlinear soil properties (strain dependence of the G/G max and hysteretic damping) are randomized. Correlation of the velocities and layer thickness variations are considered. For each base profile, source magnitude and input ground motion level, the median amplification and the standard deviation of the amplification are computed from 30 realizations of the soil profiles and material properties. These results are intended for use by the NGA developers to constrain the nonlinear scaling of the site response for the ground motion models.