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。     

Seismic response analysis of the deep saturated soil deposits in Shanghai

The quaternary deposits in Shanghai are horizontal soil layers of thickness up to about 280 m in the urban area with an annual groundwater table between 0.5 and 0.7 m from the surface. The characteristics of deep saturated deposits may have important influences upon seismic response of the ground in Shanghai. Based on the Biot theory for porous media, the water-saturated soil deposits are modeled as a two-phase porous system consisting of solid and fluid phases, in this paper. A nonlinear constitutive model for predicting the seismic response of the ground is developed to describe the dynamic characters of the deep-saturated soil deposits in Shanghai. Subsequently, the seismic response of a typical site with 280 m deep soil layers, which is subjected to four base excitations (El Centro, Taft, Sunan, and Tangshan earthquakes), is analyzed in terms of an effective stress-based finite element method with the proposed constitutive model. Special emphasis is given to the computed results of accelerations, excess pore-water pressures, and settlements during the seismic excitations. It has been found that the analysis can capture fundamental aspects of the ground response and produce preliminary results for seismic assessment.     

Evaluation of ground motion characteristics,effects of local geology and liquefaction susceptibility: the case of Itea,Corinth Gulf (Greece)

In the present paper we analyze the effect of local geology on ground motion by means of numerical calculations (numerical models) using total (TS) and effective stress (ES) methods. These numerical calculations have been applied to the site of Itea, Corinth Gulf, which was chosen based on liquefaction susceptibility criteria and field inspection. Data from seismic refraction experiments and cone penetration test N-values as well as selected records of ground motion in nearby areas were used to construct the input file for the numerical model. By means of␣dynamic analysis such characteristics of ground motion as acceleration time histories, response spectra, and amplification function were evaluated. A one-dimensional soil amplification effect was clearly shown. Liquefaction probability at the Itea site was predicted based on the safety factor and the calculation of the induced settlement at the test site. Results of the TS and ES modeling lead us to conclude: (1) the presence of soft soil at Itea caused significant amplification (almost 2.5-fold higher magnitude) of the underlying bedrock motion and, therefore, can contribute to damage; (2) the area of Itea is highly susceptible to liquefaction due to presence of silty sand deposits at depths between 2.48 m (the position of the water table) and 12 m that demonstrate the rapid growth of the excess pore water pressure (EPWP) ratio with an increase in peak ground acceleration values.     

基于ANN和GIS耦合模型的场地地震液化势评价系统研究

采用组件式GIS (COMGIS)技术开发了结合BP神经网络分析模型的场地地震液化势评价系统,调用水平成层土地震反应分析程序SHAKE91实现设定地震下地震动影响场的模拟。在VB下调用Matlab神经网络工具箱来完成场地地震液化势评价模型在COMGIS系统中的模块化;利用GIS技术对评价结果,即液化势等级进行空间复合,给出场地潜在的地层液化势空间分布图。研究表明,SHAKE91应用程序在系统菜单下可直接调用,实现地震动影响场计算的模块化;BP神经网络技术应用于场地地震液化势评价中能达到较为理想的效果;系统的GIS空间分析功能可使评价结果与场地信息进行空间匹配,实现目标场地潜在地震液化势的快速评估。     

汶川地震中甘肃清水田川黄土液化的试验研究

2008年5·12特大地震中,位于甘肃省清水县郭川乡的田川村发生了饱和黄土的液化滑移灾害。本文首先在对田川场地进行考察的基础上,综合田川在汶川地震中的震害情况以及滑移区的地形条件,将该地区在汶川地震中的烈度进行了修正。其次对田川黄土进行了物性指标测试以及室内动三轴液化试验,根据试验结果,综合考虑产生液化所需的场地及土性条件、黄土的动强度和液化特性,对田川黄土液化灾害进行了分析,并采用反应分析的方法对其进行了液化判定。研究结果证明了田川黄土液化的事实存在性,为低烈度区黄土液化提供了新的震害依据。

     

地震历史对砂土抗液化性能影响的试验研究

为探讨地震历史对饱和砂土抗液化性能的影响,通过离心机动力模型试验,观测了遭受过不同强度地震的饱和砂土水平场地在再次地震荷载作用下的响应,包括超静孔隙水压力的发展和土体的变形。研究表明,小地震有利于增强砂土颗粒之间的咬合及结构的稳定性,极大地提高了其抗液化能力,而遭受强地震并且发生液化的砂土,由于在沉积过程中形成的不稳定结构,则可能在将来强度相对较小的地震中再次液化。试验结果说明了地震历史对砂土液化性能有重大影响及进一步开展砂土细观结构研究的重要性。     

考虑Gmax附加衰减的场地地震响应离心机试验数值模拟

在常规的场地地震响应有效应力分析法中,通常只考虑Gmax随有效应力和孔隙比的变化,而最新的试验研究表明,Gmax在大应变幅振动下存在附加衰减。收集了不同类型土体在大幅振动下的298组Gmax数据,拟合并给出了对应于两种Gmax附加衰减模式的计算模型,通过编制可考虑Gmax附加衰减影响的一维场地地震响应有效应力分析程序,分别对典型场地和离心机振动台模型场地的地震响应进行分析对比。结果表明：Gmax附加衰减对场地加速度和剪应力响应均有较大影响,将加速场地液化进程和扩大场地最终液化范围;考虑Gmax附加衰减可较好地模拟动力离心模型试验场地动力响应;但在强震情况下,场地迅速液化,Gmax附加衰减对场地动力响应的影响反而减弱。     

