A procedure of strain-softening model for elasto-plastic analysis of a circular opening considering elasto-plastic coupling

A simple numerical procedure of strain-softening model for the elasto-plastic analysis of a circular opening is presented by modifying the procedure proposed by Lee and Pietruszczak (2008). The proposed procedure has two advantages over that of Lee and Pietruszczak (2008). One is that the elasto-plastic coupling can be considered, the other is that the equilibrium equation and compatibility equation need not be expressed with respect to the normalized radius. Through MATLAB programming, the accuracy and validity of the proposed procedure are demonstrated through some examples. The influence of elastic modulus and Poisson’s ratio evolution on the distribution of radial displacement and hoop stress, ground response curve (GRC) and plastic radius was investigated, and the results show that, with the decrease of elastic modulus evolutional ratio, the radial displacement, hoop stress and plastic radius all increase. With the increase of Poisson’s ratio evolutional ratio, the radial displacement, hoop stress and plastic radius all decrease. As for influence extent, the evolution of elastic modulus has great influence on the radial displacement; and the evolution of Poisson’s ratio has smaller influence on the radial displacement, both of them have very slight influence on the hoop stress near the excavation face but greater on the hoop stress near the interface of elastic and plastic region. And both of them have very slight influence on the plastic radius.     

循环荷载作用下流变性软黏土的边界面模型

为了预测交通荷载作用下流变性软黏土的长期运行沉降,提出了一个能够描述循环加载条件下饱和软黏土流变特性的弹黏塑性本构模型。本模型以边界面弹塑性理论为基础,采用滞后变形理论。模型不仅能单独考虑土体的流变效应和循环加卸载效应,还能考虑交通荷载作用下软黏土在循环荷载和流变耦合作用下的变形特性。模型概念清晰,参数少。通过多组上海软黏土循环加载流变试验结果的模拟,初步验证了本模型的合理性和有效性。     

K0固结结构性软黏土的旋转硬化规律研究

从Wheeler等S-CLAY1本构模型出发,在软黏土结构性和塑性各向异性性状前期研究成果的基础上,综合考虑软黏土的塑性各向异性、结构性及其演化规律,将传统本构模型发展为更适用于K0固结结构性软黏土的本构模型。在考虑土体结构性及其演化的基础上,进一步研究土体塑性各向异性及其演化规律,引入旋转极限曲线的概念,通过增加一个表征软黏土各向异性演化速率的参数b,分析旋转硬化对K0固结结构性软黏土受力变形性状的影响,各向异性的初始值则可由常规试验参数获得。选取典型的浙江温州软黏土和Bothkennar软黏土,对比K0固结三轴压缩和三轴拉伸的计算和试验结果,揭示结构性软黏土屈服面旋转硬化的规律,同时对旋转演化速率参数的取值方法和取值范围进行研究。     

结构性软黏土的一维弹黏塑性模型

为了研究结构性软黏土一维压缩变形的时效特征和结构破坏特征的耦合效应,首先,基于Bjerrum的等时间线体系,提出等黏塑性应变率线概念,建立非结构性软黏土的新型一维弹黏塑性模型;然后,在大量试验结果的基础上,揭示了一维压缩过程中压缩指数随孔隙比的变化规律,并提出了土体结构渐进破坏的新型描述模式;仿照特征体积,定义了"本征体积"和"本征应变"的概念,得到了结构性软黏土的"本征压缩定律",并推导了结构性软黏土的蠕变方程,构建了结构性软黏土的一维弹黏塑性模型。最后,阐述了模型参数的确定方法,所有参数均可通过试验直接确定,并用本文模型对宁波天然软黏土的常规压缩试验和长期蠕变试验、Berthierville clay、Ariake clay的一维等应变率压缩试验进行模拟,验证了该模型的有效性。研究结果表明,该模型能很好地模拟结构性软黏土一维压缩变形的时间效应和结构破坏效应,为建立结构性软黏土三维时效本构模型奠定了基础。     

K0固结软黏土的弹黏塑性本构模型

在修正剑桥模型基础上综合考虑了软黏土的各向异性及率相关性,建立了适用于K0固结软黏土的弹黏塑性本构模型。模型借鉴过应力理论的基本思想,定义了与动态加载面相对应的参考屈服面,应用径向映射准则将两者联系起来,流动函数通过分析一维情况下土体的体积蠕变速率得到。以黏塑性体积应变为硬化参数,将一维情况扩展到三维应力状态,直接用次固结系数描述土体黏性强弱,所有参数可通过压缩试验及三轴不排水剪切试验得到。分别计算了代表性等向和K0固结黏土的三轴不排水等应变率加载、不排水剪切蠕变及蠕变破坏过程,与试验结果进行对比,验证本文模型的有效可行性。     

改进屈服面和旋转硬化准则的各向异性黏土弹塑性模型

基于临界状态理论和旋转硬化法则提出了改进的各向异性模型.首先从恒定应力比加载试验结果出发,采用对数型插值函数描述土体的变形特性,通过增加参数n,推导出可以呈现多种形状的各向异性土屈服面表达式.屈服面在n>1,n<1和n=1时分别呈现泪滴形、子弹头形和椭圆形.然后提出了恒定应力比加载时旋转硬化法则边界值表达式,在各向同性...     

