循环荷载下非饱和土的各向异性弹塑性双面模型

在BBM模型及弹塑性双面模型的基础上,建立一个在静态及循环荷载作用下考虑非饱和土各向异性的弹塑性双面模型。在模型中引入反映各向异性的硬化参量,通过各向异性参量的初始值来反映初始各向异性,利用旋转硬化法则来描述循环加载过程中各向异性的变化规律。模型含有一个边界面和一个加载面,二者在应力空间中按照等向硬化、运动硬化以及旋转硬化法则来进行演化。同时,采用径向映射法则和移动的记忆中心来反映非饱和土的循环塑性特征。通过与相关试验数据的比较,模型可以较好地反映循环荷载下非饱和土的各向异性以及动态力学特征。     

循环荷载下结构性软黏土的各向异性边界面模型

在边界面塑性理论的基础上,提出了一个可以描述循环荷载作用下结构性饱和软黏土力学特性的各向异性弹塑性模型。通过在本构方程中引入表征土体结构性损伤内变量的方法,将软黏土结构损伤与累积塑性应变的发展联系起来;并引入各向异性张量及旋转硬化法则描述其各向异性及演化特征。同时,在映射准则的定义中采用映射中心可移动的方法,以反映土体在卸载再加载时的弹塑性。通过对典型结构性软黏土及上海软黏土循环三轴试验结果的模拟,初步验证了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。     

横观各向同性黏土的非正交弹塑性本构模型

为了描述土体各向异性对应力应变关系的影响,基于特征应力的概念,建立横观各向同性黏土的非正交弹塑性本构模型。建模思路如下:(1)借鉴修正应力方法将组构张量引入到特征应力概念中;(2)根据修正特征应力空间中八面体面上的摩擦规律,建立可以综合反映土体各向异性和中主应力效应的横观各向同性土的β强度准则;(3)在修正特征应力空间中,采用倾斜屈服面描述非等向固结对土体屈服特性的影响,并引入体应变相关的硬化参数;(4)通过基于分数阶微分的三维非正交塑性流动法则,确定横观各向同性黏土的塑性应变增量方向。模型仅需要6个可通过常规三轴试验确定的材料参数,所涉及3个模型参数均可通过材料参数得到。预测结果与试验数据的对比表明,本文提出的模型可以合理地描述横观各向同性黏土的强度及变形特征。     

基于统一硬化参量的原状饱和黏土的结构性本构模型

根据饱和结构性黏土特有的剪胀特点,基于统一硬化参数,对原有的超固结土UH模型进行修正,使之适用于反映结构性土的应力-应变关系,主要结论如下:(1)通过将屈服面在p-q坐标系中向左平移,得到能用来反映具有一定拉伸强度特性的屈服面,且由于屈服面左端位于原点左侧,为了能够描述随体应变发展而造成的胶结强度的损失规律,建议一个屈服面左侧移动的演化方程;(2)通过引入状态参量c来描述体变剪缩到剪胀转换时所对应的变相应力比强度特性,使得体变大小以及体变转换规律符合不同胶结程度而变化的特点;(3)对于结构性土在剪缩下的大体积剪缩特点,提出结构性应力比参量R*,并给出基于R*的结构性潜在强度的关系式。使用上述潜在强度来修正统一硬化参数,并最终得到了能反映结构性土应力-应变关系的简单模型,可以用来反映结构性土的高应力比强度特性,胶结特性随体变逐渐损失特点,大体积剪缩,伴随着胶结损失而产生的应变软化,剪缩、剪胀等主要变形特性。     

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。     

平面应变条件下原状Q3黄土的结构性演化特性研究

针对黄土工程中经常遇到的平面应变问题,通过扰动、加载和浸水充分释放土的结构势,开展Q₃原状、重塑和饱和原状黄土的无侧限压缩试验和平面应变剪切试验,依据初始结构性参数(构度)和剪切过程结构性参数(应力比结构性参数)定义了可综合描述从土的初始结构状态到平面应变剪切破坏状态的全过程结构性参数,分析了平面应变条件下,全过程结构性参数随轴向应变的衰减演化规律及结构性强度特性。研究结果表明,黄土的不同初始状态(原状、重塑和饱和)对应力 应变关系演化特性影响较大,原状黄土应力 应变曲线最高,重塑黄土次之,饱和黄土最低。随着含水率增大,构度指标不断减小,含水率小于土的塑限时,水的浸入对黄土结构的损伤较大。固结围压和含水率变化均对结构性参数及其演化规律有明显影响。建立的全过程结构性参数,可以很好的描述土的初始结构性及平面应变剪切全过程的结构损伤演化规律,亦可以反映出含水率和固结围压对于土结构性损伤演化全过程的影响规律。不同初始状态对土的强度影响显著,黏聚力随构度呈非线性增长,内摩擦角随着构度增大近似呈线性增长,但变化幅度较小。     

