基于应变空间多机构CG模型的地震液化大变形数值分析

砂土液化导致的地基侧向大变形是地震中许多重要的工程设施和建筑物破坏的主要原因之一。简要介绍了可进行液化大变形分析的散粒体材料本构模型--应变空间多机构CG模型,基于FLIP ROSE程序平台,建立了预测和研究倾斜地基砂土液化导致侧向大变形的二维有限元数值分析方法。采用该模型对相同工况的土工动态离心模型试验进行了模拟,通过对比超孔隙水压力、剪切波水平加速度以及地基侧向位移发现,数值预测与试验结果吻合良好,从而验证了该有限元数值分析模型的可靠性。最后利用该数值分析模型预测了倾斜率不同的地基受到相同剪切波作用时,倾斜地基不同深度产生的侧向位移。预测结果显示,随着地基深度的减小,倾斜率对于地震液化导致倾斜地基侧向大变形的影响越来越显著。     

离心模拟技术中运动学的物理意义

研究离心模拟中的运动学,有助于提高对离心模型试验结果的认识。笔者推导了描述离心模型中任一质点运动的整体速度和加速度的数学表达式。按照牛顿第二定律,在离心模拟中的加速度是连接运动学与动力学的重要因素。笔者描述了整体加速度表达式中各项的物理意义,指出了离心机稳定运转时的向心加速度、由角加速度效应引起的切向加速度、互补(复合向心)加速度分量、径向(原型的竖向)、环向和轴向(原型的两个水平方向)振动效应以及离心机臂伸长效应对整体加速度的贡献。分析结果表明,对于单向振动的动力离心模拟试验,选轴向(平行于转轴)为振动方向便于理解试验结果。另外,仔细检查了Schofield[1]对离心模拟中运动学的描述并发现其混淆之处。     

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.     

连云港海相软土流变特性试验及双屈服面流变模型

为研究连云港海相软土流变特性,进行了三轴流变试验。根据对试验结果的分析,可以将连云港海相软土作为弹-粘塑性体来研究;将广义Bingham模型和椭圆-抛物线双屈服面模型相结合,建立了新的流变模型来描述其流变特性,得出了各参数,并验证了该模型的适用性。     

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.     

水平多向荷载下桩-土相互作用初探

利用香港科技大学离心机双向振动台,进行了群桩基础模型的水平单向和双向振动试验;振动过程中的桩身弯矩响应表明,群桩基础在单向振动中表现出的对称性在双向振动下不复存在;并进行了桩-土相互作用的颗粒流数值模拟,初步探寻了试验观测到的“垂向加载效应”的产生机制;同时通过简单的推导,指出了当前桩-土相互作用分析方法的局限性。在试验、数值和理论分析方面对水平多向荷载下的桩-土相互作用进行了初步研究,为今后的进一步研究提供了重要参考。     

Centrifuge shaking table tests on 4 × 4 pile groups in liquefiable ground

A series of centrifuge shaking table model tests are conducted on 4?×?4 pile groups in liquefiable ground in this study, achieving horizontal–vertical bidirectional shaking in centrifuge tests on piles for the first time. The dynamic distribution of forces on piles within the pile groups is analysed, showing the internal piles to be subjected to greater bending moment compared with external piles, the mechanism of which is discussed. The roles of superstructure–pile inertial interaction and soil–pile kinematic interaction in the seismic response of the piles within the pile groups are investigated through cross-correlation analysis between pile bending moment, soil displacement, and structure acceleration time histories and by comparing the test results on pile groups with and without superstructures. Soil–pile kinematic interaction is shown to have a dominant effect on the seismic response of pile groups in liquefiable ground. Comparison of the pile response in two tests with and without vertical input ground motion shows that the vertical ground motion does not significantly influence the pile bending moment in liquefiable ground, as the dynamic vertical total stress increment is mainly carried by the excess pore water pressure. The influence of previous liquefaction history during a sequence of seismic events is also analysed, suggesting that liquefaction history could in certain cases lead to an increase in liquefaction susceptibility of sand and also an increase in dynamic forces on the piles.     

Finite element simulations of seismic effects on retaining walls with liquefiable backfills

Finite element simulations of two centrifuge tests on the same cantilever retaining wall model holding liquefiable backfill were conducted using the Biot formulation‐based program DIANA–SWANDYNE II. To demonstrate the effects due to different pore fluids in seismic centrifuge experiments, water was used as the pore fluid in one experiment whereas a substitute pore fluid was used in the second experiment. The cantilever wall model parameters were determined by comparing simulations with measurements from free‐vibration tests performed on the model wall without backfill. The initial stress conditions for dynamic analysis for the soil backfill were obtained by simulating static loads on the retaining wall from the soil backfill. Level‐ground centrifuge model results were used to select the parameters of the Pastor–Zienkiewicz mark III constitutive model used in the dynamic simulations of the soil. The effects due to different pore fluids were captured well by the simulations. The magnitudes of excess pore pressures in the soil, lateral thrust and its line of action on the wall, and wall bending strains, deflections, and accelerations were predicted well. Predictions of settlements and accelerations in the backfill were less satisfactory. Relatively high levels of Rayleigh damping were needed to be used in the retaining wall simulations in order to obtain numerically stable results, which is one of the shortcomings of the model. The procedure may be used for engineering purpose dealing with seismic analysis of flexible retaining walls where lateral pressures, bending strains and deflections in the wall are typically of importance. Copyright © 2008 John Wiley & Sons, Ltd.     