基于扰动状态概念硬化参量的结构性黏土边界面模型

在临界状态理论和边界面模型的框架内,引入结构性扰动对弹性变形、结构性屈服面大小、胶结吸力和屈服面各向异性的影响,建立一个基于扰动状态概念硬化参量的结构性黏土边界面模型。首先,通过定义3个结构扰动度R_c,R_b和R_a来定量反映塑性变形对结构性黏土的结构性屈服面、胶结吸力和屈服面各向异性的扰动程度,并对弹性特性进行各向异性和结构性影响的修正。然后,通过对天然沉积上海黏土和Vallericca硬黏土的三轴不排水剪切试验和一维固结压缩试验的模拟验证模型的预测能力。最后,通过模型与有限差分法相结合,以简单的平面应变试验模拟为例,展示模型能够描述结构扰动度的变化及其在土体中分布的模拟能力。     

非均匀强化塑性模型的统一弹塑性本构关系

介绍了三参数统一强度理论和非均匀强化塑性模型 ,并利用三参数统一强度理论对非均匀强化塑性模型进行推导 ,得出了其在强化过程中的弹塑性本构关系。所得模型考虑了材料的拉伸强度与压缩强度不等、双轴等压强度与单轴等压强度不等的情况。当取不同的参数 b和 α时 ,该本构关系退化为 Mohr- Coulomb、Tresca和双剪强度理论等在非均匀强化塑性模型下的本构体系 ,因而可以适用于各类工程材料和不同的受力情况 ,有重要的工程应用价值。     

K0超固结土的统一硬化模型

将所建立的超固结土模型推广为能够反映超固结土的初始应力各向异性的弹塑性模型。它是通过将潜在强度Mf、特征应力比M和状态应力比ηk引入到模型的统一硬化参数中,使模型具有预测超固结土的初始应力各向异性、剪缩、剪胀、硬化、软化和应力路径依赖性等基本特性的功能。采用SMP强度准则并结合变换应力方法对模型实现了三维化。通过与试验结果比较表明,提出的模型能够较好地描述初始应力各向异性超固结土的应力应变特性。     

基于SFG模型的非饱和膨胀土本构模型研究

Gens和Alonso提出了非饱和膨胀土本构概念模型(G–A模型),将土体膨胀分为微观层次和宏观层次,能够预测土体变形趋势。但G–A模型框架内建立的膨胀土模型参数多且难确定。在SFG模型的框架下,推导膨胀土的中性加载面(NL屈服面)方程,建立了膨胀土本构模型。该模型以总变形来反映土体微观层次变形,不再区分微观层次和宏观层次的结构变形,减少了非饱和膨胀土本构模型的参数。试验数据和数值模拟结果表明,中性加载屈服面和膨胀势曲线基本一致。通过与膨胀土的膨胀试验结果对比,本模型数值计算结果和试验结果较吻合,证实了所提模型的正确性。     

K0固结结构性软黏土的本构模型

本文在修正剑桥模型的基础上综合考虑了软黏土的各向异性、结构性及其演变和屈服面硬化规则中塑性剪切应变的影响,将传统模型发展为适用于K0固结结构性软黏土的本构模型。本文模型借鉴Collins等人提出的符合热力学耗散原理的本构模型,同时在描述土体结构性演变的过程中参考了Asaoka等人的次加载/超加载屈服面本构模型,并采用了具有明确物理意义的内变量。与传统的修正剑桥模型相比,增加了3个分别表征软黏土各向异性和结构性的参数(θn,R和R*)以及两个演化参数(m和a),而参数R和R*的初始值则可由结构性土的屈服应力比YSR和灵敏度St获得。本文选取了典型的浙江温州软黏土和Bothkennar软黏土,对比了三轴压缩的计算和试验结果,体现了本模型在结构性软黏土计算上相对于传统本构模型优越性。     

