Results and analysis of soil-structure interaction experiments performed in the centrifuge

In this paper a centrifuge model that is capable of realistically representing soil-structure systems subjected to earthquake-like excitation is used to create a data pool which demonstrates the influence of (i) the frequencies of the structure, (ii) the foundation embedment and (iii) the foundation shape on radiation damping and soil-structure interaction effects for a structure on a semi-infinite soil layer over bedrock. The centrifuge model used in this study was developed and validated by the authors in an earlier publication,¹ and employs an internal method of earthquake simulation, and the clay-like material, Duxseal, to absorb wave reflections at the boundary of the soil sample. The results of the experimental study are used to compute damping and stiffness values of a two-degree-of-freedom piecewise-linear numerical model of the soil-structure systems. The experimental parameter values are then compared to the values computed using classical text book formulae. The analysis demonstrates the value of the experimental data in validating and developing soil-structure interaction theory, and confirms the accuracy of classical text book formulae in the linear range.     

Seismic response of underground utility tunnels: shaking table testing and FEM analysis

Underground utility tunnels are widely used in urban areas throughout the world for lifeline networks due to their easy maintenance and environmental protection capabilities. However, knowledge about their seismic performance is still quite limited and seismic design procedures are not included in current design codes. This paper describes a series of shaking table tests the authors performed on a scaled utility tunnel model to explore its performance under earthquake excitation. Details of the experimental setup are first presented focusing on aspects such as the design of the soil container, scaled structural model, sensor array arrangement and test procedure. The main observations from the test program, including structural response, soil response, soil-structure interaction and earth pressure, are summarized and discussed. Further, a finite element model (FEM) of the test utility tunnel is established where the nonlinear soil properties are modeled by the Drucker-Prager constitutive model; the master-slave surface mechanism is employed to simulate the soil-structure dynamic interaction; and the confining effect of the laminar shear box to soil is considered by proper boundary modeling. The results from the numerical model are compared with experiment measurements in terms of displacement, acceleration and amplification factor of the structural model and the soil. The comparison shows that the numerical results match the experimental measurements quite well. The validated numerical model can be adopted for further analysis.     

Simulation of soil–foundation–structure interaction of Hualien large-scale seismic test using dynamic centrifuge test

Understanding the soil–structure interaction (SSI) mechanism is crucial in the seismic design of nuclear power plant (NPP) containment systems. Although the numerical analysis method is generally used in seismic design, there is a need for experimental verification for the reliable estimation of SSI behavior. In this study a dynamic centrifuge test was performed to simulate the SSI behavior of a Hualien large-scale seismic test (LSST) during the Chi-Chi earthquake. To simulate the soil profile and dynamic soil properties of the Hualien site, a series of resonant column (RC) tests was performed to determine the model soil preparation conditions, such as the compaction density and the ratio of soil–gravel contents. The variations in the shear wave velocity (V_S) profiles of the sand, gravel, and backfill layers in the model were estimated using the RC test results. During the centrifuge test, the V_S profiles of the model were evaluated using in-flight bender element tests and compared with the in-situ V_S profile at Hualien. The containment building model was modeled using aluminum and the proper scaling laws. A series of dynamic centrifuge tests was performed with a 1/50 scale model using the base motion recorded during the Chi-Chi-earthquake. In the soil layer and foundation level, the centrifuge test results were similar to the LSST data in both the time and frequency domains, but there were differences in the structure owing to the complex structural response as well as the material damping difference between the concrete in the prototype and aluminum in the model. In addition, as the input base motion amplitude was increased to a maximum value of 0.4g (prototype scale), the responses of the soil and containment model were measured. This study shows the potential of utilizing dynamic centrifuge tests as an experimental modeling tool for site specific SSI analyses of soil–foundation–NPP containment system.     