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。     

黏度时变性宾汉体浆液的柱-半球形渗透注浆机制研究

浆液黏度时变性对注浆扩散范围计算值影响很大。基于宾汉体浆液的流变方程与流体黏度时变性方程,建立了黏度时变性宾汉体浆液的流变方程与渗流运动方程,并依据某些假设,推导了时变性宾汉体浆液柱-半球形渗透注浆机制及探讨了半球体部分扩散半径l1与柱体部分扩散长度m的关系：m=(2l1 /3)(2n+1)。通过设计室内注浆试验对其进行了验证,结果表明：由黏度时变性宾汉体浆液的柱-半球形渗透机制计算的半球体部分扩散半径、圆柱体部分扩散长度及注浆扩散体体积的理论值与试验测量值虽分别有15%、10%及40%左右的差异,但都处于可接受误差范围内,因而,在总体上能较好地反映黏度时变性宾汉体浆液的柱-半球形注浆渗透规律,对注浆设计、施工和理论研究等方面具有一定的参考价值与指导作用     

Seismic centrifuge modelling of earth dams

Three dynamic centrifuge models were tested to obtain data for safety evaluation of the Jen-Yi-Tan Dam in Taiwan subject to a strong earthquake. In these tests, recorded 1999 Chi-Chi earthquake ground motions were modified and used on the electro-hydraulic shaking table mounted on the 400 g-ton centrifuge at the University of Colorado at Boulder. All tests were conducted under centrifugal acceleration of 150 g, and the input acceleration was scaled accordingly in order to simulate the given earthquake. A rigid container and water as pore fluid were used in the tests. In both Models 2 and 3, no sign of soil liquefaction was observed in the tests although a noticeable amount of settlements were found from the earth dam cross-section profile after testing.     

Seismic Response Analysis of Pile Foundations

A quasi-3D continuum method is presented for the dynamic nonlinear effective stress analysis of pile foundation under earthquake excitation. The method was validated using data from centrifuge tests on single piles and pile groups in liquefiable soils conducted at the University of California at Davis. Some results from this validation studies are presented. The API approach to pile response using p–y curves was evaluated using the quasi-3D method and the results from simulated earthquake tests on a model pile in a centrifuge. The recommended API stiffnesses appear to be much too high for seismic response analysis under strong shaking, but give very good estimates of elastic response.     

Application of a memory surface model to predict whole-life settlements of a sliding foundation

In this paper a novel modelling procedure is proposed to estimate whole-life settlements of tolerably mobile sliding foundations. A new kinematic hardening-critical state-state parameter constitutive model, the Memory Surface Hardening model, is implemented in a one-dimensional analysis to predict accumulated vertical settlements under drained lateral cyclic loading. The Memory Surface Hardening model performance is compared with the Modified Cam Clay and Severn-Trent Sand models. The Memory Surface Hardening model is adopted to simulate available experimental data from centrifuge tests to predict the settlement of a sliding foundation at the final stable state (i.e. no further volume changes occur).     

Discrete element analysis of the rheological characteristics of self-compacting concrete with irregularly shaped aggregate

Concrete is a heterogeneous, multiphase, composite material, and the size and shape of the coarse aggregate used have an important influence on the rheological properties of the concrete. The aggregate is usually simulated with spherical particles in the discrete element method (DEM). However, the shape of real aggregates is uncontrolled and polytropic. Therefore, spherical particles hardly reflect the actual situation. To comprehensively analyze the rheological characteristics of self-compacting concrete (SCC), experimental and simulated tests of slump-flow and L-box tests of different performative SCC are investigated. An efficient and fast random polyhedron particle generation method is proposed to simulate the real shape of the coarse aggregate, which is close to the actual state. The slump-flow and L-box tests of SCC are simulated by using the established discrete element model and the irregular generating particle method. The slump-flow test shows that the generation method could effectively simulate the flow state of concrete, and the L-box test evaluates the passing ability of SCC. The rheological characteristics of the yield stress τ₀ and plastic viscosity η are verified as Bingham model parameters, and the numerical results are perfectly consistent with the experimental results.     

