液化土层上建筑物倾斜计算要点初步分析

地震中饱和砂土地基液化会引起结构物倾斜并导致其功能丧失,但目前缺乏相应的数值模拟方法。通过振动台简单模型试验,寻找输入波-基底竖向动应力-基底孔压-结构震陷之间的关系,提出发展液化土层上建筑物倾斜数值模拟方法所需要考虑的要点和应满足的条件。结果表明：（1）分析方法中孔压增长模型应适于不规则波计算,能准确地计算出孔压增长过程,准确地计算出峰值一样但不同波形下孔压增长的差别;（2）孔压增长模型应能反映土的各向异性特性对孔压增长的影响,能合理地计算出拉、压不同应力作用下孔压增长的差异;（3）孔压增长模型应能反映非均等固结条件对孔压增长过程的影响,能合理地计算出结构底部不同固结比土体中孔压增长过程;（4）分析方法中土体变形的计算应能跟踪液化过程中的变形发展,且具备大变形计算能力。     

液化地基中桩的破坏机理研究进展

地震诱发的地基液化对桩基础的破坏极大,液化地基中桩的破坏机理是岩土地震工程中的一个重要研究课题。目前地基液化时桩土结构系统的地震性态尚没有认识充分,已有的研究内容较多局限于桩身材料的强度破坏方面,难以考虑液化土体侧向流动、基桩屈曲失稳、以及土与结构动力相互作用等复杂因素的影响。本文重点加强以下3个方面的深入探讨和研究:(1)液化地基中桩的屈曲失稳;(2)液化地基中桩基破坏的数值模拟新方法;(3)液化地基中桩-土-结构的动力相互作用分析。     

砂性地基中防波堤地震残余变形机制分析与液化度预测法

海港工程中防波堤地震残余变形的预测以及震损机制的分析是较复杂的问题。采用定义在应变空间中考虑土体动主应力轴方向偏转影响的多重剪切机构塑性模型,分别从砂性土的黏粒含量和标贯击数2个主要影响因素对防波堤地震变形的变化机制进行了有效应力分析,得出防波堤较大的残余变形是由于地震作用下土体中孔隙水压力的升高致使土体软化而产生的。然后,采用液化度单一指标从物理本质上来间接表征防波堤的残余变形,得到防波堤残余变形与液化度之间的函数预测关系,预测值与震害调查值以及数值分析值基本相符,表明所得液化度预测公式具有一定的可靠性。残余变形的液化度预测法可为类似防波堤地震灾害设计与评价提供参考依据。     

液化流动条件下隧道结构动力分布场研究

将液化流动的土体视为流体,运用流体力学原理,基于矢量符号运算法进行液化场地的动力场分析,求解出动力场解析解。采用ABAQUS/CFD进行液化场地流体动力学有限元分析,根据流动基本控制方程,计算出动力场的数值解。分析结果表明,液化土体横向流动时,隧道结构周围的应力场既包括由表面压力引起的压力阻力,也包括由剪应力引起的摩擦阻力;隧道结构周围的液化土体流动速度很小,但在隧道结构下方存在流动速度加强区;理论计算的动力场解析解大于有限元计算的数值解,但总体的分布变化趋势基本一致;隧道结构附近的应力场变化较为密集,所受到的应力主要分布在液化土体流动的迎面位置。     

液化型路堤边坡动力数值模拟分析

液化型路堤边坡动力稳定性问题涉及岩土工程与工程地震两个学科领域,是边坡工程与砂土液化的交叉课题。采用天然地震记录为输入条件,应用Finn本构关系模型,运用有限差分法,对填土+砂土+卵砾土地层组合的路堤边坡进行了全时程动力分析,探讨了地震作用下路堤边坡的液化初步规律和稳定性。数值模拟结果表明:地震作用引起了路基饱和砂土有效应力急剧减小,并导致路基砂土液化,引起路堤变形破坏。孔隙水压力的积累与消散不仅与地震记录序列存在对应关系,也与砂土所处的位置和深度有密切关系。地表变形破坏主要表现为路堤顶面发生震陷和拉裂破坏,坡底面产生挤压隆起变形。地面以下的变形破坏主要包括土体剪切破坏和深部砂土液化引起的侧向流动破坏。     

