Frequency- and pressure-dependent dynamic soil properties for seismic analysis of deep sites

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, February 2000.Includes bibliographical references.Most of the analytical techniques for evaluating the response of soil deposits to strong earthquake motions employ numerical methods, initially developed for the solution of linear elastic, small - strain problems. Various attempts have been made to modify these methods to handle nonlinear stress - strain behavior induced by moderate to strong earthquakes. However, questions arise regarding the applicability of commonly used standardized shear modulus degradation and damping curves versus shear strain amplitude. The most widely employed degradation and damping curves, are those originally proposed by Seed & Idriss, 1969. Laboratory experimental data (Laird & Stokoe, 1993) performed on sand samples, subjected to high confining pressures, show that for highly confined materials, both the shear modulus reduction factor [G /G[alpha] and the damping [zeta ] versus shear strain amplitude fall significantly outside the range used in standard practice, overestimating the capacity of soil to dissipate energy. The equivalent linear iterative algorithm also diverges when soil amplification is performed in deep soft soil profiles, due to the assumption of a linear hysteretic damping being independent of frequency. High frequencies associated with small amplitude cycles of vibration have substantially less damping than the predominant frequencies of the layer, but are artificially suppressed when all frequency components of the excitation are assigned the same value of hysteretic damping. This thesis presents a simple four - parameter constitutive soil model, derived from Pestana's (1994) generalized effective stress formulation, which is referred to as MIT-S1. When representing the shear modulus reduction factors and damping coefficients for a granular soil subjected to horizontal shear stresses imposed by vertically propagating shear [SH] waves, the results are found to be in very good agreement with available laboratory experimental data. Simulations for a series of " true" non-linear numerical analyses with inelastic (Masingtype) soils and layered profiles subjected to broadband earthquake motions, taking into account the effect of the confining pressure, are thereafter presented. The actual inelastic behavior is closely simulated by means of equivalent linear analyses, in which the soil moduli and damping are frequency dependent. Using a modified linear iterative analysis with frequency- and depth-dependent moduli and attenuation, a 1-km deep model for the Mississippi embayment near Memphis, Tennessee, is successfully analyzed. The seismograms computed at the surface not only satisfy causality (which cannot be taken for granted when using frequency-dependent parameters), but their spectra contain the full band of frequencies expected.by Dominic Assimaki.S.M

Topography effects in the 1999 Athens earthquake : engineering issues in seismology

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2004.Includes bibliographical references.It is well known that irregular topography can substantially affect the amplitude and frequency characteristics of seismic motion. Macroseismic observations of destructive earthquakes often show higher damage intensity at the tops of hills, ridges and canyons than at lower elevations and on flat areas. Systematic seismic motion amplification over convex topographies has been confirmed by instrumental studies and also predicted by theoretical and numerical simulations of wave diffraction. Nonetheless, for the most part, the former have been limited to weak motion data and the later have treated topographic asperities as simple geometric irregularities on the surface of homogeneous, linearly elastic halfspaces. Despite the qualitative agreement between theory and observations on topography effects, there is still much uncertainty concerning the actual severity of amplification near topographic irregularities, inasmuch as predictive methods are still lacking on the quantitative aspects of seismic amplification near such features. Focusing of seismic rays by convex topographies does play a significant role as shown theoretically, yet it is not the only physical phenomenon involved. On the other hand, weak motion data may not be applicable to describe topography effects for strong shaking, and indeed there exist very few- if any- well documented case studies demonstrating the severity of topographic effects for strong ground motion. In this dissertation, we find that topography and local soil conditions need to be accounted for simultaneously for the prediction of site amplification factors, especially when earthquake motions are strong enough to elicit clear nonlinear soil behavior.(cont.) We examine how local stratigraphy, material heterogeneity and nonlinear soil response can alter the focusing mechanism at the vertex of cliff-type topographies, and how the free-field response is further modified on account of soil-structure interaction. By means of a case-study from the Athens 1999 earthquake, we validate the effects of local soil conditions by comparison with weak motion data, and illustrate the effects of nonlinear soil behavior and soil-structure interaction on strong motion amplification. Our finite-element, nonlinear simulations seem to explain the uneven distribution of severe damage in the community of Adàmes that borders the crest of the Kifissos river canyon at its deepest point. They also resolve in part previously unexplained discrepancies, often observed between strong amplification during actual earthquakes and moderate values predicted by simple theoretical models. Combining our findings with earlier published results, we propose a period- and space-dependent factor, referred to as Topographic Aggravation Factor (TAF), which can be used in engineering design to modify site-specific design spectra of seismic code provisions to account for topography effects.by Dominic Assimaki.Sc.D

