Search

A subroutine incorporating soil nonlinearity is presented using SPECFEM3D Cartesian, an open-access Spectral Element Method (SEM)-based code for full waveform simulation. Specifically, the adaptive time-stepping scheme is developed in the time marching algorithm, enabling the computation to be effective. The verification of the method is first demonstrated via comparison with results calculated by DEEPSOIL. Then, applications for a trapezoid basin are implemented to study the effects of impedance ratio (IR), magnitude, and basin shape on ground motion. Finally, we focus on the nonlinear ground motion simulation of the Shidian basin in southwestern China and compare it with the linear results. [ABSTRACT FROM AUTHOR]

In this paper, a series of simple Masing rules is first proposed for best fitting the shear modulus reduction and the damping ratio curves of soil based on the extended Masing rules and hysteretic damping formulation. A new masing-type nonlinear model is established and implemented in ABAQUS software based on the Matasovic back-bone curve and the new proposed Masing rules. To validate the practical applicability of the new model, the 1-Directional 3-Component nonlinear site response analysis for station KSRH10 (in Hamanaka, Hokkaido, Japan) is conducted. The numerically predicted results of the proposed nonlinear model are in excellent agreement with the recordings. The surface peak ground acceleration obtained by ABAQUS agrees well with the recording, which has a little difference of about 8.3% on average. The comparison between the proposed nonlinear model and the DEEPSOIL model indicate that the results of the former match better with recording to some extent, in term of acceleration time histories, spectral acceleration (SA) curves, and peak ground acceleration (PGA). The mean square error of acceleration time histories and SA curves obtained by the proposed model is smaller than the model in DEEPSOIL. The agreement with recording is improved by 10.8% compared with the DEEPSOIL model. [ABSTRACT FROM AUTHOR]

Abstract The analysis of seismic response in marine engineering structures is pivotal for guaranteeing their seismic safety. Such analyses are intricate due to the complexity of fluid–structure and soil-structure interactions. This paper introduces a unified computational framework for wave motion within a water-saturated seabed-bedrock system, employing the Davidenkov model and a modified Massing rule to characterize the nonlinear properties of the saturated seabed. A comparative analysis is conducted between the nonlinear responses of marine sites subjected to linear and nonlinear free-field inputs, with a specific focus on the seismic response of bridges under seawater-seabed-structure interaction. The study demonstrates that the modulus of a nonlinear saturated seabed diminishes, leading to a decrease in the wave impedance ratio between the saturated seabed and bedrock. Consequently, the reflection and transmission coefficients from the bedrock to the saturated seabed increase, amplifying the seismic response. For deepwater bridges, under harder site conditions, the abutment’s bottom response is largely insensitive to increasing water depth, whereas the shear at the abutment’s base diminishes with increasing water depth. The proposed method is validated through two case studies, with an examination of the impact of saturated sites on the analysis results.

The analysis of seismic response in marine engineering structures is pivotal for guaranteeing their seismic safety. Such analyses are intricate due to the complexity of fluid–structure and soil-structure interactions. This paper introduces a unified computational framework for wave motion within a water-saturated seabed-bedrock system, employing the Davidenkov model and a modified Massing rule to characterize the nonlinear properties of the saturated seabed. A comparative analysis is conducted between the nonlinear responses of marine sites subjected to linear and nonlinear free-field inputs, with a specific focus on the seismic response of bridges under seawater-seabed-structure interaction. The study demonstrates that the modulus of a nonlinear saturated seabed diminishes, leading to a decrease in the wave impedance ratio between the saturated seabed and bedrock. Consequently, the reflection and transmission coefficients from the bedrock to the saturated seabed increase, amplifying the seismic response. For deepwater bridges, under harder site conditions, the abutment's bottom response is largely insensitive to increasing water depth, whereas the shear at the abutment's base diminishes with increasing water depth. The proposed method is validated through two case studies, with an examination of the impact of saturated sites on the analysis results. [ABSTRACT FROM AUTHOR]

Nonlinear dynamic properties of soil crucially influence structural responses during seismic events, highlighting the interdependence between soil and structural behavior. Incorporating soil–structure interaction (SSI) significantly increases structural vulnerability, especially in irregular conditions, compared to traditional fixed-base structures. Despite the emerging construction of reinforced concrete structures with floating columns in urban areas, their seismic performance, particularly when considering soil-structure interaction, remains largely unexplored. Therefore, this study aims to investigate the structural seismic response of mid-rise reinforcement concrete structures with and without floating columns situated on multilayered soil deposits, incorporating the effects of SSI. The nonlinearity of the soil materials was modeled using an isotropic hardening elastoplastic hysteretic constitutive model. A three-dimensional numerical investigation, employing finite element nonlinear time history analysis, was conducted to study seismic responses of structures under different configurations and base conditions. The results were presented as the ratio of structural responses with soil-structure interaction to fixed-base responses subjected to earthquake events. Structures with floating columns exhibited 1.43 times higher peak lateral storey displacement and 55% higher inter-storey drift ratio compared to those without, considering soil-structure interaction. The analysis results demonstrated a decrease in base shear values of up to 35% when accounting for SSI effects.

The choice of stiffness profile can be crucial in a site response analysis. This research aims to study site response predictions at the large-scale seismic test site in Lotung, Taiwan, employing three different approaches to choosing the stiffness profile in nonlinear and equivalent linear analyses. These approaches consider point average, layer average, and deposit average stiffness profiles. One strong and one weak earthquake event recorded at the site are simulated with these three stiffness profile approaches. Moreover, the stiffness profiles are tested under sets of seven modified real input motions (selected from the European Strong-Motion Database) at various seismic intensity levels. The results indicate that the different stiffness profiles have a minimal effect on the nonlinear site response predictions, in particular for input motions having a PGA greater than or equal to 0.05 g. The spectral acceleration values and PGA and shear strain profiles from nonlinear site response analyses change negligibly when using different approaches to derive the stiffness profile. In the equivalent linear site response analysis, the spectral acceleration predictions are strongly influenced by the stiffness profile approach, regardless of the PGA level of the input motions. The stiffness profile has a more significant role in equivalent linear site response analysis than in nonlinear site response analysis. Therefore, the point average stiffness profile should be used in equivalent linear site response analysis. [ABSTRACT FROM AUTHOR]