Centrifuge modeling of seismic response of layered soft clay

Centrifuge modeling is a valuable tool used to study the response of geotechnical structures to infrequent or extreme events such as earthquakes. A series of centrifuge model tests was conducted at 80g using an electro-hydraulic earthquake simulator mounted on the C-CORE geotechnical centrifuge to study the dynamic response of soft soils and seismic soil–structure interaction (SSI). The acceleration records at different locations within the soil bed and at its surface along with the settlement records at the surface were used to analyze the soft soil seismic response. In addition, the records of acceleration at the surface of a foundation model partially embedded in the soil were used to investigate the seismic SSI. Centrifuge data was used to evaluate the variation of shear modulus and damping ratio with shear strain amplitude and confining pressure, and to assess their effects on site response. Site response analysis using the measured shear wave velocity, estimated modulus reduction and damping ratio as input parameters produced good agreement with the measured site response. A spectral analysis of the results showed that the stiffness of the soil deposits had a significant effect on the characteristics of the input motions and the overall behavior of the structure. The peak surface acceleration measured in the centrifuge was significantly amplified, especially for low amplitude base acceleration. The amplification of the earthquake shaking as well as the frequency of the response spectra decreased with increasing earthquake intensity. The results clearly demonstrate that the layering system has to be considered, and not just the average shear wave velocity, when evaluating the local site effects.     

Shear wave velocity measurements and soil–pile system identifications in dynamic centrifuge tests

This paper presents a pre-shaking technique for measuring the $V_{s}$ profile of sand deposits and determining the natural frequencies of the sand bed and soil-structure system in a centrifuge model at an acceleration of 80 g. The pre-shaking technique is a non-destructive test. It uses a shaker as a wave generation source and a vertical array of accelerometers embedded in the sand bed and the accelerometers attached to the pile head as receivers. The pre-shaking method can be easily used for in-flight subsurface exploration ( $V_{s}$ profile measurements) and in-flight system identification of soil-structure systems (natural frequency measurements). A soil–pile centrifuge model is used to demonstrate the versatility of pre-shaking during a routine centrifuge shaking table test. This paper discusses the testing setup, testing procedures, related SI techniques, and signal processing for the soil–pile system. The natural frequencies measured by the pre-shaking tests are consistent with theory-based results. This technique can be conducted at any time before and after major earthquake events occur in a test.     

考虑土与结构相互作用效应的结构减震控制大型振动台模型试验研究

本文设计并完成了考虑土与结构相互作用的结构减震控制大型振动台模型试验。通过对四种结构形式的对比试验，探讨了土与结构相互作用（ＳＳＩ）效应对结构地震反应的影响以及调谐质量阻尼器（ＴＭＤ）在刚性和柔性地基条件下对主体结构的减震效应。通过比较同一地震动作用下主体结构在刚性和柔性两种地基条件下的地震反应，可知：ＳＳＩ效应具有降低和提高结构减震控制效果的双重作用，其综合效果与输入地震动的频谱特性、加速度峰值大小有关。由于ＳＳＩ效应在结构地震反应中发挥着双重的作用，因而使得基于刚性地基假定下设计的ＴＭＤ减震控制系统在柔性地基条件下的控制效果不太理想，甚至会出现负面效应。本文还探讨了在柔性地基条件下影响结构减震控制效果的一些因素。     

瑞利阻尼对砂土场地非线性地震响应的影响分析

在地震荷载作用下,自由场地会产生土体侧向变形和地表响应放大现象。由于土体的高度非线性,计算自由场地地震响应时,不同的阻尼比及剪切模量取值是造成其计算结果与试验结果相差较大的原因之一。目前动力计算常采用瑞利阻尼方法,其系数取值会在一定程度上影响计算结果。选用两模态简化瑞利阻尼系数计算方法,分析土体阻尼比及控制频率的取值对计算结果的影响,对比离心机模型试验,利用开源有限元平台OpenSees,采用适合于土体动力分析的多屈服面本构模型(PDMY),建立剪切梁模型模拟三维自由场地,并分析瑞利阻尼参数对自由场地地震响应和侧向变形计算结果的影响。结果表明,针对相对密度为60%的Nevada干砂,阻尼比为4%、控制频率比为5时,场地响应计算结果与试验结果较为符合。综合分析显示场地非线性响应时域计算时,应特别注意选用的瑞利阻尼参数值。     

