Comparative study on the equivalent linear and the fully nonlinear site response analysis approaches

Seismic site effect has been a major issue in the field of earthquake engineering due to the large local amplification of the seismic motion. This paper presents the importance of an appropriate soil behavior model to simulate earthquake site response and gives an overview of the field of site response analysis. Some of the well-known site response analysis methods are discussed. The objective of this paper is to investigate the influences of nonlinearity on the site response analysis by means of a more precise numerical model. In this respect, site responses of four different types of one-layered soil deposit, based on various shear wave velocities with the assumption of linear and rigid base bedrock, were analyzed by using the equivalent linear and fully nonlinear approaches. Nonlinear analyses?? results were compared with those of the linear method, and both of the similarities and differences are discussed. It is concluded that in the case of nonlinearity of soil under strong ground motions, 1-D equivalent linear modeling overestimates the amplification patterns in terms of absolute amplification level, and cannot correctly account for resonant frequencies and hysteric soil behavior. Therefore, more practical and appropriate numerical techniques for ground response analysis should be surveyed.     

天津软土场地对地震动参数的影响

天津市区的场地条件为很厚而松软的第四纪沉积物,覆盖层较厚,地貌属海积—冲积平原,对地震动参数的影响很大。本文以海河东岸的天津嘉里中心工程场地为例,通过场地地震动效应分析,研究场地特征对地震动的影响。结果表明,土层地震反应分析得到的地震动峰值和反应谱与GB50011-2001《建筑抗震设计规范》的规定有明显的差别,抗震设计中的根据场地类别调整地震动的方法不足以完全描述场地地震反应特性,也难以反映天津厚软土场地的实际情况。可以认为,对于天津软土场地,采用经场地土层地震动效应分析所得地震动参数更为合理安全。     

一维土层非线性地震反应分析的解析递推格式法

引入“结构动力学数值分析解析递推格式法”求解软土地基地震反应分析问题。就一维地基非线性地震反应分析问题,将解析递推格式方法与等效线性化方法在各种场地土的计算结果进行了比较,结果表明：(1) 对软土层地震反应分析时,两种方法计算结果差别较大; （2）入射波幅值较强时,两种方法计算结果差别较大;（3）对于中等硬度土层、入射波幅值较弱时,计算结果相似, 并验证了该方法的实用有效性。     

液化自由场地震响应大型振动台模型试验分析

开展了含上部黏土层、饱和砂土层、密实砂土层的可液化自由场地在水平地震动激励下的大型振动台模型试验研究,分析了地震动激励时饱和砂土液化后场地加速度、位移、孔压比时空响应等动力响应。试验结果表明：在小震激励时,场地动力反应较小,加速度反应自下而上不断放大,各深度处孔压比均较小,模型地基整体处于弹性反应阶段;0.3g汶川地震卧龙台地震记录输入时,孔压积累迅速,可液化土体最上部土层孔压比达到1,饱和土体液化,模型地基表现出明显的非线性反应特征,加速度反应在饱和砂土层中未有明显放大,土体卓越周期对应的反应加速度自下而上有不断增大趋势。该研究是土-群桩-上部结构体系大型振动台系列试验中可液化自由场动力反应部分,可供今后做对比分析和验证数值模拟参考。     

Nonlinear Analysis of Local Site Effects on Seismic Ground Response in the Bam Earthquake

The influence of local geologic and soil conditions on the intensity of ground shaking is addressed in this study. The amplification of the ground motion due to local site effects resulted in severe damage to dwellings in the Bam area during the 2003 Bam Earthquake. A unique set of strong motion acceleration recordings was obtained at the Bam accelerograph station. Although the highest peak ground acceleration recorded was the vertical component (nearly 1 g), the longitudinal component (fault-parallel motion) clearly had the largest maximum velocity as well as maximum ground displacement. Subsurface geotechnical and geophysical (down-hole) data in two different sites have been obtained and used to estimate the local site condition on earthquake ground motion in the area. The ground response analyses have been conducted considering the nonlinear behavior of the soil deposits using both equivalent linear and nonlinear approaches. The fully nonlinear method embodied in FLAC was used to evaluate the nonlinear soil properties on earthquake wave propagation through the soil layer, and compare with the response from the equivalent linear approach. It is shown that thick alluvium deposits amplified the ground motion and resulted in significant damage in residential buildings in the earthquake stricken region. The comparison of results indicated similar response spectra of the motions for both equivalent and nonlinear analyses, showing peaks in the period range of 0.3–1.5 s. However, the amplification levels of nonlinear analysis were less than the equivalent linear method especially in long periods. The observed response spectra are shown to be above the NEHRP building code design requirements, especially at high frequencies.     

