循环荷载下砂土变形的细观数值模拟Ⅱ:密砂试验结果

采用颗粒流理论模拟了不同排水条件下密砂的双轴试验,通过开发的细观组构统计程序记录加载不同时刻试样的细观组构演化,研究了循环荷载作用下密砂变形过程中细观配位数、接触方向、粒间接触力等的演化规律及其与试样宏观力学响应之间的内在联系。探讨了密砂在循环荷载作用下诱发各向异性、剪胀软化、循环活动性等产生的微细观机理。与松砂相比,密砂在循环荷载作用下,组构各向异性与应力各向异性彼此相协调,没有产生配位数的连续累积损失。     

Experimental study on sand anisotropy using hollow cylinder apparatus

This study investigated sand anisotropy experimentally using a hollow cylinder apparatus. The effect of the initial anisotropy on the shear behavior of sand was illustrated by conducting experiments on specimens with bedding planes and systematically varying the density, principal stress direction, and intermediate principal stress. The change in induced anisotropy during shearing was experimentally captured by re-shearing the specimens subjected to prior shear history. The experimental results revealed the following: (a) Anisotropy in sand, whether initial anisotropy developing during specimen preparation or induced anisotropy developing due to shear history, causes pseudo-density changes in the mechanical behavior, in which sand of the same density behaves as if it has a different density depending on the direction of shear. (b) The changes in induced anisotropy, due to shearing in the same direction as that of the prior shear, make the soil behave similarly to dense sand, whereas shearing in a direction perpendicular to the prior shear makes the soil behave similarly to loose sand. (c) The larger the prior shear, the more pronounced the pseudo-density changes that appear in the subsequent behavior. Moreover, the significance of induced anisotropy in liquefaction and compaction phenomena was experimentally demonstrated through single and double swing cyclic shear tests. The results obtained from the study will be useful for validating models that incorporate induced anisotropy.     

宜兴抽水蓄能电站上库堆石料的新型现场直剪试验

介绍了一种适用于大颗粒堆石材料的新型现场直剪试验法。该试验法用链条张拉剪切框使试样受剪,克服了剪切框内壁摩擦对剪切面上正应力的影响,从而能精确地测定出试样的抗剪强度。该试验法在日本应用于多个工程,江苏宜兴抽水蓄能电站上库工程中属于国内首次应用,取得了满意的结果。     

松砂不稳定行为的数值模拟研究

针对松散砂土的不稳定行为和静态液化现象，开展了一系列的双轴剪切试验离散元数值模拟，从宏细观的角度研究了松砂不稳定行为的各种影响因素及其发生机理。通过数值模拟发现，砂土的不稳定行为不仅与初始孔隙比有关，而且与土体结构的各向异性和围压有关。当加载方向与由颗粒接触法向所表征的结构各向异性主方向不共轴时，不稳定行为的发生会同时伴随着各向异性主方向的旋转。两个方向的角度差愈大，不稳定行为愈容易发生，颗粒的细观运动和细观结构重组现象也愈剧烈。不稳定状态点处的剪切荷载主要由颗粒间法向接触力承担，切向接触力仅起次要作用。     

散体颗粒介质变形局部化宏–细观机制研究

散体颗粒是自然界中广泛存在的一类特殊的介质,受内部组构影响,散体颗粒介质在外荷载作用下发生明显的变形局部化效应,这也是岩土力学领域一直研究的热点问题之一。然而目前对于其机制研究方面大都局限于细观统计参数的分析,忽视了宏观现象与细观机制的内在联系。本文以砂土颗粒介质为例,运用离散元法对其在直剪试验过程中的宏–细观力学特性及变形破坏机制进行系统分析,并取得一些有意义的认识。根据数值试验中试样在不同法向应力下剪应力比的发展,从颗粒运动角度探讨试样宏观抗摩擦特性变化趋势的细观机制;发现描述细观组构的各向异性参数及主方向与宏观上剪应力比的发展具有同步性,反映试样宏细观演化的统一性;通过对于颗粒旋转的统计分析,揭示颗粒间摩擦作用是维持细观力学结构相对稳定的重要因素;通过对力链网络的形态演化分析,发现试样大主应力方向与各向异性主方向一致,力链密集程度随剪切过程下降,试样孔隙度随之上升;提出不同法向应力下的2种主要力链结构的力学模型,通过稳定性分析与能量累积释放理论解释宏观力学参数波动情况的原因,揭示散体颗粒介质变形局部化和体应变剪胀的细观机制。     

