Effect of Depth-Dependent Shear Wave Velocity Profiles on Site Amplification

Geological conditions such as soil depth to bedrock, soil types, and shear wave velocity (V-S) profile can significantly affect the seismic response of a site. Previous studies have shown that using coarse V-S profiles could lead to considerable errors in predicting seismic responses using one-dimensional site response analysis. These errors can be avoided by applying the depth-dependent V-S gradient within a soil layer. Therefore, this paper aims to evaluate the effect of the soil V-S gradient on the PGA, short-period, and long-period site coefficients for several hypothetical sites. A series of sensibility study is performed for different soil types, soil thicknesses, soil V-S values, and impedance ratios between the soil and the underlying bedrock. In the current work, the median amplification spectra are computed for both constant and depth-dependent V-S profiles with ten selected ground-motion suites. Its mean value is calculated over the 0.1-0.5 s and 0.4-2.0 s period ranges to determine the mean F-a and the mean F-v, respectively. The study has shown that by applying depth-dependent V-S gradient within a soil layer could lead to a rise in site amplification and a reduction in the characteristic site period. The Arias intensity (I-A) obtained for the depth-dependent V-S profiles are greater than those for the constant V-S profiles. Further adjustment of the soil V-S would result in lesser change in amplification for shallower sites associated with higher impedance contrast at soil-bedrock interface. In addition, for the different site classes, a detailed comparison is drawn between the site coefficients obtained in the present study and those recommended by Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-16).