This study develops a method for estimating site amplification that combines instrumentally observed site-specific amplification factors with adjustment factors from nonlinear site-response analyses. This approach provides estimates of site response for large-strain motions based on observations and sophisticated nonlinear modeling. A database of weak-to-moderate intensity ground motions recorded in three basins of Wellington, New Zealand is used to study the observed site amplification. A subset of nine strong-motion stations was selected to perform nonlinear site-response analyses with scaled strong ground motions to assess the influence of nonlinearity on site amplification factors and demonstrate the approach. Different shear-wave velocity (VS) profiles, constitutive models, and modeling approaches (e.g. one-dimensional (1D) site-response analyses vs empirical V S 30 -based approaches) are used to quantify the sensitivity and modeling uncertainty in the nonlinear site-response analyses. It was found that for soft sites subjected to strong ground motions, there may be a decrease in spectral acceleration amplification factors for periods up to approximately 2 s, relative to the expected linear site response. For longer periods, there is little to no amplification from the effects of soil nonlinearity. However, at stiffer sites, which generally experience less basin amplification in observations, there may be moderate amplification at longer periods when nonlinearity is considered due to softening of the soil profile. Empirical ground-motion models were found to under-represent the observed amplification between basin sites and the nearby reference site, especially at intermediate to long periods, corresponding to resonant frequencies of these basin sites. In addition, the empirical nonlinear site amplification models ( V S 30 -based) were found to deviate from nonlinear analyses at large strains, where such models are poorly constrained due to such a limited number of observations. [ABSTRACT FROM AUTHOR]

Abstract After the occurrence of a large earthquake, engineering seismologists are often requested to estimate strong ground motions at a site where strong motion data were not obtained. The goal of this study was to test the ability of a class of methods that uses Fourier phase characteristics for the post-earthquake ground motion estimation, making use of the precious opportunity provide by the ESG6 Blind Prediction Steps 2&3. It was also part of the goal of this study to test the performance of the effective stress analyses to account for soil nonlinearity. In addition to the dataset provided by the organizer of the blind prediction, the author used additional accelerometric data from a nearby JMA site. To simulate ground motions for an M5.9 earthquake at the target site “KUMA”, the Fourier amplitude spectrum was estimated from the spectral ratio between KUMA and the nearby JMA site. The Fourier phase spectrum was approximated by the spectrum of another event at KUMA. Comparison between the estimated and recorded ground motions after the blind prediction revealed that the estimated ground motions were fairly consistent with the observed ground motions, indicating the effectiveness of the method when the rupture process of the target event is simple and the soil nonlinearity at the target site is not significant. To simulate ground motions at KUMA for the M6.5 foreshock and the M7.3 mainshock of the 2016 Kumamoto earthquake sequence, the author conducted effective stress analyses using a program called “FLIP” to account for soil nonlinearity. Comparison between the estimated and recorded ground motions after the blind prediction indicated that the low-frequency components were overestimated and the high-frequency components were underestimated. The strong soil nonlinearity considered in the effective stress analyses was the main cause of the discrepancy. One explanation for this result could be that the nonlinear soil behavior at KUMA during the foreshock and the mainshock was not a strong one. Another explanation could be that the effect of soil nonlinearity was already included in the records at JMA and the effect of soil nonlinearity was double counted in the results submitted by the author. Graphical Abstract

This paper represents the continuation of previous research works, in which a predictive model for spectral ordinates instrumental for design was developed. In particular, the objective of the present study is to show how to apply the model within a probabilistic seismic hazard analysis (PSHA). The model, which takes into account soil nonlinearity, is based on five parameters (i.e. peak spectral acceleration and displacement, their structural periods of occurrence and a parameter that controls the spectral ordinates at the intermediate period range), which are defined as a function of three different soil classes (i.e. rock, stiff soil, and soft soil). The outcome is a PSHA-based design spectrum (PBDS), which represents a direct tool for seismic design obtained from a PSHA. Examples of PBDS for Italy are also discussed and compared with the design spectra defined by the current Italian seismic code. [ABSTRACT FROM AUTHOR]

Based on the observed strong-motion records of K-NET and KiK-net, we utilized 1,065 earthquake observation stations installed throughout Japan, gathering more than 140,000 recordings for weak motions with a peak ground acceleration (PGA) of less than 100 cm/s2 and more than 5,300 records for strong motions with a PGA of more than 100 cm/s2. These 1,065 sites detected at least one recording with a PGA value greater than 100 cm/s2. These recordings were used to quantify the horizontal-to-vertical spectral ratio (HVSR) difference between weak and strong shakings. Based on the observations, the discrepancy in HVSR between weak and strong shakings can be depicted as a shift both in the frequency and amplitude axes in the logarithmic coordinate system. This kind of shift can be favorably interpreted by an established diffused field theory of the HVSR of earthquakes. Based on the concept of shifts, new empirical functions are proposed to modify the averaged HVSR in linear cases in order to acquire the HVSR in nonlinear regimes. Subsequently, an improved vertical amplification correction function, which considers the time-averaged shear wave velocity down to 30 m, was used to convert the modified HVSR into the horizontal site amplification factor (HSAF) in nonlinear regimes. The new methodology adopted in this study, which does not require detailed soil layer information, is convenient for obtaining the HSAF by considering empirically obtained nonlinear soil behaviors. [ABSTRACT FROM AUTHOR]

Forward engineering applications often use modulus reduction and damping ratio curves (MRD) to represent the non-linear response of the soil as a strain dependent equivalent-linear response. While a multitude of such models are available for the shear-modulus, G, only a handful of studies have attempted to constrain MRD models for the compressional-related constrained modulus, M. In this study, nonlinearity associated with the constrained modulus was studied using an extensive analysis of records from 27 KiK-net seismic stations. To represent the response of dry soil, stations with at least 10 m ground water depth (GWD) were analysed. Using frequency shifts, the modulus degradation of the horizontal (H), the vertical (V) and the vertical P-wave window (PV) components were characterized. Degradation models for the constrained modulus, for a coupled and uncoupled analysis with the shear component are suggested. We find that for PGA>0.1 g, the H component has 91% probability of developing non-linear response, while for the V and PV components it is only 55% and 43%, respectively. We further find that within those records in which nonlinearity was observed, the PGA threshold for H, V, and PV, is 0.1 g, 0.08 g, and 0.03 g, respectively. Finally, the differences we observe between the full vertical component and the extracted P-wave window suggest that the V component includes a combination of P and SV waves, implying that vertical site-response analysis should include both shear and compression-related soil properties. [ABSTRACT FROM AUTHOR]

