Pile response in submerged lateral spreads: Common pitfalls of numerical and physical modeling techniques

A three dimensional dynamic numerical methodology is developed and used to back-analyze experimental data on the seismic response of single piles in laterally spreading slopes. The aim of the paper is not to seek successful a-priori (Type A) predictions, but to explore the potential of currently available numerical techniques, and also to get feedback on modeling issues and assumptions which are not yet resolved in the international literature. It is illustrated that accurate simulation of the physical pile–soil interaction mechanisms is not a routine task, as it requires the incorporation of advanced numerical features, such as an effective stress constitutive soil model that can capture cyclic response and shear-induced dilation, interface elements to simulate the flow of liquefied ground around the pile and proper calibration of soil permeability to model excess pore pressure dissipation during shaking. In addition, the “conventional tied node” formulation, commonly used to simulate lateral boundary conditions during shaking, has to be modified in order to take into account the effects of the hydrostatic pore pressure surplus that is created at the down slope free field boundary of submerged slopes. A comparative analysis with the two different lateral boundary formulations reveals that “conventional tied nodes”, which also reflect the kinematic conditions imposed by laminar box containers in centrifuge and shaking table experiments, may underestimate seismic demands along the upper part of the pile foundation.     

A simple displacement model for response analysis of EPS geofoam seismic buffers

A simple displacement-type block model is proposed to compute the compression–load–time response of an idealized seismic buffer placed against a rigid wall and used to attenuate earthquake-induced dynamic loads. The seismic buffer is modelled as a linear elastic material and the soil wedge shear surface by a stress-dependent linear spring. The model is shown to capture the trends observed in four physical reduced-scale model shaking table tests carried out with similar boundary conditions up to a base excitation level of about 0.7g. In most cases, quantitative predictions are in reasonable agreement with physical test results. The model is simple and provides a possible framework for the development of advanced models that can accommodate more complex constitutive laws for the component materials and a wider range of problem geometry.     

Numerical simulation on the seismic absorption effect of the cushion in rigid-pile composite foundation

In order to quantitatively study the seismic absorption effect of the cushion on a superstructure, a numerical simulation and parametric study are carried out on the overall FEA model of a rigid-pile composite foundation in ABAQUS. A simulation of a shaking table test on a rigid mass block is first completed with ABAQUS and EERA, and the effectiveness of the Drucker-Prager constitutive model and the finite-infinite element coupling method is proved. Dynamic time-history analysis of the overall model under frequent and rare earthquakes is carried out using seismic waves from the El Centro, Kobe, and Bonds earthquakes. The different responses of rigid-pile composite foundations and pile-raft foundations are discussed. Furthermore, the influence of thickness and modulus of cushion, and ground acceleration on the seismic absorption effect of the cushion are analyzed. The results show that: 1) the seismic absorption effect of a cushion is good under rare earthquakes, with an absorption ratio of about 0.85; and 2) the seismic absorption effect is strongly affected by cushion thickness and ground acceleration.     

高土石坝振动台模型试验理论、技术与应用

对土石坝振动台模型试验理论和技术进行系统阐述,提出基于原型和模型坝料静、动力特性试验的模型相似设计方法和不同强度地震动递进输入(白噪声微振-设计地震-校核地震-破坏试验)的振动试验方法。基于1g大型振动台和ng超重力离心机振动台设备性能现状,结合高土石坝的结构特点和动力试验相似模拟要求,对土石坝振动台模型试验的优势及局限进行深入讨论。结合已有的工程实践,对土石坝振动台模型试验在工程中的应用进行总结,并以某实际高面板堆石坝为例研究面板坝生命周期内经历多次地震情况下结构动力特性的演化规律。     

Experimental seismic investigation of Sefid‐rud concrete buttress dam model on shaking table

Owing to the devastating M7.6 earthquake of 20 June 1990 that occurred in the northern province of Iran, Sefid‐rud concrete buttress dam located near the epicenter was severely shaken. The crack penetrated throughout the dam thickness near slope discontinuity, causing severe leakage, but with no general failure. In this study, nonlinear seismic response of the highest monolith with empty reservoir is investigated experimentally through model testing. A geometric‐scaled model of 1:30 was tested on a shaking table with high‐frequency capability to study dynamic cracking of the model and serve as data for nonlinear computer model calibration. Three construction joints are set up in the model to simulate effects of construction aspects. The experimental results are then compared with smeared crack and damage mechanics finite‐element simulations using nonlinear concrete constitutive models based on fracture mechanics. The crack patterns obtained from numerical models are in good agreement with those obtained from shaking table tests for the case of including construction joint effects and rigid foundation. Copyright © 2008 John Wiley & Sons, Ltd.     

