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Seismic Site Response Analysis with Pore Water Pressure Generation: Guidelines (2024)

Chapter: Chapter 2 - Site Response Analysis Overview

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Suggested Citation:"Chapter 2 - Site Response Analysis Overview." National Academies of Sciences, Engineering, and Medicine. 2024. Seismic Site Response Analysis with Pore Water Pressure Generation: Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27536.
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Suggested Citation:"Chapter 2 - Site Response Analysis Overview." National Academies of Sciences, Engineering, and Medicine. 2024. Seismic Site Response Analysis with Pore Water Pressure Generation: Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27536.
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Page 6
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Suggested Citation:"Chapter 2 - Site Response Analysis Overview." National Academies of Sciences, Engineering, and Medicine. 2024. Seismic Site Response Analysis with Pore Water Pressure Generation: Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27536.
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Page 7

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5   2.1 General Site-specific evaluation of design earthquake ground motions considers the source effects, path effects, and site effects, as schematically illustrated in Figure 2-1. The source and path effects are evaluated at the seismic hazard assessment stage of the project and are, at a minimum, reported as the hypothetical bedrock outcrop acceleration response spectrum. Depending on site location and analysis type, several spectra might be required. These spectra are then termed the “target spectra” and are, in addition to other seismic hazard parameters not discussed herein, used for the development of a suite of design acceleration histories (accelerograms). In developing the design accelerograms, geotechnical engineers identify site class and, depend- ing on the approach followed (see next section), appropriate site factors. The results of these evaluations may be used in several ways, including for design of foundations, embankments, or soil retaining structures. Depending on the approach, the results may also be used to evaluate soil-foundation-structure-interaction (SFSI) effects. 2.2 Approaches for Evaluation of Site Effects There are three general approaches for the evaluation of site effects on strong ground shaking: (i) the ground motion model (GMM)4 approach, (ii) the code-factor approach, and (iii) the seismic site response analysis approach: • The first approach uses statistically developed GMMs5 that consider local site conditions and effects. Older GMMs distinguished only between soil and rock. Modern GMMs can be used to estimate ground motions as a function of time-averaged shear wave velocity (Vs) over the upper 30 m (Vs30) and, in some cases, depths to shear wave isosurfaces (e.g., depth to Vs = 1 km/s, which is denoted Z1, or depth to Vs = 2.5 km/s, which is denoted Z2.5). In this approach, acceleration response spectra are calculated for an average site condition and can be used directly for design. If needed, design ground motions (accelerograms) for structural design can be selected to match the ground surface spectrum or to match the spectrum at the predominant period of the structure. Some structural engineers prefer using a reduced set of accelerograms developed in a process called “spectral matching.” • The second approach for assessing soil effects on design ground motions is the code-based approach (e.g., AASHTO, 2020; AASHTO, 2011; ASCE/SEI, 2022). This approach is, in gen- eral, based on the results of probabilistic seismic hazard analysis performed for the National Earthquake Hazard Reduction Program (NEHRP) Site Class B/C (the boundary between “very dense soil and soft rock” and “rock”) and generic soil amplification factors that are used to modify select spectral ordinates (e.g., at 0, 0.2, and 1.0 s). This approach is used only when the first (i.e., statistically developed GMMs) or the third approach (i.e., detailed site response C H A P T E R 2 Site Response Analysis – Overview

