Hypoplastic and viscohypoplastic models for soft clays with strength anisotropy

This paper presents a new viscohypoplastic model for soft clays accounting for their typical features—strength anisotropy and rate dependency. The model is based on the hypoplastic model for clays enhanced by the anisotropic shape of the asymptotic state boundary surface. It has been shown that if the surface is skewed, the model predicts different ultimate strength in compression and in extension. Additional enhancement makes the tensor L bilinear in the strain rate, which more realistically predicts the stress paths of K₀ consolidated samples. The new model has been evaluated by simulating laboratory experiments on soft marine clays (Singapore and Bangkok clays). The model can be easily calibrated using only undrained triaxial and odometer tests. The model is subsequently enhanced by the rate effects. The resulting viscohypoplastic model has been evaluated using experiments of remolded kaolin clay and St. Herblain clay. It is shown that the enhanced model can predict important features of soil viscous behavior, such as rate dependency of strength and preconsolidation pressure, relaxation, and creep.     

Analysis of the compression behavior of artificial lumpy composite materials

The landfills are dumped without any compaction and have a relatively open structure, which is similar to that of the granular materials. However, the original dumped material might be gradually transformed into a lumpy composite structure because of the influence of the climate. As a result, the lumps are randomly distributed in the reconstituted soil. In the presented study, the compression behavior of the lumpy composite soils was analyzed within the homogenization framework. Firstly, the volume of the composite soil was divided into four individual components. The inter‐lump porosity was introduced to account for the evolution of the volume fractions of the constituents, and it was formulated as a function of the overall porosity and those of its constituents. A homogenization law was then proposed based on the analysis of the lumpy structure together with a numerical method, which gives a relationship for tangent stiffnesses of the lumpy soil and its constituents. Finally, a simple compression model was proposed for the composite lumpy material, which incorporates both the influence of the soil structure and the volume fraction change of the reconstituted soil. The predictions of the model were validated against the test results, and the stress distribution within the lumpy composite was assessed. Copyright © 2016 John Wiley & Sons, Ltd.     

Numerical simulation of lumpy soils using a hypoplastic model

The lumpy soil is a by product of the open-pit mining. A composite-lumpy material (in which, the lumps are randomly distributed in the reconstituted soil) is being created due to the degradation of the initial granular structure. In the present study, the compression and failure behaviour of an artificial lumpy material with randomly distributed inclusions are investigated using the finite element method. The computation results show that the stress ratio, defined as the ratio of the volume average stress between the lumps and the reconstituted soil within the inter-lump voids, is significantly affected by both the volume fraction and the preconsolidation pressure of the lumps under an isotropic compression path, while the volume fraction of the lumps plays a minor role under a triaxial compression path. Based on the simulation results, a homogenization law was proposed utilizing the secant stiffnesses.     

爆炸挤淤作用对海相软黏土压缩特性的影响

浙江漩门三期围垦项目中采用爆炸挤淤置换法进行筑堤工程,为了对爆炸置换后海堤的固结沉降预测及稳定性评价提供可靠的试验参数,取爆炸前后地基范围内的海相软黏土,进行一维压缩和各向等压固结试验,并对该地区软土的压缩变形与结构特性进行初步研究。试验结果表明,海相软黏土存在一定的结构特性,表现出与其重塑土不同的性质;通过引入孔隙指数的概念,研究该地区软黏土的结构性,证实土体结构性的存在是其压缩特性不同于重塑样压缩曲线的内在原因,在高压力范围内其压缩曲线趋近于固有压缩曲线。爆炸挤淤作用对海相软黏土产生较强的扰动效应,也对其基本物理性质与压缩特性均产生重要影响;扰动效应造成软黏土的结构屈服应力降低,结构强度下降,采用新定义的结构破坏比来定量评估爆炸挤淤作用对海相软黏土结构性的破坏程度,可为爆炸挤淤置换法处理软基的海堤工程提供一定的理论基础。     

