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.     

Experimental and theoretical investigation on the compression behavior of sand-marine clay mixtures within homogenization framework

In this paper, the compression behavior of sand-marine clay mixtures was investigated, both experimentally and theoretically. The test data reveal that the Normal Compression Line of a sand-clay mixture depends on both the sand fraction and the initial water content of the clay matrix. The local stress in the clay matrix σ′_c is approximately close to the overall stress of the sand-clay mixture σ′ for a sand mass fraction of 20%. The stress ratio, σ′_c/σ′, falls significantly with increasing overall stress for a sand fraction of 60%, which may be attributed to the formation of clay bridges between adjacent sand particles. A compression model was formulated within the homogenization framework. First, a homogenization equation was proposed, which gives a relationship between the overall stiffness E and that of the clay matrix Ec. Then, a model parameter ξ was incorporated considering the sensitivity of the structure parameter on the volume fraction of the clay matrix. Finally, a simple compression model with three model parameters was formulated using the tangent stiffness. Comparisons between the experimental data and simulations reveal that the proposed model can well represent the compression curves of the sand-marine clay mixtures observed in the laboratory.     

A compression model for sand–silt mixtures based on the concept of active and inactive voids

A compression model for sand–silt mixtures is needed in geotechnical engineering, for example in the analysis and prediction of deformation of levees and embankments due to internal erosion. In this paper, we introduce a novel concept of dividing the voids of a granular material into two hypothetical fractions: active and inactive voids. The active voids are kinematically available to the compression process. The inactive voids are kinematically unavailable to the compression process. The volume of active voids is dependent on the initial density and effective stress level. The volume of inactive voids is dependent on the amount of fines in the mixture. The current paper considers 1-D and isotropic compression behavior of sand–silt mixtures at stress levels lower than 2 MPa, so no substantial particle breakage is expected to occur. To successfully predict the void ratio for a sand–silt mixture during compression loading, we need (1) a mathematical expression for the evolution of the active void ratio during compression and (2) a relationship between the inactive void ratio and fines content of the mixture. For sand–silt mixtures with any amount of fines, the proposed model requires five material parameters, which are determined from two compression tests, and four minimum void ratio tests on sand–silt mixtures with different fines content. The performance of the proposed model is verified for six different types of sand–silt mixture with various fines contents, by comparing the predicted void ratios with the measured data from the experiments. The comparisons show a good agreement between the predictions and the measured data and prove the suitability of the proposed model for the prediction of compressibility of sand–silt mixtures with any amount of fines.     

Model for analysis of fiber-reinforced clayey soil

Fiber-reinforced soils have many applications in improving the response of clayey soils. In this paper, an analytical model for the analysis of fiber-reinforced soil in the framework of modified cam clay model is presented. The analytical model is verified using experimental results from the standard undrained triaxial tests with pore water pressure measurements. Tests have been conducted on clayey soil specimens reinforced with randomly oriented discrete coir fibers with different percentages of fiber contents. Numerical simulations of triaxial compression tests on fiber-reinforced clay specimens were also performed. Results are presented in the form of stress vs. strain curves for plain soil as well as fiber-reinforced soil for various fiber contents based on the model developed. The results demonstrate the applicability of proposed analytical model in predicting the stress strain response of fiber-reinforced soils.     

盐溶液饱和黏土的力学行为模拟

由于黏性土表面带有丰富的负电荷,孔隙水溶液化学状态的变化对黏性土的物理力学特性存在明显影响。随着化学-力学耦合的相关岩土工程问题日益突出,进行有效的化学-力学耦合行为的数值分析评价显得尤为重要。因此,建立一个简单有效的考虑化学-力学耦合的本构模型是非常关键的。基于传统的修正剑桥模型,提出了一个简单的化学-力学耦合模型。该模型采用渗透吸力π描述孔隙水的化学状态,建立了前期屈服应力,临界状态线斜率M和弹性刚度与渗透吸力π之间的关系式,从而实现了模型对盐溶液饱和黏性土的变形和强度特性的有效模拟。通过与试验数据的对比和分析,说明该模型能有效地模拟孔隙盐溶液饱和黏性土的等向压缩行为、 状态下压缩行为以及 状态下化学-力学循环加载行为。此外,通过对黏性土三轴压缩试验的模拟,说明该模型能反映黏性土三轴应力状态下的基本力学特征。     