Application of Micromechanics Model to Study Anisotropy of Soils at Small Strains

A micromechanics model is used to analyse the stiffness anisotropy of soils at small strains. Five material constants for a cross-anisotropic elastic material are related to micromechanics variables such as fabric anisotropy, contact stiffness, particle radius, and the number of contacts in a given volume of particulate assembly. The analytical results from the model are compared with the published experimental data on small-strain stiffness anisotropy in order to estimate typical soil fabric conditions of sands and clays. The relationship between the small-strain shear modulus obtained from triaxial tests and shear tests is examined using the micromechanics model. The analysis shows that, when a soil is stiffer in the horizontal direction, the shear modulus evaluated from the conventional triaxial drained tests underestimates G_vh and G_hh. The opposite is true when a soil is stiffer in the vertical direction. When a soil is sheared in undrained condition, the measured shear modulus is closer to G_vh than G_hh, especially when the soil is stiffer in the horizontal direction. The effect of soil anisotropy on the stiffness measured from different stress paths in triaxial condition is investigated.     

Direct shear behavior of gravel-rubber mixtures: Discrete element modeling and microscopic investigations

In this paper, a newly developed 3-dimentional discrete element model (DEM) for gravel-rubber mixtures (GRMs), namely DEM4GRM, that is capable of accurately describing the macro-scale shear response (from small to large deformation) of GRMs in a direct shear box apparatus is presented. Rigid gravel grains are modelled as simple multi-shape clumps, while soft rubber particles are modeled by using deformable 35-ball body-centered-cubic clusters. Mixtures are prepared with different volumetric rubber content (VRC) at 0, 10, 25, 40 and 100%, statically compressed under 30, 60 and 100 kPa vertical stress and then sheared, by closely simulating a reference laboratory test procedure. The variation of micro-scale factors such as fabric, normal and tangential force anisotropy is carefully examined throughout the shearing process and described by means of novel micro-mechanical relationships valid for GRMs. Moreover, strong-force chains are scrutinized to identify the transition from rigid to soft granular skeleton and gain insights on the load transfer and deformation mechanisms of GRMs. It is shown that the development of the fabric and force anisotropy during shearing is closely related to the macro-scale shear strength of GRMs, and strongly depends on the VRC. Besides, strong-force chains appear to be primarily formed by gravel-gravel contacts (resulting in a rigid-like mechanical behavior) up to VRC = 30%, and by rubber-rubber contacts (causing a soft-like mechanical response) beyond VRC = 60%. Alternatively, at 30% < VRC < 60%, gravel-rubber contacts are predominant in the strong-force network and an intermediate mechanical behavior is observed. This is consistent with the behavioral trends observed in the macro- and micro-mechanical responses.     

Investigation into macroscopic and microscopic behaviors of bonded sands using distinct element method

This paper presents an investigation into the inter-particle bonding effects on the mechanical behaviors of structured sands using the distinct element method (DEM) incorporating measured inter-particle mechanical behaviors. The inter-particle mechanical behaviors are first studied by testing on idealized bonded granules under designed loading paths, which demonstrates a linear pre-failure force–displacement relationship and normal force-dependent shear strength of bonded particles. Then a modified contact model is proposed by employing different force–displacement laws for pre-failure and post-failure bonded particles, in which a failure criterion is introduced to describe the inter-particle debonding. The third part deals with the DEM numerical simulation of isotropic and biaxial compression tests to investigate the bonding effects on the mechanical behaviors of bonded sands, where the proposed model has been verified capable of capturing the main mechanical behaviors of bonded sands. In addition, the investigation into the microscopic responses quantitatively figures out the effects of inter-particle cementation on the mechanical behaviors and the loss of soil structure.     