Effects of Local Soil Conditions on the Topographic Aggravation of Seismic Motion: Parametric Investigation and Recorded Field Evidence from the 1999 Athens Earthquake

During the 1999 Athens earthquake, the town of Adàmes, located on the eastern side of the Kifissos river canyon, experienced unexpectedly heavy damage. Despite the particular geometry of the slope that caused significant motion amplification, topography effects alone cannot explain the uneven damage distribution within a 300-m zone parallel to the canyon’s crest, which is characterized by a rather uniform structural quality. In this article, we illustrate the important role of soil stratigraphy and material heterogeneity on the topographic aggravation of surface ground motion. For this purpose, we first conduct an extensive time-domain parametric study using idealized stratified profiles and Gaussian stochastic fields to characterize the spatial distribution of soil properties, and using Ricker wavelets to describe the seismic input motion; the results show that both topography and local soil conditions significantly affect the spatial variability of seismic motion. We next perform elastic two-dimensional wave propagation analyses based on available local geotechnical and seismological data and validate our results by comparison with aftershock recordings

Quantifying Nonlinearity Susceptibility via Site-Response Modeling Uncertainty at Three Sites in the Los Angeles Basin

The effects of near-surface soil stratigraphy on the amplitude and frequency content of ground motion are accounted for in most modern U.S. seismic design codes for building structures as a function of the soil conditions prevailing in the area of interest. Nonetheless, currently employed site-classification criteria do not adequately describe the nonlinearity susceptibility of soil formations, which prohibits the development of standardized procedures for the computationally efficient integration of nonlinear ground response analyses in broadband ground-motion simulations. In turn, the lack of a unified methodology for nonlinear site-response analyses affects both the prediction accuracy of site-specific ground-motion intensity measures and the evaluation of site-amplification factors when broadband simulations are used for the development of hybrid attenuation relations. In this article, we introduce a set of criteria for quantification of the nonlinearity susceptibility of soil profiles based on the site conditions and incident ground-motion characteristics, and we implement them to identify the least complex ground response prediction methodology required for the simulation of nonlinear site effects at three sites in the Los Angeles basin. The criteria are developed on the basis of a comprehensive nonlinear site-response modeling uncertainty analysis, which includes both detailed soil profile descriptions and statistical adequacy of ground-motion time histories. Approximate and incremental nonlinear models are implemented, and the limited site-response observations are initially compared to the ensemble site-response estimates. A suite of synthetic ground motions for rupture scenarios of weak, medium, and large magnitude events (M 3.5–7.5) is next generated, parametric studies are conducted for each fixed magnitude scenario by varying the source-to-site distance, and the variability introduced in ground-motion predictions is quantified for each nonlinear site-response methodology. A frequency index is developed to describe the frequency content of incident ground motion relative to the resonant frequencies of the soil profile, and this index is used in conjunction with the rock-outcrop acceleration peak amplitude (PGA_(RO)) to identify the site conditions and ground-motion characteristics where incremental nonlinear analyses should be employed in lieu of approximate methodologies. We show that the proposed intensity-frequency representation of ground motion may be implemented to describe the nonlinearity susceptibility of soil formations in broadband simulations by accounting both for the magnitude-distance-orientation characteristics of seismic motion and the profile stiffness characteristics. The synthetic ground-motion predictions are next used for the development of site-amplification factors for the alternative site-response methodologies, and the results are compared to published site factors of attenuation relations. For the site conditions investigated, currently established amplification factors compare well with synthetic simulations for class C and D site conditions, while long-period amplification factors are overestimated by a factor of 1.5 at the class E site, where site-specific nonlinear analyses should be employed for levels of PGA_(RO)>0.2g

Site Amplification and Attenuation via Downhole Array Seismogram Inversion: A Comparative Study of the 2003 Miyagi-Oki Aftershock Sequence

