侧向流动地基单桩基础离心机振动台试验研究

该文在清华大学土工离心机振动台上开展了倾斜可液化地基中刚性单桩基础的试验研究, 观测了地基的加速度、位移、孔压和桩基础的弯矩等响应。研究结果显示:地震中地基浅层部分较快达到液化状态, 不同深度超静孔隙水压力的累积与消散并不相同;砂土的液化使得地基的加速度响应发生变化, 有效地过滤了地震波的部分高频段;地震过程中桩身弯矩峰值出现在浅层地基达到初始液化后, 且最大弯矩出现在桩头;倾斜地基使得桩身产生了较大的残余弯矩。由此表明在侧向流动地基中, 液化前、液化后大变形和地震后永久位移三个不同阶段桩的受力并不相同。在进行可液化地基中桩基础的研究中, 应考虑这三个不同阶段桩基础的受力要求。该研究加深了对倾斜液化地基中桩基础的地震响应的理解, 有助于进一步的揭示地震过程中可液化地基桩基础的震损机理。     

饱和砂土中直群桩及土体地震动力响应特征研究

为了更好地研究地震作用下饱和砂土中群桩及土体动力响应特征,设计了饱和砂土液化场地2×2直群桩动力响应离心机振动台试验,获得承台、土体加速度以及孔压动力时程曲线。为了更深入地分析群桩及土体地震动力响应特征并满足对比研究的需要,在试验基础上,基于ABAQUS有限元软件平台,通过引入砂土液化大变形本构模型,采用有限元网格自适应调整技术克服大变形畸变问题,建立液化场地群桩基础静动耦合非线性相互作用的二维有限元模型进行数值模拟分析,并与试验结果进行对比验证。结果表明:在峰值加速度0.3g El-Centro地震波工况下离心机振动台试验饱和砂土地基液化速度非常快,直群桩基础承台加速度相比较输入波明显缩小,0.3g大震作用下地基浅层加速度显著衰减,地基液化区域由浅入深逐渐发育;饱和砂土地基超静孔隙水压力发展影响土体和桩基承台加速度响应,土体液化直接导致加速度数值减小;数值模拟加速度结果与试验的加速度动力响应特性略有区别,数值模拟加速度地基浅层出现先放大后缩小的规律,深层土与输入波形基本一致;数值模拟超静孔隙水压力与超静孔压比与试验基本一致,而模拟得到的承台位移结果相较于试验偏于保守。     

可液化地基上桩基基础小高宽比隔震结构体系振动特性试验研究

为研究地基液化对小高宽比隔震结构体系振动特性的影响，通过振动台模型试验再现了桩基基础小高宽比隔震结构体系在液化场地上的地震反应过程，分析了地基液化过程中基础及隔震层振动特性和隔震结构地震响应规律。结果表明：地基液化后小高宽比隔震结构的一阶自振频率较刚性地基时大幅增加，阻尼比较刚性地基时也明显增加；液化地基上隔震层对群桩基础水平向加速度反应起明显的放大作用，隔震效能消失，但隔震层对桩基承台转动角加速度反应起显著的减震作用；液化地基上小高宽比隔震结构楼层加速度峰值放大系数的分布规律与非液化地基上隔震结构相比也具有明显差异，呈现出弯曲放大的特点，隔震结构顶层加速度峰值放大系数增大尤为明显；液化地基上小高宽比隔震结构的最大层间位移反应远超刚性地基上隔震结构在强震作用下的最大层间位移反应，可能导致基于刚性地基假定设计的隔震结构在地基液化时不满足抗震设计要求。     

堤防地基地震液化的数值模拟

基于Biot动力耦合理论,采用一个经过广泛验证的砂土循环弹塑性本构模型,建立了平面应变条件下土堤地基的地震液化数值模拟方法。以某河流堤防的实际工程为例,应用该方法对土堤地基的地震液化进行了数值模拟,并对地震作用下的超孔隙水压力、加速度、位移等动力反应进行了计算分析,得到了一些对于抗震分析有用的结果。     

不同土层条件下斜直交替群桩-土-结构地震响应特性研究

为研究地震作用下斜直交替群桩-土-结构特性,采用El Centro地震波作为动荷载,FLAC3D软件为研究工具,分别针对黏土-砂土、黏土-砂土-黏土中建立了数值模型并对这两种工况下土层加速度、桩基受力与位移、上部结构位移等进行了分析。研究结果表明:砂土层对加速度峰值的放大作用比黏土层大,砂土层中加速度峰值比动力荷载峰值的出现时刻明显滞后;弯矩最大值均出现在桩与承台交界处;三层土模型的斜桩最大弯矩比两层土模型小。不同模型之间直桩弯矩的差别较小,桩端土层的构造对桩身弯矩的影响较大,桩端非液化黏土层的嵌固作用能够显著减小斜桩桩身最大弯矩。斜桩与直桩的水平位移基本相同,桩端非液化土层的存在对斜桩竖向位移具有显著影响并对桥墩顶部水平位移具有减缓作用。     