液化地基自由场振动台模型试验研究

进行了液化场地自由场振动台试验,试验采用柔性容器以减小边界影响,采用上覆黏土层的饱和砂土作为模型土。试验中再现了液化场地土的震害现象。得出的主要规律有:随着振次的增加,地基的频率迅速降低,阻尼比迅速增大;砂土对地震动起滤波作用;土体的加速度峰值反应在高度上呈"K"形分布;当最初加速度峰值到达前,砂土层中的孔压比存在负值;震后土中振动孔隙水压力不一定随振动的停止而立即开始消散,在短期内可能继续增长。     

考虑水-饱和土场地-结构耦合时的沉管隧道地震反应分析

用理想流体介质模拟水层、流体饱和多孔介质模拟饱和土地层,在理想流体介质与流体饱和多孔介质相连接边界的连续条件基础上,结合已有的对理想流体介质、流体饱和多孔介质进行动力反应分析的显式有限元方法,开考虑地层与结构的动力相互作用,建立了进行水与场地、结构耦合动力分析的方法。利用该方法对沉管隧道的地震响应进行了研究,重点分析了水深、地质条件等因素对沉管隧道地震反应的影响,并从中得出了一些可供相关人员进行沉管隧道抗震分析时参考的结论。     

Seismic behaviors of earth-core and concrete-faced rock-fill dams by dynamic centrifuge tests

The investigation on the seismic behavior of dams becomes crucial but is limited to lack of experimental or field data. This paper aims to experimentally simulate two major dam types of earth-core rock-fill dam and concrete-faced rock-fill dam by dynamic centrifuge tests to investigate the seismic response of the dam. A series of staged centrifuge tests was performed by applying real earthquake records from 0.05 to 0.5g. The distributions of amplification ratio differed depending on the magnitude of earthquake loading and the zoning condition. The amplification ratio at the crest increased in the bedrock acceleration that exceeds 0.3g and strongly influenced by the loosening behavior of the upper part. The residual settlements and horizontal displacement at the dam crest were small. Shallow surface sliding was dominant failure. The maximum tensile stress on the face slab by dynamic loading occurred at a height of around 4/5 near the upstream water level. Finally, two-dimensional numerical simulations were performed in an effort to simulate the centrifuge models. The centrifuge tests and numerical analysis obtained mostly comparable results, thus confirming that centrifuge modeling reasonably simulates the seismic behavior of dams.     

基于土-结构体系对建筑结构响应的减震效果评估

提出一种基于土-结构体系地震记录的土-结构相互作用(SSI)的减震评估方法。该方法采用简化的SSI模型,通过系统辨识确定模型参数。将上部建筑结构地震反应的SSI减震效应分解为惯性相互作用和运动相互作用,同时还提出由惯性相互作用和运动相互作用单独降低结构响应的方法。将2011年东北地震太平洋沿岸期间两栋中层建筑用此方法进行分析,结果表明:当建筑物结构响应进入非弹性范围时,惯性相互作用的减震效果降低。     

An identification procedure of soil profile characteristics from two free field acclerometer records

In this paper, an analytical, numerical and experimental approach for identifying soil profile characteristics by using system identification and free field records, is presented. First, a theoretical soil amplification function for two sites is defined and expressed in terms of the different parameters of the layers constituting the soil profiles (thickness, damping ratio, shear wave velocity and unit weight). Then, this function is smoothed with an analogous function obtained from experimental data by using the least squares minimization technique. The identification of the parameters is performed by solving, numerically, a non-linear optimisation problem. To demonstrate the numerical efficiency and the validity of this approach, two examples are treated. The first one consists in the identification of characteristics of a given uniform soil layer. The second example consists in the experimental validation of this approach with the data recorded within the Garner Valley Down Hole Array (GVDA). Finally, this approach is applied to identify, simultaneously, soil profile characteristics of sites from only a single soil acceleration record at free surface of each site. This procedure is utilised to identify soil profile characteristics of sites by using strong ground motions data recorded during the recent Boumerdes earthquake of May 21, 2003.     