Constitutive approach for rate‐sensitive anisotropic structured clays

This paper describes a constitutive approach to model the behavior of rate‐dependent anisotropic structured clay. Rate‐sensitivity is modeled using overstress viscoplasticity. Clay structure is treated as a viscous phenomenon whereby the viscosity of the undisturbed structured clay is initially very high and the viscosity degrades or decreases with plastic straining until the intrinsic or residual viscosity is reached. A microstructure tensor approach is used to make the structured viscosity anisotropic; whereas, the intrinsic viscosity is assumed to be isotropic. The behavior of the constitutive model is compared with the measured response of two clays (Gloucester and St. Vallier clay) from Eastern Canada during triaxial compression tests on specimens trimmed at different orientations to the vertical. The comparisons show that the constitutive framework is able to describe the anisotropic and rate‐sensitive response of both clays. The response of the model is also examined for the more general case of anisotropic consolidated triaxial compression and extension. Copyright © 2010 John Wiley & Sons, Ltd.     

SANICLAY: simple anisotropic clay plasticity model

SANICLAY is a new simple anisotropic clay plasticity model that builds on a modification of an earlier model with an associated flow rule, in order to include simulations of softening response under undrained compression following K_o consolidation. Non‐associativity is introduced by adopting a yield surface different than the plastic potential surface. Besides, the isotropic hardening of the yield surface both surfaces evolve according to a combined distortional and rotational hardening rule, simulating the evolving anisotropy. Although built on the general premises of critical state soil mechanics, the model induces a critical state line in the void ratio–mean effective stress space, which is a function of anisotropy. To ease interpretation, the model formulation is presented firstly in the triaxial stress space and subsequently, its multiaxial generalization is developed systematically, in a form appropriate for implementation in numerical codes. The SANICLAY is shown to provide successful simulation of both undrained and drained rate‐independent behaviour of normally consolidated sensitive clays, and to a satisfactory degree of accuracy of overconsolidated clays. The new model requires merely three constants more than those of the modified Cam clay model, all of which are easily calibrated from well‐established laboratory tests following a meticulously presented procedure. Copyright © 2006 John Wiley & Sons, Ltd.     

Undrained Triaxial Behavior of Cement Treated Marine Clay

Clays treated with lower cement contents often exhibit behaviour similar to stiff clays with planar failure surface under triaxial compression. In the present work the behaviour of a marine clay treated with 5 % cement, subjected to undrained triaxial compression tests is studied. The pre-consolidation pressure of the cemented clay due to the cementation bonding is observed to be very high. It is attempted to model the behaviour of cement treated clay using a bounding surface plasticity formulation as the plastic behaviour of the cemented clays within the yield surface has to be considered. The effect of cementation is included in the model as the pre-consolidation pressure obtained from consolidation tests. The tensile strength due to cementation bonds is included in the equation of the bounding surface. Simulations of the undrained triaxial compression tests on cemented clays are carried out and the results are validated with the experimental results.     

基于扰动状态概念的结构性土压缩特性分析

由于土结构性的影响天然沉积土的结构是亚稳定的,屈服后的压缩变形阶段必然伴随着结构的破损。将附加孔隙比?e作为表征原状土的一个结构状态参数,基于扰动状态概念,引入定量分析附加孔隙比?e随固结压力变化关系的方法,得到一维扰动状态概念(DSC)压缩模型,以描述结构性土压缩损伤现象,并通过结构破损指数b来刻画压缩过程中的结构破损速率。根据该模型,对太湖湖沼相典型的天然沉积软黏土、粉质黏土和硬黏土不扰动试样的一维压缩试验结果进行模拟和分析,并结合试验数据提出了模型参数的测定方法。分析结果验证了该模型能够描述具有不同结构形式土样的压缩特性及其实际变形过程中表现出的非线性,这给结构性土非线性固结与沉降计算提供了一定的理论基础。     

A hypoplastic constitutive model for clays

This paper presents a new constitutive model for clays. The model is developed on the basis of generalized hypoplasticity principles, which are combined with traditional critical state soil mechanics. The positions of the isotropic normal compression line and the critical state line correspond to the Modified Cam clay model, the Matsuoka–Nakai failure surface is taken as the limit stress criterion and the non‐linear behaviour of soils with different overconsolidation ratios is governed by the generalized hypoplastic formulation. The model requires five constitutive parameters, which correspond to the parameters of the Modified Cam clay model and are simple to calibrate on the basis of standard laboratory experiments. This makes the model particularly suitable for practical applications. The basic model may be simply enhanced by the intergranular strain concept, which allows reproducing the behaviour at very small strains. The model is evaluated on the basis of high quality laboratory experiments on reconstituted London clay. Contrary to a reference hypoplastic relation, the proposed model may be applied to highly overconsolidated clays. Improvement of predictions in the small strain range at different stress levels is also demonstrated. Copyright © 2005 John Wiley & Sons, Ltd.     

