Nonlinear Soil-Site Effects in Probabilistic Seismic-Hazard Analysis

Abstract

This study presents effective probabilistic procedures for evaluating ground-motion hazard at the free-field surface of a nonlinear soil deposit located at a specific site. Ground motion at the surface, or at any depth of interest within the soil formation (e.g., at the structure foundation level), is defined here in terms either of a suite of oscillator-frequency-dependent hazard curves for spectral acceleration, , or of one or more spectral acceleration uniform-hazard spectra, each associated s S (f) a with a given mean return period. It is presumed that similar information is available for the rock-outcrop input. The effects of uncertainty in soil properties are directly included. This methodology incorporates the amplification of the local soil deposit into the framework of probabilistic seismic hazard analysis (PSHA). The soil amplification is characterized by a frequency-dependent amplification function, AF(f), where f is a generic oscillator frequency. AF(f) is defined as the ratio of to the spectral s S (f) a acceleration at the bedrock level,. The estimates of the statistics of the ampli-s S (f) a fication function are obtained by a limited number of nonlinear dynamic analyses of the soil column with uncertain properties, as discussed in a companion article in this issue (Bazzurro and Cornell, 2004). The hazard at the soil surface (or at any desired depth) is computed by convolving the site-specific hazard curve at the bedrock level with the probability distribution of the amplification function. The approach presented here provides more precise surface ground-motion-hazard estimates than those found by means of standard attenuation laws for generic soil conditions. The use of generic ground-motion predictive equations may in fact lead to inaccurate results especially for soft-clay-soil sites, where considerable amplification is expected at long periods, and for saturated sandy sites, where high-intensity ground shaking may cause loss of shear strength owing to liquefaction or to cyclic mobility. Both such cases are considered in this article. In addition to the proposed procedure, two alternative, easier-to-implement but approximate techniques for obtaining hazard estimates at the soil surface are also briefly discussed. One is based on running a conventional PSHA with a rockattenuation relationship modified to include the soil response, whereas the other consists of using a simple, analytical, closed-form solution that appropriately modifies the hazard results at the rock level.

Sensitivity of site response analysis on the number of ground motion records and implications for PSHA

“…Bazzurro and Cornell (2004a;2004b) performed regressions of lnAF(f) on lnS a r (f) based on a large dataset of records. However, they suggest that in practice, given the relatively small values of σ lnAF(f) for a given value of S a r (f), only 10 soil response analyses are sufficient to achieve a level of accuracy of ±10% in estimating the median AF(f).…”

“…Similarly, if more than one spectral period is of interest for the application, then records with a good range of accelerations need to be considered for more than one spectral period as well. For this reason this paper evaluates the sensitivity of the Bazzurro and Cornell (2004b) method to suites of different numbers of records which span over a wide acceleration and period range. The entire set of 120 records is used to obtain a benchmark site amplification function; subsequently, records are selected to form 11 suites of 10, 15 and 20 records.…”

“…using only the median value of the site factor and ignoring its uncertainty. However, this procedure underestimates ground motions evaluated probabilistically (Bazzurro and Cornell 2004b;Goulet and Stewart 2009). In order to overcome this shortcoming, site-specific PSHA needs to be performed that considers the nonlinear site effects within the hazard calculations, either using an appropriate groundmotion prediction equation (e.g.…”

“…Power et al 2008) or using methods that allow the proper incorporation of site-specific effects (e.g. Bazzurro and Cornell 2004b;Baturay and Stewart 2003). This paper investigates the minimum number of ground motion records required for a robust estimation of the site-specific amplification function and its incorporation within the hazard calculations.…”

“…Site response analyses are performed using both equivalent linear and nonlinear methodologies in order to identify the number of records required for the stable estimation of the site amplification function and the surface hazard curve in each case. The methodology proposed by Bazzurro and Cornell (2004b) involving the transformation of a rock ground-motion prediction equation (GMPE) into a site-specific one is employed.…”