不同密度粗粒料强度特性的大型真三轴试验

干密度对粗粒料的强度特性有重要影响,特别是三维应力状态下影响更为显著。通过不同密度粗粒料大型真三轴等小主应力等中主应力系数（b=0.25）加载试验和大三轴试验研究了干密度对粗粒料强度特性的影响。结果表明,大型真三轴试验的应力曲线基本呈爬升型,高于和陡于大三轴应力曲线,表现出较强的硬化性;粗粒料强度基本随初始干密度的增大或小主应力的增大呈线性增大;大型真三轴试验强度比大三轴试验强度增大20%～97%,且小主应力越小,强度增大幅度越大;令粗粒料黏聚力为0,则内摩擦角随初始干密度的增大基本呈线性增大,随小主应力的增大而减小;破坏应力比随初始干密度的增大呈线性增大,随小主应力的增大而呈线性减小,大型真三轴试验的破坏应力比小于大三轴试验的破坏应力比。     

Modelling the time‐dependent behaviour of granular material with hypoplasticity

This paper presents a constitutive model for time‐dependent behaviour of granular material. The model consists of 2 parts representing the inviscid and viscous behaviour of granular materials. The inviscid part is a rate‐independent hypoplastic constitutive model. The viscous part is represented by a rheological model, which contains a high‐order term denoting the strain acceleration. The proposed model is validated by simulating some element tests on granular soils. Our model is able to model not only the non‐isotach behaviour but also the 3 creep stages, namely, primary, secondary, and tertiary creep, in a unified way.     

粒状介质三维复杂应力加载离散元数值试验方法

实际荷载条件下（如交通、地震荷载）,粒状岩土材料常受到三维复杂应力路径作用。目前,多数粒状岩土材料的本构理论和模型都基于简单应力路径加载条件下的物理试验提出,在更加复杂应力路径下的适用性则需要进一步验证。但受机械控制的限制,物理试验中无法实现很多客观存在的三维复杂应力路径加载。为了能够再现并分析三维复杂应力路径下粒状介质的力学响应,提出了一种离散元数值试验方法,该方法采用球形数值试样,通过直接控制试样边界应力达到对3个主应力大小和方向的任意控制,从而可以实现诸多物理试验中无法实现的复杂应力路径。通过与目前常见的一些物理试验进行定性对比,论证了该数值试验方法通过高精度的加载控制和测量能够再现已有物理试验现象。在此基础上,进一步开展了应力主轴的三维旋转,分析了在这种实际存在却无法通过物理试验再现的加载条件下粒状介质的变形规律,初步显示了提出的数值试验方法在深入研究三维复杂应力路径下粒状介质力学响应方面所拥有的能力和优势。     

粗粒土组构之颗粒运动研究

土的结构性是决定土的力学特性的一个最根本的因素,正是由于土的结构性的存在,连续体力学理论用于岩土材料存在明显不足,沈珠江将土体结构性的研究提到“21世纪土力学的核心问题”的高度,但由于土的结构性的复杂性,缺乏测试方法,还没有找到土结构性的定量指标。选择结构特征相对简单的粗粒土作为研究对象,探索建立其力学体系的方法和途径。粗粒土的结构性（称之为组构）主要体现在颗粒的排列特征方面,采用CT测试技术和DDA数值分析方法初步解决了粗粒土组构的量化问题,对粗粒土多种受力过程进行了CT三轴试验和数值模拟,取得了粗粒土的组构信息及其变化,为构成合理的组构量、研究粗粒土组构量与宏观力学响应之间的相互关系、探讨粗粒土的变形机理和力学特征奠定了基础,并侧重介绍粗粒土CT三轴试验成果,展示三轴试验过程中颗粒运动规律。     

Fractional order model for granular soils under drained cyclic loading

The theory of fractional calculus has been successfully applied to model the triaxial behaviour of soils under static loading conditions. However, limited work has been carried out in using the fractional calculus to describe the cyclic behaviour of granular soils. In this paper, a fractional order constitutive model for granular soils under drained cyclic loading is proposed by incorporating the concept of fractional rate for strain accumulation. The fractional rate for strain accumulation is obtained from the analysis of the experimental data by utilizing the fractional calculus. Comparison between the test results and model predictions is presented. The key feature of the proposed model is that it can reasonably characterise the cyclic deformation of granular soils under both low and high loading cycles. Copyright © 2016 John Wiley & Sons, Ltd.     