地震载荷作用下液化土中输流管道动态响应研究

将地震载荷作用下的液化区埋土管道模拟成受到液化土弹性力作用下的直梁模型,将管道两端约束等效化为两端弹性支承,考虑管-土间的相互作用和管内流体与管道之间的流固耦合作用,采用梁模型一般振型函数实施模态叠加法对液化区埋地管道进行地震响应的动态分析,探讨了管道、流体和液化土参数对管道上浮反应的影响。数值仿真结果表明:埋土管道在地震作用下砂土液化时的上浮位移,随液化区管道长度、管外径、管内流体流速、液化砂土的密度和管截面受到的轴向压应力的增大而增大,随管内输送流体的密度、液化土的相对弹性系数、管材的粘弹性系数和管截面受到的轴向拉应力的增大而减小,地震加速度幅值对管道上浮位移的影响相对较小。     

运用FLAC~(3D)对水泥土桩加固液化砂土地基的分析

该文运用三维有限差分软件FLAC3D(Fast Lagrangian Analysis of Continua)对水泥土桩加固的液化砂土地基建立模型并进行地震响应分析,从计算机数值模拟的角度对水泥土桩加固模型地基竖向位移、超静孔隙水压力、孔压比、剪应变增量、接触面剪力及表面加速度作了较系统地分析,总结了这些参数的变化规律,得到了一些有价值的结论,对实际工程有一定的指导意义。     

液化侧向扩展场地刚性排水管桩群桩振动台试验研究EI北大核心CSCD

地震作用下土体发生液化侧向扩展对建筑物极具破坏性,特别是对建筑物的桩基、高架桥梁等,消除和减小土体液化扩展引起的对结构安全的危害具有极大的意义。刚性排水管桩由圆形空心刚性桩与排水体结合而成,其在具有排水功能的同时,又具有较大的承载力,但是目前针对刚性排水管桩群桩抗液化性能的研究仍十分有限。基于振动台试验,开展了桩顶承台竖向荷载作用下刚性排水管桩群桩与普通桩群桩处理液化侧向扩展场地的振动响应对比研究,分析了地基土的超孔压比、加速度、平均沉降、承台位移、挡板位移以及桩身弯矩等。研究结果表明:刚性排水管桩地基与普通桩地基相比,超孔压、桩身弯矩、地基沉降、承台位移、岸壁位移明显减小,而加速度增大,充分表明刚性排水管桩的抗液化效果显著。     

液化场地钢筋混凝土桥墩残余位移分析

为探究场地液化对钢筋混凝土(ReinforcedConcrete,RC)桥墩残余位移影响机理，首先基于OpenSees有限元计算平台阐述了液化场地-结构有限元数值建模方法，通过与离心机试验结果对比验证了建模方法的可靠性。其次以一座实际工程中的单桩单柱式桥墩为原型，发展了液化场地-RC桥墩建模方法，建立非液化场地-桩-RC桥墩数值模型(模型1)和液化场地-桩-RC桥墩数值模型(模型2)，对两模型输入近断层地震动进行非线性动力时程分析。讨论了模型2场地液化情况，并对模型1与模型2的场地位移、桩身位移、桩身最大屈服曲率延性系数、墩顶残余位移及墩顶残余位移随地震动峰值加速度(PGA)变化情况进行对比。结果表明：地震过程中，模型2场地各深度出现不同程度液化现象；且土体中上部均达完全液化状态，土体水平极限抗力损失最大，对桩身侧向约束严重降低。液化导致场地震后残余位移显著增加，场地对桩身震后约束增强，导致桩顶残余位移增大。另外，由于地震过程中中上部液化土体对桩身侧向约束严重降低导致桩身塑性变形显著增大、桩身最大塑性变形位置明显下移以及桩身塑性变形区扩展，引起桩身更为严重的塑性损伤，进而增大桩顶残余位...     

