基于PFC-COMSOL联合开展软土固结细观机理研究

基于细观颗粒离散元方法建立土体细观结构性模型,依据室内试验匹配模型细观参数,采用COMSOL软件进行土体内部渗流场求解,实现PFC-COMSOL联合开展软土固结研究,最终将分析方案与室内固结试验数据进行对比。研究结果表明:PFC使用聚粒模拟絮凝状软土能较真实地再现海相软土的细观结构特性;结构性蠕变对软土固结沉降量有一定影响,软土压缩量应考虑蠕变的影响;软土固结过程中首先出现不均匀压缩,超孔压完全消散后,土体恢复均匀性。     

软土结构性及结构破损机理

通过对漳州和黄石地区2种结构性软土进行单向与等向压缩试验,根据原状样和重塑样的压缩曲线确定了先期固结压力和结构强度.研究表明,漳州软土结构应力比在1.1~1.2之间,黄石软土在1.2~1.3之间,说明在沉积过程中产生的结构屈服压力与上覆压力基本成正比.结构强度随埋深、试验的加荷间隔、加荷时间的增加而增大;由单向压缩试验曲线得到的结构强度大于等向压缩试验曲线得到的结构强度;卸载再加载过程对结构性强弱影响不大.计算得到漳州软土结构破损系数为17.7%,黄石软土结构破损系数为14%,说明黄石软土结构性略强于漳州软土.从微观角度解释了土的结构破损主要是由颗粒间的连接方式的改变与孔隙变化所引起的.     

考虑软土结构性高速公路软基变形有限元分析

软土的结构性对沉降有着很大的影响.详细介绍了软土的结构性模型,Plaxis 程序的特点以及其在计算路基沉降方面的应用.通过工程现场采集的数据与计算结果对比,认为在基于软土结构性模型的基础上对Plaxis程序的计算结果进行修正、选择合理加载步长、符合土体的边界条件以及初始条件,Plaxis 程序能够很好的模拟土体在荷载作用下的受力变形过程,计算结果能够满足工程精度要求.可以用Plaxis程序来预测高速公路软土路基的变形.     

风积土本构模型参数修正方法

目的修正邓肯-张E-B模型参数并建立新的风积土本构模型.方法采用应变控制式三轴压缩仪,通过对风积土的结构性试验研究,进行固结排水试验并引入围压损伤比的概念.结果试验通过观测围压在0.1 MPa、0.2 MPa、0.3 MPa时对风积土结构的损伤,得到考虑土体结构性影响的邓肯-张E-B模型的参数修正方法.试验中,在多组围压下发现土体内摩擦角φ值逐渐减小,黏结力C值逐渐增大;在卸载-再加载条件下,原状土的结构性被破坏.因此,将邓肯—张模型中参数C、φ、Rf、K、n、Kb、m、Kur、nur进行了修正.结论提出的围压损伤比对土体的模型参数修正方法具有一定的理论意义和实际应用价值,建立修正后的风积土本构模型为其他同类工程地区风积土工程数值计算提供了参考依据.     

不同填料顺序下路基沉降分析

针对杨林船闸原状淤泥质土,进行一维压缩试验,研究一次加荷和多次加荷试验下的固结特性。结合典型路基断面,运用ABAQUS软件建立时间硬化蠕变模型和D-P弹塑性模型,得到不同填料顺序下杨林船闸附近高速公路路基工后沉降。试验表明,多次加荷试验下,主、次固结的分界点时间变大,即多次加荷试验主固结完成时间大于一次加荷试验主固结完成时间。模拟结果显示,未考虑蠕变情况下路基沉降量小于考虑蠕变情况下路基沉降量。把蠕变考虑进来时一次填料计算的沉降量大于多次填料计算的沉降量,同时也验证了多次加荷可有效降低次固结沉降。     

逐级改变有限元计算参数分析软土地基的稳定性

目的提出综合处理后的软土地基在分级加载作用下逐级改变有限元计算参数,分析软土地基稳定性的计算方法和模型参数．方法利用理论计算或现场监测结果确定下一级荷载加栽前软土地基相对于最终沉降量所达到的固结度,根据固结压力与固结度,利用室内试验结果获得计算参数,输入有限元计算软件中相应的施工步骤中,作为评价下一级荷载时软土地基的稳定性．结果将逐级改变计算参数和采用固定不变计算参数的计算结果与实测结果比较,逐级改变计算参数的计算结果表明,竖向位移、侧向位移与实测结果很接近．而采用固定不变的计算参数计算结果竖向位移最大差值达42．5cm,侧向位移最大差值达8cm,与实际工程不符．结论逐级改变计算参数时更接近工程中的实际情况．所提出的计算方法和试验所确定的模型参数为今后类似工程的设计、计算提供参考依据．     

软土地基一维非单调压缩固结性状分析

针对储油罐等结构物在使用期内常对下部地基产生往复性变化荷载等实际情况,考虑土体在加载和卸载时压缩系数不同的特性,研究了软土地基一维非单调压缩固结问题.基于现有的成层地基一維线性固结解析理论,采用半解析法求解,得到了相应的半解析解并编制了计算程序.通过系列计算和分析对比,全面地研究了当考虑和不考虑非单调压缩因素时,软土地基固结性状的区别以及不同参数对非单调压缩固结性状的影响.结果表明,压缩系数比、荷载比和卸载速率等对地基的固结均有显著影响;在考虑了非单调压缩的影响因素后,软土地基中超静孔压的消散将滞后于沉降的发展.     