渗透系数对砂土液化震陷影响的数值研究

饱和砂土在地震荷载的作用下往往会产生液化变形,包括竖向震陷和侧向扩展。砂土由于液化的作用,其渗透系数会发生改变,而目前描述砂土液化变形的本构模型均采用常量渗透系数,这是造成自由场地的震陷数值模拟结果低于试验观测值的重要原因之一。利用开源有限元平台Open Sees对饱和砂土的自由场地震陷进行模拟分析,对比离心机模型试验,分析了渗透系数对饱和砂土液化震陷的影响。为进一步提高数值模拟的准确性,采用了适合于动力分析及液化模拟的边界面模型。与固定渗透系数模型相比,最终提出的变渗透系数模型允许液化状态的渗透系数升高为初始值的数倍,该模型模拟结果较好,可以作为从渗透模型角度提高数值模拟精度的近似考虑。通过一系列的模拟和分析发现,采用合理的变渗透系数模型,可在一定程度上提高砂土自由场地地震液化震陷的数值模拟精度。     

可液化倾斜场地的侧向扩展机制分析

针对已完成的液化水平自由场大型振动台试验,采用OpenSees有限元平台,建立了振动台试验的数值分析模型,并验证了该模型的可靠性。基于此,建立了地基整体倾斜的自由场数值模型,重点讨论了场地土体的非循环地震响应和液化侧向扩展机制。结果表明,建立的数值模型可以有效地模拟可液化场地的地震反应。倾斜场地中,可液化松砂与上覆非液化层界面处具有显著的相对位移,饱和砂土的应变累积从松砂层浅层开始,逐步向深层发展,超孔隙水压力增长和土体非循环应变累积未表现出耦合的现象,场地的中部土体控制非循环横向位移的发展。另一方面,土体液化过程中,当沿滑动面的剪切应力小于初始静剪应力时,液化侧向扩展启动。此时饱和松砂层的剪应力比在0.04～0.06范围内,略小于初始静剪应力比。此外,还发现液化诱导侧向扩展需要场地具有一定的倾斜度（大于0.5°）;土的侧向位移符合余弦分布模式;随着场地倾斜度的增大,可液化深层土对整体侧向位移的贡献更显著。     

饱和砂土液化后大变形机制的离散元细观分析

采用颗粒流软件模拟了饱和砂土在不排水条件下的循环剪切试验,研究了不同因素对液化的影响,并进一步分析了饱和砂土液化后宏观变形的基本规律。在此基础上,从孔隙分布角度解释了砂土液化后的大变形的细观物理机制。通过自编程序对颗粒排列和孔隙分布的演化过程进行定量描述,给出孔隙率标准差作为液化后体积收缩势的度量,并研究了孔隙率标准差与液化后大变形的关系。离散元细观数值模拟再现了室内试验中的宏观现象,证实了室内试验中饱和砂土液化后的有限剪切大变形是客观真实的材料响应。土体体积收缩势的累积所导致的孔隙均匀化以及土颗粒间自由空隙增大正是饱和砂土液化后循环剪应变逐渐增大的细观机制。孔隙率标准差作为孔隙均匀化的量化指标,与循环剪应变各周次幅值有良好的相关性。     

地震历史对各深度土体抗液化性影响的振动台试验研究

地震引起的液化会对岩土结构造成重大破坏,而震后伴随的余震可能导致砂土再次发生液化。为研究自由场地下地震历史对各深度饱和砂土的抗液化能力的影响,设计并开展了一系列振动台试验。试验中对砂土输入了4次不同加速度的震动事件,细分为7次小的事件。通过计算对比每次振动事件的超静孔压比、加速度响应以及土体沉降,以研究不同地震历史下各深度土体抗液化能力的变化规律。试验结果表明：输入的地震波加速度大小与加速度响应系数呈正相关关系;饱和砂土遭受轻微和中等地震时液化更容易发生在浅层而非表层;表层土体中,强烈地震后的中等余震使得土体对地震烈度的敏感性降低,孔隙水压力在峰值加速度后消散迅速,液化时间缩短;有强震历史的土体会降低浅层土体的抗液化性能,深层土体抗液化性反而会在经历强地震后增强,影响深度范围取决于地震的烈度;通过数据拟合得到了不同等级的循环地震在各深度土体的抗液化提升比的量化公式。试验结果可以反映在地震实际中各等级的地震历史对各深度土体的不同影响。     

水平动载下饱和砂土地基液化区扩展

对一侧受沿深度分布的水平振动载荷时 ,半无限饱和砂土地基液化区域的扩展进行了数值分析。结果表明 :在水平振动载荷作用下 ,砂土地基液化区域从载荷端由近到远逐渐发展 ;随载荷强度和频率的增加 ,以及土体模量的减小 ,液化发展逐渐加快。在考虑的频率范围内 ,随着载荷频率的增加 ,土体表面的变形和土中的孔隙压力上升速度增加。     