After the occurrence of a large earthquake, engineering seismologists are often requested to estimate strong ground motions at a site where strong motion data were not obtained. The goal of this study was to test the ability of a class of methods that uses Fourier phase characteristics for the post-earthquake ground motion estimation, making use of the precious opportunity provide by the ESG6 Blind Prediction Steps 2&3. It was also part of the goal of this study to test the performance of the effective stress analyses to account for soil nonlinearity. In addition to the dataset provided by the organizer of the blind prediction, the author used additional accelerometric data from a nearby JMA site. To simulate ground motions for an M5.9 earthquake at the target site "KUMA", the Fourier amplitude spectrum was estimated from the spectral ratio between KUMA and the nearby JMA site. The Fourier phase spectrum was approximated by the spectrum of another event at KUMA. Comparison between the estimated and recorded ground motions after the blind prediction revealed that the estimated ground motions were fairly consistent with the observed ground motions, indicating the effectiveness of the method when the rupture process of the target event is simple and the soil nonlinearity at the target site is not significant. To simulate ground motions at KUMA for the M6.5 foreshock and the M7.3 mainshock of the 2016 Kumamoto earthquake sequence, the author conducted effective stress analyses using a program called "FLIP" to account for soil nonlinearity. Comparison between the estimated and recorded ground motions after the blind prediction indicated that the low-frequency components were overestimated and the high-frequency components were underestimated. The strong soil nonlinearity considered in the effective stress analyses was the main cause of the discrepancy. One explanation for this result could be that the nonlinear soil behavior at KUMA during the foreshock and the mainshock was not a strong one. Another explanation could be that the effect of soil nonlinearity was already included in the records at JMA and the effect of soil nonlinearity was double counted in the results submitted by the author. [ABSTRACT FROM AUTHOR]

Earthquake in the sea area is an important factor affecting the safety of marine engineering construction, seabed site seismic response analysis is an important preliminary work for marine engineering construction. Based on a fluid-solid weak coupling model which could simulate seawater-seabed interaction, four typical borehole sections along the proposed tunnel at Qiongzhou strait are selected to establish layered seabed models for studying the seabed site seismic responses affected by the seawater, seabed soft sediments and bedrock earthquake motion under bidirectional seismic excitation, in which the dynamic nonlinearity of the seabed soft soil is simulated by a generalized non-Masing constitutive model (DCZ model). The result shows: the suppression effect of seawater on seabed seismic motion exists only in the shallow range of seabed (< 50 m), and the suppression effect on the vertical seismic response is higher than that along the horizontal direction; the suppression effect of seawater on the seabed surface seismic motion and the frequency response phenomenon of “high frequency suppression, low frequency amplification” of seabed seismic response is positively correlated with seawater depth; The mean lines of the horizontal and vertical spectrum β obtained by numerical calculation are higher than the design spectrum in the land code within several period ranges, and the possibility of adverse effects induced by seawater and seabed soft sedimentation on the seismic resistance of marine engineering should be considered.

High-frequency motion is often observed in small-scale experimental works carried out in flexible containers under simplified seismic loading conditions when single harmonic sine input motions are introduced at the base of a soil specimen. The source of the high-frequency motion has often been sought in experimental inaccuracies. On the other hand, the most recent numerical studies suggested that high-frequency motion in the steady-state dynamic response of soil subjected to harmonic excitation can also be generated as a result of soil elastic waves released in non-linear hysteretic soil upon unloading. This work presents an example of a finite element numerical study on seismic soil–structure interaction representative of an experimental setup from the past. The results show how high-frequency motion generated in soil in the steady-state response, apparently representative of soil elastic waves, affects the steady-state response of a structure, that is, it is presented how the structure in the analysed case resonates with the soil elastic waves. The numerical findings are verified against the benchmark experimental example to indicate similar patterns in the dynamic response of the structure.

A large‐scale physics‐based simulation is conducted to investigate ground motion distribution in the Mw 7.0 2016 Kumamoto earthquake, Japan. The model simulates the earthquake scenario from fault rupture to wave propagation, and localized site response with consideration of the combined effect of soil nonlinearity and topographic amplification. The simulation domain is 51 km × 43 km × 25 km, and the obtained ground motion time histories are compared satisfactorily with recordings from KiK‐net and K‐NET. Ground motion distribution considering nonlinear soil response and topographic amplification is presented. A 3D equivalent linear model is developed to mimic the soil nonlinearity, and it is demonstrated that neglecting soil nonlinearity could over‐predict peak ground acceleration (PGA) and underestimate peak ground velocity (PGV) near the fault. The topographic amplification factors (TAFs) of PGA and PGV are found between 0.5 and 2.0, with a correlation coefficient of 0.7 between them. Predictive equations are proposed to correlate TAFs of PGA and PGV with topographic features, which are represented by relative heights obtained at different length scales. Finally, major earthquake damages are summarized with reference to the obtained ground motion intensity map. [ABSTRACT FROM AUTHOR]

Soil supporting foundations behave in a nonlinear manner at loading close to its bearing capacity. This behavior affects soil pressure distribution and the corresponding stresses in the raft foundation, which differs significantly from considering the soil as elastic linear material. The present research uses the finite element method to study the effect of soil nonlinearity on the behavior of raft foundations subjected to concentrated loads. The analysis is based on the Winkler type of foundation. The analysis considered different values of the average soil pressure to its bearing capacity. The supporting soil was modeled as elastic perfectly plastic material. The effects of soil nonlinearity on the intensity and distribution of soil pressure, punching shear force, and bending moments in the raft are investigated. The effects of the foundation stiffness evaluation factor L·λ on the behavior of rectangular raft foundations were studied. This parameter combines the effects of the modulus of subgrade reaction, raft thickness, modulus of elasticity of concrete, and column spacing, as these parameters are interrelated. The results indicated that considering the subgrade soil as a linear elastic material underestimates punching shear and bending moments compared to modeling the soil as an elastoplastic material.