Soil-structure interaction analysis of the Hualien containment model

This paper presents results from forced vibration tests, microtremor observations and earthquake response analysis of a nuclear reactor containment model constructed on stiff soil in Hualien, Taiwan. The dynamic behavior of the soil-structure system is simulated successfully with two numerical models: a sway-rocking model, whose soil parameters are evaluated on the basis of the continuum formulation method, and a finite element model, using the program SASSI with the flexible volume substructuring approach. The dependences of the soil parameters of both models on the amplitudes of the different dynamic excitations are investigated in detail. An original numerical simulation of microtremor is performed. Comparison with results of a previous study involving a rigid tower on a soft soil site in Chiba, Japan is offered.     

Vertical earthquake response of megawatt‐sized wind turbine with soil‐structure interaction effects

The dynamic response of a wind turbine on monopile is studied under horizontal and vertical earthquake excitations. The analyses are carried out using the finite element program SAP2000. The finite element model of the structure is verified against the results of shake table tests, and the earthquake response of the soil model is verified against analytical solutions of the steady‐state response of homogeneous strata. The focus of the analyses in this paper is the vertical earthquake response of wind turbines including the soil‐structure interaction effects. The analyses are carried out for both a non‐homogeneous stratum and a deep soil using the three‐step method. In addition, a procedure is implemented which allows one to perform coupled soil‐structure interaction analyses by properly tuning the damping in the tower structure. The analyses show amplification of the ground surface acceleration to the top of the tower by a factor of two. These accelerations are capable of causing damage in the turbine and the tower structure, or malfunctioning of the turbine after the earthquake; therefore, vertical earthquake excitation is considered a potential critical loading in design of wind turbines even in low‐to‐moderate seismic areas. Copyright © 2015 John Wiley & Sons, Ltd.     

Equivalent linearization of non-linear soil-structure systems

The concept of equivalent linearization, in which the actual nonlinear structure is replaced by an equivalent linear single-degree-of-freedom (SDOF) system, is extended for soil-structure systems in order to consider the simultaneous effects of soil-structure interaction (SSI) and inelastic behavior of the structure on equivalent linear parameters (ELP). This is carried out by searching over a two-dimensional equivalent period–equivalent damping space for the best pair, which can predict the earthquake response of the inelastic soil-structure system with sufficient accuracy. The super-structure is modeled as an elasto-plastic SDOF system whereas the soil beneath the structure is considered as a homogeneous half-space and is replaced by a discrete model. An extensive parametric study is carried out for a wide range of soil-structure systems subjected to a suite of 59 ground motions. The effect of SSI on ELP is studied through introducing a set of non-dimensional key parameters, which define the soil-structure system. It is shown that ELP of soil-structure systems result from a trade-off between SSI effect and nonlinear behavior of the structure. The contribution of each of these two factors depends on the characteristics of the soil-structure system which, in turn, are defined by the introduced non-dimensional key parameters. Moreover, the reliability of the predicted response of soil-structure systems and its sensitivity to deviation from optimal ELP is studied in detail, which sheds light on the consequences of using improper pairs of ELP for interacting systems in the framework of performance-based design of structures. Copyright © 2012 John Wiley & Sons, Ltd.     

接触对桩-土-结构动力相互作用体系的影响

为了考察桩-土接触效应对结构地震反应的影响,利用有限元软件ABAQUS建立了土-桩-框架二维有限元模型,分别采用损伤塑性模型和动力粘塑性记忆型嵌套面模型模拟混凝土和土体,利用rebar单元模拟混凝土内的钢筋,取得了较好的计算效果.计算分析中采用19条不同频谱的地震波记录,考虑了地震动强度、桩径、摩擦系数等因素,以层间位移角和桩顶最大位移为主要评价指标,揭示相互作用体系的动力响应特性.分析认为,计算结果对桩、土摩擦系数的取值不敏感;不考虑土-桩接触时,近场土体的动力反应与实际情况存在一定的误差,且上部结构和桩基的动力反应会被低估,应该考虑桩-土动力接触效应;地震动强度增加时,随着结构进入塑性状态,低估程度减小;桩径增加时,低估程度没有显著变化,虽然桩基和上部结构的反应都有所减小.     