爆破地震作用下CFG桩复合地基动力特性的数值分析

该文以CFG桩复合地基在爆破地震作用下的现场动力试验为背景,应用有限元软件建立有限元动力分析的数值模拟模型,从水平加速度、竖向加速度、水平位移、竖向位移及应力几个方面对复合地基同一位置不同深度和同一深度不同位置的动力响应进行了分析,其分析结果与试验结果吻合较好。通过数值模拟对动力响应的影响因素进行了比较研究。并与天然地基的动力响应结果进行对比,得出了有指导意义的结论。为今后探讨CFG桩复合地基的承载力、变形和动力特性提供了研究基础。     

Effect of fabric method on instability behavior of granular material

It is well known that the specimen preparation method and the resulting sand fabric significantly affect sand behavior and sand liquefaction resistance. In many cases, the fabric and behavior of reconstituted sand samples do not represent those of in-situ deposits. Therefore, understanding the influence of specimen preparation and sand fabric on its behavior, particularly at the critical state, is important for relating the behavior of laboratory reconstituted specimens to in-situ soil response. In this study, the effect of sand fabric and specimen preparation method on the shearing behavior of sand is studied using triaxial shear tests. Dry funnel pluviation (DFP) and wet tamping (WT) are used to prepare the specimens. The results from instability lines and stress–strain curves indicate that the liquefaction resistance of specimens prepared with the DFP method is more than specimens reconstituted by the WT method.     

可液化场地中复杂异跨地铁地下车站结构的地震反应分析

地基液化是导致地铁地下车站结构在地震中发生严重破坏的重要威胁之一,然而目前对可液化场地中地铁地下车站结构地震反应的研究较少,尤其是针对大型复杂异跨地铁地下车站结构地震反应的研究更是少见。通过引入砂土液化大变形本构模型,采用有限元网格自适应调整技术克服土体网格大变形的畸变问题,建立了可液化场地土一复杂异跨地铁车站结构静动耦合非线性相互作用的有限元数值模型,分析了该相互作用体系的场地液化分布特征、异跨车站结构上浮特征、周围场地位移沉降及矢量特征、结构侧向变形和地震损伤破坏特征等,初步揭示了该相互作用体系的地震反应规律及液化地基中大型异跨地下结构的地震破坏特征,研究成果可为提高可液化场地中异跨地铁车站结构地震反应的认识及完善其抗震设计方法提供合理的参考。     

爆炸荷载作用下礁沙地基钢塔架结构动力响应试验探究

爆炸荷载作用下礁砂地基钢塔架结构动力响应的研究,对我国的国防和岛礁工程建设具有重要的参考和现实意义.通过模型试验对不同礁砂地基类型、药量、爆心距等工况下钢塔架结构加速度动力响应信号进行测试,结合量纲分析与π定理,得到钢塔架结构模型的加速度动力响应计算公式,并利用试验所采集的数据对计算公式进行参数拟合,得出了在礁沙地基下...     

Stress-induced anisotropy in sand under cyclic loading

The anisotropy of a granular material’s structure will influence its response to applied loads and deformations. Anisotropy can be either inherent (e.g. due to depositional process) or induced as a consequence of the applied stresses or strains. Discrete element simulations allow the interactions between individual particles to be explicitly simulated and the fabric can be quantified using a fabric tensor. The eigenvalues of this fabric tensor then give a measure of the anisotropy of the fabric. The coordination number is a particle scale scalar measure of the packing density of the material. The current study examines the evolution of the fabric of a granular material subject to cyclic loading, using two-dimensional discrete element method (DEM) simulations. Isotropic consolidation modifies and reduces the inherent anisotropy, but anisotropic consolidation can accentuate anisotropy. The ratio of the normal to shear spring stiffness at the particle contacts in the DEM model affects the evolution of anisotropy. Higher ratios reduce the degree of anisotropy induced by anisotropic consolidation. The anisotropy induced by cyclic loading depends on the amplitude of the loading cycles and the initial anisotropy.     

重型设备-桩箱基础-砂卵石地基体系动力稳定性振动台试验

设计并完成了重型设备-桩箱基础-砂卵石地基体系地震反应的大型振动台模型试验,分析了设备结构位移和转角反应特征以及桩箱基础和砂卵石地基的动力稳定性。试验结果表明:设备结构位移反应峰值呈上大、下小的三角形分布,位移和转角反应受结构振动方向第一振型控制;振动过程中基底两侧土反力增量不均,由上部重型结构水平惯性力引起的基础转动效应明显,这也是基础发生倾斜的主要原因;箱形基础振动对周围地基土产生挤密作用,基础转动显著影响侧墙土压力分布形态;强震作用下,砂卵石地基持力层振动孔压比能达到0.3~0.5,振动孔压的累积和消散可引起基础附加沉降。试验结果可为砂卵石地基箱形基础抗震设计提供参考和借鉴。     

Micro-scale modeling of anisotropy effects on undrained behavior of granular soils

This paper presents a micro-scale modeling of fabric anisotropy effects on the mechanical behavior of granular assembly under undrained conditions using discrete element method. The initial fabrics of the numerical samples engendered from the deposition under gravity are measured, quantified and compared, where the gravitational field can be applied in different directions to generate varying anisotropy orientations. The samples are sheared under undrained biaxial compression, and identical testing conditions are applied, with samples having nearly the same anisotropy intensities, but with different anisotropy directions. The macroscopic behaviors are discussed for the samples, such as the dilatancy characteristics and responses at the critical state. And the associated microstructure changes are further examined, in terms of the variables in the particulate scale, with the focus on the fabric evolution up to a large deformation reaching the critical state. The numerical analysis results compare reasonably well with available experimental data. It is also observed that at critical state, in addition to the requirements by classical critical state theory, a unique fabric structure has also been developed, and might be independent of its initial fabric. This observation is coincided with the recent theoretical achievement of anisotropic critical state theory. Finally, a general framework is introduced for quantifying and modeling the anisotropy effects.     