厚软覆盖层场地地震反应分析方法研究

遭遇地震动作用时,厚软覆盖层场地由于其剪切模量低,极易产生较大的剪应变,采用等效线性化方法进行土层反应分析计算得到的响应高频成分被低估,峰值加速度偏低,对于工程抗震设防偏于不安全。本文给出了基于移动平均线和Parzen谱窗的频率相关等效线性化方法,并和有实际强震记录的厚软场地进行了对比分析,结果表明本文方法比等效线性化方法能更有效地模拟地震波在厚软覆盖层场地中的传播规律。最后采用本文方法和等效线性化方法对天津滨海新区某一典型厚软场地进行了地震反应计算,并对计算结果进行了对比分析。     

Evaluation of a nonlinear seismic geotechnical site response analysis method subjected to earthquake vibrations (case study: Kerman Province, Iran)

Equivalent static load and dynamic analyses methods are usually used for designing structures under and subjected to earthquake excitations. Estimation of site response from an earthquake is fundamental step to anticipate the possible damages and then to try to mitigate them. In this paper, the effect of nonlinearity on site response analyses summarized and evaluating ground surface response taking into account the local soil and subsurface soils properties for the proposed bridge over the river at Sirdjan Boulevard road subjected to earthquake vibration and provokes with assumption of rigid (viscoelastic) and elastic half space bedrock and quantify the site effect on the surface over a number of geotechnical areas has been notified. First, by field investigation, the required data were collected and by primary processing the acceptable data were selected. Then, in nonlinear analysis, for elastic and rigid half space bedrock, standard hyperbolic model was selected and executed, and then the results were compared to each other. The critical point of this work was to develop and use a computer code by the authors, named the “Abbas Converter”, with several advantages, such as work and quick installation, operating as a connecter function between the used softwares and generating the input data corresponding to defined format for them. Its output results can easily be exported to the other used softwares in this study. This code can make and render this study more easily than the previous softwares have done, and take over the encountered problem. This study clearly showed the applicability of the “Abbas Converter” for evaluation of site response with bedrock-type assumption on soil behavior under the earthquake excitations. The proposed scheme is used to analyze the ground motion data from the Bam earthquake in Kerman Province, Iran (2003, M_w 6.5).     

巨厚沉积土夹火山岩场地非线性地震反应特性

地震基岩深度和土体动力本构模型的选取对核岛场地地震效应评价结果的合理性具有重要影响。以拟建某沿海核电厂深度470 m沉积土夹火山岩层场地的3个钻孔剖面为研究对象,采用一维等效线性波传分析（ELA）法、基于Matasovic本构模型和Davidenkov-Chen-Zhao（DCZ）本构模型的一维非线性分析（NLA）法,选取不同剪切波速的5个岩土层作为地震基岩,研究了输入地震动特性、地震基岩深度和土体动力本构模型的选取对巨厚沉积土夹火山岩层场地非线性地震反应特性的影响。结果表明：（1）以浅层硬岩夹层或深部土层作为地震基岩,NLA法计算的5%阻尼比的地表谱加速度SA的短周期部分较之ELA法的计算值大,但两者计算的地表SA谱的长周期部分几乎一致;（2）基于Matasovic模型和DCZ模型的NLA法计算的地表SA谱谱形和峰值加速度随深度的变化趋势基本一致;（3）从NLA法计算的地表峰值加速度和累积绝对速度而言,以剪切波速约2 500 m/s的浅层硬岩夹层作为地震基岩是适宜的。     

刚度和阻尼频率相关的等效线性化方法

在实际工程中发现,在强地震动作用下等效线性化方法低估了地表地震动的峰值加速度反应。通过与场地效应竖向台阵观测记录的对比分析证实了这一现象,其主要原因是等效剪应变水平的取值高于高频段实际剪应变水平,从而抑制了土层地震反应中的高频成份。若干分析表明,动力反应过程中土体的剪应变是随频率而变化的,且试验表明土体的刚度和阻尼是与剪应变相关的,据此假设土体刚度和阻尼是与频率相关的。基于这一假设,对传统等效剪应变的选取如何使得土层对基底地震动输入中的高频成分产生滤波效应的原理进行探讨,且通过频率相关的等效线性化方法对该原理进行证明。将该方法应用于Port Island场地效应竖向台阵的分析,并与实际地震动记录进行了对比,结果表明,较之传统等效线性化方法,能更加合理地考虑地震动输入中的高频成分。     