第四纪地层中断层同震错动行为的离心机试验研究

运用试验地球物理学的原理和方法来研究认识当地震发生时在第四纪地层中断层同震错动行为的有关特征,为减轻地震灾害相关问题进行基础研究.原创了在试验模型中预制断层的方法来模拟第四纪地层中存在的断层,用离心机模拟试验研究第四纪地层中不同活动年代、不同上断点深度断层的同震错动行为,特别是地表破坏(地表形变和破裂)特征,取得了新的...     

Liquefaction potential and strain dependent dynamic properties of Kasai River sand

The availability of efficient numerical techniques and high speed computation facilities for carrying out the nonlinear dynamic analysis of soil-structure interaction problems and the analysis of ground response due to earthquake loading increase the demand for proper estimation of dynamic properties of soil at small strain as well as at large strain levels. Accurate evaluation of strain dependent dynamic properties of soil such as shear modulus and damping characteristics along with the liquefaction potential are the most important criteria for the assessments of geotechnical problems involving dynamic loading. In this paper the results of resonant column tests and undrained cyclic triaxial tests are presented for Kasai River sand. A new correlation for dynamic shear damping (D_s) and maximum dynamic shear modulus (G_max) are proposed for the sand at small strain. The proposed relationships and the observed experimental data match quite well. The proposed relationships are also compared with the published relationships for other sands. The liquefaction potential of the sand is estimated at different relative densities and the damping characteristics at large strain level is also reported. An attempt has been made to correlate the G_max with the cyclic strength of the soil and also with the deviator stress (at 1% strain) from static triaxial tests.     

Constitutive model for soil amplification of ground shaking: Parameter calibration,comparisons, validation

A one-dimensional constitutive model, developed for the nonlinear ground response analysis of layered soil deposits, is calibrated and validated experimentally in this paper. The small number of parameters renders the model easily implementable, yet quite flexible in effectively reproducing almost any type of experimentally observed hysteretic soil behavior. In particular, the model generates realistic shear modulus and damping curves as functions of shear strain, as well as stress–strain hysteresis loops. The model is calibrated against three sets of widely-used published shear modulus and damping (G : γ and ξ : γ) curves and a library of parameter values is assembled to facilitate its use. The model, along with a developed explicit finite-difference code, NL-DYAS, for analyzing the wave propagation in layered hysteretic soil deposits, is tested against established constitutive models and numerical tools such as Cyclic1D [12] and SHAKE [42], and validated against experimental data from two centrifuge tests. Emphasis is given on the proper assessment of the V_s profile in the centrifuge tests, on the role of soil nonlinearity, and on comparisons of two inelastic codes (NL-DYAS and Cyclic1D) with equivalent linear (SHAKE) analysis.     

Observation and numerical analysis of soil-structure interaction of a reinforced concrete tower

A vast amount of earthquake response records of an observation tower are used together with microtremor data to investigate various aspects of the dynamic behaviour of the soil-structure system. It is found that separation of the soil from the structure occurs under large dynamic loads, leading to changes in the predominant frequency of the system. As a result of the decreasing of the soil support at the side walls of the foundation, the stress caused by the structural weight on the bottom soil increases during earthquakes. With regard to its practical applicability, a linear sway-rocking model is applied for numerical modelling of the soil-structure system. Alterations in the soil support as a result of soil non-linearity and separation of the structure from the soil are investigated by comparing recorded and simulated structural response. The influence of each of these factors on the softening of the soil support is distinctly assessed. An empirical relationship between the peak ground velocity and the soil constants for earthquake excitations of different magnitude is presented.