Micromechanical analyses of the effect of rubber size and content on sand-rubber mixtures at the critical state

This study presents a series of monotonic drained triaxial tests consisting of mixtures of sand (stiff) and rubber (soft) particles simulated by the discrete element method (DEM). Sets of mixtures were prepared with different rubber size ratio of 1, 2.5 and 5 and contents ranging from 0% to 30% by weight. The numerical samples were sheared up to large axial strains to reach a critical state. The slope of the critical state lines is strongly affected when the rubber size is the same as sand. When rubber size increases, the critical state lines shift downward with little effect on the slope. While it is generally accepted that, for the given range of rubber contents used in the study, the sand-rubber mixture strength increases when adding rubber particles, the results from this study suggest that said strength diminishes as rubber size is incremented. Micro-scale information, including coordination number, geometrical and mechanical anisotropy, was obtained for all the tests. Regardless of the rubber particle size, rubber-sand contacts represent an important contribution to the overall strength of the material; however, the rubber particle size dictates how said contribution takes place. These findings highlight the importance of understanding new geomaterials to practicing engineers that different size ratios and rubber contents have positive or negative effect of strength and deformability and the choice of a sand-rubber mixture has to be based on the project nature.     

剪胀性砂土地震后流滑的机理和模拟

不均匀地层中超静孔压的扩散、重分布过程会导致孔隙水集中渗流累积至局部区域,导致该区域土体强制吸水,从而引发剪胀性砂土边坡地震后侧向流动变形甚至失稳。通过分析边坡流动变形过程中砂土的应力路径特征及强制吸水条件下的体变平衡条件,指出预测剪胀性砂土边坡流动变形的关键是描述砂土保持常剪应力和当前峰值应力比时的剪胀特性和震后再固结体变特性。通过常偏应力下的三轴剪切吸水试验,观察了砂土在流动变形过程中的吸水量与剪应变发展的关系,给出了基于强制吸水体变预测流动剪应变的状态剪胀模型。基于剪切后再固结试验,得出了再固结体变的变化规律和数学描述。基于所提出的机制和数学描述,给出了基于有限差分法的边坡流动变形发展过程直至失稳破坏的模拟方法。     

常压至高压下砂土强度、变形特性试验研究

 砂土材料常压至高压下的强度、变形特性是构建砂土模型的首要问题。开展3种粒组砂土8 MPa围压范围内的等向压缩试验以及0.2～6.4 MPa围压范围内的三轴剪切试验,将砂土常压至高压范围内的力学特性进行系统分析,以获得能够将常压至高压范围内的强度、变形特性进行统一描述的力学参数。通过研究发现：(1) 砂土在高压下出现一定量的颗粒破碎,改变了砂土的剪切耗能机制,使得砂土三轴压缩剪切由剪胀软化特征向剪缩硬化特征转变;(2) 砂土材料的三轴压缩剪切峰值应力比受砂土粒径、围压共同影响,M-C强度准则在高压条件下不再适用;而残余应力比则基本不受粒径、围压的影响,是典型的无黏性摩擦型岩土力学参数,应作为砂土基本力学特性指标;(3) 砂土材料在常压至高压范围内的剪切过程中存在较明显的临界状态现象,临界状态曲线与等向压缩曲线形态相同均呈指数衰减型并在高压条件下产生交叉,两者共同构成砂土材料的状态区间能够体现常压至高压范围内的剪胀与剪缩特征。     

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。     

Applicability of discrete element method with spherical and clumped particles for constitutive study of granular materials