黄土和泥岩的动力学特性及微观损伤效应

 为了系统研究天水地区黄土和泥岩的动力学特性和微细观损伤机制,采用循环荷载试验、反复剪试验和SEM电镜扫描技术,获取岩土体的动力学行为特征和微观参量。研究结果表明：黄土和泥岩的动弹性模量随围压的增加逐渐增大,呈非线性关系,而动弹性模量与阻尼比呈反比关系;动剪应力比大于某阈值时,岩土体在相应周期动应力下发生破坏;地震强度以及岩土体自身材料对其动力学强度影响较大;反复剪切次数增加,岩土体的抗剪强度降低,最终达到残余强度,但随围压的增大,岩土体咬合程度增强,抗剪强度增加;静力剪切使土颗粒变细,属颗粒剪切磨碎模式,土颗粒排列更平整,定向性好,颗粒间孔隙变密实;循环动荷载作用下,随着剪切幅值的增加,颗粒剪切磨碎,排列更平整,孔隙明显减少,剪切面表现为“板结”。泥岩微观孔隙的定量化分析与定性研究一致性较好。研究结果对黄土高原地区工程灾害的预测和控制具重要的理论意义和实用价值。     

砂土直剪力学性状的非圆颗粒模拟与宏细观机理研究

基于PFC2D非圆颗粒单元的二次开发,对砂土直剪力学过程进行了非圆颗粒仿真模拟,分析了数值试样的应力–剪胀关系并与实际砂土进行对比,探讨了颗粒位移与颗粒旋转特征及其与剪切带演化的内在关联,研究了主应力与主应变增量的非共轴效应,揭示了细观组构各向异性的演化规律及其与宏观剪切强度之间的宏细观关联。研究结果表明,数值试验能够较好的模拟实际砂土的应力–剪胀关系和剪切过程主应力与主应变增量的非共轴效应;剪切带的演化与颗粒位移和颗粒旋转密切相关,颗粒形状影响剪切带的厚度;试样宏观的剪切强度主要受控于粒间法向接触力的分布及其各向异性演化;整个加荷过程中,剪切带内大主应力的偏转方向与法向接触力各向异性的主方向保持了良好的一致性。     

基于能量原理的木质素固化粉土剪切特性研究

 为研究木质素固化粉土剪切过程中的应力–应变特征,基于能量守恒原理,通过对剪切过程不同阶段土体所需能量进行分析,建立用以描述固化土剪切特性的理论模型,分析模型中所含参数的意义和确定方法,根据室内试验结果验证模型的有效性,并探讨模型的适用性。研究结果表明,具有胶结特性的固化土发生剪切所需能量主要由破坏土颗粒间胶结的能量、颗粒滚动的能量和颗粒滑动的能量3个部分组成;模型考虑胶结作用、颗粒形状和土体剪切破坏形式等因素;木质素固化粉土的直剪试验结果验证了模型对其剪切特性的准确描述。所提模型可为固化土强度特性的研究提供新的途径。     

土工合成材料大型直剪界面作用宏细观研究

利用大型直剪模型试验设备，在不同竖向压力下进行一系列的土工合成材料直剪试验，应用数码可视化跟踪技术，结合土体变形无标点量测技术来研究双向土工格栅与砂土直剪界面作用的宏细观特性，同时分析界面附近土压力分布规律，并研究界面颗粒运动变化规律和细观组构演化特征与宏观特性的关联。分析结果表明，直剪筋土界面附近竖向压力分布从前端依次向后端减少；直剪界面位移达25 mm时，形成了稳定的剪应变集中带；在筋土界面（6～8）D₅₀粒径厚度范围内，界面颗粒以旋转和平动方式同时位移，该范围外颗粒以平动方式沿剪切方向位移，且位移较小；在剪切过程中，界面颗粒发生旋转，土体发生剪胀，孔隙率增大，平均接触数减小，颗粒重新被压密，孔隙率减小，平均接触数增多，颗粒长轴排列趋于水平方向，各细观组构处于相对稳定状态。     

Ultrasonic imaging of microscale processes in quartz gouge during compression and shearing