Assessment of soil liquefaction potential based on numerical method- a case study of an urban railway project

Paying special attention to geotechnical hazards such as liquefaction in huge civil projects like urban railways especially in susceptible regions to liquefaction is of great importance. A number of approaches to evaluate the potential for initiation of liquefaction, such as Seed and Idriss simplified method have been developed over the years. Although simplified methods are available in calculating the liquefaction potential of a soil deposit and shear stresses induced at any point in the ground due to earthquake loading, these methods cannot be applied to all earthquakes with the same accuracy, also they lack the potential to predict the pore pressure developed in the soil. Therefore, it is necessary to carry out a ground response analysis to obtain pore pressures and shear stresses in the soil due to earthquake loading. Using soil historical, geological and compositional criteria, a zone of the corridor of Tabriz urban railway line 2 susceptible to liquefaction was recognized. Then, using numerical analysis and cyclic stress method using QUAKE/W finite element code, soil liquefaction potential in susceptible zone was evaluated based on design earthquake.     

Estimating inelastic sediment deformation from local site response simulations

Significant insight into the dynamic local site response of a horizontally layered sediment deposit to seismic excitation can be gained from numerical simulations. In this paper we use a nonlinear local site response analysis code SPECTRA to estimate the coseismic sediment deformation at a seismically active site in Lotung, Taiwan. We address some basic issues relevant for interpreting the simulation results, including the impact of noise and baseline offsets present in the input ground motion. We also consider the sensitivity of the predicted deformation responses to statistical variations of sediment constitutive properties. Finally, we apply a suite of hypothetical strong ground motions to the base of the sediment deposit to better understand the pattern of inelastic deformation likely to result from strong seismic shaking.     

Numerical and analytical investigation of compressional wave propagation in saturated soils

In geotechnical earthquake engineering, wave propagation plays a fundamental role in engineering applications related to the dynamic response of geotechnical structures and to site response analysis. However, current engineering practice is primarily concentrated on the investigation of shear wave propagation and the corresponding site response only to the horizontal components of the ground motion. Due to the repeated recent observations of strong vertical ground motions and compressional damage of engineering structures, there is an increasing need to carry out a comprehensive investigation of vertical site response and the associated compressional wave propagation, particularly when performing the seismic design for critical structures (e.g. nuclear power plants and high dams). Therefore, in this paper, the compressional wave propagation mechanism in saturated soils is investigated by employing hydro-mechanically (HM) coupled analytical and numerical methods. A HM analytical solution for compressional wave propagation is first studied based on Biot’s theory, which shows the existence of two types of compressional waves (fast and slow waves) and indicates that their characteristics (i.e. wave dispersion and attenuation) are highly dependent on some key geotechnical and seismic parameters (i.e. the permeability, soil stiffness and loading frequency). The subsequent HM Finite Element (FE) study reproduces the duality of compressional waves and identifies the dominant permeability ranges for the existence of the two waves. In particular the existence of the slow compression wave is observed for a range of permeability and loading frequency that is relevant for geotechnical earthquake engineering applications. In order to account for the effects of soil permeability on compressional dynamic soil behaviour and soil properties (i.e. P-wave velocities and damping ratios), the coupled consolidation analysis is therefore recommended as the only tool capable of accurately simulating the dynamic response of geotechnical structures to vertical ground motion at intermediate transient states between undrained and drained conditions.     

Analysis of pile behaviour in liquefying sloping ground

In this paper, a numerical procedure based on the finite element method is outlined to investigate pile behaviour in sloping ground, which involves two main steps. First a free-field ground response analysis is carried out using an effective stress based stress path model to obtain the ground displacements, and the degraded soil stiffness and strength over the depth of the soil deposit. Next a dynamic analysis is carried out for the pile. The interaction coefficients and ultimate lateral pressure of soil at the pile–soil interface are calculated using degraded soil stiffness and strength due to build-up of pore pressures, and the soil in the far field is represented by the displacements calculated from the free-field ground response analysis. Pore pressure generation and liquefaction strength of the soil predicted by the stress path model used in the free-field ground response analysis are compared with a series of simple shear tests performed on loose sand with and without an initial static shear stress simulating sloping and level ground conditions, respectively. Also the numerical procedure utilised for the analysis of pile behaviour has been verified using centrifuge data, where soil liquefaction has been observed in laterally spreading sloping ground. It is demonstrated that the new method gives good estimate of pile behaviour, despite its relative simplicity.     