Discrete element method(DEM)has been intensively used to study the constitutive behaviour of granular materials.However,to what extent a real granular material can be reproduced by virtual DEM simulations remains unclear.This study attempts to answer this question by comparing DEM simulations with typical features of experimental granular materials.Three groups of models with spherical and clumped particles are investigated from four perspectives:(i)deviatoric stress and volumetric behaviour;(ii)critical state behaviour;(iii)stress-dilatancy relationship;and(iv)the evolution of principal stress ratio against axial strain.The results demonstrate that DEM with spherical or clumped particles is capable of qualitatively describing macroscopic deviatoric stress responses,volumetric behaviour,and critical state behaviour observed in experiments for granular materials.On the other hand,some qualitative deviations between experiments and the investigated DEM simulations are also observed,in terms of the stress-dilatancy behaviour and principal stress ratio against axial strain,which are proven to be critical for constitutive modelling.The results demonstrate that DEM with spherical or clumped particles may not necessarily fully capture experimental features of granular materials even from a qualitative perspective.It is thus encouraged to thoroughly validate DEM with experiments when developing constitutive models based on DEM observations.     

聚丙烯纤维加筋砂土的剪胀特性

随机分布的聚丙烯纤维在土体中彼此交错形成网状,可显著提高砂土的抗剪强度,减少其峰值强度折减。描述土体剪胀规律的剪胀方程是建立纤维加筋土的本构模型的核心。以聚丙烯纤维加筋南京细砂为主要研究对象,通过不同围压下的常规三轴压缩试验,研究了不同纤维质量百分比含量对南京细砂强度、变形以及剪胀特性的影响。对比聚丙烯纤维加筋砂土与纯砂土的应力应变关系和强度特性,分解出聚丙烯纤维对砂土的内在约束应力。将外力克服剪胀阻力需要做的额外功分解为由土颗粒运动和纤维变形两部分所消耗的能量,利用纤维对土的内在约束应力表示与加筋纤维有关的耗散能,基于最小比能原理建立了聚丙烯纤维加筋砂土的剪胀方程,揭示了聚丙烯纤维加筋砂土的特殊剪胀特性。经过对比发现常规三轴压缩试验结果与模型结果具有良好的一致性。     

Microstructural investigation on mechanical behavior of soil-geosynthetic interface in direct shear test

Interface shear strength between soil and geosynthetics mainly depends on the mechanical and physical properties of soil, geosynthetics and the normal stress acting at the interface. This paper presents results of an extensive experimental investigation carried out on sand-geosynthetic interface using modified large direct shear box. The study focusses on the shearing mechanism at the sand-geosynthetic interface and the effect of different parameters on the shearing mechanism. Smooth HDPE geomembrane, nonwoven needle punched geotextile and two types of sand having different mean particle size, have been used in the present study. Microstructural investigation of deformed specimen through Field Emission Scanning Electron Microscope (FESEM) reveals the shearing mechanism which includes interlocking and fiber stretching for sand-geotextile while sliding, indentation and plowing for sand-geomembrane interface. The shearing mechanism for sand-geomembrane interface highly depends on the normal stress and degree of saturation of sand. The critical normal stress that demarcates the sliding and plowing mechanism for sand-geomembrane interface is different for dry and wet sand. The amount of scouring (or plowing) of the geomembrane surface reduces with increase in the mean particle size of sand. FESEM images revealed that the sand particles get adhered to the geotextile fibers for tests involving wet sands. The present microstructural study aided in understanding the shearing mechanism at sand-geosynthetic interface to a large extent.     

砂土三维多重机构边界面模型

以土的临界状态和边界面塑性理论为基础,引入状态参数,考虑砂土的剪胀特性,提出一个新型三维多重机构边界面模型。模型将复杂的宏观变形行为分解为一个宏观体应变机构和一系列空间分布的虚拟一维微观剪切机构。每个微观剪切机构包含一个微观剪应力–应变关系和一个微观应力–剪胀关系。利用三轴压缩试验中的应力条件,建立典型宏微观参数之间的关系。模型包含13个参数,多数可通过具有明确物理意义的土性参数来确定。通过对砂土三轴压缩试验和空心圆柱扭剪试验结果的数值模拟,表明模型不但能够合理反映在排水或不排水条件下砂土的硬化及软化特性,而且能在不增加任何参数条件下预测应力主轴旋转产生的变形累积特性和应变增量主轴与应力主轴之间的非共轴特性。     