The high-frequency spectral-decay parameter κ has been used in the field of engineering seismology for several decades. The zero-distance κ value (κ 0) is a site parameter in ground motion models and strong motion synthesis. Using surface-borehole pairs of strong motion records from the strong-motion seismograph network KiK-net, we explored the effect of site soil on κ 0 , specifically the contribution from ground motion directionality. In the target area (138°E−143°E, 36°N–40°N), and records from earthquakes at 50 pairs of stations were collected. All horizontal orthogonal records were rotated from 0° to 180° with an increment of 10° to capture the difference between the average κ 0 on two horizontal components and the median value of the rotated κ 0 at surface and borehole stations. The same process was applied to Δκ 0 , the difference between surface station κ 0 and borehole station κ 0 , to measure the contribution by soil column. The nonlinear behavior of soils led to large shear strains, reduced stiffness, and decreased resonance frequency. Thus, the effect of soil nonlinearity on κ is investigated. In the same area, strong ground motion records with peak ground acceleration >100 cm/s2 were selected to identify the soil nonlinearity by the moving time window deconvolution method, and the temporal variation of time-average shear wave velocity and κ at the 50 surface stations were assessed. • The influence from ground motion directionality on κ 0 and Δκ 0 cannot be ignored. • An automated method to measure κ is proposed. • The temporal changes of V S and κ is observed by moving time window deconvolution. [ABSTRACT FROM AUTHOR]

Soft sediment layers can significantly amplify seismic waves from earthquakes. Large dynamic strains can trigger a nonlinear response of shallow soils with low strength, which is characterized by a shift of resonance frequencies, ground motion deamplification, and in some cases, soil liquefaction. We investigate the response of marine sediments during earthquake ground motions recorded along a fiber‐optic cable offshore the Tohoku region, Japan, with distributed acoustic sensing (DAS). We compute AutoCorrelation Functions (ACFs) of the ground motions from 105 earthquakes in different frequency bands. We detect time delays in the ACF waveforms that are converted to relative velocity changes (dv/v). dv/v drops, which characterize soil nonlinearity, are observed during the strongest ground motions and exhibit a large variability along the cable. This study demonstrates that DAS can be used to infer the dynamic properties of the shallow Earth with an unprecedented spatial resolution. Plain Language Summary: Seismic waves from earthquakes are amplified by shallow and soft sediment layers of the Earth. This amplification is linear for weak seismic waves, but can become highly nonlinear during strong ground motions. Nonlinear soil response, which can lead to a complete failure of the ground through soil liquefaction, threatens the safety of human‐made constructions and needs to be accurately characterized. We study the response of marine sediments offshore the Tohoku region in Japan using earthquake data recorded along a fiber‐optic cable with distributed acoustic sensing (DAS). We use an autocorrelation approach to analyze the ground motions from 105 earthquakes recorded by thousands of DAS channels. We detect a clear nonlinear behavior of shallow sediments during the strongest ground motions. Moreover, we show that soil nonlinearity significantly varies along the cable. Our methodology could easily be applied to earthquake DAS data recorded in populated and seismically active regions to help understand better the dynamic behavior of shallow soils. Key Points: AutoCorrelation Functions (ACFs) of earthquakes recorded by a distributed acoustic sensing (DAS) experiment exhibit phase delays with increasing ground motionsACF phase delays are converted to relative velocity drops in the medium that characterize soil nonlinearityDAS is used to infer the nonlinear behavior of soils with an unprecedented spatial resolution [ABSTRACT FROM AUTHOR]

In the course of developing ground-motion models (GMMs) of peak ground acceleration (PGA) for Japanese megathrust earthquakes, we discovered that empirical nonlinear site effects from these events are very different from those found for shallow crustal earthquakes in California and other global active tectonic regions. In particular, nonlinearity in site response is much less prominent in Japan than these other regions, which is consistent with other empirical and theoretical ground-response studies using recordings from both crustal and subduction earthquakes. In this study, we used the residuals from Japanese megathrust GMMs that account for linear site response to develop a nonlinear site-response term that uses the value of PGA on rock as a measure of ground-motion severity and the value of time-averaged shear-wave velocity in the top 30 m of a site (VS30) as a measure of site properties. The resulting residuals are unbiased and trendless over all PGA values of interest. We also investigated whether soil nonlinearity can be modeled using proxies for shear stress and shear strain. We found deviations from expected linearity unrelated to nonlinear site effects at small values of strain—believed to be due to network accelerograph trigger thresholds—and at large earthquake magnitudes, believed to be due to the shape of the source spectrum. [ABSTRACT FROM AUTHOR]

Several sites located between Road No.28 and Akitsu River in downtown Mashiki were liquefied during the mainshock of the 2016 Kumamoto earthquake. According to the building damage survey results, only a few buildings were damaged in areas proximate to the Akitsu River, where liquefaction occurred, however, serious building damage occurred in neighboring regions. Therefore, the effect of soil liquefaction on strong ground motions in Mashiki should be ascertained. Moreover, the distribution of visible and invisible liquefaction is required to be estimated as well. In this study, the distribution of depth of groundwater level in Mashiki was studied, which decreased from 14 to 0 m from northeast to southwest. Thereafter, the nonlinearities of the shallow layers at four borehole drilling sites were identified from the experimental data using the Ramberg–Osgood relationship. Subsequently, the dynamic nonlinear effective stress analysis of the one-dimensional soil column was performed to 592 sites in Mashiki between the seismological bedrock and ground surface to estimate the distribution of strong ground motions during the mainshock. First, the ground motions estimated by the nonlinear analysis corresponded to the ground motions observed at the Kik-net KMMH16. Second, the soil nonlinearity of shallow layers was considerably strong in the entire target area especially in the southern Mashiki, and the PGV distribution was similar to the building damage distribution after the mainshock. Furthermore, the estimated distribution of the soil liquefaction site was similar to the observed results, whereas certain invisible-liquefaction sites were estimated in the north and middle of the target area. [ABSTRACT FROM AUTHOR]

