Site Response Analysis Research Articles

Impact of Liquefiable Layers on Site Response Analysis

Impact of Liquefiable Layers on Site Response Analysis

Simplified site response analysis for regional seismic risk assessments

Seismic risk assessment on a regional scale requires an estimation of ground motion intensity and its spatial variation over large geographical areas. This task is particularly challenging in the presence of very soft soil deposits where significant amplification occurs at specific frequencies that are often referred to as local resonances. It is often not feasible to conduct a nonlinear site response analysis at thousands of sites of interest across a region such as an urban area due to the computational effort involved and the required detailed information on soil profiles at each site. Thus, in regional seismic risk analyses the hazard is typically represented by a field of response spectral ordinates estimated by ground motion models. This study proposes a simplified site response analysis procedure to improve the characterization of spectral ordinates to be used in regional seismic risk assessments. In the proposed procedure, reduced-order models are used to conduct site response analysis using very few parameters to transform response spectra at rock outcrop into response spectra at the surface of each of the sites. A particular emphasis is given to soft-soil sites characterized by low shear-wave velocities overlaid on rock or firm soils. A non-uniform continuous shear beam model with a parabolic variation of shear modulus along the depth and modal damping is used in combination with Random Vibration Theory. It is shown that the proposed approach provides better estimates than those of ground motion models or physics-based ground motion simulation models which often cannot capture the local resonances. Site-specific validation of the shear beam model and overall simplified site response analysis is performed at four soft-soil sites in San Francisco, California. Despite its simplicity, it is shown that the proposed procedure adequately captures the amplitudes and spectral shapes of the motions. It is also shown that in conjunction with ground motion models or physics-based models, the proposed simplified site response analysis procedure can be used to efficiently simulate the seismic hazard on a regional scale through an example of a regional seismic hazard analysis of 160 softer soil sites in Downtown San Francisco during the Loma Prieta earthquake.

Three-Dimensional Site Response Analysis of Clay Soil Considering the Effects of Soil Behavior and Type

To understand changes in bedrock motion at the ground surface, frequency effects, and spatial distribution within the soil, it is important to look at how a site responds to earthquakes. This is important for soil–structure interaction in structural and geotechnical earthquake engineering. This study deals with the effect of classifying clays according to shear wave velocity (stiff/medium/soft) and nonlinearity in behavior (linear/nonlinear) on the analysis of the site response. A 3D soil model with a combination of free fields and quiet boundaries and advanced constitutive models for soil to obtain accurate results was used to conduct this study. A strong TABAS earthquake was used to excite the compliant base of the model after converting the velocity record of TABAS to an equivalent surface traction force using a horizontal force–time history proportional to the velocity–time history. This study reveals that the site response analysis is affected by the type of clay soil and the soil material behavior, with soft clay soil causing higher PGV and PGV values in the linear case and lower values in the nonlinear case due to soil yielding, which causes soil response attenuation. This results in extremely conservative and expensive building designs when linear soil behavior is adopted. On the other hand, the applied earthquake exhibits greater attenuation at longer frequencies and greater amplification at mid and short frequencies. However, at frequencies near the applied earthquake frequency, neither attenuation nor amplification occurs. Furthermore, nonlinear soil behavior is crucial for soil evaluation and foundation design due to higher octahedral shear strain and settlement values, especially in softer soils, resulting from extensive plastic deformation.

Ground Motion Simulation of Scenario Earthquakes in the Nakdonggang Delta Region using a Broadband Hybrid Method and Site Response Analysis

Ground Motion Simulation of Scenario Earthquakes in the Nakdonggang Delta Region using a Broadband Hybrid Method and Site Response Analysis

Spurious Accelerations in Finite Element Analysis of Geotechnical Problems: Cause and Remedy

Present study investigates the response of the two distinct geotechnical problems under earthquake loading. These two problems are: 1) nailed soil slope and 2) 1-D site response analysis. The aim of the study is to understand the cause of spurious accelerations and suggest a remedy. Two distinct interface conditions were considered in modelling the nailed soil slope. In first, the soil nail interface was considered to be perfectly bonded. In second case, the sliding at the interface was allowed. The analysis resulted into the physically acceptable deformations. However, the dumbbell shaped spurious acceleration pattern was observed in perfect bonding case. Further, the maximum acceleration was found to span from 4 g to 13 g. Such extremely high accelerations are unphysical and hence, unacceptable. In case of 1-D site response analysis, misleading acceleration spike of magnitude twice that of true acceleration was noted. The introduction of the numerical damping successfully removed the spurious accelerations and resulted in physically acceptable response for both problems.