Seismic displacements on shear surfaces in cohesive soils

The paper is concerned with the earthquake-induced displacements on pre-existing shear surfaces in cohesive soils. Results from ring shear tests have shown that during fast shearing the strength of such surfaces depends on displacement and rate of shearing. The test results have been used in a numerical analysis to assess the displacement of a rigid block sliding on a plane surface. The results from the analysis show that the earthquake-induced displacements on pre-existing shear surfaces are influenced significantly by the soil behaviour under earthquake loading conditions. The results are consistent with the field performance of pre-existing slides in cohesive soils during earthquakes.     

A practical numerical method for large strain liquefaction analysis of saturated soils

Accurate prediction of the liquefaction of saturated soils is based on strong coupling between the pore fluid phase and soil skeleton. A practical numerical method for large strain dynamic analysis of saturated soils is presented. The u–p formulation is used for the governing equations that describe the coupled problem in terms of soil skeleton displacement and excess pore pressure. A mixed finite element and finite difference scheme related to large strain analysis of saturated soils based on the updated Lagrangian method is given. The equilibrium equation of fluid-saturated soils is spatially discretized by the finite element method, whereas terms associated with excess pore pressure in the continuity equation are spatially discretized by the finite difference method. An effective cyclic elasto-plastic constitutive model is adopted to simulate the non-linear behavior of saturated soils under dynamic loading. Several numerical examples that include a saturated soil column and caisson-type quay wall are presented to verify the accuracy of the method and its usefulness and applicability to solutions of large strain liquefaction analysis of saturated soils in practical problems.     

Practical applications of a nonlinear approach to analysis of earthquake-induced liquefaction and deformation of earth structures

Seismic stability, liquefaction, and deformation of earth structures are critical issues in geotechnical earthquake engineering practice. At present, the equivalent linear approach is considered the ‘state of practice’ in common use. More recently, dynamic analyses incorporating nonlinear, effective-stress-based soil models have been used more frequently in engineering applications. This paper describes a bounding surface hypoplasticity model for sand [Wang ZL. Bounding surface hypoplasticity model for granular soils and its applications. PhD Dissertation for the University of California at Davis, U.M.I. Dissertation Information Service, Order No. 9110679; 1990; Wang ZL, Dafalias YF, Shen CK. Bounding surface hypoplasticity model for sand. ASCE, J Eng Mech 1990;116(5):983–1001; Wang ZL, Makdisi FI. Implementing a bounding surface hypoplasticity model for sand into the FLAC program. In: Proceedings of the international symposium on numerical modeling in geomechanics. Minnesota, USA; 1999. p. 483–90] incorporated into a two-dimensional finite difference analysis program [Itasca Consulting Group, Inc. FLAC (Fast Lagrangian Analysis of Continua), Version 4. Minneapolis, MN; 2000] to perform nonlinear, effective-stress analyses of soil structures. The soil properties needed to support such analyses are generally similar to those currently used for equivalent linear and approximate effective-stress analyses. The advantages of using a nonlinear approach are illustrated by comparison with results from the equivalent linear approach for a rockfill dam. The earthquake performance of a waterfront slope and an earth dam were evaluated to demonstrate the model's ability to simulate pore-pressure generation and liquefaction in cohesionless soils.     

