Finite element analyses of bridge abutments on firm clay

In finite element analysis of soil-structure interaction problems involving firm to stiff overconsolidated clay, there have been difficulties in modelling the stress-strain response of the soil. Non-linearity and anisotropy of the soil depend on the inherent anisotropy of its particle structure and the induced anisotropy of its stress history and current stress path. In CRISP modelling of the centrifuge test of an abutment wall and its backfill of sand on the surface of a firm to stiff overconsolidated kaolin, the clay foundation was divided into 6 broad zones in accordance with the stress history and stress path. Undrained movements of the abutment and its subsoil were closely modelled in two analyses; one with a non-linear elastic model and the other with the Schofield model with shear modulus G assigned to the foundation zone in accordance with the estimated strain level as well as stress history and stress path. In the prediction of consolidation movement, there is a difficulty in the current critical state soil model in CRISP. The fe solution incorrectly predicted that substantial horizontal movement would accompany settlement due to consolidation, whereas the centrifuge test showed mainly vertical movement. This is attributable to the pronounced anisotropy separately observed in element tests.     

Finite element analyses of two-tier geosynthetic-reinforced soil walls: Comparison involving centrifuge tests and limit equilibrium results

This study presents the procedure and results of the finite element (FE) analyses of a series of centrifuge tests on geosynthetic-reinforced soil (GRS) two-tier wall models with various offset distances. The objectives of this study were to evaluate the applicability of FE for analyzing GRS two-tier walls with various offset distances and to investigate the performance and behavior of GRS two-tier walls in various stress states. The FE simulations were first verified according to the centrifuge test results by comparing the locations of failure surfaces. The FE results were then used to investigate the effective overburden pressure, mobilization and distribution of reinforcement tensile loads, and horizontal deformation at the wall faces. The interaction between two tiers was investigated based on the FE results, which were also used to examine the modeling assumption of reinforcement tensile loads in limit equilibrium (LE) analysis and to evaluate the design methods in current design guidelines. This study demonstrated favorable agreement between FE and the centrifuge model in locating the failure surface. The FE results indicated that as the offset distance increased, the reinforcement tensile load and wall deformation decreased in both the upper and lower tiers, suggesting the attenuation of interaction between the two tiers. The maximum tensile loads of all reinforcement layers at the wall failure predicted using FE analysis and LE method assuming uniform distribution of reinforced tensile loads were comparable. Compared with the FE results, the Federal Highway Administration (FHWA) design guidelines are conservative in determining the effect of overburden pressure, required tensile strength, location of maximum tension line (for designing the reinforcement length), and the critical offset distance. Furthermore, the FHWA design guidelines do not account for the influence of the lower tier on the upper tier that was observed in this study.     

无加筋、加筋砂土蠕变特征的有限元分析

根据分级加载条件下的蠕变试验,分析研究了无加筋砂土和土工格栅加筋砂土的蠕变特征,发现蠕变变形与分级加载时的应力水平、初始蠕变应变速率有很大关系,且蠕变后以恒定应变速率重新加载时呈现出刚度很大、近似弹性的行为。针对无加筋砂土和土工格栅加筋砂土提出一种弹黏塑性有限元计算方法。有限元计算过程中,砂土和土工格栅均采用统一的3要素弹黏塑性本构模型。该方法能够对含多个蠕变段的恒定应变速率加载全过程进行模拟。通过试验结果与有限元计算结果的比较,表明所提出的弹黏塑性有限元计算方法能较好地模拟无加筋砂土和土工格栅加筋砂土的蠕变特征,特别是蠕变后重新加载时的刚度很大、近似弹性的行为。     