Numerical modeling of site response at large strains with simplified nonlinear models: Application to Lotung seismic array

Abstract Site response analyses at large strains are routinely carried out neglecting the shear strength of soil and the stiffness degradation due to the increase in pore pressures, leading to unrealistic predictions of the seismic response of soil deposits. The study investigates the performance of a simplified nonlinear (NL) approach, implemented in the Deepsoil code, constituted by coupling a hyperbolic model incorporating shear strength with a strain-based semi-empirical pore pressure generation model. The first part of the study, based on a large one-dimensional parametric study, shows that above a shear strain of 0.1%, it is necessary to include shear strength in the site response modelling to get more realistic results. Then, the approach has been evaluated with reference to the well-known downhole Large-Scale Seismic Test array located in Lotung (Taiwan): numerical results have been compared with recordings in terms of acceleration response spectra and pore water pressure time histories at different depths along the soil profiles. The comparison shows that the NL simplified model is characterized by an accuracy comparable with more sophisticated advanced elasto-plastic NL analyses adopting essentially the same input data of the traditional equivalent linear approaches(shear modulus and damping curves) and simple physical-mechanical properties routinely determined during geotechnical surveys (i.e., shear strength, relative density, fine content). This approach is therefore recommended for site response analyses reaching large strains (i.e., soft soil deposits and moderate-to-high input motions).

Site Response Analysis Considering Site-Affects Leading to Seismic Microzonation Map of Lahore

Site Response Analysis Considering Site-Affects Leading to Seismic Microzonation Map of Lahore

Seismic site characterization baseline data for microzonation and site response analysis of Otuasega Town, Bayelsa State, Niger Delta region of Nigeria

This study presents the findings of a comprehensive geotechnical and seismic site investigation conducted at Otuasega Town located in Bayelsa State within the Niger Delta region of Nigeria. Subsurface exploration involved advancing 10 boreholes to 30 m depth using hollow stem auger drilling. Continuous disturbed and undisturbed soil sampling was performed at 1.5 m intervals for detailed geotechnical testing. Laboratory tests on the recovered soil samples established the index properties, classification, densities and consistency limits of the stratified deposits. The subsurface profile comprised alternating layers of clay, silt and sand typical of deltaic sediments, with the clay fractions exhibiting medium to high plasticity. Shear wave velocity (Vs) profiling using Multichannel Analysis of Surface Waves (WASW) techniques categorised the site predominantly as Site Class C and D based on international standards. The Standard Penetration Test (SPT) N-values ranged from 5 to 10, indicating soft normally consolidated clay conditions typical of the Niger Delta region. Predictive empirical models developed from the field and lab data showed strong correlations for estimating key geotechnical parameters such as SPT blow count, Vs and liquefaction resistance. Ground response analyses using the Vs and SPT data indicated significant site amplification potential, with peak ground accelerations up to 1.5 times the bedrock motion. Liquefaction analysis based on the empirical SPT-based methods revealed a high potential for liquefaction in the sandy layers, especially under strong earthquake shaking. The study characterized the complex sedimentology and provided baseline information for seismic microzonation and site-specific ground response analyses to advance understanding of geohazards in this delta environment.

Nonlinear seismic response analysis of liquefiable sites based on effective stress method

In this study, the one-dimensional nonlinear Matasovic model was extended to the two- and three- dimensional stress space by replacing the shear strain with the generalized shear strain. The extended Matasovic model was subsequently combined with Dobry’s excess pore water pressure generation model, and the reliability of the proposed loosely coupled effective stress analysis model was verified through undrained cyclic triaxial tests and a one-dimensional site response analysis. Finally, this method was applied to an engineering site in China to evaluate the nonlinear seismic response of liquefiable sites. The results show that the proposed effective stress analysis method can capture the dynamic behavior of the soil and the generation of pore water pressure during strong shaking. Moreover, there are differences between the proposed effective stress analysis method and the total stress analysis method for liquefiable sites, which indicates the need for careful selection in practical applications.