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。     

超固结土热黏弹塑性本构关系

热黏弹塑性本构模型是描述土在温度（热）和时间（黏）耦合作用下的应力-应变关系的本构模型。在一些新型岩土工程诸如高放核废料地质处置、地热资源开发与贮存的建设中,需要同时考虑温度和时间对土的影响,所以建立一个热黏弹塑性本构模型具有理论和实际意义。将温度变化对黏土体积和强度参数的影响引入笔者之前提出的超固结土等向应力-应变-时间关系,建立了一个等向应力条件下的应力-应变-时间-温度关系。随后,基于该关系推导了屈服面硬化定律,并将其与超固结土统一硬化模型的屈服方程和流动法则结合,建立了超固结土的热黏弹塑性本构模型。最后,使用新模型预测室内试验,证明新模型能够反映时间和温度对土体积、一维压缩曲线和前期固结压力的耦合影响。     

Evaluation of allowable withdrawn volume of groundwater based on observed data

To control land subsidence due to groundwater withdrawal, it is important to estimate allowable withdrawn volume of groundwater in a soft deposit. This technical note presents a simple approach for estimating the allowable withdrawn volume of a deposit. A regression analysis method was used based on measured land subsidence and recorded net withdrawn volume. This approach was proposed based on the principle of soil compression at different effective stresses, i.e. the soil compression is small when the consolidation stress is lower than the yield stress of the deposit, but large when the consolidation stress is higher than the yield stress. Two case studies are presented in this technical paper to demonstrate how to use the simple approach to estimate the allowable withdrawn volume.     

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.     

A New Concept on the Compressibility of Mixed Soils: Experimental and Numerical Approach

In this article, a new concept on the compressibility of mixed soils is proposed. Six characteristic structures of mixed soils are recognized, defined and described based on the percentage of the ground material in the mixture. The compressibility mechanism and the deformational behavior of each structure are extensively studied by both laboratory and numerical experiments. Two series of compressibility tests are initially conducted in the laboratory, using artificial mixtures of sand and clay at different ratios and subjected to typical compressibility tests using the oedometer apparatus. Then, a numerical approach is employed, based on the finite element method and Monte-Carlo simulations, in order to reproduce the conditions of the laboratory tests and to further study the compressibility of each characteristic structure. From the results obtained, it is concluded that the deformational behavior of mixed soils depends strongly on the percentage of the ground in the mixture and on the mechanical properties of the components of this mixture. Furthermore, it is shown that the estimated deformations and stress states can be highly unrealistic when the mixed soil is not properly modelled and is assumed to be governed by the properties of its weaker component.     

Numerical study of localization in soil systems

A numerical study of the mechanical behavior of heterogeneous soil systems, consisting of a bulk of sand with embedded stiff gravel inclusions or soft clay inclusions, is performed. A solution scheme using parallel computing is employed when analyzing two different categories of problems. First, a homogenization problem is studied, where use of a single representative volume element subjected to plane strain compression offers the possibility to investigate the coupling between the response at a local scale and at a global scale. Second, a plane strain footing problem with different heterogeneous soil systems is analyzed using a traditional finite element formulation. The material model utilized for the soil is a large deformation formulation of non-associated elasto-plasticity with an isotropic hardening law, able to represent dilation. It was found that the shape of the gravel or clay inclusions in the systems had no significant effect on the global responses, whereas the strain localizations in the two different soil systems, sand–gravel and sand–clay, were found to have different character. The effect of the initial density on the response was clearly observed in the localization patterns.     