Kerman Clay Improvement by Lime and Bentonite to Be Used as Materials of Landfill Liner

Kerman province, located in the south eastern Iran, is dominated with clays which can be used in different projects. The liner system within a landfill is constructed to control leachate migration and can be constructed by low permeable natural soils or plastic lining materials, environmentally however, natural materials is preferred that usually need to be amended in order to meet requirements recommended by environmental agencies. This research examines the possibility of using the Kerman collapsible clay as a liner layer material. A set of laboratory test was conducted on pure soil samples and additive treated samples. The moderate collapse potential of the used soil is decreased with wet compaction and under the effect of additive-soil reactions. Laboratory investigations showed that lime and bentonite treatment improved the hydraulic conductivity. The results revealed hydraulic conductivities on the order of 10^?8 m/s. The obtained values met the 1.0E?07 m/s criterion required by Iranian standards. Unconfined compression tests were also performed on pure soil and additive amended samples. The unconfined compression strength values demonstrated gradual decreases with the addition of bentonite and considerable increases with adding lime such that with adding 1% lime the unconfined compression strength increased by 75%. This study verified that the Kerman collapsing clay can be used as a liner material using lime and bentonite as additives.     

An anisotropic elastoplastic model for soft clays based on logarithmic contractancy

A new constitutive model for soft structured clays is developed based on an existing model called S‐CLAY1S, which is a Cam clay type model that accounts for anisotropy and destructuration. The new model (E‐SCLAY1S) uses the framework of logarithmic contractancy to introduce a new parameter that controls the shape of the yield surface as well as the plastic potential (as an assumed associated flow rule is applied). This new parameter can be used to fit the coefficient of earth pressure at rest, the undrained shear strength or the stiffness under shearing stress paths predicted by the model. The improvement to previous constitutive models that account for soil fabric and bonding is formulated within the contractancy framework such that the model predicts the uniqueness of the critical state line and its slope is independent of the contractancy parameter. Good agreement has been found between the model predictions and published laboratory results for triaxial compression tests. An important finding is that the contractancy parameter, and consequently the shape of the yield surface, seems to change with the degree of anisotropy; however, further study is required to investigate this response. From published data, the yield surface for isotropically consolidated clays seems ‘bullet’ or ‘almond’ shaped, similar to that of the Cam clay model; while for anisotropically consolidated clays, the yield surface is more elliptical, like a rotated and distorted modified Cam clay yield surface. © 2015 The Authors. International Journal for Numerical and Analytical Methods in Geomechanics published by John Wiley & Sons Ltd.     

A hypoplastic model for clay and sand

The theory of hypoplasticity was developed initially for non-cohesive soils. However, sand and clay have many common properties; therefore arose the idea to extend the hypoplastic model to clay. The proposed model is able to describe the behaviour of cohesive soils with the incorporation of an appropriate structure tensor into the constitutive equation. This tensor is a stress-like internal parameter, also called back stress. This enables us to describe the behaviour of cohesive soils with the same material parameters for several states of consolidation and also to model barotropy and pycnotropy of sand. Numerical simulations of element tests are performed in order to check the performance of this hypoplastic model. Experimental data obtained with normally and overconsolidated clay and sand specimens with various densities are taken for comparison, and it is shown that the model is capable of describing the material behaviour of clay and sand. The determination of the material constants, the calibration method, is also presented in this paper.     