紫坪铺面板坝堆石料颗粒破碎试验研究

采用大型三轴仪对紫坪铺面板坝堆石料进行了单调和循环荷载下的固结排水剪切试验,研究了不同孔隙比情况下颗粒破碎及剪胀的变化规律。试验表明：（1）单调和循环荷载条件下,堆石料颗粒破碎率与塑性功之间存在一致的双曲线关系;（2）峰值应力处剪胀率与颗粒破碎率在半对数坐标中呈近似线性关系;（3）峰值应力处主应力比与相应的剪胀率呈近似线性关系,且上述结果受初始孔隙比的影响不大。研究成果有助于进一步了解堆石料的颗粒破碎特点,对建立复杂应力条件下考虑颗粒破碎和状态相关性的弹塑性本构模型,分析紫坪铺面板堆石坝汶川地震破损机制是十分有益的。     

A numerical investigation of the incremental behavior of crushable granular soils

The mechanical behavior of granular materials is characterized by strong nonlinearity and irreversibility. These properties have been differently described by a variety of constitutive models. To test any constitutive model, experimental data relative to the nature of the incremental stress–strain response of the material is desirable. However, this type of laboratory data is scarce because of being expensive and difficult to obtain. The discrete element method has been used several times as an alternative to obtain incremental responses of granular materials. Crushable grains add one extra source of irreversibility to granular materials. Crushability has been variously incorporated into different constitutive models. Again, it will be helpful to obtain incremental responses of crushable granular materials to test these models, but the experimental difficulties are increased. Making use of a recently introduced crushing model for discrete element simulation, this paper presents a new procedure to obtain incremental responses in discrete analogs of granular crushable materials. The parallel probe approach, previously used for uncrushable discrete analogs, is here extended to account for the presence of crushable grains. The contribution of grain crushing to the incremental irreversible strain is identified and separately measured. Robustness of the proposed method is examined in detail, paying particular attention to aspects such as dynamic instability or crushing localization. The proposed procedure is later applied to map incremental responses of a discrete analog of Fontainebleau sand on the triaxial plane. The effect of stress ratio and granular state on plastic flow characteristics is highlighted. Copyright © 2016 John Wiley & Sons, Ltd.     

A comparative study of the hypoplasticity and the fabric‐dependent dilatant double shearing models for granular materials

In this paper, we consider the mechanical response of granular materials and compare the predictions of a hypoplastic model with that of a recently developed dilatant double shearing model which includes the effects of fabric. We implement the constitutive relations of the dilatant double shearing model and the hypoplastic model in the finite element program ABACUS/Explicit and compare their predictions in the triaxial compression and cyclic shear loading tests. Although the origins and the constitutive relations of the double shearing model and the hypoplastic model are quite different, we find that both models are capable of capturing typical behaviours of granular materials. This is significant because while hypoplasticity is phenomenological in nature, the double shearing model is based on a kinematic hypothesis and microstructural considerations, and can easily be calibrated through standard tests. Copyright © 2006 John Wiley & Sons, Ltd.     

Three-dimensional numerical modelling of the behaviour of a pile subjected to cyclic lateral loading

This paper presents a finite element simulation of the behaviour of a vertical pile subjected to a cyclic lateral loading. First, the choice of a suitable constitutive model is discussed. The analytical solution of the model equations for a monotonic triaxial compression is given, and the model is compared with monotonic and cyclic triaxial tests on dry Fontainebleau sand. Simulations of tests carried out on a model pile with the geotechnical centrifuge of the French Public Works Research Laboratory (LCPC) were then performed with the model parameters derived from the triaxial tests. Results are in good agreement with experiments.     

Constitutive model for cyclic behaviour of cohesionless sands

This paper presents a constitutive model for describing the stress-strain response of sands under cyclic loading. The model, formulated using the critical state theory within the bounding surface plasticity framework, is an upgraded version of an existing model developed for monotonic behaviour of cohesionless sands. With modification of the hardening law, plastic volumetric strain increment and unloading plastic modulus, the original model was modified to simulate cyclic loading. The proposed model was validated against triaxial cyclic loading tests for Fuji River sand, Toyoura sand and Nigata sand. Comparison between the measured and predicted results suggests that the proposed modified model can capture the main features of cohesionless sands under drained and undrained cyclic loading.     