土石混合体压缩性的三维颗粒力学研究

土石混合体是由作为骨料的砾石或卵石与作为填充料的黏土或砂土组成的特殊工程地质材料,无论天然形成或人工合成,其力学特性与均质土体或岩体相比均有较大差异。以昆明新机场高填方中大量采用土石混合体填料为工程背景,在三维颗粒流程序中对特定的土石混合体试样的侧限压缩试验进行数值模拟,比较了不同级配条件下土石混合体模型的微观结构及基本力学物理性能。通过比较侧限压缩模量和压缩后孔隙率,研究了含石率对土石混合体骨架效应等结构性的影响,并提出了工程中建议采用的土石比例区间,在颗粒流方法用于实际工程方面进行了有益的探索。     

结构性土UH模型

在描述重塑土的统一硬化模型（UH模型）基础上,以动态地移动正常固结线（MNCL）代替静态的正常固结线（NCL）作为参考线来确定参考应力,将土的结构性衰减体现在MNCL的演化中,从而把UH模型扩展为可考虑土结构性的结构性土UH模型。结构性土UH模型可以合理地并连续光滑地描述结构性土的等向压缩规律、应变硬化、应变软化、剪切体积收缩、剪切体积膨胀、不排水剪切减压软化以及结构性和密度耦合影响等力学特性。相比于UH模型,结构性土UH模型新增3个参数,分别用于描述结构性的程度、衰减速度、以及塑性流动规律。通过3种天然土的试验结果与模型预测对比验证了结构性土UH 模型的合理性。     

A simple hypoplastic model for normally consolidated clay

The paper presents a simple constitutive model for normally consolidated clay. A mathematical formulation, using a single tensor-valued function to define the incrementally nonlinear stress–strain relation, is proposed based on the basic concept of hypoplasticity. The structure of the tensor-valued function is determined in the light of the response envelope. Particular attention is paid towards incorporating the critical state and to the capability for capturing undrained behaviour of clayey soils. With five material parameters that can be determined easily from isotropic consolidation and triaxial compression tests, the model is shown to provide good predictions for the response of normally consolidated clay along various stress paths, including drained true triaxial tests and undrained shear tests.     

An elasto‐plastic three phase model for partially saturated soil for the finite element simulation of compressed air support in tunnelling

This paper presents a fully coupled finite element formulation for partially saturated soil as a triphasic porous material, which has been developed for the simulation of shield tunnelling with heading face support using compressed air. While for many numerical simulations in geotechnics use of a two‐phase soil model is sufficient, the simulation of compressed air support demands the use of a three‐phase model with the consideration of air as a separate phase. A multiphase model for soft soils is developed, in which the individual constituents of the soil—the soil skeleton, the fluid and the gaseous phase—and their interactions are considered. The triphasic model is formulated within the framework of the theory of porous media, based upon balance equations and constitutive relations for the soil constituents and their mixture. An elasto‐plastic, cam–clay type model is extended to partially saturated soil conditions by incorporating capillary pressure according to the Barcelona basic model. The hydraulic properties of the soil are described via DARCY 's law and the soil–water characteristic curve after VAN GENUCHTEN . Water is modelled as an incompressible and air as a compressible phase. The model is validated by means of selected benchmark problems. The applicability of the model to geotechnical problems is demonstrated by results from the simulation of a compressed air intervention in shield tunnelling. Copyright © 2009 John Wiley & Sons, Ltd.     

考虑压缩模量深度效应的深厚软土桩基沉降计算

对于深厚软土桥梁桩基础的沉降计算,土层的压缩模量是一个极为重要的参数。针对京沪高速铁路桥梁桩基沉降,以DK152工点处的土工试验数据和现场测试成果为基础,通过拟合分析得到与土层深度有关的天然状态下压缩模量计算公式。在该基础上,进一步运用神经网络建立土层压缩模量与桩基沉降之间的映射关系,对不同土层在不同深度的压缩模量进行反演分析,相关反演结果与经验公式计算值基本一致,得到的桩基沉降量与现场监测位移吻合良好,说明文中提出的压缩模量计算公式的准确性和实用性,相关模型概化和反演计算方法也是合理的,对深厚软土地基下高速铁路桥梁深长桩基的沉降计算有一定的参考价值。     