Site-specific earthquake response study for hazard assessment in Kolkata city,India

The assessment of local site effects on seismic ground motions is of great importance in earthquake engineering practice. Several destructive earthquakes in the past have demonstrated that the amplification of ground motion and associated damage to structures due to local site conditions is a significant consideration in earthquake hazard analysis. A recent paper published in this journal highlights the hazard posed by earthquakes in the megacity of Kolkata in India due to its seismic and geological settings. The seismic hazard assessment study speculates that the deep alluvial deposit in the city may increase the seismic hazard probably due to the amplification of the seismic energies. This paper focuses on the seismic response studies of the various soil strata (i.e. for local subsurface conditions) obtained from various construction sites in the city for predicted earthquake. It is very well recognized that site response studies (a part of seismic microhazard zonation for urban areas) are the first step towards performance-based foundation design or seismic risk analysis and mitigation strategy. One of the problems for carrying out site-specific study in Kolkata is the lack of recorded strong motion data in the city. Hence, this paper outlines a methodology to carry out site-specific study, where no strong motion data or seismic data are available. The methodology uses wavelet-based spectrum compatibility approach to generate synthetic earthquake motions and equivalent linear method for seismic site response analysis. The Mega City of Kolkata has been considered to explain the methodology. Seismic hazard zonation map by the Bureau of Indian Standards classifies the City of Kolkata as moderate seismic zone (Zone III) with a zone factor 0.16. On the other hand, GSHAP(Global Seismic Hazard Assessment Program) map which is based on 10% probability of exceedance in 50 years specifies a maximum peak ground acceleration (PGA) of 1.6 m/s² (0.163 g) for this region. In the present study, the seismic response has been carried out based on GSHAP. The results of the analysis indicate the amplification of ground motion in the range of 4.46–4.82 with the fundamental period ranging from 0.81 to 1.17 s. Furthermore, the maximum spectral accelerations vary in the range of 0.78–0.95 g.     

Seismic demands and analysis of site effects in the Marmara region during the 1999 Kocaeli earthquake

The characteristics of the strong ground motion accelerograms from the 1999 Kocaeli earthquake are investigated in detail in this study. The emphasis is on the comparison of the response spectra for the fault normal (FN) and fault parallel (FP) components of the ground motions. The results show that the near-fault records with directivity effects characterize themselves with increased base shear demands rather than increased displacement demands for both the FN and FP components and a narrower velocity sensitive region for the FN component. This study also shows that the effectiveness of base isolation may vary from site to site and for a given site, from component to component. The site effects in the Marmara region during the 1999 Kocaeli earthquake are examined. Site amplifications are predicted by the classical spectral ratio (CSR) and the receiver function (RF) methods. The CSR method gives higher estimates for the site amplifications compared to the RF method and is in better conformity with the observed damage during the Kocaeli earthquake. The districts of Istanbul that are especially susceptible to site amplification hazard are determined. It is apparent from the results that the site amplification hazard risk is the highest for Avcilar and Bakirkoy districts. This study also shows that for sites which have the risk of soil amplification for long-period structures, liquefaction may not be beneficial as a natural base isolator, and may result in shifting the eigenperiod of the low- and mid-rise structures to the critical periods with high site amplifications. This may be especially the case for Avcilar and Bakirkoy districts. In Fatih, Bakirkoy, and Cekmece districts, the predominant period of the ground motion is calculated to be very close to the eigenperiods of the typical residential buildings. Therefore, these three districts are expected to experience heavier damages in future earthquakes due to resonance effects.     

场地条件对地震动和震害影响的研究进展与建议

场地条件对地震动和震害有重要影响。本文简要回顾了场地条件对地震动和震害影响的研究历史,总结和评述了这一领域的研究进展。重点对地形地貌、岩土类型、覆盖层厚度、土层结构、地下水、岩土动力性质及物理地质现象等对地震动和震害的影响进行了总结和评述。提炼和概括了目前场地条件对地震动和震害影响研究中比较一致的几点认识。在此基础上,提出了这一领域当前应进一步研究的问题,主要包括地震现场资料调查、局部地形几何尺度、场地分类、覆盖层安全厚度界限、近断层地震动参数分布规律、活断层避让距离,以及强震资料积累等关键问题。     