Only a seismic analysis of tunnels at a single-phase site can be performed using the commonly used finite element software owing to the lack of a dynamic two-phase element. The seismic analysis of tunnels at a saturated site is primarily based on OpenSees or self-compiled programmes, the majority of which are limited to uniform or horizontal-layered sites owing to the related issues of the inconvenience of modelling and the limited computation scale. In this study, based on ABAQUS, a finite element model for the earthquake response analysis of twin tunnels at a complex saturated site is established by calling the user-defined element, which is developed for the dynamic analysis of two-phase media. The wave propagation towards infinity is simulated by the viscous-spring boundary, the seismic wave is inputted in the form of the equivalent nodal force, and the soil nonlinearity is considered via the equivalent linear method. Differences between the earthquake responses of tunnels at a complex saturated site and a complex single-phase site as well as those between tunnels at a complex saturated site and a horizontal-layered saturated site are investigated to emphasise the effects of dynamic solid-fluid coupling and complex topography and geology. [ABSTRACT FROM AUTHOR]

Higher train speeds and heavier axle loads trigger elevated stresses and vibrations in the track, potentially increasing track deterioration rates and maintenance costs. Alternative track forms made of combinations of reinforced concrete and asphalt layers have been developed. A thorough understanding of the slab and asphalt tracks is needed to investigate track performance. Thus, analytical and numerical models have been developed and validated by many researchers. This paper reviews numerical models developed to investigate railway track performance. The synthesis of major finite element models is described in detail, highlighting the main components and their outputs. For slab track models, the use of a structural asphalt layer within the railway track remains an active research topic and firm conclusions on its efficacy are not yet available. It can be expected that slab track structures will also be affected by train-induced ground vibrations. There is thus a gap in the literature regarding the measurement of dynamic effects on high-speed railway lines, and further research is needed to investigate the dynamic behaviour of slab–asphalt track systems. In this review, novel solutions for mitigating the vibrations in high-speed rail are discussed and compared. The use of asphalt material in railways appears to have beneficial effects, such as increasing the bearing capacity and stiffness of the structure and improving its dynamic performance and responses, particularly under high-speed train loads. [ABSTRACT FROM AUTHOR]

A computationally efficient semianalytical framework for obtaining the consolidation settlement of flexible foundations such as beams and strip footings resting on nonlinear, saturated, poroelastic, and multilayered continua (soil) is developed. Two-dimensional (2D) plane strain is assumed, and Biot's consolidation theory is used in the analysis. The differential equations governing the displacements and excess pore pressure dissipation of the beam-soil system are developed using the variational principles of mechanics in which the soil is modeled as a simplified continuum and the foundation is modeled as an Euler-Bernoulli beam. The developed differential equations are coupled and are solved following a unique iterative algorithm using one-dimensional (1D) finite-element analysis. The numerical solution is less expensive than conventional 2D numerical methods. A distinct feature of the developed framework is that the effect of soil stress-strain nonlinearity is considered in the calculation of the consolidation settlement, which is usually not considered in Terzaghi's and Biot's consolidation theories. It is observed that soil nonlinearity and foundation flexibility can significantly impact the foundation settlement and consolidation rate. The developed framework provides a fast, easy-to-use, and accurate method for estimating the consolidation settlement of flexible shallow foundations. [ABSTRACT FROM AUTHOR]

This paper presents the numerical analysis of the lateral response of hollow circular caisson embedded in nonlinear soils. The caisson–soil system is modeled by a three-dimensional nonlinear finite-element method. The caisson is considered as a linear elastic body, whereas the nonlinear behavior of the adjoining soil is defined by kinematic hardening-based multiyield surface plasticity model. The interface between the caisson and adjoining soil is modeled by a zero thickness contact element which is defined by the constitutive relationships capable of describing the relative sliding and the separation at the interface. The present study discusses the lateral response of the hollow caisson, which consists of the lateral load versus lateral displacement (P–Y) curves, mechanism of deformation of the caisson–soil system, and deformation profile of the hollow caisson. It also presents the effect of the D/B ratio, relative stiffness of the caisson–soil system, and the effect of the vertical load on the lateral response of the hollow caisson embedded in nonlinear soils. The present study indicates that the D/B ratio, relative stiffness of the caisson–soil system, and the vertical load have a significant effect on the lateral response of the caisson–soil system. This study reveals that there occurs elastoplastic deformation during the lateral loading of the caisson–soil system. Further, a simplified transitional plasticity-based constitutive law is proposed to model the lateral response of a caisson–soil system. The proposed model requires only three parameters, which can be calibrated using the lateral load versus lateral displacement curve obtained from the finite-element analysis or the field test. Furthermore, mathematical relationships between the model parameters and the material parameters defining the caisson–soil system are established using the least-squares regression analysis. [ABSTRACT FROM AUTHOR]

Soil effects are collectively referred to the influences that local geology and morphology of soil deposits have on ground motions coming from bedrock. The quantification of soil effects in building codes has a great impact in the design of structures because soil factors amplify the reference seismic input, which is typically defined for rock‐type soil class from probabilistic seismic hazards analysis. This topic, within earthquake engineering, represents one of the typical interfaces between structural engineering, geotechnical engineering and engineering seismology. In fact, this issue is generally addressed with different approaches depending on the background of the researchers. This article investigates soil effects combining the viewpoints of structural and geotechnical engineers and seismologists. It is shown that the averaging technique adopted for the definition of ground motion predictive models for spectral ordinates does not play a significant role in defining the results of rock‐like soil classes. However, for other soil classes characterized by soil effects, different methods of averaging the spectral ordinates produce significant differences. The main result of this study is the proposal of a new analytical formulation for the quantification of soil amplification factors for both acceleration and displacement response spectra. The proposed formulation is based on a database of real ground motions and simulated accelerograms. The latter have been obtained through stochastic ground response analyses, with the propagation of natural ground motions on rock‐like soil class through randomly generated soil deposits representative of different soil classes of Eurocode 8 and the Italian Building Code. The comparison between real and simulated data revealed the crucial role of soil nonlinearity in the definition of soil effects, which is also in relation with the variation of magnitude, fault distance and the intensity measures expected on outcropping rock. [ABSTRACT FROM AUTHOR]