复杂加载下K0超固结黏土的本构模型

卸载后的K₀超固结黏土同时具有初始各向异性以及超固结特性,其特点有如下三方面：(1)当大主应力沿着K₀固结沉积面法线方向加载时,此时的剪切模量较等方向固结的剪切模量更高。(2)由于初始偏压固结导致三轴压缩下K₀固结黏土的临界状态应力比相较等方向固结的更大。(3)循环加载导致超固结特性以及应力诱导各向异性更为显著。基于超固结土UH模型,引入反映初始各向异性的边界面转轴参量ζ,通过倾斜边界面来提高其塑性模量。分析其剪胀应力比特征,提出状态应力比来替换统一硬化参数中的普通应力比,用来反映应变硬化、软化现象,引入旋转硬化规则用来反映复杂加载路径下的应力诱导各向异性性质,同时修正统一硬化参数,用来反映循环加载下的塑性体应变累积特性,塑性偏应变的滞回圈与棘轮特征,卸载路径的塑性变形特性。利用基于t准则的变换应力方法将上述新模型转变为三维本构模型,通过与一系列K₀超固结黏土在不排水及循环加载等复杂加载路径下的试验与预测结果进行对比,结果表明：新模型可简单方便地应用于K₀超固结黏土在复杂加载路...     

考虑各向异性影响的循环弹塑性模型

软黏土在动荷载作用下具有显著的非线性、滞回性和变形累积性等动力特性,因此采用弹塑性模型来描述土的动应力应变关系更为合理。基于混合硬化塑性模量场理论,建立了一个总应力形式的、屈服面连续变化的饱和黏土不排水增量弹塑性动本构模型。模型克服了一般嵌套塑性模型因在模量场中存在共轭点而具有奇异性的不足,不需要记忆应力反向面,也不需要判断当前屈服面是否超过应力反向面进而引入零圆心位置的过渡应力反向面,极大地简化了模型和数值计算。由于现有弹塑性动本构模型通常不能考虑各向异性固结对土体变形和破坏的影响,通过在破坏面方程中引入各向异性偏应力张量来反映各向异性的影响。对试样动三轴试验的结果与文献模型计算结果的对比分析表明:模型能够合理地描述土的主要动力特性及应力诱发各向异性对土体变形和破坏的影响。     

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。     

A Multilaminate Model with Destructuration Considering Anisotropic Strength and Anisotropic Bonding

Many constitutive models have been developed to describe the behaviour of soils, but only more recently a limited number of them have paid attention to include effects of structure and consequently destructuration due to volumetric and/or deviatoric straining. Most of them consider isotropic behaviour for strength and structure of soil. In this paper a multilaminate type of constitutive model is presented in which anisotropic strength and anisotropic structure are taken into account. According to the mathematical formulation of the multilaminate framework, this kind of model is capable of taking into account induced anisotropy intrinsically. Inherent anisotropy can be introduced to the model by varying the model properties over sampling planes using a so-called microstructure tensor. Only mechanical destructuration, namely degradation of structure with deformation is considered in this paper. The proposed model is capable to consider anisotropy in bonding and preconsolidation pressure as well as inherent anisotropy in strength thus the hardening of the yield surface and strength parameters can be considered direction dependent. In this paper the drained and undrained behaviour of natural clays is simulated and the effect of different assumptions with respect to structure degradation and anisotropy is evaluated.     

软土应力各向异性及其对深基坑工程的影响

天然软粘土一般都具有应力各向异性.在有些大型深基坑工程中,忽略了土体应力各向异性的影响将可能无法保证分析结果的准确性.作者首先回顾了有关土体应力各向异性对基坑抗隆起稳定性影响的研究现状,包括极限平衡法、常规弹塑性位移有限单元法以及极限分析有限单元法等.对于理论分析方法,尤其是有限单元法,均需要合理描述应力各向异性对土体应力-应变-强度特性的影响.为此通过对Wheeler各向异性模型进行改进以得到一个更为合理的模型,并与Boston Blue粘土的有关三轴试验结果进行了比较,验证了模型的合理性.     