Finite element analyses of negative skin friction on a single pile

This paper presents finite element analyses of negative skin friction on a single pile under various conditions. Negative skin friction is a common problem if a pile is designed in a highly compressible soil. There are two most important parameters in estimating the load caused by negative skin friction: (1) the distribution and magnitude of skin friction and (2) the location of the neutral plane. The neutral plane is the location where the pile and soil settle the same amount or have no relative displacement. Negative skin friction is a very complex phenomenon influenced by many factors. In this paper, a two-dimensional axisymmetric model is built in the finite element program, ABAQUS. The model is first verified with a known case history. A systematic parametric analysis is performed to investigate the influence on both the neutral plane and the magnitude and distribution of negative skin friction along the pile length of various influencing factors, including the consolidation time, the properties of pile/soil interface, the lateral earth pressure coefficient, pile-soil limiting displacement, the intensity of surcharge, and soil stiffness. Based on the analyses, it is found that the location of the neutral plane is significantly influenced by the consolidation time and the stiffness of bearing layer. The distribution and magnitude of negative skin friction is influenced mainly by the pile/soil interface, soil compressibility, and the surcharge intensity. Based on the field measurements from literature and this investigation, a simple design procedure is proposed for estimating the pile load caused by negative skin friction.     

阶梯式钻头挤土机理的有限元分析

为了分析阶梯钻头冲击挤密作用下土体的变形机理,将土体视为DP材料,服从Drucker-Prager屈服准则,通过钻具与土体之间的接触以及施加冲击载荷来模拟钻进过程,建立了潜孔锤冲击挤密钻进的有限元模型。以非线性瞬态动力学有限元分析为手段,利用有限元软件ANSYS9.0对阶梯钻头冲击挤密作用下土体的变形机理进行了模拟分析。结果表明,在冲击载荷作用下,冲击功主要用于竖向压缩土体,水平方向的挤压作用相对较小。阶梯钻头对土体的破坏作用可以看作是每个台阶的微剪切作用,当台阶布置合理时,土体沿等效锥面整体破坏,破坏后的土体在等效锥面的作用下被挤向孔壁四周,挤密效果较好。     

基坑复合土钉支护的有限元分析

针对基坑复合土钉支护，讨论了类似问题的有限元分析方法，主要是卸载条件下土体变形模式，模型参数的选用以及开挖过程的模拟等。通过计算分析了水泥搅拌桩与土钉联合形式的复合土钉支护的工作性能，并对其设计提出初步看法。     

地道桥结构与土相互作用的有限元分析

借助有限元软件ANSYS来分析地道桥结构与土的相互作用,采用三维实体建模方法模拟空间范围内的地道桥结构及其周围土体.分析有无列车荷载等两种不同工况下地道桥结构的位移、应力分布规律以及由其引起的地表沉降和土体的应力变化规律.通过有限元分析可以求出结构和土体的位移、应力峰值及其空间位置,为地道桥结构设计提供可靠的分析依据.     

Finite element analyses of rate-dependent thermo-hydro-mechanical behaviors of clayey soils based on thermodynamics

Clayey soils in the vicinity of energy geostructures may be exposed to long-term periodic thermal cycles. The creep and consolidation behaviors of the clayey soils can be both rate-dependent and temperature-dependent, and the underlying physical mechanisms are merely investigated theoretically. In this study, based on the theory of thermodynamics, a fully coupled thermo-hydro-mechanical (THM) finite element (FE) program for saturated soils is developed for this purpose. The FE formulation accounts for the combined effect of rate and temperature through the novel concept of granular temperature. Simulations of THM coupled validation cases and a series of experimental observations on the soft Bangkok clay are carried out. The obtained numerical results exhibit good agreement with analytical solutions and laboratory measurements. It is found that three fundamental physical mechanisms contribute to the irreversible thermal contraction observed for normally consolidated and lightly overconsolidated clays under drained thermal cycles: (1) the thermal creep excited by mass exchange from adsorbed water to free water; (2) the mechanical creep induced by confining stresses; and (3) the increase in granular packing caused by the thermal expansion of soil particles. The thermal contraction generally stabilizes within a few thermal cycles, as a result of the noticeable reduction in the thermal creep rate. It is further demonstrated that the transient heat transfer and the heating rate can greatly influence the deformation of clays subjected to thermal cycles.

     

Numerical simulation of secondary consolidation of soil: Finite element application

A rheological model has been developed for simulating the secondary consolidation of soils. Numerical procedures have been incorporated into a coupled consolidation program using results from a representative oedometer test. A solution of a two-dimensional problem has also been performed. The technique is numerically stable and has provided satisfactory predictions for the consolidation settlements and the dissipation of pore water pressure within soils.     