倾斜土层地震液化诱发大变形数值分析方法

基于有限元理论,进行了缓坡土层地震液化引起大变形的数值方法研究,即采用二维有效应力动力有限元方法进行分析,在液化分析过程的每一时段考虑地震液化和振动软化得到土单元的模量,通过非线性静力方法计算每时段地震液化引起的大变形,得到土层各深度处的水平和竖向位移。由算例分析了地震动和土层坡度等因素的影响,通过对比分析表明了该方法的有效性,可为工程场地地震地质灾害评价提供参考数据。     

Flow analysis of liquefied soils based on smoothed particle hydrodynamics

To overcome the disadvantages of traditional flow analysis methods for liquefied soils that exhibit fluidization and large deformation characteristics, Smoothed particle hydrodynamics (SPH) is adopted in this study to analyze the flow processes of liquefied soils. Bingham model with the use of the Mohr–Coulomb yield criterion, the concepts of equivalent Newtonian viscosity, and the Verlet neighbor list method are introduced into the framework of SPH to build an algorithm for the analysis of flowing liquefied soils. This modeling involves a simulation of physical model test of flowing liquefied soils that can be compared with numerical results. In addition, a shaking table test is selected from the literature for SPH analysis to verify the validation of the SPH method and extend its applications. The SPH simulation can reproduce the flow processes of liquefied soils and constrain estimates of the horizontal displacement, vertical displacement, and velocity of soils after liquefaction. According to the dynamic behaviors of the materials involved, designs can be implemented to improve the seismic safety of structures.     

Numerical simulation of flow processes in liquefied soils using a soil–water-coupled smoothed particle hydrodynamics method

Liquefaction can result in the damage or collapse of structures during an earthquake and can therefore be a great threat to life and property. Many site investigations of liquefaction disasters are needed to study the large-scale deformation and flow mechanisms of liquefied soils that can be used for performance assessments and infrastructure improvement. To overcome the disadvantages of traditional flow analysis methods for liquefied soils, a soil–water-coupled smoothed particle hydrodynamics (SPH) modeling method was developed to analyze flow in liquefied soils. In the proposed SPH method, water and soil were simulated as different layers, while permeability, porosity, and interaction forces could be combined to model water-saturated porous media. A simple shear test was simulated using the SPH method with an elastic model to verify its application to solid phase materials. Subsequently, the applicability of the proposed SPH modeling method to the simulation of interaction forces between water and soil was verified by a falling-head permeability test. The coupled SPH method produced good simulations for both the simple shear and falling-head permeability tests. Using a fit-for-purpose experimental apparatus, a physical flow model test of liquefied sand has been designed and conducted. To complement the physical test, a numerical simulation has been undertaken based on the soil–water-coupled SPH method. The numerical results correspond well with the physical model test results in observed configurations and velocity vectors. An embankment failure in northern Sweden was selected so that the application of the soil–water-coupled SPH method could be extended to an actual example of liquefaction. The coupled SPH method simulated the embankment failure with the site investigation well. They have also estimated horizontal displacements and velocities, which can be used to greatly improve the seismic safety of structures.     

砂土液化流动变形的简化方法

已有的液化砂土流动特性试验结果表明,砂土在液化流动状态下是剪切稀化非牛顿流体,可以用幂函数表示其剪应力-剪应变率的关系,从而建立了砂土液化流动的本构方程。基于FLAC3D程序的二次开发平台,将液化流动本构方程开发到FLAC3D中,建立了液化流动变形的简化分析方法。通过倾斜场地的液化流动变形分析,发现倾斜场地的液化变形曲线可以用正弦函数曲线描述,这与Towhata的理论分析成果一致,验证了本方法的合理性。分析了液化层坡度、稠度系数、流动指数以及弹性参数等变量对液化变形的影响。计算结果表明,液化变形随液化层坡度的增大而逐渐增大,液化砂土的稠度系数和流动指数对液化流动变形有重要的影响,而弹性参数对变形基本无影响,因此,在实际工程分析中,需要对流动模型参数进行深入研究。     