Weak-motion geotechnical array recordings at 38 stations of the Japanese strong-motion network KiK-Net from the 2003 M_w 7:0 Miyagi-Oki aftershock sequence are used here to quantify the amplification and attenuation effects of near-surface formations to incident seismic motion. Initially, a seismic waveform optimization algorithm is implemented for the evaluation of high-resolution, low-strain velocity (V_s), attenuation (Q_s), and density (ρ) profiles at the sites of interest. Based on the inversion results, V_s versus Q_s correlations are developed, and scattering versus intrinsic attenuation effects are accounted for in their physical interpretation. Surface-to-downhole traditional spectral ratios (SSR), cross-spectral ratios (c-SSR), and horizontal-to-vertical (H/V) site-response estimates are next evaluated and compared, while their effectiveness is assessed as a function of the site conditions classified on the basis of the weighted average Vs of the upper 30 m (V_(s30)) of the formations. Single and reference-station site-response estimates are successively compared to surface-to-rock outcrop amplification spectra and are evaluated by deconvolution of the downhole records based on the inversion results; comparison of the observed SSR and estimated surface-to-rock outcrop amplification spectra illustrates the effects of destructive interference of downgoing waves at the downhole instrument level as a function of the site class. Site amplification factors are successively computed in reference to the National Earthquake Hazards Reduction Program (NEHRP) B–C boundary site conditions (V_(s30) = 760 m/sec), and results are compared to published values developed on the basis of strong-motion data and site-response analyses. Finally, weak-motion SSR estimates are compared to the mainshock spectra, and conclusions are drawn for the implications of soil nonlinearity in the near surface. Results presented in this article suggest that currently employed site classification criteria need to be reevaluated to ensure intraclass consistency in the assessment of amplification potentials and nonlinearity susceptibility of near-surficial soil formations

Site Effects and Damage Patterns

A set of observations on site effects and damage patterns from the M_w 8.8 Maule, Chile, earthquake is presented, focusing on identification of structural damage variability associated with nonuniform soil conditions and subsurface geology. Observations are reported from: (1) the City of Santiago de Chile (Américo Vespucio Norte Ring Highway, Ciudad Empresarial business park), (2) the Municipality of Viña del Mar, and (3) the City of Concepción, extending over 600 km along the Chilean coast. Reconnaissance information and ground motion recordings from the megathrust event are combined with site investigation data in the regions of interest. Comparisons against macroseismic observations related to uneven damage distribution from the M_w 8.0 1985 Valparaíso earthquake are discussed. Complexities associated with identifying the mechanics and underlying physical processes responsible for the manifestation of these effects are elucidated

Modified Topographic Amplification Factors for a Single Faced Slope due to Kinematic Soil-Structure Interaction

In this paper, we evaluate the additive effects of topography, soil nonlinearity, and soil-structure interaction SSI along the crest of an idealized 40 m high cliff-type topographic feature with slope inclination 300, where excessive damage was observed during the Athens 1999 earthquake. The objective of this paper is to investigate the relative contribution of topographic amplification, and kinematic SSI as a function of the incident motion frequency content and geotechnical site conditions for a surface and an embedded structure located at the cliff crest. For this purpose, we perform elastic parametric and nonlinear site-specific two-dimensional finite element simulations using three profiles and six input motions. We illustrate the role of SSI in altering the response at the location of peak topographic amplification potential behind the crest, the effects of incident motion incoherency on the transient structural response, and the beneficial contribution of structural embedment. We finally suggest that empirical models for base-slab averaging of shallow foundations, developed as a function of site conditions, structural dimensions and center line location, could be combined with topographic amplification factors to predict realistic design spectra for structures located on irregular topographic features

Site- and ground motion dependent nonlinear effects in seismological model predictions

We investigate the empirical relationship between site response nonlinearity, soil properties, and ground motion characteristics, as a first step to enable efficient integration of nonlinear analyses in broadband ground motion simulations. For this purpose, we use 24 downhole array sites with detailed geotechnical information and subject the profiles to broadband ground motion synthetics. We quantify the extent of soil nonlinearity in site-specific response analyses by estimating the divergence between linear and nonlinear ground surface predictions. We show that the parameters controlling the nonlinear response are V_(S30) (weighted averaged shear wave velocity in the top 30 m of the soil profile), the site amplification at the fundamental frequency (Amp), the peak ground acceleration (PGA), and the frequency index (FI), a quantitative measure we define to characterize how well the incident motion can “see” the near-surface soil layers. Using the synthetic results, we quantitatively describe the error introduced in ground motion predictions when nonlinear effects are not accounted for, and show that the error is both site and ground motion dependent. Our study indicates that to characterize the susceptibility of a soil profile to nonlinear effects, V_(S30) should be complemented with measures of the soil–rock impedance contrast, as well as measures of the ground motion intensity and frequency content