土结构性对天然软黏土压缩特性的影响

天然软黏土普遍受到土结构性的影响,明确结构性对软黏土压缩特性的影响机制显得尤为重要。对两个天然软黏土原状样进行室内一维固结试验,得到的压缩曲线与已有文献中搜集的39个原状样压缩曲线进行比较。压缩曲线在对数坐标下均呈现倒S形,并存在明显的结构屈服压力。采用双对数坐标处理后,压缩曲线可由双直线较好地表示,易于确定固结屈服压力。41个原状样屈服后的压缩指数和屈服点含水率之间的定量关系,与基于重塑土定量公式计算的压缩指数与固结屈服压力下重塑土含水率的定量关系相一致。对比结果表明：天然软黏土屈服后的压缩特性取决于屈服时的含水率,而与土结构性无关,解释了天然土压缩曲线位于重塑土上方是由于对应屈服点含水率不同而引起的。     

结构性粘土的堆砌体模型

天然粘土一般都具有结构性, 其变形过程必然伴随着结构的破坏。 提出了一种新型的堆砌体模型, 用以描述这种结构破坏现象。 这一模型把变形过程中的结构性土看作不同大小土块的集合体, 总的变形将由土块的弹性变形、土块之间滑动引起的塑性变形和土块破碎引起的损伤变形三部分组成。 塑性变形常用屈服函数描述, 损伤变形则可以引入一种类似的损伤函数加以描述。推导了相应的应力应变关系式并提出了模型参数的测定方法。     

Directional response of a reconstituted fine‐grained soil—Part II: performance of different constitutive models

In this paper, the performance of different advanced constitutive models for soils is evaluated with respect to the experimentally observed behaviour of a soft reconstituted clay subject to a wide range of loading directions, see (presented in the companion paper). The models considered include a three‐surface kinematic hardening elastoplastic model; the CLoE hypoplastic model; a recently proposed K‐hypoplastic model for clays, and an enhanced version of the same model incorporating the concept of intergranular strain. A clear qualitative picture of the relative performance of the different models as a function of the loading direction is obtained by means of the incremental strain response envelopes. The definition of suitable error measures allows to obtain further quantitative information in this respect. For the particular initial conditions and loading programme considered in this study, the kinematic hardening and the enhanced K‐hypoplastic models appear to provide the best performance overall. Copyright © 2006 John Wiley & Sons, Ltd.     

Sensitive bounding surface constitutive model for structured clay

The main purpose of the paper is to present a relatively simple, yet realistic, constitutive model for simulations of structured sensitive clays. The proposed constitutive model can simulate 1‐D and isotropic consolidation, and drained and undrained shear response of sensitive structured clay. The proposed sensitive bounding surface model is based on concepts from the modified Cam clay model 8 and bounding surface plasticity 27 , with the addition of a simple degradation law. The key material parameters are M, λ, κ, and ν from the modified Cam clay framework, h from the bounding surface framework to model a smoothed elasto‐plastic transition, and ω_v, ω_q, and S_sr to model softening associated with destructuration. The model has separate parameters to model destructuration caused by volumetric strain and deviatoric strain. The model is capable of modeling unusual behavior of strain softening during 1‐D compression (i.e., a reduction of effective stress as void ratio decreases). A good match between test results and the model simulation is demonstrated. Copyright © 2016 John Wiley & Sons, Ltd.     

上海软土的流变特性试验研究

本文利用无侧向变形的单向压缩试验仪,对原状和重塑上海软土进行了压缩试验和流变试验,研究了不同压力和超固结比对次固结系数的影响以及压缩特性和次固结变形特性的关系。试验结果表明,原状上海软土的次固结系数随着压力而变化,而重塑上海软土的次固结系数随压力变化不大。上述研究得到的结论及相关参数可为工程实践、上海软土流变模型的建立提供参考数据。     

超固结粘土的二元介质模型

为了分析超固结粘土边坡的变形和稳定的需要,在岩土破损力学的框架内建议了一个适用于这类土的二元介质模型。该模型考虑了土体内在的不均一性,把它看作由结构块和结构带组成的复合体,两者共同分担外荷载,模型包含10参数,并拟定了这些参数的测定方法。通过模拟三轴试验的计算表明,该模型可以反映London粘土的应力-应变特征。