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。     

文克尔地基模型下液化土桩基水平振动响应分析

在桩基顶部承受竖向荷载作用的条件下,将完全液化后的上层土体视为流体,将桩基等效为欧拉-伯努利梁模型,探讨了桩底嵌固时桩基顶部的水平振动阻抗。运用流体动力方程模拟顶部液化土层的运动,运用文克尔地基模拟下部非液化分层土的运动。结合传递矩阵法,利用液化土与非液化分层土交界面处的位移、转角和内力连续条件,得到桩基顶部和底部的相关位移?内力表达关系式。根据桩基底部的嵌固条件,求得桩顶阻抗的表达式。与已有文献解进行对比,验证了分析过程的正确性。对阻抗影响条件进行参数分析,表明液化深度、轴力和流体密度大小对桩顶阻抗有不同的影响。     

Test Investigation on Liquefied Deformation Structure in Saturated Lime-Mud Composites Triggered by Strong Earthquakes

Three identical model boxes were made from transparent plexiglass and angle iron. Using the method of sinking water and according to the sedimentary rhythm of saturated calcium carbonate(lime-mud) intercalated with cohesive soil,calcites with particle sizes diameters of ≤ 5 μm,10–15 μm and 23–30 μm as well as cohesive soil were sunk alternatively in water of three boxes to build three test models,each of which has a specific size of calcite. Pore water pressure gauges were buried in lime-mud layers at different depths in each model,and connected with a computer system to collect pore water pressures. By means of soil tests,physical property parameters and plasticity indices(Ip) were obtained for various grain-sized saturated lime-muds. The lime-muds with Ip ranging from 6.3 to 8.5(lower than 10) are similar to liquid saturated silt in the physical nature,indicating that saturated silt can be liquefied once induced by a strong earthquake. One model cart was pushed quickly along the length direction of the model so that its rigid wheels collided violently with the stone stair,thus generating an artificial earthquake with seismic wave magnitude greater than VI degree. When unidirectional cyclic seismic load of horizontal compression-tension-shear was imposed on the soil layers in the model,enough great pore water pressure has been accumulated within pores of lime-mud,resulting in liquefaction of lime-mud layers. Meanwhile,micro-fractures formed in each soil layer provided channels for liquefaction dewatering,resulting in formation of macroscopic liquefaction deformation,such as liquefied lime-mud volcanoes,liquefied diapir structures,vein-like liquefied structures and liquefied curls,etc. Splendid liquefied lime-mud eruption lasted for two to three hours,which is similar to the sand volcano eruption induced by strong earthquake. However,under the same artificial seismic conditions,development of macroscopic liquefied structures in three experimental models varied in shape,depth and quantity,indicating that excess pore water pressure ratios at initial liquefaction stage and complete liquefaction varied with depth. With size increasing of calcite particle in lime-mud,liquefied depth and deformation extent increase accordingly. The simulation test verifies for the first time that strong earthquakes may cause violent liquefaction of saturated lime-mud composed of micron-size calcite particles,uncovering the puzzled issue whether seafloor lime-mud can be liquefied under strong earthquake. This study not only provides the latest simulation data for explaining the earthquake-induced liquefied deformations of saturated lime-mud and seismic sedimentary events,but also is of great significance for analysis of foundation stability in marine engineering built on the soft calcium carbonate layers in neritic environment.     

强震作用下液化场地群桩动力响应及p-y曲线

为研究液化场地中群桩在强震作用下的动力响应特征及桩侧土抗力-桩土相对位移（p-y）曲线规律,依托海文大桥实体工程,基于振动台模型试验,开展了0.15g～0.35g地震动作用饱和粉细砂土层不同埋置深度下的砂土孔压比、桩身弯矩及p-y曲线动力响应研究。结果表明：地震动强度达到0.25g时,不同埋置深度下的饱和粉细砂土层孔压比均大于0.8,产生液化现象,且随埋置深度增加,孔压比增长时刻明显滞后;不同埋置深度下,桩身弯矩最大值均位于液化土层和非液化土层分界面处;同一埋置深度时,随地震动强度的增大,p-y曲线所包围的面积逐渐增大,其整体斜率逐渐变小,说明桩-土相互作用动力耗能逐渐增大,桩周土体刚度逐渐减小;随埋置深度增加,p-y曲线所包围的面积逐渐减小,其整体斜率逐渐增大,说明桩-土相互作用动力耗能逐渐减小,桩周土体刚度逐渐增大。因此,液化场地桥梁群桩抗震设计时,应综合考虑液化土层与桩基础的相互位置关系,确保桩基础在液化土层与非液化土层分界处的抗弯承载能力。