6 Seismic Site Response Analysis with Pore Water Pressure Generation: Guidelines analysis based on the actual site conditions and using specialty software—see following bullet) is not justifiable for economic or other reasons. On January 1, 2023, AASHTO abandoned this approach in favor of the first approach (approach based on Vs30). • The third approach, explained in greater detail herein, is the only approach that can cap- ture impedance ratio, nonlinear, and resonance effects and account for soil liquefaction. It involves detailed site response analysis based on the actual site conditions and using spe- cialty software. A key input into the analysis is the ground motion represented by a suite of acceleration histories that conform to a target spectrum, as defined in Section 2.1. The 1D total-stress site response analysis approach for evaluation of site effects on ground motions is widely used in transportation engineering practice (e.g., see the survey by Matasovic and Hashash, 2011; 2012). The site response approach is often favored by geotechnical earthquake engineers relative to the other two approaches because it takes into account the unique geo- technical characteristics (i.e., the seismic signature) of the site. Through back analysis of numerous case histories (including those presented in this docu- ment), the third approach has been shown to work well in design when (i) the GMM approach is not appropriate (e.g., where a significant impedance contrast is present in the profile), (ii) material and model parameters are established through a reasonable site characterization effort (e.g., Kwok et al., 2006; 2008), and (iii) a suite of design (e.g., bedrock outcrop) acceleration histories has been adequately established. Similarly, the approach has been shown, with few exceptions, to provide meaningful estimates of site response for site periods (Ts) ranging from zero to those equal to at least 1.5 times the softened Ts. For more details about the limitations of 1D SRA beyond 1.5 times softened Ts, refer to Stewart et al. (2014) and Stewart and Afshari (2021). Information and evaluations presented in this study follow the third approach, with the addi- tional stipulation that the site response analysis presented and discussed herein is an ESA with provisions for excess PWP generation and dissipation. Figure 2-1. Framework of site response analysis and basic definitions.

Site Response Analysis – Overview 7   2.3 Applications of Site Response Analysis There are many conditions under which a site response analysis is desirable. Such an analysis is commonly performed when (i) soil characteristics cannot be reasonably categorized into one of the standard site profiles encompassed by GMMs (e.g., soil softening due to PWP buildup or soil liquefaction); (ii) code-based analysis is mandated, but empirical site factors are not available, or their use is not appropriate (e.g., such as for the NEHRP, Site Class F, or for sites with significant impedance contrast in the top 30–60 m of the profile/short stiff soil columns); (iii) impacts of ground failure govern the design (e.g., seismic settlement or lateral spreading, and slope stability); (iv) the objective of such a site response analysis is to obtain ground motions considered to be more representative of the local geologic and seismic site conditions than motions obtained from the first two approaches; or (v) nonlinear SFSI analysis is to be undertaken in which ground motion time series at different depths are required. 2.4 Classification of Methods for Site Response Analysis Site response analysis methods can be classified based on the domain in which calculations are performed (frequency domain or time domain), the sophistication of the constitutive model employed (linear/bilinear, equivalent-linear, or nonlinear), whether or not effects of PWP gen- eration and dissipation are neglected or considered [total stress analysis (TSA) and ESA, respec- tively], and the dimensionality of the space in which analysis is performed (e.g., 1D, 2D, and 3D). Other considerations for classifying site response analysis methods include options for modeling cyclic stiffness/strength reduction and degradation in a total-stress mode (the deg- radation index approach). This study focuses on 1D ESA of soil profiles with significant presence of saturated cohesion- less soils that are susceptible to significant generation of excess PWP during strong ground shaking. In support of ESA, including to model cohesive materials in the profile and for cali- bration and validation purposes, bilinear, equivalent-linear, and nonlinear models were also considered.

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One-dimensional (1D) equivalent-linear total-stress site response analysis (SRA) is the de facto standard for state department of transportation (DOT) highway facilities at locations where site-specific ground response analyses are conducted. However, many users and various DOTs have concerns about the applicability of equivalent-linear analyses for the cases where site-specific SRA is most relevant.

NCHRP Research Report 1092: Seismic Site Response Analysis with Pore Water Pressure Generation: Guidelines, from TRB's National Cooperative Highway Research Program, presents guidelines for the selection and use of methods for 1D nonlinear seismic SRA with excess pore water pressure generation and dissipation.

Supplemental to the report is NCHRP Web-Only Document 383: Seismic Site Response Analysis with Pore Water Pressure Generation: Resources for Evaluation.

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