Adaptation of an existing constitutive model for describing mechanical behaviour of cemented granular soils

In this paper, an existing elastoplastic constitutive model, originally developed for granular soils, is adapted to describe the stress–strain behaviour of cemented granular soils. The existing model (CJS), due to its modular formulation, can be easily developed to take into account different supplementary behavioural aspects in soil mechanics. In the present study, the failure mechanism of the CJS model is modified by introducing the essential aspects in the behaviour of cemented granular soils in its formulation. All of the model parameters have clear physical meaning and can be identified using classical laboratory tests. A set of direct relations between model parameters and famous mechanical parameters of soils such as internal friction angle and cohesion at peak and residual states is presented. In order to validate the model, the results of triaxial and uniaxial tests in the compression and extension performed on cemented granular materials are used. The validation results indicate the good capability of the proposed model.     

基于分数阶微积分的粗粒料静动力边界面本构模型

分数阶微分理论在土体静力黏弹性本构模型中得到了广泛应用,然而,其在动力弹塑性模型中的应用尚不多见。为此,基于分数阶微积分理论分析了粗粒料在循环荷载下的变形特性,提出了粗粒料在循环荷载下的分数阶应变率;并以此为基础,进一步建立了粗粒料受静动力荷载作用下的边界面塑性力学本构模型。所提出模型包含10个参数,均可以运用常规三轴试验获得。为了验证所提出模型,选取了几种已有不同文献中的不同粗粒料试验数据进行了模拟,发现,所提出的模型可以较好地模拟粗粒料在静动力加载下的应力-应变行为,对于循环荷载下的长期变形也能较好地预测。     

颗粒级配对粒状材料不排水力学特性的影响

为了研究颗粒级配对粒状材料不排水力学特性的影响,采用两种形状自相似度很高的材料（玻璃球和Hostun砂）进行了一系列传统三轴不排水压缩试验。对于每种材料,保持相近的相对密实度制作了6个不同颗粒级配（不均匀系数 1.1～20）的试样,然后进行剪切。试验结果表明,颗粒级配对粒状材料的不排水力学性能存在明显影响：两种材料试样的偏应力水平随着不均匀系数的增加而降低,抗剪强度随着材料的不均匀系数 的增加而降低,并在 5后呈现出趋稳势态。基于不排水剪切试验中颗粒集合体二阶功的演化规律,对材料受荷载过程中出现不稳定性的条件进行分析。结果表明,在相同密实度下,粒状材料的静态液化潜能随着材料不均匀系数 的提高而增强,高不均匀系数的材料变得不稳定。     

A constitutive model for sand based on non-linear elasticity and the state parameter

The paper describes the development of a constitutive model for a poorly graded sand, which was used in geotechnical experiments on buried pipes (reported elsewhere). The sand was tested extensively in the laboratory to determine the state parameter constants. Triaxial tests on the sand included conventional drained triaxial compression tests, as well as more specialized shearing tests at constant mean effective stress and others at constant volume. Single element simulation of the triaxial tests was performed to validate the proposed constitutive model. The adopted model allowed non-linear elastic behaviour prior to yielding. After yielding of the sand, the state parameter-based model for the sand permitted non-associated plastic flow. Dilation and frictional strength were both dependent on the current value of the state parameter. The combination of laboratory testing and single element modelling resulted in the selection of a single set of material constants for the soil, which adequately described the full range of triaxial tests. Subsequently the model was applied to the problem of a plate loading test on the sand and the model predictions were compared with the test data.     

BEYOND FAILURE IN GRANULAR MATERIALS

Recent investigations on the hypoplastic constitutive model for granular materials show that the failure surface can be surpassed by some stress paths. This is contradictory to the conventional definition of failure surface in plasticity, according to which the stress is allowed to move on the failure surface but never across it. In the present paper, the interrelations among the different constitutive models are discussed with special reference to failure and stability. For the hypoplastic constitutive equation, the accessible stress states and the stable stress states are found to be enclosed by a bound surface and a stability surface in the stress space, respectively. Theoretical findings about the bound surface and the stability surface are verified qualitatively by presenting results of triaxial tests on dry sand. © 1997 by John Wiley & Sons, Ltd.     