Site response analysis by generating a new 3d mesh design with surface topography: a 3d site response analysis of northwest Turkey

Predicting soil behavior under dynamic load due to earthquakes is pivotal for engineering structures and human life. Due to various limitations, such as insufficient computers and difficulties in generating models, the third-dimension effect is generally neglected in many studies. Conversely, the third-dimension effect in regions with high topographic differences, deep basins, three-dimensional heterogeneous and anisotropic environments, and alluvium is at a level that cannot be neglected. This study created a three-dimensional model of the northwest of Turkey for the first time by including surface topography. Soil properties were added to this model, and dynamic analysis was performed. This new model aims to increase the accuracy of ground motion predictions in Northwest Turkey. The accuracy of this model was analyzed using real earthquake data recorded in the study area. In addition, a new software (SiteEffect3D) with various features has been developed to create a three-dimensional mesh with topography using digital elevation model data and to perform dynamic analysis more effectively. This software has been tested comparatively with “Plaxis 3D” software using synthetic terrain models. The importance of this study is that in addition to its contributions to site response analysis and seismic hazard assessment, new software has been developed that can be used in similar studies. The findings will provide valuable information for seismic design and construction practices and facilitate the development of more effective strategies to reduce the potential damage from earthquakes in the region.

An Assessment of the Seismic Performance of EC8-Compliant CBFs Taking into Account the Role of Soil: A Case Study

Based on criticisms raised in the past by researchers about the effectiveness of the design rules reported in the European seismic code for the design of concentrically braced frames, a new design procedure has been proposed and included in the upcoming version of Eurocode 8. The upcoming version of Eurocode 8 is in the enquiry stage. Hence, it is important to evaluate the effectiveness of the design procedure reported in the code using accurate numerical models and seismic inputs. In the present paper, a four-story building with concentrically braced frames in the chevron configuration is designed according to the upcoming version of Eurocode 8. A seismic performance assessment is carried out by the means of multiple-stripe analyses performed on refined numerical models. The seismic input is defined based on one-dimensional local site response analyses. The numerical analyses prove that the use of local site response analysis to properly account for the soil-filtering effects is of paramount importance, and that the design procedure reported in the upcoming version of Eurocode 8 for chevron concentrically braced frames leads to reasonably low probabilities of exceeding the considered limit states.

Combined Effect of A-Priori Information and Varying Source Offset in Minimizing Propagation of Near-Field Effects in MASW Testing and Subsequent Site Response Analysis

Combined Effect of A-Priori Information and Varying Source Offset in Minimizing Propagation of Near-Field Effects in MASW Testing and Subsequent Site Response Analysis

Temporal variations of the ‘in-situ’ nonlinear behaviour of shallow sediments during the 2016 Kumamoto Earthquake sequence

SUMMARY Strong ground shaking has the potential to generate significant dynamic strains in shallow materials such as soils and sediments, thereby inducing nonlinear site response resulting in changes in near-surface materials. The nonlinear behaviour of these materials can be characterized by an increase in wave attenuation and a decrease in the resonant frequency of the soil; these effects are attributed to increased material damping and decreased seismic wave propagation velocity, respectively. This study investigates the ‘in-situ’ seismic velocity changes and the predominant ground motion frequency evolution during the 2016 Kumamoto earthquake sequence. This sequence includes two foreshocks (Mw 6 and Mw 6.2) followed by a mainshock (Mw 7.2) that occurred 24 hr after the last foreshock. We present the results of the seismic velocity evolution during these earthquakes for seismological records collected by the KiK-net (32 stations) and K-NET (88 stations) networks between 2002 and 2020. We analyse the impulse response and autocorrelation functions to investigate the nonlinear response in near-surface materials. By comparing the results of the impulse response and autocorrelation functions, we observe that a nonlinear response occurs in near-surface materials. We then quantify the velocity reductions that occur before, during, and after the mainshock using both approaches. This allows us to estimate the ‘in-situ’ shear modulus reduction for different site classes based on V$_{S30}$ values (V$_{S30}\lt 360$ m s−1, $360\lt $V$_{S30}\lt 760$ m s−1 and V$_{S30}\gt 760$ m s−1). We also establish the relationships between velocity changes, shear modulus reduction, variations in predominant ground motion frequencies and site characteristics (V$_{S30}$). The results of this analysis can be applied to site-specific ground motion modelling, site response analysis and the incorporation of nonlinear site terms into ground motion models.