Investigation of the ultimate particle size distribution of a carbonate sand

A series of ring shear tests were conducted to investigate the ultimate particle size distribution of a carbonate sand. The tests were carried out under different stress levels, on three types of specimens: 1) uniformly graded specimens made of dry natural sand 2) remoulded specimens of the crushed sand after first shearing to large strains 3) specimens made of natural sand grains but with the same grading as in (2). The first series of tests on type (1), carried out to very large strains, led to apparently stable gradings, distinct for each stress level. Only limited additional particle breakage could be induced by remoulding the specimens after shearing (type (2)) and subjecting them to more shearing. Tests on specimens created at the apparently stable gradings (type (3)) but from the intact sand particles however led to significantly greater breakage. For the three types a stable, fractal grading was achieved. Analyses of the soil particles’ shape showed that the aspect ratio, sphericity and circularity reach a steady value at large strains, in parallel to reaching a stable grading. The mobilized angle of shearing resistance however was not significantly different in the different types of samples, suggesting the final grading dominates the behaviour.     

Effects of particle shape and size distribution on the shear strength behavior of composite soils

The effects of particle shape and size distribution on the constitutive behavior of composite soils with a wide range of particle size were investigated. Two comparable sets of specimens were prepared: (1) mixtures of fines (clay and silt) and an ideal coarse fraction (glass sand and beads), and (2) mixtures of fines and natural coarse fraction (river sand and crushed granite gravels). Direct shear box testing was undertaken on 34 samples and the structure of the shear surfaces, change in volume and water content and the particle shape coefficient of the sheared specimens were examined. The results indicate that the contraction/dilation a specimen exhibits is restrained within the shear zone while the outer zones remain unchanged during shearing. An increased coarse fraction leads to an increase in constant volume shear strength. In addition, increasing elongation or decreasing convexity of the coarse fraction increases the constant volume friction angle. The overall roughness of the shear surface at constant volume state is negatively related to particle smoothness (convexity) and positively related to the area of the shear surface occupied by particles with particular shapes. Two equations are proposed for the estimation of constant volume friction angle based on the proportion and shape coefficient of the coarse fraction. It is hoped this will assist in considering the shear strength of mixed soils when the size of the coarse fraction makes laboratory testing difficult.     

On the modelling of stress-dilatancy behavior in weakly cemented sands

A comprehensive study on the stress-dilatancy behavior of cemented sand and its modeling is presented. The effect of confining pressure, relative density, and cement content on stress-dilatancy behavior are studied from the published experimental results and an additional series of experiments performed in this study. To facilitate a contrast and comparison of stress-dilatancy behavior between these datasets, a normalized stress ratio is proposed which removes the effect of mineralogy and morphology of parent sand. A set of key insights were obtained from this comparative study which aided in improving the stress-dilatancy relation; for example, the effect of initial conditions on stress-dilatancy behavior was found to be captured by the ratio of cohesion intercept (or tensile strength) and mean effective stress before shearing. The limitations of stress transformation, often used in modelling of cemented sand, were also systematically studied by a set of carefully designed experiments; it was found to be only applicable before gross yielding of cementation. After gross yielding, it is necessary to take in account of the breakage of bonds/cementation. The gross yield locus was identified from 70 experimental datasets and a cohesion/bond degradation model was formulated to model the stress-dilatancy behavior of cemented sand. The efficacy of stress-dilatancy relations (after including the gross yield locus and bond degradation behavior) is evaluated from the experimental results; the Rowe's stress-dilatancy relation was found to be most suitable with the proposed bond/cohesion degradation model.     