下穿隧道开挖引起的挤土桩沉降控制分析方法

随着地下空间开发利用的立体式发展,地下近接工程中结构相互作用问题日益突显。针对盾构隧道下穿既有挤土桩问题,提出了基于岩土介质小孔扩张（收缩）理论的分析方法。通过基于统一砂黏土本构模型和大应变假设推导的小孔扩张-收缩排水解析解,建立了隧-桩相互作用力学模型;提出了隧道开挖影响下桩基的承载力衰减因子,并采用荷载-沉降曲线预测了桩基沉降。由分析结果可知,隧道地层损失引起的桩侧承载力减小、桩端承载力减小和桩端刚度损失,三者共同作用促使桩基发生沉降失稳,并提出了桩基的承载力控制准则、稳定控制准则和变形控制准则。此外,研究得到了桩基失稳时隧道地层损失与各因素之间的相关关系。其结果为揭示隧道与邻近结构相互作用机制、保障地下结构稳定性提供了有效的分析手段和必要的理论依据。     

考虑气相硬化的准饱和黏土弹塑性模型

假定准饱和黏土由饱和基质（土骨架和水）和封闭气体组成,采用饱和基质塑性体应变和气体塑性体应变作为硬化参数以反映准饱和黏土的塑性硬化行为,在综合考虑气体溶解系数随温度和水中含盐量变化、预固结应力随封闭气体的变化等关键因素的基础上,基于临界状态土力学理论提出了可以反映封闭气体气压变化对准饱和黏土力学影响的弹塑性本构模型。模型共计10个材料参数,均可通过等向压缩试验和三轴剪切试验获得。通过与已有试验数据对比分析,结果显示本模型可以较好地模拟准饱和黏土在不排水应力路径下的应力-应变关系、孔隙水压力发展以及饱和度的演变规律;所建立模型形式简单,参数易于确定,为准饱和黏土地基中岩土工程问题的设计、计算及灾变控制提供了重要的理论基础。     

The combined effect of clay and moisture content on the behavior of remolded unsaturated soils

The behavior of unsaturated clayey soil is highly influenced by the coupled interaction between water and clay content. Various aspects of the behavior of artificial clay–sand mixtures with variable water content were experimentally studied. Laboratory tests were utilized for the determination of consistency limits, the stress–strain relationship, strength parameters, hydraulic conductivity, and volume change characteristics for various combinations of water and clay content in soil mixtures.

Results presented for various clay–sand mixtures include: new normalized consistency limits; the combined effect of clay content and water content on the stress–strain relationship and on the strength parameters (c and φ); and the effect of clay content on hydraulic conductivity and swelling potential. The cohesion of clayey sand is found to increase with increasing water content to a certain limit, above which it decreases. The angle of internal friction for clayey sand is found generally to decrease with increasing water content. The degree of saturation is found to be better than the water content in explaining the strength behavior. The hydraulic conductivity sharply decreases with increasing clay content up to 40% beyond which the reduction becomes less significant. Simple empirical equations are proposed for predicting the swelling potential of clayey soils as a function of either the clay content or plasticity index.     

Development of synthetic soil mixture for experimental shaking table tests on building frames resting on soft soils

In this study, a synthetic soil mixture has been developed and proposed for experimental soil-structure interaction shaking table tests on building frames with shallow foundations resting on soft soil deposits. The proposed mix provides adequate undrained shear strength to mobilise the required shallow foundation bearing capacity underneath the structural model while meeting both criteria of dynamic similarity between the model and the prototype to model soft soils in shaking table tests. To find the most appropriate mixture, different mixes with different proportions of mix components were examined in the soils laboratory. Performing bender element tests, the shear wave velocity of the soil specimens was acquired at different cure ages and the results were examined and compared. Based on the test results, a synthetic clay mixture consisting of kaolinite clay, bentonite, fly ash, lime and water has been proposed for experimental shaking table tests.