This study uses recorded ground motions at soil sites over a range of shaking intensities to investigate the effects of soil nonlinearity on the high-frequency spectral decay, as quantified by the parameter κ. Equivalent-linear site response analyses indicate that κ should increase significantly with increasing shear strain and ground motion intensity due to increases in soil damping. However, using more than 2500 motions from 32 sites, this study shows that κ does not vary systematically with the induced shear strain but instead remains at its small-strain value. This observation indicates that high-frequency components of motion are consistent with small-strain damping, rather than the strain-compatible damping used in site response analysis. It is demonstrated that equivalent-linear site response analyses for large strains can be modified to generate surface motions with more realistic high-frequency content by scaling the predicted surface motion to fit the small-strain κ or by employing frequency-dependent soil properties that account for the frequency dependence of the induced strains. [ABSTRACT FROM AUTHOR]

Physical-scaled model testing under 1 g conditions is carried out in obtaining the vertical response of fixed head floating-inclined single piles embedded in dry sand. Practical pile inclinations of 5° and 10° besides a vertical pile (0°) subjected to static and dynamic vertical pile head loadings are considered. To account for the effects of soil nonlinearity as well as the soil–pile interface nonlinearity on the response of piles, a range of low-to-high magnitude of pile head displacements is considered for the static case while a varying amplitude of harmonic accelerations for a wide range of frequencies is considered for the dynamic case. Experimental results are obtained in the form of pile head stiffnesses and strains generated in the pile under both the static and dynamic loadings. Results suggest that the nonlinear behavior of soil as well as the nonlinearity generated at the interface between the soil and the pile as the result of applied loading considerably affect the response of piles. The soil–pile interface nonlinearity that governs the slippage of pile shows a clear influence on the pile head stiffnesses by providing two distinct values of stiffnesses corresponding to the push and the pull directional movement of piles; the two values are significantly different. Axial and bending strains generated in the piles show expected dependency on the amplitude of applied loading; the pile head-level bending strain increases almost linearly with the increase in the angle of pile inclination. [ABSTRACT FROM AUTHOR]

Copyright of Engineering Mechanics / Gongcheng Lixue is the property of Engineering Mechanics Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Strong vertical ground motion has significant influences on the seismic response of engineered structures. However, the soil nonlinearity of vertical ground motion has received less attention compared with that of horizontal ground motion. In this study, seismic observations from KiK-net were used to investigate the soil nonlinearity of vertical ground motion. A new fitting formula was proposed to estimate the global constrained modulus degradation curve considering the influence of groundwater. The proposed formula was verified by the higher goodness of fit compared with a widely used fitting formula and the consistency with the theoretical solution from a former study. The order of the magnitude of the normal strain linear threshold was approximately 10−6 which is the same as that of the shear strain. Furthermore, different from the shear modulus, the constrained modulus of the selected stations decreased quickly when normal strain was between 10−5 and 10−4, and the reduction speed slowed down when it was above 10−4. This result was consistent with the theoretical result based on the laboratory test data from a previous study. Using the fitted relationships between the soil strain and peak ground acceleration (PGA), we found the PGA threshold for vertical nonlinearity could be as low as 30 cm/s2 and was generally higher than that for horizontal nonlinearity. Moreover, the difference in reduction between the shear modulus and constrained modulus after a strong earthquake was investigated which could be related to the change in Poisson's ratio. [ABSTRACT FROM AUTHOR]

A number of vertical array records during eight destructive earthquakes in Japan are utilized, after discussing criteria for desirable requirements of vertical arrays, to formulate seismic amplification between ground surface and outcrop base for seismic zonation. A correlation between peak spectrum amplification and Vs (S-wave velocity) ratio (base Vs/surface Vs) was found to clearly improve by using Vs in an equivalent surface layer wherein predominant frequency or first peak is exerted, though the currently used average Vs in top 30 m is also meaningful, correlating positively with the amplification. We also found that soil nonlinearity during strong earthquakes has only a marginal effect even in soft soil sites on the amplification between surface and outcrop base except for ultimate soil liquefaction failure, while strong nonlinearity clearly appears in the vertical array amplification between surface and downhole base. Its theoretical basis has been explained by a simple study on a two-layered system in terms of radiation damping and strain-dependent equivalent nonlinearity.

The numerical modeling of soil–structure interaction systems for cracked foundations laying on nonlinear clay soils is performed by solving line spring model, using the method of exact series solutions. This semi-analytical approach is well known in fracture mechanics practice for the computation of structural responses near the crack tips, as well as for the calculation of normalized stress intensity factors (SIF). The effects of the crack topologies and soil material nonlinearity are analyzed especially on SIF and biaxiality rates (SBR) tendencies. A parametric study based on the variations of equivalent (SIF) for cracked foundations is discussed to investigate the prediction of failure occurrences for several cases of crack extensions and positions along shallow spread footing span. The numerical finding of this study can be potentially employed to direct engineers and researchers to optimal solutions for revamping design works, as well as in the improvement of mechanical reliability of cracked foundations mainly the ones of strategic industrial parts of petrochemical and nuclear infrastructures. [ABSTRACT FROM AUTHOR]

Abstract For circular tunnels, two widely used analytical solutions are well suitable for seismic design, given that they provide simple formulas for conveniently estimating the internal force and the deformation of the tunnel. The assessment of the applicability of these analytical solutions has been an issue of interest. The companion paper (Part I) provides a 2-D finite element-indirect boundary element (FE-IBE) coupling method for seismic analysis of tunnels in water-saturated, poroelastic half-space with soil nonlinearity. This coupling method has been proved to be accurate and efficient for addressing problems involving dynamic soil-tunnel interaction. This paper (Part II) uses the results obtained by the coupling method as benchmark to evaluate the applicability of two widely used analytical solutions for seismic design of tunnels with soil nonlinearity. The comparison between the analytical solutions and the coupling method is performed within a wide range of flexibility ratios. Besides, different seismic loading intensities as well as tunnel buried depths are also considered. The purpose of this paper is not to conduct validation of the analytical solutions, but rather to investigate the effect of the dynamic soil-tunnel interaction, which the analytical solutions cannot account for, thus to further recognize the limitations of the analytical solutions. Moreover, for tunnels in water-saturated soils, emphasis is on the effect of interaction between solid frame and pore water, which has not been investigated in previous related studies. The result shows that the analytical solutions considerably underestimate the lining internal force and the lining deformation of the tunnel, which may lead to unsafe design of underground tunnels. The difference between the analytical solutions and the coupling method in this paper may exceed 50%, highlighting the significance of more realistic and rigorous simulation of the soil-tunnel interaction, especially for a water-saturated soils scenario. Highlights • The applicability of two widely used analytical solutions to engineering practice. • Different flexibility ratios, seismic loading intensities and tunnel buried depths. • Effect of dynamic soil-tunnel interaction. • Effect of interaction between solid frame and pore water. • Analytical solutions lead to 50% underestimation in seismic response of tunnels. [ABSTRACT FROM AUTHOR]