考虑横向惯性效应时桩侧土-管桩-土塞纵向耦合振动特性研究

与实心桩相比,由于土塞与管桩内壁复杂的相互作用,开口管桩的桩-土动力相互作用问题更为复杂。因此,需深入研究考虑土塞效应时的管桩-土体动力相互作用问题。首先,考虑管桩的横向惯性效应及其黏性性质,采用Rayleigh-Love动力杆件模型和附加质量模型建立了桩侧土-管桩-土塞系统的纵向振动控制方程。进一步,采用积分变换和阻抗函数传递技术,分别得到了任意荷载形式下管桩桩顶速度频域响应的解析解以及半正弦脉冲激励作用下桩顶速度时域响应的半解析解。最后,通过与现有解及模型试验的对比研究验证了理论解的合理性,并研究了管桩设计参数对桩顶纵向振动特性的影响。结果表明,当管桩其他设计参数不变时,管桩截面尺寸越大或长度越小时,应力波传播过程中的弥散效应越明显。对于同样外半径的管桩,壁厚越大时,越容易检测到土塞顶部界面及桩尖传来的反射信号,就能对管桩的施工质量进行更为合理的评价。     

Finite element analyses of layered visco‐elastic system under vertical circular loading

Analyses for the response of a linear visco‐elastic system subjected to axi‐symmetric vertical circular loading are presented. Hankel transforms with respect to the radial spatial coordinate are used to reduce the three‐dimensional problem to that involving only a single spatial dimension, which is then discretized using the finite element method. Three techniques are employed to handle the time factor in the visco‐elastic material: (i) direct time integration; (ii) Fourier transforms; and (iii) Laplace transforms. These methods are compared and evaluated through their numerical results. Copyright © 2007 John Wiley & Sons, Ltd.     

刚性桩复合地基抗震性能的有限元分析

近年来刚性桩复合地基在工程中的应用日益广泛,但是,对该种复合地基在动力荷载下,尤其在是地震作用下的动力响应性状却研究很少。考虑地震作用下土-桩-上部结构之间的相互作用,采用三维有限元方法,对刚性桩单桩复合地基的地震响应进行时域内的模拟计算与分析研究,分析了这种复合地基与桩基础地震响应的差异,并对影响刚性桩复合地基抗震性能的一些因素进行了研究,得出了一些对刚性桩复合地基的抗震设计有参考价值的结论。     

土工织物加固软土路堤的有限元分析

采用有限元方法对土工织物加固软弱地基路堤的力学机理进行了研究.通过比较路基加筋和未加筋两种情况研究了土工织物的加固机理,结果表明土工织物对于提高路堤填筑高度、减少路堤侧向位移、均化路堤沉降以及加速超孔隙水压力消散有显著的效果.加筋路堤中土工织物的轴力受填筑高度、超孔压消散速度和接触面系数等因素的影响,路堤填筑高度和施工进度的控制有利于土工织物抗拉强度的发挥.对路堤安全系数的研究分析表明传统的极限平衡法验算路堤稳定性偏于保守,采用有限元方法能较好地反映土工织物的加筋作用,可以优化工程设计.     

圆弧滑动有限元土坡稳定分析

提出一种基于有限元应力变形计算的边坡稳定分析方法,仍假定滑动面形状为圆弧形,有限元网格由一组同心圆作为纬线,一组竖向线为经线构成。对两相邻圆弧线所夹的弧形带分析滑动力和抗滑力,建立平衡方程,确定安全系数,其中小值安全系数对应的弧形带为可能的滑动带。变化圆心位置用优选方法寻找最小安全系数对应的圆心,从而得出真正的滑动面。算例分析表明,该方法计算所得安全系数与Bishop法接近,是合理的,其突出优点是由有限元计算直接得出滑面上的应力,而不须作近似的插值处理,因而应力更准确。该方法可以考虑土的非线性变形特性,也更符合土的实际情况。此外,用有限元计算得出位移,亦可将稳定分析和变形联系起来,为现场通过位移监测来估计边坡的稳定性提供了可能性,同时也为膨胀土边坡的稳定分析中考虑膨胀性的影响提供了可能性。     