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。     

A continuum‐discrete hydromechanical analysis of granular deposit liquefaction

A coupled continuum‐discrete hydromechanical model was employed to analyse the liquefaction of a saturated loose deposit of cohesionless particles when subjected to a dynamic base excitation. The pore fluid flow was idealized using averaged Navier–Stokes equations and the discrete element method was employed to model the solid phase particles. A well established semi‐empirical relationship was utilized to quantify the fluid–particle interactions. The conducted simulations revealed a number of salient micro‐mechanical mechanisms and response patterns associated with the deposit liquefaction. Space and time variation of porosity was a major factor which affected the coupled response of the solid and fluid phases. Pore fluid flow was within Darcy's regime. The predicted response exhibited macroscopic patterns consistent with experimental results and case histories of the liquefaction of granular soil deposits. Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

Physical and Numerical Modeling of Seismic Soil-Structure Interaction in Layered Soils

The structural response of buildings subjected to seismic loads is affected by local site conditions and the interaction between the structure and the supporting soil media. Seismic centrifuge model tests were conducted on two layered clay soil profiles at 80 g field to investigate soil-structure interaction and dynamic response of foundation. Several earthquake-like shaking events were applied to the models using an electro-hydraulic shaking table to simulate linear and nonlinear soil behavior. Results showed that the foundation input motion was significantly amplified in both models, especially for weak earthquake motions. Seismic soil-structure interaction was found to have an important effect on structure response by increasing the amplification of foundation input motion. A 3D finite difference numerical model was also developed to simulate the response of centrifuge model tests and study the parameters that affect the characteristics of earthquake at the base of the structure. The results indicated that the stiffness and stratification of the soil profiles had a significant effect on modifying the foundation input motion.     

Liquefaction hazard mapping at the town of Edessa,Northern Greece

The town of Edessa is located on Northern Greece at a region that is characterized as low seismicity zone due to the fact that few moderate events of M < 6 occurred during the last century. According to the Greek Seismic Code, the expected acceleration having a 10% probability of being exceeded in 50 years is equal to 0.16g. However, an amplification of ground motion is likely to occur due the local geology that is consisted of Holocene fluvio-torrential deposits. The basic aim of this paper is to evaluate the site amplification due to geological conditions and to assess the liquefaction hazard. In order to achieve this, 1-D site response analyses were performed. The data that were employed for the construction of the numerical models have been collected from borings with standard penetrations tests (SPT) that were drilled for construction purposes. Afterward, the liquefaction potential of the subsoil layers was evaluated taking into consideration two seismic scenarios. The first scenario was based on the seismic parameters, earthquake magnitude and PGA, assigned by the Greek Seismic Code. On the second seismic model, we employed the values of acceleration, resulted from the 1-D analyses and the earthquake magnitude as it was defined by the Greek Seismic Code. In order to compile the liquefaction hazard maps, we initially estimated the liquefaction potential index (LPI) of the soil columns using the parameters provided by SPT, for both seismic loadings, and afterward we correlated these values with the proposed classification of the severity of liquefaction-induced deformations. In addition, having computed the value of probability based on the LPI, liquefaction manifestations probability maps were compiled for both scenarios. The result of this study was that liquefaction-induced ground disruptions are likely to occur at the center of the city, among the branches of Voda River, only when the amplified values of acceleration are taken into account to the computation of liquefaction potential.     

A comparative study on linear and nonlinear site response analysis

Site response analysis is usually the first step of any seismic soil-structure study. Geotechnical earthquake engineers and engineering geologist have been trying to find both practical and most appropriate solution techniques for ground response analysis under earthquake loadings. The paper attempts to give a critical overview of the field of site response analysis. In this paper, the influences of nonlinearity on the site response analysis summarized and were evaluated with a numerical example. Site response of a two layered soil deposit with the assumption of linear and rigid base bedrock (or viscoelastic half-space) was analyzed by using linear and nonlinear approaches. The amplification spectrum of the soil column is computed between the top and the bottom of this soil deposit. Nonlinear analysis was compared with the linear method of analysis. Steps involved in ground response analyses to develop site-specific response spectra at a soil site are briefly summarized. Some of the well-known site response analysis methods are summarized and similarities and differences between linear and nonlinear methods are compared by a numerical example.     