大型振动台试验土质边坡模型材料相似性及评价

大型振动台模型试验是边坡动力响应和破坏模式研究的重要手段,其中相似材料的选择是决定试验能否成功的关键。以黄土地区2类典型边坡为研究对象,在对滑坡体原状土样开展室内土动力学测试的基础上,提出6种相似比条件下振动台模型试验相似材料的配比方法;对2种相似比条件下相似材料的参数进行量纲分析,依据相似判据、相似准则的约束,以模糊数学理论进行优化,并提出大型土质边坡振动台试验的材料相似性评价体系。     

Simplified dynamic analysis to evaluate liquefaction-induced lateral deformation of earth slopes: a computational fluid dynamics approach

Lateral deformation of liquefiable soil is a cause of much damage during earthquakes, reportedly more than other forms of liquefaction-induced ground failures. Researchers have presented studies in which the liquefied soil is considered as viscous fluid. In this manner, the liquefied soil behaves as non-Newtonian fluid, whose viscosity decreases as the shear strain rate increases. The current study incorporates computational fluid dynamics to propose a simplified dynamic analysis for the liquefaction-induced lateral deformation of earth slopes. The numerical procedure involves a quasi-linear elastic model for small to moderate strains and a Bingham fluid model for large strain states during liquefaction. An iterative procedure is considered to estimate the strain-compatible shear stiffness of soil. The post-liquefaction residual strength of soil is considered as the initial Bingham viscosity. Performance of the numerical procedure is examined by using the results of centrifuge model and shaking table tests together with some field observations of lateral ground deformation. The results demonstrate that the proposed procedure predicts the time history of lateral ground deformation with a reasonable degree of precision.     

Seismic analysis and design of rigid bridge abutments considering rotation and sliding incorporating non-linear soil behavior

A two-dimensional (2D) finite element analytical model is developed to analyze the seismic response of rigid highway bridge abutments, retaining and founded on dry sand. A well verified finite element code named FLEX is used for this purpose. The proposed model has the following characteristics: (1) The soil (dry sand in this study) is modeled by a 2D finite element grid; (2) The bridge abutment is molded as a rigid substructure; (3) The strength and deformation of the soil are modeled using the viscous cap constitutive model. This model consists of a failure surface and hardening cap together with an associated flow rule. The cap surface is activated for the soil under the wall to represent compaction during wall rocking. In addition, viscoelastic behavior is provided for representing the hysteretic-like damping of soil during dynamic loading; (4) Interface elements are used between the wall and the soil (at the backface of the wall and under its base) to allow for sliding and for debonding/recontact behavior; (5) The finite element grid is truncated by using an absorbing boundary approximation. Using this boundary at both sides of the grid simulates the horizontal radiation of energy scattered from the wall and the excavation. Shear beams are placed adjacent to the lateral boundaries from each side which give the far-field ground motion, for comparison with those computed adjacent to the boundaries. The analytical model is verified comparing predictions to results from dynamic centrifuge tests, with satisfactory agreement. The proposed model is used to study the dynamic response of an 8.0 m high and 3.0 m wide rigid bridge abutment (proportioned using the traditional approach to design) for different sinusoidal and earthquake acceleration input motions. The results from the analysis show that outward tilting of rigid bridge abutments is the dominant mode of response during dynamic shaking and that these abutments end up with a permanent outward tilt at the end of shaking. The results from all the analyzed cases of the 8.0 m high gravity retaining wall together with those from the analysis of the tilting wall centrifuge tests are discussed and used for proposing a practical method for evaluating the seismic response of rigid abutments during earthquakes.     

Seismic failure modeling of concrete dams considering heterogeneity of concrete

Study on the failure process of high concrete dams subjected to strong earthquakes is crucial to reasonable evaluation of their seismic safety. Numerical simulation in this aspect involves dynamic failure analysis of big bulk concrete dam subjected to cyclic loading. The Rock Failure Process Analysis (RFPA) proposed by C.A. Tang, with successful applications to failure modeling of rock and concrete specimens mainly subjected to static loading, is extended for this purpose. For using the proposed model, no knowledge on the cracking route needs to be known beforehand, and no remeshing is required. Simulation of the whole process of elastic deformation, initiation and propagation of microcracks, severe damage and ultimate failure of concrete dams in earthquakes with a unified model is enabled. The model is verified through a shaking table test of an arch dam. Finally a practical gravity dam is employed as a numerical example. Considering the uncertainty in ground motion input and concrete material, typical failure process and failure modes of gravity dam are presented. Several small cracks may occur due to tension particularly at dam neck, dam faces and dam heel, and a few of them evolve into dominant ones. Relatively smaller earthquake may cause damage to the dam neck while a bigger one may bring on cracks at lower parts of the dams. Cracking at the dam bottom may incline to a direction almost perpendicular to the downstream face after propagating horizontally for a certain distance when the shaking is strong enough.     