裂隙贯通率对双重孔隙介质热-水-应力耦合现象影响的有限元分析

为探讨裂隙的贯通率对于耦合的温度场、渗流场和应力场的作用,应用所建立的遍有节理岩体双重孔隙-裂隙介质热-水-应力耦合模型,以一个假定的位于非饱和地层中的高放废物地质处置库为算例,针对裂隙和孔隙的贯通率不同的4种工况进行了二维有限元数值分析,考察了围岩中的温度、负的孔隙和裂隙水压力、地下水流速、孔隙及裂隙的渗透系数修正因子和应力的变化、分布情况。结果显示,由于裂隙贯通率的差异使得双重介质的刚度不同,引起岩体中应力状态及水平的改变,从而影响到孔隙、裂隙的孔隙率及渗透系数的量值,并导致孔隙水和裂隙水的压力大小、分布以及水流速度的变化     

预应力混凝土管桩桩土相互作用的有限元分析

以软土地区某桥梁基桩动载试验为背景,采用有限单元法对桩土动力相互作用进行了分析。采用等效线性模型模拟了土体材料的非线性,并考虑桩的材料阻尼以及桩土接触界面处状态非线性对动力响应的影响,并分析了桩的材料参数和土的参数对桩顶动位移的影响。分析表明,基桩弹性模量和材料阻尼对桩顶动位移有一定的影响,但基桩材料阻尼的影响很小,桩顶位移随桩侧土压缩模量和阻尼的增大而减小。     

动荷载作用下碎石渣路基动力响应的有限元分析

针对土资源缺乏地区利用施工造成的碎石弃渣作填料的高速公路碎石渣路基,采用有限元软件PLAXIS动力分析模块,计算碎石渣路基在车辆动荷载作用下的动位移、加速度等动力响应特性,并与传统的填土路基进行对比分析,验证了碎石渣路基具有较高强度和较强变形恢复能力等优点[1-9]。     

不同的层状岩体抗剪强度表达式计算效果的有限元分析

为考察不同的各向异性抗剪强度表达式对于数值计算结果的影响,以一个假定的位于层状岩体中的矩形地下洞室为背景,分别使用笔者和Pietruszczak各自提出的材料摩擦系数和黏聚力随方向变化的公式,进行3种工况的弹塑性二维有限元模拟,对比了围岩中的应力、位移及塑性区。结果显示,笔者提出的经验表达式与和相应的Pietruszczak一阶表达式在计算中的精度基本相当,而与Pietruszczak的二阶表达式相比有较明显的差别。     

成层土中倾斜荷载作用下桩承载力有限元分析

利用有限元方法对现场单桩水平载荷试验进行模拟,在此基础上,分析了成层土中桩在倾斜荷载作用下其竖向分量的有利作用和横向土抗力分布特点。计算结果表明,在地面下一定范围内,倾斜荷载作用下的桩侧摩阻力比水平荷载作用下的桩侧摩阻力大。在土层分界处土抗力分布有明显的跳跃。达到一定深度后,横向土抗力主要是静止土压力,而由荷载引起的横向土抗力很小。承台能有效减小土体及桩的水平位移。模拟的灌注桩和钢管桩桩顶在地面以上的自由长度较小,竖向分量由于桩身挠曲变形而产生的P-Δ效应较小,所以就算例中的灌注桩和钢管桩而言,荷载倾斜度不大时,荷载竖向分量提高了桩的侧阻并由此增大桩侧土竖向应力,对桩水平承载力总体上起到了有利的作用。     

基坑开挖对邻近不同刚度建筑物影响的三维有限元分析

受各种因素的影响,基坑邻近建筑物的刚度差异显著。为了解基坑开挖对邻近不同刚度建筑物的影响,在考虑土体小应变刚度行为的基础上,对基坑邻近不同刚度建筑物的变形展开精细化分析。算例结果表明:对于任意刚度的建筑物,当其跨越坑外沉降槽最低点以及上凸曲率最大点时,墙体所产生的拉应变最为显著,即此时对于任意刚度的建筑物来说,均为最为不利位置。随着建筑物刚度的增大,墙体挠度值与拉应变值呈对数曲线下降。当建筑物整体刚度较差时,其墙体拉应变更主要取决于坑外沉降幅度,而受自身刚度影响较小。当建筑物刚度较大时,在基坑变形的影响下,建筑物更主要表现为刚体运动,而自身内部变形则相对较小。