Macro deformation and micro structure of 3D granular assemblies subjected to rotation of principal stress axes

This paper presents a numerical investigation on the behavior of three dimensional granular materials during continuous rotation of principal stress axes using the discrete element method. A dense specimen has been prepared as a representative element using the deposition method and subjected to stress rotation at different deviatoric stress levels. Significant plastic deformation has been observed despite that the principal stresses are kept constant. This contradicts the classical plasticity theory, but is in agreement with previous laboratory observations on sand and glass beads. Typical deformation characteristics, including volume contraction, deformation non-coaxiality, have been successfully reproduced. After a larger number of rotational cycles, the sample approaches the ultimate state with constant void ratio and follows a periodic strain path. The internal structure anisotropy has been quantified in terms of the contact-based fabric tensor. Rotation of principal stress axes densifies the packing, and leads to the increase in coordination numbers. A cyclic rotation in material anisotropy has been observed. The larger the stress ratio, the structure becomes more anisotropic. A larger fabric trajectory suggests more significant structure re-organization when rotating and explains the occurrence of more significant strain rate. The trajectory of the contact-normal based fabric is not centered in the origin, due to the anisotropy in particle orientation generated during sample generation which is persistent throughout the shearing process. The sample sheared at a lower intermediate principal stress ratio \((b=0.0)\) has been observed to approach a smaller strain trajectory as compared to the case \(b=0.5\), consistent with a smaller fabric trajectory and less significant structural re-organisation. It also experiences less volume contraction with the out-of plane strain component being dilative.     

Pushover and shaking table tests on a rocking-governed column-footing model on dry dense sand

Footings are normally used on firm ground. Previous studies have indicated that footings with large vertical bearing capacity have small settlement and recentering characteristics during excitation. The rocking foundation design is, therefore, considered quite suitable for a firm ground condition; however, some 1 g shaking table tests have shown that rocking structures still might have stability problems leading to toppling. In this study, we conducted pushover and shaking table tests on a rocking-governed column-footing model on dry dense sand to investigate the performance of a rocking structure on sturdy ground. Even in a dense soil condition, the structure had a longer period response than in the fixed-base condition, but this did not necessarily reduce the seismic demand on the structure. The major source of rocking isolation is that the maximum acceleration response of the structure is limited by the ultimate moment capacity of the footing; however, it is important to note that extensive rocking may induce a larger permanent rotation, which may influence the functionality of structures, and also may lead to structure instability, such as foundation twisting observed in this study.     

三维各向异性砂土UH模型

为了全面地描述砂土在三维空间不同方向上的本构特性,应当综合考虑各向异性和中主应力的影响。该文在砂土UH（统一硬化）模型的基础上,采用各向异性的变换应力方法,发展了三维各向异性砂土UH模型。该方法通过调整不同方向应力分量的相对大小,将各向异性砂土等效为各向同性砂土,并且在变换应力空间将偏平面上的破坏或屈服包线变成圆形。用各向异性的变换应力张量代替真实应力张量,能够方便地得到同时考虑各向异性和中主应力影响的砂土UH模型。与试验数据的对比表明,三维各向异性UH模型能够合理地预测真三轴应力状态下各向异性砂土的应力-应变关系。     

ALE方法在沉箱码头地震液化分析的应用

沉箱码头作为一种常用的结构形式,在地震荷载作用下由于地基液化可能发生沉降、倾斜等灾害形式。在强震作用下,传统有限元方法可能由于液化大变形导致网格畸变而不能正确反映沉箱码头的地震响应。基于算子分裂技术,该文发展了一种适用于饱和砂土液化大变形分析的ALE有限元方法并将其用于两种地基处理形式的沉箱码头在三种地震荷载作用下的地震液化分析。结果表明:强震作用下,ALE方法能够提供精度可靠的沉箱码头地震响应并保证整体的网格质量;沉箱的残余位移与地震荷载峰值近似呈线性关系,且对低频地震荷载较为敏感;在置换区和回填区存在高孔压连通区,可能会加大沉箱码头的残余位移,可考虑设置阻断置换区和回填区孔压转移的措施。