Active ultrasonic monitoring in rock joints and gouge materials has the potential to detect the signatures of shear failure for a wide range of sliding modes, from slow and stable movements to fast and unstable sliding. While these collected measurements currently are being used to identify the seismic precursors to shear failure in rock joints and gouge materials, the underlying physical processes and contact scale mechanisms that control the changes in ultrasonic wave attributes are still poorly understood. To address this knowledge gap, this paper aims to investigate the relationship between the variations in ultrasonic wave attributes and the underlying particle scale mechanisms during both compression and shearing. Our double direct shear experiments were coupled with ultrasonic wave propagation measurements on granular quartz gouges, in which the gouge layers were sheared under different sliding velocities and constant normal stress conditions. Simultaneously, ultrasonic waveforms were continuously recorded during shearing with a fast data acquisition system and three pairs of ultrasonic wave transducers embedded at the two sides of the gouge layers. Different particle comminution mechanisms were observed from the non-uniform distribution of normal and shear stresses through the changes in ultrasonic transmissivity and scanning electron microscope (SEM) images. Our results show that the signatures of the geometry- and time-dependent variations of the inter-particle contact quality and pore volume changes with sliding velocity and slip accumulation were clearly captured from the variations in the transmitted wave amplitude and the dominant frequency, respectively. In addition, we found that variations in dominant frequency corresponded to dilation and compaction of the granular gouge layer during compression as well as stable and unstable sliding. Our results therefore confirmed that non-destructive acoustic techniques are capable of capturing a variety of micromechanical processes during fault gouge deformation and may prove useful in natural settings.     

Influence of the Anisotropy of Confining Stress on the Sand/Steel Interface Behaviour in a Cylinder Shear Apparatus

According to the type of works and to the consolidation of the ground, soil-structure interfaces are orientated in various ways with respect to the stress tensor in the soil. For instance, before shearing, the normal stress applied to the interface could be the major stress in the soil or the minor one or some intermediate value. This paper shows how the anisotropy of confining stresses can influence the behaviour of sand/steel interface. To this end, a series of tests was conducted with the cylinder shear apparatus. The cylinder shear apparatus is dedicated to the study of the shear behaviour of ground/structure interfaces. This device controls the radial confining stress and an average axial confining stress. One is particularly interested in estimating the influence of anisotropy of confining stresses and to look for empirical laws clarifying this influence on the peak shear, the post-peak shear, the displacement needed for full mobilization. The influences of the roughness of the interface, of the density of the sample and of the level of confining stresses are also studied.     

砂土单调剪切特性的非圆颗粒模拟

针对纯圆颗粒模拟的缺陷,利用颗粒流"团颗粒"方法开发了形状近似椭圆的"椭圆团颗粒",研究了颗粒形状变化对数值试样宏观剪切特性的影响及其细观机理;分析了加荷过程中平均接触数的变化规律以及颗粒形状对临界接触数的影响;对比了纯圆颗粒和椭圆团颗粒的不同颗粒旋转特性,并初步探讨其机理;分析了剪切过程中椭圆团颗粒试样的组构各向异性演化规律。结果表明,颗粒形状对试样的强度和变形特性具有重要影响,其在细观机理上与平均接触数有关;剪切荷载作用下椭圆团颗粒试样呈现明显的组构各向异性,颗粒长轴逐渐趋于水平定向,接触法向逐渐偏于加荷的大主应力方向。     

The inter-particle coefficient of friction at the contacts of Leighton Buzzard sand quartz minerals

A series of micro-mechanical tests was carried out in order to investigate the inter-particle coefficient of friction at the contacts of quartz minerals of Leighton Buzzard sand. For this purpose, a custom-built inter-particle loading apparatus was designed and constructed, the main features of which are described briefly in this paper. This apparatus is capable of performing shearing tests at the contacts of soil minerals of a particle–particle type in the range of very small displacements, from less than 1 μm to about 300 μm, and very small normal loads, between about less than 1 N and 15 N. The laboratory data showed that the effects of the normal force and the sliding velocity on the coefficient of dynamic friction are not significant, while dry and saturated surfaces had similar frictional characteristics. The steady state sliding was mobilized within a range of 0.5–3.0 μm of horizontal displacement, and the coefficient of static friction was very similar to the corresponding coefficient during constant shearing. Repeating the inter-particle shearing tests on the same particles and following the same shearing track indicated a small reduction in the inter-particle coefficient of friction after the first shearing, which is possibly related to plastic deformation and damage to the asperities.     