软岩三轴卸荷流变力学特性及本构模型研究

以典型软岩－泥质粉砂岩为研究对象,进行了不同应力水平下的恒轴压、分级卸围压室内流变试验。试验结果表明,软岩在卸荷条件下的轴向及侧向流变变形较大,各向异性显著。在恒定围压较高时,轴向流变变形较大,而随着围压逐级降低,侧向流变变形发展较轴向更快,试样在破裂围压下的侧向变形远大于轴向。通过深入分析软岩卸荷流变试验成果可知,线性Burgers流变模型能较好地反映各级围压下流变曲线的衰减流变及稳态流变阶段,但对于破裂围压下的非线性加速流变阶段无法描述。基于上述分析,建立一个非线性损伤流变模型,采用Levenberg-Marquardt非线性优化最小二乘法对试验结果进行拟合,并获得软岩的非线性流变参数。经比较,拟合计算与试验曲线吻合程度很高,说明该流变模型能较好地反映软岩卸荷流变各阶段的曲线特征。     

Dynamic Properties of Soft Clay and Loose Sand from Seismic Centrifuge Tests

Appropriate evaluation of shear modulus and damping characteristics of soils subjected to dynamic loading is key to accurate seismic response analysis and soil modeling programs. Dynamic centrifuge experiments were conducted at C-CORE (Memorial University of Newfoundland) centrifuge center to investigate the dynamic properties and seismic response of soft clay and dry loose sand strata. Soft clay with shear strength of about 30 kPa and well graded silica sand at about 35% relative density were employed in a rigid container to simulate local site effects. Several earthquake-like shaking events were applied to the model to evaluate variation of shear modulus and damping ratio with shear strain amplitude and confining pressure, and to assess their effects on site response. The estimated modulus reduction and damping ratio were compared to the predictions of empirical formulae and resonant column tests for both soft clay and loose sand. The evaluated shear modulus and damping ratio were found to be dependent on confining pressure in both soil types. Modulus variation in both soils agreed well with the empirical curves and resonant column test results. However, the sand modulus values were slightly higher than the empirical relations and resonant column tests. This discrepancy is attributed to higher stress and densification of sand during large amplitude shaking applied to the model. The damping ratio at shear strains lower than 0.5% was in reasonable agreement with the empirical curves and the resonant column tests in both clay and sand models, but was generally higher at shear strain larger than 0.5%.     

Flow regimes in debris flow

The Bingham model has been suggested for the rheological behaviour of debris flow. Velocity plotted against yield strength uniquely defines turbulent and laminar flow in published experimental data on flow of Bingham materials. These regimes are not defined by a unique value of the Bingham number even though it includes both of the critical parameters, yield strength and velocity. Laminar flow in at least the final stages natural debris flows is suggested by sediments with parallel-to-flow or inclined clast fabric, preservation of delicate clasts, blocks projecting from the top of the deposit(?), and an absence of flutes.     

Numerical modelling of the effects of foundation scour on the response of a bridge pier

Foundation scour can have a detrimental effect on the performance of bridge piers, inducing a significant reduction of the lateral capacity of the footing and accumulation of permanent settlement and rotation. Although the hydraulic processes responsible for foundation scour are nowadays well known, predicting their mechanical consequences is still challenging. Indeed, its impact on the failure mechanisms developing around the foundation has not been fully investigated. In this paper, numerical simulations are performed to study the vertical and lateral response of a scoured bridge pier founded on a cylindrical caisson foundation embedded in a layer of dense sand. The sand stress–strain behaviour is reproduced by employing the Severn-Trent model. The constitutive model is firstly calibrated on a set of soil element tests, including drained and undrained monotonic triaxial tests and resonant column tests. The calibration procedure is implemented considering the stress and strain nonuniformities within the samples, by simulating the laboratory tests as boundary value problems. The numerical model is then validated against the results of centrifuge tests. The results of the simulations are in good agreement with the experimental results in terms of foundation capacity and settlement accumulation. Moreover, the model can predict the effects of local and general scour. The numerical analyses also highlight the impact of scouring on the failure mechanisms, revealing that the soil resistance depends on the hydraulic scenario.

     

一种新型板桩码头结构的离心模拟

卸荷式地连墙板桩结构码头是国内自主创新的深水码头结构,为了深入认识这一板桩码头结构在设计荷载下的工作性状和土与结构共同作用机制,结合某一拟建码头泊位的卸荷式地连墙板桩码头结构设计方案验证需要,进行了3组大型土工离心模型试验研究。其中2组为重复性的模型试验,以验证设计方案的可行性;第3组则模拟了无卸荷平台结构的地连墙板桩码头结构设计方案,以比较有无卸荷平台的板桩码头来揭示全直桩卸荷平台结构的卸荷效果。并且介绍了码头结构离心模拟技术,包括模型设计、模型制作和模型测量及试验步骤等关键内容。     

液化土层地震动模拟计算方法及验证

针对目前土层反应分析方法难以模拟液化土层地震动时程的缺欠,在工程力学研究所原有有效应力分析程序基础上提出了一个改进的计算方法并进行了验证。改进方法中,通过每个应力循环模拟土的非线性进程以及液化导致的土刚度衰减过程,并引入了新建立的适于非均等固结随机地震荷载作用下的孔压增长模型。将计算方法与实际地震记录及大型振动台实验结果进行了对比,结果表明：在峰值加速度、液化后波形变化、时频曲线及加速度反应谱等主要特征上,计算结果均与现场实际记录及振动台实验基本一致;在液化引起地震动特征变化上,计算出的液化较非液化土层加速度反应谱的增量与实验结果一致。说明所提出的改进方法可以用于液化土层地震动的模拟。