考虑软土流变和损伤的比奥固结有限元分析

软土的变形受到流变性和结构性这两种基本特性的双重影响,计算分析中应予以同时考虑。本文基于已有的软土流变模型,提出了一种可以同时考虑这两种特性影响的软土粘弹塑性损伤本构模型,并能定量的描述原状土结构性逐步破损过程和反映施工预处理引起的初始损伤对于土体后期变形的影响。将该模型耦合Biot固结理论,导出了有限元求解方程并介绍了求解方法,编制了相应的有限元程序。结合工程实例,采用本文模型和不考虑软土流变及损伤影响的双屈服面模型分别对路基变形进行对比计算分析,验证了模型的合理性,揭示了软土地基考虑流变性及其结构性损伤在固结变形中的综合作用。     

交通荷载引起的软土地基累积沉降计算方法研究

软土地基在交通荷载下的沉降计算和稳定性保证,是修建低路堤公路的重要环节,而科学的计算方法是其前提和基础.文章以新河（青潍界）至辛庄子高速公路寿光段为例,通过利用循环荷载作用下软粘土不排水累计变形模型-Chai-Miura模型,结合利用Flac3D有限差分程序计算地基中的动、静应力,对软土地基的累计沉降进行了计算.结果表明:对修建在软土地基上的低路堤,反复的交通荷载引起的累计沉降很大,同时变形随深度衰减很快,当深度大于8m后,变形与地表的相比小得多;路基沉降速率与运行时间关系较大,且随时间非线性增长,其中累积沉降及总沉降在通车初期累计沉降发展较快,且累积沉降远大于固结沉降,占总沉降的90％多.     

宁波软土流变试验及经验模型

为了研究宁波软土的流变特性及经验模型,进行一维固结流变试验和三轴排水流变试验,并采用西原模型拟合模型参数.试验对比分析表明,固结试验的侧限作用会阻碍试样流变的发展,应力-应变等时曲线偏向应力轴,而三轴流变试验试样的流变能够得到充分地发展,应力-应变等时曲线偏向应变轴,能够更好地体现软黏土的流变性状.将一维西原模型应力-应变关系扩展到三维,采用修正剑桥模型屈服函数描述材料的塑性屈服.推导荷载恒定时土样的轴向应变计算公式,根据三轴排水流变试验结果进行曲线拟合,确定西原模型参数.结果表明,该模型能够较好地符合宁波软土的流变规律.     

双标量描述下土的弹性非线性损伤模型

根据土的变形特点,采用双标量描述土的各向同性弹性损伤,将土看成是由两种材料组成的复合体,一种材料的柔度矩阵仅与土的弹性模量有关,而另一种材料的柔度矩阵仅与土的剪切模量有关。分别定义各自的损伤变量,并根据等效应力原理建立各自的损伤本构方程,最后通过合成获得了土的弹性非线性损伤本构方程,并根据土的三轴试验结果确定损伤演化方程,在此基础上建立了一个土的弹性非线性各向同性损伤模型。将其运用到地基沉降计算中,与实测结果进行比较,验证了本模型的正确性。     

混凝土四相复合模型的三维细观破坏模拟

考虑到混凝土内部细微裂隙的存在对混凝土强度和变形的重要影响,提出将混凝土视为砂浆、骨料、界面和随机缺陷组成的四相复合材料模型。混凝土细观单元服从应变空间内增量形式的弹塑性损伤本构,采用破坏单元网格消去法模拟混凝土裂纹扩展,数值模拟了三维混凝土单轴压缩和不同均质度试样的单轴拉伸弹塑性损伤破坏过程。结果表明：文中建立的混凝土四相复合材料模型中增加随机缺陷的模拟,一定程度上符合混凝土成形实际情况;随机缺陷的体积百分比是决定弱化混凝土宏观强度程度的主要因素;均质度低的单轴拉伸试样裂纹咬合明显,均质度高的试样断裂更清晰,数值模拟结果与试验结果较吻合。     

砌块结构地震破损反应研究

本文研究了软化负刚度恢复力模型对砌块结构在强震作用下的破损反应影响,讨论了由于软化负刚度效应引起的结构系统不稳定反应的一些理论和数值计算上的问题。文中还以一座七层配筋粉煤灰砌块结构模型在模拟地震作用下的反应为例,进行了理论计算结果和振动台试验结果的比较。结果表明,本文的计算模型能够较好地模拟砌块结构在强震作用下的破损反应。     

碎石土边坡稳定性及影响因素的数值模拟研究

结合工程实例,采用有限差分强度折减法对碎石土边坡进行稳定性数值模拟求得安全系数,并与传统分析方法进行计算比较.同时,分析边坡土性及计算模型参数对稳定性的影响.结果表明,FLAC3D分析边坡的稳定性可行且具有较好的优越性,内聚力、内摩擦角及土体密度对边坡稳定性的影响更加显著.     