DEM analysis of constrained modulus nonlinearity and damping factor of dry granular soils

The seismic site response analysis requires the dynamic soil properties (i.e., the modulus and damping). While it is well understood that the shear modulus and damping ratio are nonlinearly shear-strain dependent, the knowledge on the constrained modulus and damping ratio of compressional waves is still very limited due to lack of laboratory testing equipment. This study aims to simulate cyclic tests of constrained compression for granular specimens by discrete element method (DEM) to understand the dynamic soil properties of compressional waves, i.e., the nonlinearity in constrained modulus and damping ratio with the compression strain. The evolution in microstructure of granular specimens is revealed to provide micromechanical interpretations for the compressional soil nonlinearity. The results show that the dependency of constrained modulus on compression strain is different in compression and extension stages, and the modulus reduction is only observed in the extension path. The damping ratio of samples under cyclic constrained compression is smaller than that of cyclic shear. The nonlinear soil behavior is more obvious at lower confining pressure and for dense sample. The nonlinearity of constrained modulus depends on the coordination number, contact normal force and fabric anisotropy, while the associated damping ratio is only related to fabric anisotropy.

A Seismic Site Factor Adjustment Framework for Sloping Grounds and Its Application to Charleston, South Carolina

ABSTRACT Ground motions and seismic site factors for the design and analysis of infrastructure systems are typically computed based on one-dimensional (1-D) site response analysis, assuming the ground surface is horizontal and the earthquake waves propagate vertically near the ground surface. The 1-D results may not be applicable for sloping ground surfaces and layer interfaces, and therefore, the 1-D results must be adjusted for sloping grounds. In this study, a general framework for developing seismic site factors for sloping grounds and its application for Charleston, SC is presented. The two-dimensional (2-D) site response analysis results of sloping ground were compared with the 1-D horizontal ground results to develop slope adjustment factors. A total of 385 2-D nonlinear finite element simulations of sloping grounds were performed considering seven ground inclinations, 11 shear wave velocity profiles, and five ground motions representative of the area. The results show that the surface responses computed from sloping ground analyses significantly differ from those of horizontal ground analyses. By comparing the 1-D and 2-D results, a set of slope adjustment factors was developed and expressed as an easy-to-use chart. The slope adjustment factor at a project site can be obtained from the proposed chart using site-specific VS30 and ground inclination at a selected period.

Quantifying near-fault motion effects on soil liquefaction through effective stress site response analysis

Near-fault (NF) pulse-like ground motions have a significant impact on the performance of structures, but their effects on soil liquefaction potential have been relatively understudied. This paper uses 1D effective stress site response analysis capable of modeling porewater pressure (PWP) generation to investigate the seismic site response under NF and general ground motions. The aim is to assess the effects of NF motions on soil liquefaction by considering different soil models and PWP generation models. The results show that NF ground motions generally induce larger ground responses in terms of PWP generation, especially when the durations of the motions are short. This is due to the higher cumulative absolute velocity associated with NF ground motions compared with general ground motions due to a high-velocity pulse under the same peak ground acceleration. However, this effect diminishes as the duration of motion increases. To avoid underestimating seismic demand, especially for small to moderate earthquakes, a preliminary magnitude scale factor modified for the NF effect is suggested for use in conventional soil liquefaction triggering analysis.

Periodic foundation piles for the seismic protection of structures

This study introduces a novel solution for seismic protection of structures based on resonant metamaterials, named Periodic Foundation Piles, which can be integrated with deformable soil deposits in order to filter shear waves in a specific frequency range. A Periodic Foundation Pile consists of an array of vertical periodic structural elements containing internal resonators suitably tuned to one or more natural frequencies of the soil deposit. The proposed system is equivalent to a discontinuous foundation pile and can be installed in the ground with approximately the same procedure for piles, thus making it suitable also for applications in existing structures. The performance of the Periodic Foundation Piles in reducing the amplitude of a seismic motion has been shown through non-linear site response analyses carried out using real acceleration records as input motion. A soil deposit with an increasing shear wave velocity profile has been considered; the nonlinear behaviour of the soil has been modelled through the Iwan model, calibrated using a normalized secant shear modulus curve accounting for the influence of the confining pressure on the stress-strain relationship. Numerical results, presented in terms of peak acceleration, horizontal displacement and shear deformation profiles, as well as in terms of Fourier amplitude and response spectra, indicate that Periodic Foundation Piles may noticeably reduce the amplitudes of seismic motion in a suitable frequency range, hence potentially representing an innovative strategy to mitigate the effects of earthquakes on structures.