天然结构性黏土的各向异性边界面模型

提出了一个可同时描述结构性和各向异性特征的适用于天然黏土的边界面本构模型。该模型具有如下特点:①采用了一个同时含有各向异性张量和结构性参数的边界面;②通过初始固结应力状态来确定各向异性张量的初始值以反映初始各向异性,通过各向异性张量随塑性应变的演化规律来描述荷载作用下的诱发各向异性;③通过结构性参数取一个较大的初始值得到一个扩张的边界面,从而反映结构性土的初始高刚度和强度峰值;④通过引入结构性参数随塑性应变的累积逐渐减小的演化规律来模拟土的结构的损伤以及应力应变曲线的软化过程。通过对几组典型的结构性黏土的试验进行模拟初步验证了模型的合理性和有效性。     

弱胶结结构性软黏土力学特性的试验研究

对取自不同地区的两种弱胶结结构性软黏土原状(undisturbed)样、重塑(remolded)样和泥浆(reconstituted)样进行了单向压缩和三轴剪切试验,分别得到土样的压缩曲线和应力–应变曲线。试验结果表明:原状样的压缩曲线为陡降型曲线,而不同制样土样的压缩曲线存在明显的差异;由于孔隙比和孔径分布对土体抗剪强度的综合影响,不仅导致相同围压下三轴剪切时孔隙比不同的重塑样和原状样强度差异较大,且孔隙比相近的不同土样的强度也存在不同程度的差异;若同一孔隙比下,两种软黏土的不同制样土样的强度关系均为原状样的强度最高,重塑样的强度最低,并可通过相近孔隙比下孔径大于0.2μm的孔隙体积量和孔径分布均匀性可合理地解释3种制样土样强度高低的关系。由于不同制样土样的孔径分布的差异不会随固结压力的增大而消失,用参考孔隙比e_(10)~*,简单表示土的孔隙比和孔径分布(即组构)参数,对压缩和剪切试验结果进行归一化整理后,发现不同土样的试验结果可归一化为相关度高的e/e_(10)~*-σv曲线和e_f/e_(10)~*-qf曲线,证明结构屈服应力后,不同土样变形和强度差异主要是由孔隙比及孔径分布(即组构)的不同引起的,用参考孔隙比e_(10)~*简单表示土的组构参数是有效的。     

Stiffness and strength parameters for hardening soil model of soft and stiff Bangkok clays

A comprehensive set of experimental data on Bangkok subsoils from oedometer and triaxial tests are analysed in this paper in order to determine the stiffness and strength parameters for Hardening Soil Model. The parameters determined are the Mohr–Coulomb effective stress strength parameters together with the stiffness parameters; tangent stiffness for primary oedometer loading, secant stiffness in undrained and drained triaxial tests, unloading/reloading stiffness and the power for stress level dependency of stiffness. The oedometer data are obtained from three different Bangkok soil layers: soft clay at 6–8 m depths; medium clay at 12–14 m depths; and stiff clay at 15.5–18 m depths. The triaxial tests data are carried out for soft and stiff clays at depths of 5.5–6 m and of 16–18 m under both undrained and drained conditions, respectively. Finally, two sets of parameters for soft and stiff Bangkok clays are numerically calibrated against undrained and drained triaxial results using PLAXIS finite element software.     

软黏土不排水循环应力应变响应的弹塑性模拟

为描述循环荷载作用下饱和软黏土不排水应力应变特性,以Mises屈服准则构建边界约束面方程,利用硬化模量插值方法及映射中心移动方法,在偏应力空间中构建硬化模量场,建立了一个总应力形式的增量弹塑性边界面模型。该模型依据径向映射法则建立弹塑性模量插值函数,并认为加卸载过程中映射中心随加卸载路径变化而不断移动,通过弹塑性模量的插值及映射中心的移动,使得硬化模量场的演化规律表述简单,跟踪循环应力路径所需记忆的参数较少。另外,模型通过在弹塑性模量插值函数中引入反映剪应变累积速率和大小的参数控制累积剪应变随循环次数的变化规律,不仅可描述循环加载时应力应变曲线的非线性、滞回性、应变累积性等基本特性,还可预测不同初始应力水平、循环应力水平下土单元的应力应变响应。最后阐述了利用循环三轴压缩试验确定模型参数的方法,并对循环三轴拉伸试验结果进行了预测,通过预测结果与试验结果的对比,验证了所建模型的合理性。     