Abstract Currently, finite element method is the most widely used method for seismic analysis of underground tunnels. However, the finite element method has an apparent difficulty in modelling unbounded soils, therefore, artificial boundaries are often used to solve this problem, but it becomes much more complicated when the unbounded soils are water-saturated, poroelastic media. This paper provides an alternative technique for seismic analysis of tunnels in unbounded water-saturated, poroelastic soils, with excellent accuracy and efficiency. Based on the Biot's theory of wave propagation in a poroelastic medium, this paper proposes a 2-D finite element-indirect boundary element (FE-IBE) coupling method for seismic analysis of tunnels in water-saturated, poroelastic half-space. The soil nonlinearity is dealt with equivalently linear approach, while the tunnel lining remains linear. One particular advantage of the proposed coupling method is that it allows separate computation for the FE subdomain and the IBE subdomain while avoids iterative process, which is well suitable for parallel computation. Another desirable feature of the proposed coupling method is that it can account for nonlinear soil effect for both the FE subdomain (near field) and the IBE subdomain (far field). In the first part of this paper, the FE-IBE coupling method is presented in detail and the accuracy of this coupling method is verified extensively through comparisons with published results. Besides, the capability of the proposed coupling method is further demonstrated by applying it to analyze seismic responses of tunnels in water-saturated, poroelastic half-space, with emphasis on the effects of soil nonlinearity and interaction between twin tunnels. In the second part of this paper, using results obtained by the FE-IBE coupling method as benchmark, the applicability of two widely used analytical solutions to seismic design of tunnels with soil nonlinearity is investigated. Of particular interest is the influence of dynamic soil-tunnel interaction and interaction between solid frame and pore water. Highlights • A 2-D FE-IBE coupling method for linearized nonlinear seismic analysis of tunnels. • Biot's theory for wave propagation in water-saturated, poroelastic half-space. • Equivalently linear approach for soil nonlinearity. • 35% amplification of tunnel lining deformation due to soil nonlinearity. • 50% amplification of lining internal force due to interaction between twin tunnels. [ABSTRACT FROM AUTHOR]

Abstract This paper provides an efficient and rigorous model for 3D seismic analysis of long lined tunnels considering wave passage effects. By taking advantages of the 2.5D model, computation effort is considerably reduced. The water-saturated poroelastic half-space is modelled as two-phase media and soil nonlinearity is considered via the equivalent linear approach. This improved model is realized by a finite element-indirect boundary element (FE-IBE) coupling method. The study reveals that the wave passage effect causes significant longitudinal deformation of the tunnel, and differences in the tunnel seismic responses between the two-phase half-space case and the single-phase half-space case are not negligible. [ABSTRACT FROM AUTHOR]

Evaluating soil nonlinearity during cyclic loading is one of the most significant challenges in ground response analysis, especially when dealing with the inverse problem of deconvolution. Different schemes have already been developed for dynamic ground response analysis, both in the time and the frequency domain. The most accurate method to account for soil nonlinearity is the nonlinear dynamic analysis in the time domain. This approach is based on nonlinear constitutive models capable of accurately simulating highly nonlinear problems like soil liquefaction. However, the time-domain analysis is suitable only for the convolution analysis to define the ground motion at the free surface of a soil deposit from the bedrock motion. The frequency-domain analysis is the most common solution for the inverse problem called deconvolution, which is used to define the bedrock motion from the free surface ground motion. A well-known approach developed in the frequency domain for ground response analysis is the equivalent-linear method (EQL). This approach adopts an iterative procedure to define elastic shear modulus and damping ratio compatible with the induced strain level. Still, it presents some limitations, especially for highly nonlinear soil response, due to the use of strain-compatible but constant soil properties. This article presents a new scheme to conduct truly nonlinear dynamic analysis in the frequency domain based on the new concept of the short-time transfer function. Unlike the EQL method, which uses a constant transfer function, the proposed approach, called the "Equivalent-Nonlinear" method (EQNL), defines a soil transfer function evolving in time, depending on the shear stress and strain demands. The EQNL method approximates the response of a nonlinear system as an incrementally changing viscoelastic system and could represent a valuable tool for nonlinear deconvolution. This article shows the analytical formulation and the first set of validations of the EQNL approach, with detailed comparisons with the EQL and NL methods and vertical array data. These comparisons show the potentialities of the EQNL approach to reproduce the results of the nonlinear dynamic analysis. The EQNL approach has been implemented in MATLAB, and the source code is provided as supplementary material for this article. A more comprehensive validation is underway, aiming to better characterize the limitations and the capabilities of the method. • New method for nonlinear dynamic analysis in the frequency domain. • Nonlinear ground response analysis: convolution and deconvolution. • Proposal of the equivalent-nonlinear method for ground response analysis. • Comparisons between nonlinear, equivalent-linear and equivalent-nonlinear analysis. • Vertical array ground motions used as a benchmark for the analyses. [ABSTRACT FROM AUTHOR]

Summary: Bounding surface plasticity models based on one‐dimensional hardening functions are broadly accepted as a valid approach to represent the multiaxial cyclic behavior of undrained cohesive soils. However, under certain conditions, these models may exhibit deviations from the expected stress path. This makes them inadequate to meet traditional hysteretic rules. Current solutions to this problem impose thresholds to help adjust the stress path by introducing additional memory variables. This article presents a formulation that achieves the same goal without the need of such additional variables. The proposed formulation operates on a generic hardening function under multiaxial loading while preserving the simplicity inherited from pure deviatoric bounding surface models. In addition, the approach presented here allows the implementation of Masing‐type rules, as well as the use of reduction factors to mitigate the overdamping effects of large hysteresis loops. The formulation is tested using well‐known hyperbolic backbone functions under radial and nonradial multiaxial loading cycles, and it is shown to have good agreement with reference solutions. [ABSTRACT FROM AUTHOR]