Deep soil seismic hazard assessment: analyzing the impact of local site effects by stochastic site response analysis—a comparative approach

Deep soil seismic hazard assessment: analyzing the impact of local site effects by stochastic site response analysis—a comparative approach

Impact of Randomized Soil Properties and Rock Motion Intensities on Ground Motion

Seismic site response is inevitably influenced by natural variability of soil properties and anticipated earthquake intensity. This study presents the influence of variability in shear wave velocity (Vs) and amplitude of input rock motion on seismic site response analysis. Monte Carlo simulations were employed to randomize the Vs profile for different scenarios. A series of 1-D equivalent linear (EQL) seismic site response analyses were conducted by combining the randomized Vs profile with different levels of rock motion intensities. The results of the analyses are presented in terms of surface spectral acceleration, amplification factors (AFs), and peak ground acceleration (PGA). The mean and standard deviation of these parameters are thoroughly discussed for a wide range of randomized Vs profile, number of Vs randomizations, and intensities of input rock motions. The results demonstrate that both the median PGA and its standard deviations across different number of Vs profile realization exhibit a slight variation. As few as twenty Vs profile realizations are sufficient to compute reliable response parameters. Both rock motion intensity and standard deviation of Vs variability cause significant variation in computed surface parameters. However, the variability in the number of records used to conduct site response has no significant impact on ground response if the records closely match the target spectrum. Incorporating the multiple sources of variabilities can reduce uncertainty when conducting ground response simulations.

Applicability of 1D site response analysis to shallow sedimentary basins: A critical evaluation through physics‐based 3D ground motion simulations

AbstractOne‐dimensional site response analysis (1D SRA) remains the standard practice in considering the effect of local soil deposits and predicting site‐specific ground motions, although its range of applicability to realistic seismic wavefields is still in question. In this 1D approach, horizontal and vertical ground shaking are assumed to be induced by vertically propagating shear and compressional waves, respectively. A recent study based on analytical two‐dimensional (2D) plane waves and simple point source earthquake simulations has shown two mechanistic limitations in this 1D modelling technique for general inclined seismic waves, that is, systematic over‐prediction of the vertical motion and wave trapping in the 1D soil column. In this article, we evaluate in detail the applicability of this 1D modelling approach to realistic three‐dimensional (3D) simulated seismic wavefields in shallow sedimentary basins. Linear‐viscoelastic 1D SRA predictions using two types of input motions that are commonly used in practice—rock outcrop and in‐column motions, are compared with the reference true site response results from 3D earthquake simulations in terms of various measures in the frequency and time domain. It is shown that the horizontal motion in the 3D seismic wavefield exhibits dominant shear wave propagation phenomenon, while the vertical motion is a combined effect of compressional and shear waves and can be over‐predicted by the 1D approach when the incident seismic waves are inclined. Direct evidence of the wave refraction process that leads to the vertical motion over‐prediction is provided. 1D SRA with in‐column inputs can yield motions that have significantly longer duration compared to the true 3D site response solution due to trapped waves, casting in doubt the frequent need for increased soil damping in existing site studies to compensate for wave attenuation due to scattering alone. Sensitivity investigation on the increase of soil profile damping by a multiplier Dmul shows Dmul values compatible with those found in the literature for both horizontal and vertical motions. It is shown that the level of Dmul optimized for a best match of the spectral acceleration is dependent on the characteristic of the input motion and a larger Dmul is typically required for the vertical component. In contrast, 1D SRA with outcrop motions predicts motions with shorter significant duration due to its inability to capture the basin‐edge generated surface waves. A suite of ground motion simulations was performed to assess the sensitivity of the observations to the basin geologic structure including the velocity gradient, rock‐basin impedance contrast and basin depth. The analysis results show that the accuracy of the simplified 1D procedure is dependent on the wavefield composition of both the input motions and the true 3D site response solution. While the horizontal motions in shallow sedimentary basins can, to the first order, be reasonably captured by the simplified 1D approach, 1D SRA for the vertical component is in general not reliable and contributions from inclined shear waves should be accounted for in site‐specific evaluation of the vertical design ground motion.