Liquefaction analysis and damage evaluation of embankment-type structures

The increasing importance of performance-based earthquake engineering analysis points out the necessity to assess quantitatively the risk of liquefaction of embankment-type structures. In this extreme scenario of soil liquefaction, devastating consequences are observed, e.g., excessive settlements, lateral spreading and slope instability. The present work discusses the global dynamic response and interaction of an earth structure-foundation system, so as to determine quantitatively the collapse mechanism due to foundation’s soil liquefaction. A levee-foundation system is simulated, and the influence of characteristics of input ground motion, as well as of the position of liquefied layer on the liquefaction-induced failure, is evaluated. For the current levee model, its induced damage level (i.e., induced crest settlements) is strongly related to both liquefaction apparition and dissipation of excess pore water pressure on the foundation. The respective role of input ground motion characteristics is a key component for soil liquefaction apparition, as long duration of mainshock can lead to important nonlinearity and extended soil liquefaction. A circular collapse surface is generated inside the liquefied region and extends toward the crest in both sides of the levee. Even so, when the liquefied layer is situated in depth, no significant effect on the levee response is found. This research work provides a reference case study for seismic assessment of embankment-type structures subjected to earthquake and proposes a high-performance computational framework accessible to engineers.     

饱和砂土地层中隧道结构动力离心模型试验

饱和砂土地层中隧道结构可能会因地震地基液化而发生破坏。通过对可液化地层中地铁隧道结构的地震反应进行动力离心模型试验,研究了饱和松砂地基在地震作用下的反应特性、可液化地层中地铁隧道结构的上浮及变形特性和设置截断墙对限制隧道结构上浮的效果等问题。研究结果表明,地基液化引起的隧道衬砌上的附加变形内力以及隧道上浮量主要受地基液化时土水压力的变化影响。截断墙的设置限制了隧道两侧土体向隧道下方流动的趋势,有效减小了隧道结构的上浮量     

Liquefaction susceptibility mapping using geotechnical laboratory tests

The soil liquefaction potential has been evaluated for the Ariana Region because of its important socio-economic interest and its location. Liquefaction susceptibility mapping is carried out using a decisional flow chart for evaluation of earthquake-induced effects, based on available data such as paleoliquefaction, geological, groundwater depth, seismotectonic, sedimentary features and geotechnical parameters in particular laboratory testing like grain size analyses and state parameters. Survey results showed that some of these localities are considered as possible sites to soil liquefaction. Indeed, Quaternary alluvium deposits, paleo beaches and recent deposits that edge the lake and the sebka constitute the most susceptible locations to liquefaction. In the east and the west sides of the studied zone, Quaternary deposits are less susceptible to the liquefaction due to the groundwater level deepening and to the relatively old age of the deposits. Elsewhere sedimentary formations are classified as non-liquefiable as they are heavily compacted and old.     

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。     

饱和砂土地震液化判别的可拓聚类预测方法

基于可拓学的物元模型和聚类分析原理,提出了饱和砂土地震液化判别的可拓聚类方法。选取地震烈度、震中距、砂层埋置深度、地下水位、标贯击数、平均粒径、不均匀系数和动剪应力比等8个影响因素,作为饱和砂土地震液化的评价因子,构建了经典域物元和节域物元。应用物元理论和可拓集合中的关联函数,建立预测模型,通过聚类分析得到饱和砂土地震液化的判别结果。实例研究表明,该模型能客观地反映砂土的液化规律,可拓聚类预测方法应用于饱和砂土地震液化判别是有效可行的。     

可液化场地微型桩地震响应特性研究

可液化场地微型桩的地震响应分析是确保工程安全和优化抗震设计的前提。应用动态离心机试验和三维有效应力数值分析方法,研究了微型单桩桩台的侧向变形和加速度、不同埋深桩身弯矩、可液化场地的加速度及超孔隙水压力等响应特征。首先开展了相对密实度为57%饱和土层、输入波是频率为1 Hz和峰值加速度为1.516 m/s2正弦波的微型桩40 g动态地震响应离心机试验,进而应用基于多重剪切机构塑性模型和液化前缘状态面概念的三维有效应力分析方法,反演了试验结果,并进行了对比分析,结果表明,数值模拟与离心机试验结果吻合,液化场地特性控制着建于其中微型桩的地震响应特征,微型桩桩台的水平变形和残余变形可达78、30 mm,桩身最大弯矩和最大残余弯矩呈现向桩身底部迁移特点,同时表明,基于动态土工离心机试验和数值分析相结合的研究方法,分析可液化场地微型桩地震响应特性是有效可行的,研究结论为可液化场地微型桩的抗震设计提供了可靠的依据和参考。     

砂土地震液化预测的GA_SVM_Adaboost模型

为快速准确地对砂土液化情况作出预测,选取地震烈度、地下水位、覆盖厚度、标贯击数、平均粒径、地貌单元、土质及不均匀系数为主要影响因素,运用相关性分析和因子分析模型对其进行分析和属性约减,采用遗传算法（GA）对支持向量机（SVM）的参数寻优,结合Adaboost迭代算法,建立预测砂土地震液化的GA_SVM_Adaboost模型。选用唐山地震砂土液化现场勘察资料中的329组数据对模型进行训练,利用该模型对剩余68组砂土液化数据进行预测。最后,将预测结果与GA_SVM和SVM模型预测结果进行比较。结果表明,3个模型的平均预测准确率分别为100%、98.04%、89.71%,基于因子分析的GA_SVM_Adaboost模型的预测准确性优于GA_SVM模型和SVM模型,是一种解决砂土地震液化预测问题的有效方法,具有一定的应用参考价值。