Geosynthetic-encased stone columns in soft clay: A numerical study

This paper presents the findings of a series of numerical studies on the contribution of geosynthetic encasement in enhancing the performance of stone columns in very soft clay deposits. In this study, the imposed loading is from a fill embankment, and the stone columns act like reinforcements. Observed settlement of a trial embankment built on very soft clay strengthened with stone columns indicated that the stone columns alone were not adequately effective in reducing settlement because the very softy clay could not provide adequate confining stress to the stones. An alternative system utilizing geosynthetic encasement was examined numerically. As the primary issue is the development of settlement with time after the completion of stone column installation, a fully coupled analysis was performed. To reduce the computational effort, a unit cell idealization was adopted. This study showed that the use of geosynthetic encasement has the potential of significantly enhancing the effectiveness of stone columns in very soft clay and the simplified analysis presented in earlier work is valid. Furthermore, the predicted performance was found to be insensitive to assumed stiffness parameters of the compacted stone. However, it was found to be dependent on the locked-in stress in the geosynthetic encasement induced during installation.     

A novel finite volume based formulation for the elasto-plastic analysis of plates

In this paper, a novel finite volume formulation for the elasto-plastic analysis of Mindlin–Reissner plates is proposed. A layered approach is adopted which enables to monitor the evolution of the through-thickness plasticity. For the solution of the discretized equations, two different incremental solution algorithms are implemented. The proposed method is validated through a series of benchmark comparisons. It is observed that the results obtained are in good agreement with the reference results. It is also demonstrated that the proposed finite volume based formulation has good capabilities in dealing with the plastic modeling of very thin to moderately thick plates.     

Strength and stiffness parameters for hardening soil model of rockfill materials

In recent years, an increasing number of rockfill dams have been constructed in many developing countries around the world. One difficulty in the engineering design and construction of rockfill dams is the characterisation of the rockfill materials. Thanks to advancements in computational geomechanics, a finite element analysis is usually employed in geotechnical engineering design. To perform such analyses, a proper constitutive model with relevant parameters that reflect the stress–strain-strength characteristics of rockfill materials are required. This study presents a procedure to determine the strength and stiffness parameters of rockfill materials that can be used with the Hardening Soil (HS) model to characterise the behaviour of rockfill materials. Based on triaxial and oedometer test results, a set of HS model parameters can be determined. To implement and validate the procedure, six rockfill materials from the Nam Ngum 2 Dam construction project in Laos PDR are employed in this study. The influences of stress level and particle breakage on the stress–strain/volumetric-strain behaviours of rockfill materials are discussed. Finally, the procedure to determine the material parameters for rockfill materials presented in this study can also be applied to other rockfill materials.     

上海软土小应变三轴试验及本构模拟

软土地区新建地下工程为了减少对周边的影响,周边土体的变形通常要求控制在小应变范围内(0.001%~0.1%)。近年来随着环境要求的不断提高,软土在小应变范围内的力学特性日益受到关注。目前国内在三轴试验中测量软土小应变力学特性的研究相对较少,也缺乏相应的试验数据。利用安装有LVDT局部位移传感器的三轴仪,对上海(2)~(6)层软土进行了K0固结不排水剪切试验,成功获得了上海软土从0.001%小应变到20%大应变范围内的剪切模量变化规律。通过分别用初始剪切模量和有效平均主应力来对剪切模量进行归一化处理,揭示了上海软土的非线性特征、土体剪切模量的衰减规律等。考虑土体应力状态、孔隙比和超固结比的经验公式能够合理地描述上海软土层初始剪切模量;经典的骨干曲线模型能较好地拟合各土层的剪切模量衰减规律。     

Laboratory model studies on unreinforced and geogrid-reinforced sand bed over stone column-improved soft clay

Results from a series of laboratory model tests on unreinforced and geogrid-reinforced sand bed resting on stone column-improved soft clay have been presented. The diameter of stone column and footing has been taken as 50 mm and 100 mm, respectively for all the model tests carried out. Load was applied to the soil bed through the footing until the total settlement reached at least 20% of footing diameter. As compared to unimproved soft clay, the increase in load-carrying capacity under different improved ground conditions has been observed. Influences of the thickness of unreinforced as well as geogrid-reinforced sand bed and the size of geogrid reinforcement on the performance of stone column-improved soft clay bed have also been investigated. Significant improvement in load-carrying capacity of soft soil is observed due to the placement of sand bed over stone column-improved soft clay. The inclusion of geogrid layer within sand bed further increases the load-carrying capacity and decreases the settlement of the soil. Due to the placement of sand bed, the bulge diameter of stone column reduces while the depth of bulge increases. Further reduction in the bulge diameter and increase in bulge depth are observed due to application of geogrid layer. The optimum thickness of unreinforced sand bed is twice the optimum thickness of geogrid-reinforced sand bed. Under specific material properties and test conditions, it is further observed that the optimum diameter of geogrid layer is thrice the diameter of footing.