应力状态对粗粒料力学特性影响的大型真三轴试验

土石坝所处的应力状态较接近平面应变应力状态或三维应力状态,而常规三轴试验低估了粗粒料的力学性能。应用大型真三轴仪对常规三轴应力状态、平面应变应力状态和真三轴应力状态（ 0.25, 为中主应力系数, 、 、 分别为大、中、小主应力）下粗粒料的力学特性进行了压缩试验研究。试验结果表明：相同小主应力下,常规三轴试验、平面应变试验、真三轴试验的大、小主应力之差与大主应力方向应变的关系曲线依次变高变陡。某一试验加载条件下,体应变随球应力的增大而增大,初始剪切阶段增加较慢,随后呈线性增大,不同小主应力的体应变曲线较为接近。平面应变试验强度和真三轴试验强度比常规三轴试验强度有较大增长,真三轴试验强度增加百分比大于平面应变试验强度增加百分比。初始弹性模量随小主应力的增大呈线性增大。平面应变状态下中主应力系数随大主应力方向应变的增大而增大,初始剪切阶段增长较缓,之后近似呈线性增大。相同小主应力下,从常规三轴应力状态至平面应变应力状态再到真三轴应力状态,小主应力方向应变均为膨胀变形,且膨胀变形依次增大。同一试验加载条件下,小主应力较小时,应力比（偏应力与球应力之比）与偏应变的关系曲线位于上方。     

Constitutive modeling of inherent anisotropy in a strain space multiple mechanism model for granular materials

Consideration of fabric anisotropy is crucial to gaining an improved understanding of the behavior of granular materials. This paper presents a constitutive model to describe the sand behavior associated with fabric anisotropy within a framework of a strain space multiple mechanism model. In the proposed model, a second-order fabric tensor is extended by incorporating a new function that represents the effect of inherent (or initial fabric) anisotropy, along with three additional parameters: two of them, a₁ and a₂ , control the degree of anisotropy, and the second mode of inherent anisotropy can be expressed by introducing the parameter a₂ as well as the first mode by the parameter a₁ . The third parameter, θ₀ , expresses the principal direction of inherent anisotropy (eg, the normal vector direction of bedding planes relative to horizontal axis). The formulation of the dilative component of dilatancy (ie, positive dilatancy) is also extended to consider the effect of inherent anisotropy based on the interlocking mechanism. Experimental data on the complex anisotropic responses of Fraser River sand and Toyoura sand under monotonic loading is used to validate this model. The proposed model is shown to successfully capture anisotropic responses, which become contractive or dilative depending on different principal-stress directions, with a single set of anisotropy parameters; thus, the model is considered to possess the capability to simulate the anisotropic behaviors of granular materials. In addition to different loadings on the same fabric, the effects of different fabric anisotropies upon the sand behavior under the same loadings are also investigated.     

堆石料Gudehus-Bauer亚塑性本构模型改进及参数确定方法

研究了Gudehus-Bauer亚塑性本构模型和模型参数的求取方法。采用侧限压缩试验曲线求取模型参数颗粒硬度hs和指数n。根据模型方程的推导,建立了拟合指数 和 与围压之间的关系,并提出了新的拟合参数。考虑到堆石料具有明显的剪胀、剪缩性,在Gudehus-Bauer模型线性项中增加了主要控制体积应变项 ,以改善模型对堆石料体积应变曲线的描述。采用堆石料大型侧限压缩试验、常规三轴试验分别验证了新的拟合参数和改进后的Gudehus-Bauer亚塑性本构模型。与堆石料试验成果比较,提出的新拟合参数与改进后的Gudehus-Bauer亚塑性本构模型可以较好地模拟堆石料的应力-应变特性,并较好地改善了堆石料体变曲线的模拟结果。对改进后的模型作了常规三轴加、卸载模拟,模拟结果反映了改进的Gudehus-Bauer亚塑性本构模型具有一定的卸载适应性。     

A critical state sand plasticity model accounting for fabric evolution

Fabric and its evolution need to be fully considered for effective modeling of the anisotropic behavior of cohesionless granular sand. In this study, a three‐dimensional anisotropic model for granular material is proposed based on the anisotropic critical state theory recently proposed by Li & Dafalias [2012], in which the role of fabric evolution is highlighted. An explicit expression for the yield function is proposed in terms of the invariants and joint invariants of the normalized deviatoric stress ratio tensor and the deviatoric fabric tensor. A void‐based fabric tensor that characterizes the average void size and its orientation of a granular assembly is employed in the model. Upon plastic loading, the material fabric is assumed to evolve continuously with its principal direction tending steadily towards the loading direction. A fabric evolution law is proposed to describe this behavior. With these considerations, a non‐coaxial flow rule is naturally obtained. The model is shown to be capable of characterizing the complex anisotropic behavior of granular materials under monotonic loading conditions and meanwhile retains a relatively simple formulation for numerical implementation. The model predictions of typical behavior of both Toyoura sand and Fraser River sand compare well with experimental data. Copyright © 2013 John Wiley & Sons, Ltd.