密肋复合墙体的静力弹塑性分析

针对新型密肋壁板结构中的复合墙体建立了Push-over分析简化计算模型，采用Push-over分析方法，利用大型结构分析程序SAP2000n对复合墙体进行了罕遇地震作用下的计算，并与试验结果进行了比较．分析表明Push-over分析是对这种新型复合墙体进行弹塑性分析和性能评估的一种有效的方法，由于复合墙体独特的构造特点，使其承力构件（砌块、框格）能够分阶段释放地震能量，从而可有效提高结构的耗能能力和抗倒塌能力．     

Settlement calculation for long-short composite piled raft foundation

The mechanism of long-short composite piled raft foundation was discussed. Assuming the relationship between shear stress and shear strain of the surrounding soil was elasto-plastic, shear displacement method was employed to establish the different explicit relational equations between the load and the displacement at the top of pile in either elastic or elasto-plastic period. Then Mylonakis ＆ Gazetas model was introduced to simulate the interaction between two piles or between piles and soil. Considering the effect of cushion, the flexible coefficients of interaction were provided, With the addition of a relevant program, the settlement calculation for long-short composite piled raft foundation was developed which could be used to account for the interaction of piles, soil and cushion. Finally, the calculation method was used to analyze an engineering example. The calculated value of settlement is 10.2 ram, which is close to the observed value 8.8 mm.     

A flexible various-scale approach for soil-structure interaction and its application in seismic damage analysis of the underground structure of nuclear power plants

In simulations of geotechnical engineering, interface elements are versatile tools and are widely used in the modeling of the relative displacements between soils and structures. To consider the case of a local failure adjacent to a soil-structure interaction region, a partial mesh refinement should be performed. In this study, a three-dimensional (3D) interface element with an arbitrary number of nodes is developed as a new technique to reduce the complexity and difficulty of managing the various scales between soil and structure. An asymmetric number of nodes is permissible on the two sliding surfaces. In this manner, soil and structure can be discretized independently, and the various-scale model is established conveniently and rapidly. The accuracy of the proposed method is demonstrated through numerical examples. The various-scale approach is employed in an elasto-plastic seismic damage analysis of a buried concrete drainage culvert of a nuclear power plant. The results indicate that by applying the proposed method, the number of elements decreased by 72.5%, and the computational efficiency improved by 59% with little influence on accuracy. The proposed method is powerful for local damage evolution analyses of both soil and structure and possesses great practical significance and the potential for further application, especially for nonlinear analysis of large-scale geotechnical engineering.     

Time-dependent lateral response of pile embedded in elasto-plastic soil

A two-dimensional (2D) finite element analysis was carried out to assess the time-dependent behavior of single vertical pile embedded in elasto-plastic soil. The finite element analyses were carried out using the linear elastic model for the structure of the pile, while the Mohr-Coulomb model was used for representing the soil behavior surrounding the pile. The study includes cohesionless and cohesive soil to assess the lateral response of pile in the two types of soil. The whole geotechnical model is suitable for problem of piles to determine the design quantities such as lateral deformation, lateral soil stress and its variation with time. The model is verified based on the results of published cases and there is good comparison between the results of published case and the present simulation model. It is found that, the pile in cohesionless soil has more resistance in the rapid loading and less one in the long term loading. On the other hand, the pile in cohesive soil shows opposite behavior.     

Vertical bearing capacity of pile based on load transfer model

The load transfer analytical method is applied to study the bearing mechanism of piles with vertical load in this paper. According to the different hardening rules of soil or rock around the pile shaft, such as work-softening, ideal elasto-plastic and work-hardening, a universal tri-linear load transfer model is suggested for the development of side and tip resistance by various types of soil （rock） with the consideration of sediment at the bottom of the pile. Based on the model, a formula is derived for the relationship between the settlement and load on the pile top to determine the vertical bearing capacity, taking into account such factors as the characteristics of the stratum, the side resistance along the shaft, and tip resistance under the pile tip. A close agreement of the calculated results with the measured data from a field test pile lends confidence to the future application of the present approach in engineering practice.     

考虑应变强化的高压缩性黏土修正双曲线模型

研究了武汉地区高压缩性黏土弹塑性本构模型。基于Duncan E～v双曲线模型,对等压固结条件下CTC应力路径三轴实验数据进行了数据拟合,得到了该条件下的非线性应力应变关系,并将卸载-再加载试验曲线的平均斜率作为卸载模量,引入了加卸载函数作为判定加卸载的准则。将该模型嵌入有限元程序对三轴压缩试验进行数值模拟,并与Duncan模型数值解及试验曲线作对比。结果表明,该模型较Duncan模型能更好的体现正常固结黏土在CTC路径下的应力应变关系,特别是加载后期的应变强化效应,与试验曲线的最大相对误差不超过5％。