Parametric analysis of single pile response in laterally spreading ground

Extensive damage to pile-supported structures has been witnessed in several recent earthquakes (Chi-Chi, 1999; Kobe, 1995, etc.), as a result of liquefaction-induced lateral spreading of slightly sloping ground or free-face topographic irregularities. This paper presents a parametric analysis of the basic pile and soil parameters, as well as the pile-soil interaction mechanisms affecting the response of single piles subjected to such lateral spreading, based on numerical simulation with the nonlinear P-y method. In parallel, a set of design charts and analytical relations is established, for approximate computation of maximum pile deflections and bending moments, using a “theory guided” multi-variable statistical analysis of the numerical predictions. Three different combinations (design cases) of pile head constraints and soil conditions were considered, which are commonly encountered in practice. The overall accuracy of the proposed analytical relations is evaluated against experimental results from seven centrifuge and five large shaking table experiments.     

Seismic failure modeling of concrete dams considering heterogeneity of concrete

Study on the failure process of high concrete dams subjected to strong earthquakes is crucial to reasonable evaluation of their seismic safety. Numerical simulation in this aspect involves dynamic failure analysis of big bulk concrete dam subjected to cyclic loading. The Rock Failure Process Analysis (RFPA) proposed by C.A. Tang, with successful applications to failure modeling of rock and concrete specimens mainly subjected to static loading, is extended for this purpose. For using the proposed model, no knowledge on the cracking route needs to be known beforehand, and no remeshing is required. Simulation of the whole process of elastic deformation, initiation and propagation of microcracks, severe damage and ultimate failure of concrete dams in earthquakes with a unified model is enabled. The model is verified through a shaking table test of an arch dam. Finally a practical gravity dam is employed as a numerical example. Considering the uncertainty in ground motion input and concrete material, typical failure process and failure modes of gravity dam are presented. Several small cracks may occur due to tension particularly at dam neck, dam faces and dam heel, and a few of them evolve into dominant ones. Relatively smaller earthquake may cause damage to the dam neck while a bigger one may bring on cracks at lower parts of the dams. Cracking at the dam bottom may incline to a direction almost perpendicular to the downstream face after propagating horizontally for a certain distance when the shaking is strong enough.     

Study of pore pressure variation during liquefaction using two constitutive models for sand

Numerical analyses of liquefiable sand are presented in this paper. Liquefaction phenomenon is an undrained response of saturated sandy soils when they are subjected to static or dynamic loads. A fully coupled dynamic computer code is developed to predict the liquefaction potential of a saturated sandy layer. Coupled dynamic field equations of extended Biot's theory with u–P formulation are used to determine the responses of pore fluid and soil skeleton. Generalized Newmark method is employed for integration in time. The soil behavior is modelled by two constitutive models; a critical state two-surface plasticity model, and a densification model. A class ‘B’ analysis of a centrifuge experiment is performed to simulate the dynamic response of level ground sites. The results of the numerical analyses demonstrate the capability of the critical sate two-surface plasticity model in producing pore pressures that are consistent with observations of the behavior of liquefiable sand in the centrifuge test.     

不同厚度层状边坡动力响应的模型试验研究

采用振动台物理模拟试验方法,以4种不同覆土厚度的层状边坡模型为研究对象,水平输入振幅逐渐增大的正弦波加速度,分析了结构面上覆不同厚度土层对动力作用下边坡的稳定影响.研究了在动力作用下边坡的破坏位置和性质、破坏形式及最危险覆土厚度,验证了坡面放大效应与高程的关系,采用MIDAS/GTS软件对模型试验进行振型分析,分析了模型边坡的自振频率与覆土厚度的变化关系.试验结果表明：①模型破坏时最先出现的裂缝在边坡的中上部,且6 cm覆土厚度的模型对振动的响应最大,对应到实际工程中时12m厚度土层覆盖的边坡是最应该注意防护的.②不同厚度的土层破坏的形式不同：当土层厚度较薄时模型破坏较迅速,基本沿结构面发生整体滑动破坏,且滑动呈现一定的流体特性;当覆土较厚时裂缝先在模型中上部出现,随着振动的持续裂缝继续发展,最后发生整体性崩塌.③随着高程的增加峰值加速度总体呈放大趋势,但最大值出现在边坡中上部而非坡顶,说明不仅均质边坡有加速度的高程放大效应,层状边坡也具有加速度的高程放大效应.     