沉积岩的一种时间效应模型

提出沉积岩的一种时间效应模型。沉积岩的固有各向异性用一个标量各向异性参数描述,引入一个损伤因子来考虑时间效应,材料的单轴抗压强度是时间、微结构张量和应力的函数。建立了一个描述各向异性沉积岩变形过程的全塑性模型。用提出的模型对一些试验进行了模拟,结果表明该模型能有效地描述沉积岩的固有各向异性及其时间效应。     

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。     

UPPER-BOUND SOLUTIONS FOR BEARING CAPACITY OF STRIP FOOTINGS OVER ANISOTROPIC NONHOMOGENEOUS CLAYS

All available solutions for the problem of bearing capacity on clays of anisotropic and nonhomogeneous strength are based on the assumption of either circular or Prandtl-type failure mechanisms. Although these solutions are rigorous within the concept of limit analysis or limit equilibrium methods, the formulations are too mathematical and exceedingly cumbersome and the bearing capacity value can only be determined through numerical optimization for each given combination of soil and failure mechanism parameters. By means of the upper bound approach of limit analysis, and adopting translational failure mechanisms, this paper presents analytical solutions for the bearing capacity of surface strip footings on clays of strength anisotropic and linearly increasing with depth. Closed-form expressions for bearing capacity factors in the case of smooth and rough footings, have been derived by considering two newly introduced kinematically admissible translational failure mechanisms with varying wedge angles. It was remarkable to find that the two mechanisms would render closed-form expressions for the bearing capacity factor if and only if the deformed region underneath the footing is set to be bounded by two vertical discontinuity surfaces. The derived formulas are expressed in terms of degree of strength anisotropy and a nondimensional parameter that reflects strength nonhomogeneity. Besides being the only closed-form solutions yet available for the bearing capacity of strip footings on clays with anisotropic and nonhomogeneous strength, the derived expressions have been found to not only provide upper bound values for the bearing capacity factor that compared favorably with available solutions, but also yield the best upper bound values when strength increase with depth becomes predominant.     

基于微观破损规律的结构性土本构模型

在岩土破损力学理论的基础上,通过考虑土体结构性破损的微观机理,建立了结构性土的本构模型。通过定义与重塑土屈服面几何相似的结构性屈服面,模型可以反映结构性对土体力学特性的影响;通过引入表征结构性损伤的破损参数,确定了结构性土加载过程中的硬化规律。该破损参数基于岩土破损力学中应力应变分担的概念而提出,具有明确的力学意义;模型的硬化规律同时考虑了塑性体应变及塑性主应变的影响,可以更好地反映土体结构性损伤过程。将该本构模型用于结构性土室内固结试验及三轴压缩试验结果的模拟,初步验证了该模型的合理性。     

Geo-mechanical behavior of clay soils stabilized at ambient temperature with fly-ash geopolymer-incorporated granulated slag

Geopolymer is a cementitious material that can replace ordinary Portland cement in several geotechnical engineering applications, such as soil stabilization, with the advantages of much lower harmful emissions and energy consumption. This paper presents a rigorous evaluation of the geo-mechanical behavior of different types of clay soils treated with geopolymer, including the influence of soil characteristics and mineralogy. Two natural clay soils in addition to a commercially available kaolin clay were used for this investigation. Laboratory experiments were performed including unconfined compressive strength (UCS) and consolidated undrained (CU) triaxial compression tests under different confining pressures. The UCS and triaxial tests indicated that the addition of geopolymer considerably increased the yield stress and initial stiffness of all examined clays. With the increase of geopolymer content, the stress–strain behavior of treated clays was found to develop progressively from ductile response into a post-peak brittle fashion. The CU tests also demonstrated that the addition of geopolymer changed the initial characteristics of remolded clays from quasi-over-consolidated to heavily over-consolidated, rendering high yield surface and more effective shear strength parameters (i.e., cohesion and friction angle). Moreover, although the overall qualitative stress–strain and stress path responses of the clays were similar, significant quantitative differences were observed, particularly in terms of the attainable yield strength, stiffness, and shear strength. These differences can be attributed mainly to the heterogeneity associated with the soil mineralogy and the corresponding differences in the interaction between the clay/non-clay minerals and geopolymer.