Ground motion amplification due to shallow cavities in nonlinear soils

Subsurface cavities or stiff inclusions represent mechanical discontinuities for seismic waves propagating in soils. They modify the propagation pattern of seismic waves and alter soil response in correspondence to the ground level or building foundations. In the literature, different analytical and numerical solutions have been proposed to account for the effect of underground cavities or inclusions on the motion generated by P, S or R waves. In these former studies, the subsoil was assimilated to a homogeneous, isotropic and linear elastic halfspace containing one or more cavities. In the present study, the effect of subsurface cavities on ground motion amplification has been analysed accounting for soil stiffness degradation and associated damping increase with increasing levels of shear strains, a fundamental aspect of soil behaviour under earthquakes. The analysed model was inspired to a real case represented by the village of Castelnuovo (Italy), which during the 2009 Abruzzo earthquake suffered huge damage. The main shock (6 April 2009) caused the collapse of 50 % of the whole built environment. The historical centre of Castelnuovo rises on a hill. In its subsoil, there are many cavities with roofs 2–3 m below the ground level. The longitudinal NW–SE section of the hill has been investigated by 2D nonlinear site response analyses. A preliminary site response analysis was performed without modelling cavities, to identify ground motion amplification due to mere stratigraphic and topographic factors. The numerical model was later refined inserting: (1) a single cavity below the hilltop, (2) multiple cavities placed below the ground surface of the hill and (3) multiple cavities filled with concrete (inclusions). The performed study highlights the important role exerted by underground cavities on the ground motion computed at the hill surface. This effect should be properly considered for both microzonation studies and the correct determination of the seismic actions on specific buildings.     

远场大地震作用下大尺度深软场地的非线性地震效应分析

基于ABAQUS软件自行研制的并行计算显式算法集群平台,针对苏州城区典型地层剖面,建立了大尺度深软场地的二维精细化非线性有限元分析模型,对人工地震波和大地震远场地震动作用下深软场地的非线性地震效应进行了比较研究。研究结果表明：（1）与人工地震波作用时深软场地的地表峰值加速度放大效应相比,大地震远场地震波作用时的放大效应尤为显著,由于土介质的横向不均匀性及其非线性,使不同地表的峰值加速度放大效应存在明显的变异性。（2）深软场地对周期小于0.3 s的高频地震波均具有显著的滤波效应;大地震远场地震波作用时,深软场地对周期0.85～1.65 s的长周期地震波的放大效应非常显著,但对2.5～7.0 s的长周期地震波呈现出明显的滤波效应。（3）地震动峰值加速度PGA值沿土层深度和横向的分布形态呈现出明显的高低起伏现象,在不同成因的土层更迭面附近及土介质横向不均匀性显著的区域,地震波的局部聚焦放大和过滤减小现象尤为明显,且大地震远场地震动作用时,20 m以浅土层的PGA值呈现出非常显著的放大效应。（4）地震波的频谱特性、土层的横向不均匀性对深软场地地表加速度反应谱? 谱的谱形有显著影响;给出了描述加速度反应谱沿土层深度变化特征的三维谱形曲线,可以直观地展示出深软场地中细长地下结构地震反应可能存在类共振现象的土层深度。     

Development of an inverse analysis framework for extracting dynamic soil behavior and pore pressure response from downhole array measurements

Observations from earthquakes over the past several decades have highlighted the importance of local site conditions on propagated ground motions. Downhole arrays are deployed to measure motions at the ground surface and within the soil profile, and also to record the pore pressure response within the soft soil profiles during excitation. The degradation of soil stiffness as excess pore pressures are generated during earthquake events has also been observed. An inverse analysis framework is developed and demonstrated to directly extract soil material behavior including pore water pressure (PWP) generation from downhole array measurements that can then be readily used in 1D nonlinear site response analysis. The self‐learning simulations (SelfSim) inverse analysis framework, previously developed for total stress site response analysis, is extended to extract PWP generation behavior of the soil in addition to cyclic response during ground shaking. A Neural Network based constitutive model is introduced to represent PWP generation during cyclic loading. A new analysis scheme is introduced that can use data from co‐located piezometer and accelerometer sensors. The successful performance of the proposed framework is demonstrated using four synthetic vertical array recordings. Copyright © 2013 John Wiley & Sons, Ltd.     

土层非线性地震反应一维时域分析

等效线性化方法在工程中经常被用于土层非线性地震反应一维数值分析,但考虑大震作用将较大峰值加速度时程作为输入时,该方法会引起过大的“共振”效应,造成对地表地震反应的误估。为了更合理的估计非线性地震反应,提出了一种考虑拟合阻尼比、刚度比试验曲线的时域分析方法,并用于响嘡台阵场地土层地震一维非线性反应分析,通过数值模拟结果与实际记录的比较,验证了该方法的可行性。最后,通过数值模拟证明该方法对大震情况下对地表地震反应的估计比等效线性化方法更为合理。