离散元中破碎自组织对颗粒破碎影响研究

从数学领域的阿波罗填充法入手,建立了4种破碎自组织,并借助线性膨胀法保证破碎前后质量守恒。在此基础上,开展了不同破碎自组织的数值试验,研究了破碎自组织对级配演化以及材料的宏细观力学特性等的影响。结果表明:颗粒级配曲线的分形维数和颗粒间的平均应力随破碎自组织中颗粒数目增多而下降,而相对破碎率B_r和材料的压缩性随自组织中颗粒数目增多而增大。加载过程中的法向接触和接触力玫瑰图表明,自组织中颗粒数目愈多,材料的各向异性程度愈低,颗粒法向接触数目愈多,而法向接触力愈小。另外,配位数及接触力的概率密度也与破碎自组织存在密切联系。     

土的应力方向依赖性(I):概念与现象

土的应力方向依赖性是指土的力学特性随应力方向改变而改变的性质,包含两层含义:一是材料的方向性,即土的各向异性,是应力方向依赖性产生的内在原因;二是应力的方向性,即通常所说的“应力主轴旋转效应”,是应力方向依赖性的外部客观条件。以材料各向异性主轴和应力主轴的位置关系分类,相关试验研究可划分成“应力不转、试样旋转”和“旋转应力、试样不转”两类。诸多证据表明,材料的方向性与应力的方向性之间存在某种密切的关联。随着各向异性主轴和应力主轴发生相对旋转,试样所表现出的力学性质存在显著的差异。旋转角度相同时的剪切试验研究表明(包括直剪试验、三轴试验、定轴剪切试验等),旋转角度、潜在滑移面位置和试样的强度与变形之间存在某种特定的关系。通过对比梳理两类试验的试验原理与重要结论,明晰了土应力方向依赖性的试验研究框架,为相关研究提供一些新的思路,以期抛砖引玉,引起广大岩土工作者对该问题的思考。     

不排水循环荷载条件下胶结砂土宏微观力学性质离散元模拟研究

天然或人工胶结的存在能够提高砂土的抗液化能力,从宏微观尺度对其动力学性质进行研究具有重大意义.将已有的三维完整胶结接触模型引入到三维离散元程序中,对胶结砂土不排水循环三轴剪切试验进行三维离散元模拟,研究颗粒间胶结、循环应力比对离散元试样宏微观力学性质的影响.研究结果表明,胶结的存在能够抑制轴应变和孔压的发展,提高砂土的...     

Small strain dynamic behavior of two types of carbonate sands

The study reports on the small-strain dynamic behavior of two types of carbonate sands from Western Australia and the Philippines. Basic characterization of the soils was performed in terms of specific gravity, grading information, angle of shear strength at critical state, particle shape characterization and composition analysis. Piezo-element inserts were utilized to carry out the dynamic tests. For the Western Australia (WA) carbonate sand, both bender and extender element tests were performed, thus the shear modulus, the Young’s modulus and the Poisson’s ratio were examined. Both vertical and lateral bender element tests were performed on a set of specimens from the Philippines (PH) carbonate sand to study the shear modulus and, from which, no fabric anisotropy of the reconstituted specimens was found. It was observed that the overconsolidated specimens had higher stiffness than those during the first loading stages for both carbonate sands. In the pressure range of the study, grain breakage was small and its effect on the behavior of the samples was almost negligible. Empirical equations in the literature proposed from quartz sands could not predict the stiffness of the carbonate soils satisfactorily. In this regard, a preliminary study was carried out adopting the assumed void ratio that only considers the inter-particle voids instead of the summation of inter- and intra-particle voids; based on this concept, the predicted and measured stiffness (including both small-strain shear modulus, G_max and small-strain Young’s modulus, E_max) were found to be satisfactorily close.