To define the seismic input in non-liquefiable soils, current seismic standards give the possibility to treat local site effects using a simplified approach. This method is generally based on the introduction of an appropriate number of soil categories with associated soil factors that allow modifying the shape of the elastic acceleration response spectrum computed at rocky (i.e. stiff) sites. Although this approach is highly debated among researchers, it is extensively used in practice due to its easiness. As a matter of fact, for standard projects, this method represents the driving approach for the definition of the seismic input. Nevertheless, recent empirical and numerical studies have risen doubts about the reliability and safety of the simplified approach in view of the tendency of the current soil factors of Italian and European building codes to underestimate the acceleration at the free surface of the soil deposit. On the other hand, for certain soil classes, the current soil factors seem to overestimate ground amplification. Furthermore, the occurrence of soil nonlinearity, whose magnitude is linked to both soil type and level of seismic intensity, highlights the fallacy of using constant soil factors for sites with a different seismic hazard. The objective of this article is to propose a methodology for the definition of hazard-dependent soil factors and simultaneously quantify the reliability of the coefficients specified in the current versions of Eurocode 8 (CEN 2005) and Italian Building Code (NTC8 2008 and revision NTC18 2018). One of the most important outcome of this study is the quantification of the relevance of soil nonlinearity through the definition of empirical relationships between soil factors and peak ground acceleration at outcropping rock sites with flat topological surface (reference condition). [ABSTRACT FROM AUTHOR]

Abstract This paper explores the generation of harmonics of a monochromatic excitation frequency in the stress response of an excited soil deposit. The relationship between the level of nonlinearity in the soil and the generation of harmonics is investigated in terms of a simple hysteretic stress-strain model. Centrifuge tests of soil deposits subjected to base excitation are used to confirm numerical inferences. Effects of harmonics generation were also analyzed in nonlinear and equivalent-linear numerical simulations of an excited soil deposit. It is shown that the harmonics are present in both physical experiments and nonlinear numerical simulations, but cannot be captured by equivalent-linear analyses. Highlights • Generation of frequency harmonics in the stress response of an excited soil deposit. • Centrifuge experiments are used to validate numerical inferences. • Generation of harmonics is explored in numerical simulations. [ABSTRACT FROM AUTHOR]

The prediction of the seismic response of sedimentary basins and its incorporation into seismic building codes is of prime interest to the engineering community, since highly populated areas and infrastructures are often located within such geological configurations. The additional effect of the two-dimensional (2D) basin response above the corresponding one-dimensional (1D) response, expressed as the ratio between 2D and 1D acceleration response spectra at the basin surface and commonly referred to as “aggravation factor”, might be an engineering feasible way to incorporate such effects into building codes. The majority of the studies are based on numerical simulations for modeling the seismic behaviour of complex subsurface geology and thus for the estimation of aggravation factors, usually taking place under the assumption of homogeneous viscoelastic sediments, ignoring the fact that sedimentary deposits may be very heterogeneous in both the horizontal and the vertical directions, as well as the effect of soil nonlinearity, which for strong seismic excitation may be crucial. The present work is an extension of our work [1], in which extensive numerical analyses of the linear viscoelastic response of homogeneous sedimentary basins were performed, in order to explore the potential additional effects of sediments inhomogeneity and nonlinearity on aggravation factors. To this end, a generic relationship between shear wave velocity and depth was developed based on experimental shear wave velocity data from well-documented sites worldwide. To further model the nonlinear behaviour of sediments material, a nonlinear kinematic hardening constitutive model was used, combined with the Von Mises failure criterion and an associated plastic flow rule. All analyses were conducted for trapezoidal sedimentary basins using the generic finite element code ABAQUS. The results of the analyses revealed that the replacement of an inhomogeneous soil with an equivalent homogeneous might lead to underestimation of aggravation in the vicinity of the edges and overestimation of aggravation at the flat part of the basin, while consideration of soil nonlinearity may result in a decrease of aggravation factors for the flat part of the basin. These outcomes may be useful for the proposal of simple recommendations to the engineering community for the introduction of basin effects in the seismic design of structures. [ABSTRACT FROM AUTHOR]

Strong vertical ground motions recorded during earthquakes are critical to structure designs. Similar to horizontal motions, vertical motions can be amplified by the local site condition. However, the amplification behavior of vertical motion is different from that of horizontal motion because of distinct propagation mechanisms. In this study, three component records of five downhole arrays, considering different geological conditions, ground water tables, and intensity of motions, are analyzed to evaluate the differences of wave propagation in the vertical and horizontal directions. The amplification behavior of the two directions is characterized by the transfer function of the surface and downhole measurement. It is found that the location of ground water table highly influences the amplification of vertical motion (e.g., amplitude and nonlinearity) but does not affect the amplification of horizontal motion. These variances should be considered in the dynamic site response analysis. [ABSTRACT FROM AUTHOR]

Scouring causes the exposure of structure foundations, which may influence their seismic performance. In this study, we develop a theoretical model to investigate the influence of foundation exposure caused by scouring on the displacement ductility capacity of fixed-head piles. The proposed theoretical model is based on consideration of a long pile in cohesive soil. The analytical solutions of the displacement ductility capacity and system overstrength ratio of scoured piles in linear soil are derived. The ultimate state of the pile is set as the state in which the pile-head moment reaches its ultimate capacity or when in-ground plastic hinging begins to occur. The proposed solutions are validated through numerical pushover analyses. A pile with a large scour depth is prone to in-ground plastic hinging. If no in-ground plastic hinging occurs, the displacement ductility capacity increases until an upper limit as the scour depth increases; however, the displacement ductility capacity decreases with increasing scour depth when in-ground plastic hinging occurs. The proposed model for linear soil provides a lower-bound estimate of the displacement ductility capacity. Soil nonlinearity can increase a pile's ductility; however, this effect becomes negligible for large scour depths. The solutions for linear soil are modified for nonlinear soil by using an equivalent linear soil model. [ABSTRACT FROM AUTHOR]