Seismic behavior of breakwaters on complex ground by numerical tests: Liquefaction and post liquefaction ground settlements

A large number of breakwaters have been constructed along coasts to protect humans and infrastructures from tsunamis.There is a risk that foundation soils of these structures may liquefy,or partially liquefy during the earthquake preceding a tsunami,which would greatly reduce the structures’capacity to resist the tsunami.It is necessary to consider not only the soil’s liquefaction behavior due to earthquake motions but also its post-liquefaction behavior because this behavior will affect the breakwater’s capacity to resist an incoming tsunami.In this study,numerical tests based on a sophisticated constitutive model and a soil-water coupled finite element method are used to predict the mechanical behavior of breakwaters and the surrounding soils.Two real breakwaters subjected to two different seismic excitations are examined through numerical simulation.The simulation results show that,earthquakes affect not only the immediate behavior of breakwaters and the surrounding soils but also their long-term settlements due to post-earthquake consolidation.A soil profile with thick clayey layers beneath liquefied soil is more vulnerable to tsunami than a soil profile with only sandy layers.Therefore,quantitatively evaluating the seismic behavior of breakwaters and surrounding soils is important for the design of breakwater structures to resist tsunamis.     

Canyon topography effects on ground motion at Feitsui damsite

The free-field accelerograms along Feitsui Canyon are analyzed and modeled by a numerical scheme to study the effect of canyon topography. Since six strong-motion accelerometers (SC1–SC6) were deployed along the Feitsui Canyon in 1991; there are 14 earthquakes (4.9≤M_L≤6.6) recorded by these stations until June 1996, but only five triggered all six stations. The maximum PGA value is 68.6 cm s⁻² recorded at station SC1. According to the present data, the effect of the dam on the ground motions at canyon stations can be negligible. The amplitude of ground motion on the slopes of the canyon is bigger than that at its trough. The integral equation method is applied to a two dimensional model of Feitsui Canyon to study the effects of the canyon topography. We choose the ground motion of SC3 or SC4 station at the trough of the canyon as the input motion for the model, which is then used to predict the ground motion at the other five stations. Apart from the earthquake close to the damsite, the simple model can reproduce the observed accelerations at all frequencies below 4 Hz. Overall, the numerical method can well predict the ground motion along the canyon, although the high-frequency simulation is underestimated.     

三层地铁车站振动台试验的数值模拟

进行了三层地铁车站大型振动台试验,获得了可靠的试验数据。通过室内实验获取了模型材料和土体材料的力学参数。基于ABAQUS有限元计算平台,建立了振动台试验的二维有限元模型,处理了混凝土本构模型及其参数的选取、土体本构模型及其参数的选取、阻尼设置、边界条件设置等问题。对多种工况下的试验结果和模拟结果进行了对比分析。结果表明,按照本文建议的建模方法,可以很好地重现振动台试验,数值模拟结果无论在趋势上还是数值上都和试验结果符合得很好。     

Earthquake loss estimation for the New York City Metropolitan Region

This study is a thorough risk and loss assessment of potential earthquakes in the NY–NJ–CT Metropolitan Region. This study documents the scale and extent of damage and disruption that may result if earthquakes of various magnitudes occurred in this area. Combined with a detailed geotechnical soil characterization of the region, scenario earthquakes were modeled in HAZUS (Hazards US), a standardized earthquake loss estimation methodology and modeling program. Deterministic and probabilistic earthquake scenarios were modeled and simulated, which provided intensities of ground shaking, dollar losses associated with capital (the building inventory) and subsequent income losses. This study has also implemented a detailed critical (essential) facilities analysis, assessing damage probabilities and facility functionality after an earthquake. When viewed in context with additional information about regional demographics and seismic hazards, the model and results serve as a tool to identify the areas, structures and systems with the highest risk and to quantify and ultimately reduce those risks.