Laboratory characterization and modelling of the thermal-mechanical properties of binary soil mixtures

Binary soil mixtures are extensively used in the construction of geothermal-related earth structures such as geothermal energy piles (GEP), ground source heat pumps (GSHP) and earth air tunnel heat exchangers (EATHE). An evaluation of the binary soil’s thermal-mechanical properties is the key process in determining the final performance of geothermal-related projects. Therefore, the thermal-mechanical properties of binary soil mixtures were systematically investigated in this paper. A series of thermal and mechanical property tests was conducted on five sand-kaolin clay mixtures with sand contents ranging from 0% to 100% by dry weight. The experimental results indicated that the sand-clay mixtures achieved the theoretically densest state when the sand content reached the critical threshold. The further the binary mixture’s sand content was from the critical threshold, the lower the mixture’s density was. As the sand content increased, the shear stress-strain curves gradually shifted from strain-softening behavior to strain-hardening behavior due to the decrease in suction stress. The relationship between the sand content and the shear strength of the mixtures exhibited an “S” shape, which is attributed to the interaction between the sand and clay particles and varied with the sand contents. The shear wave velocity of the sand clay mixtures was found to decrease continuously with the increase in sand content until the sand skeleton had formed. In addition, the thermal conductivity of the binary mixed soil changed linearly with the sand content, and the upper bound of the critical threshold interval (77%) was found to separate the two different heat conduction modes. Finally, an elastic shear modulus (G₀) model, which correlated to the tangent elastic modulus of the binary mixture (E_m), and a more generalized thermal conductivity (K) model were formulated for the binary sand-clay mixtures, and the effectiveness and feasibility of the proposed models were validated by comparing the values predicted with the model and the experimental data.     

砂土的压剪效应与渐近状态准则

以往的研究结果表明:砂土在沿着等应变增量比路径加载时存在一个稳定的应力状态,即渐近状态;渐近状态随应变增量比的变化而变化;渐近状态是比临界状态更为广泛的一个概念,临界状态是渐近状态在体应变增量为零时的特殊情况。在一系列应变路径试验的基础上揭示了渐近状态随应变增量比的变化而变化的物理机制是加载过程中的压缩变形与剪切变形的所占比例不断调整的结果,将压缩变形与剪切变形耦合变化的力学行为及其对土的应力应变响应的影响定义为压剪耦合效应(简称压剪效应),在此基础上,提出定量描述渐近状态应力比随应变增量比变化的二维和三维渐近状态准则,并且通过试验结果验证了准则的有效性。     

高频循环剪切下结构与松砂接触面的土体特性

应用颗粒离散元分析理论,深入地研究了高频循环剪切荷载下松砂与结构的接触面土体响应特性,对各种颗粒因素和剪切条件对接触面土体孔隙率和体应变的影响进行详细分析.数值仿真分析结果表明,松砂颗粒摩擦系数和颗粒刚度越大,接触面附近土体剪缩性越不明显;松砂的颗粒刚度比对其力学性质影响不大;边界围压越大,松砂的剪缩性越明显;高频循环...     

Particle Crushing and Deformation Behaviour

Particle breakage occurs in granular materials with various engineering applications, such as when driving piles (especially where the strength of the particles is low) and in debris flows (where the energy levels are high), and the influence of this breakage on the mechanical behaviour of soils should be given proper consideration in a constitutive model for soils. Particle breakage results in an increase in the number of fine particles and broadens the grading of particle sizes, and the primary effect of broadening the grading is to lower the critical state line and other characteristics of the volumetric response in the compression plane. In our study, an existing constitutive model, the Severn-Trent sand model, in which the critical state line plays a central role as the locus of asymptotic states, has been extended to include the effects of particle breakage. Severn-Trent sand is a frictional hardening Mohr-Coulomb model described within a kinematic hardening, bounding surface framework. The central assumption is that strength is seen as a variable quantity, dependent on the current value of the state parameter (volumetric distance from the critical state line) which varies with changes in density and stress levels. If the critical state line falls as a result of broadening grading, the state parameter tends to increase and the soil feels looser.     

石英砂砾破碎过程中粒径分布的分形行为研究

荷载作用下粒状土的颗粒破碎改变土的粒径分布,从而影响其力学特性。试验证据显示随着颗粒破碎的增加,任何初始分布的土粒都将趋向一种自相似的分形分布。为了揭示土的粒径分布的分形转变机制,利用侧限压缩试验研究高压应力下石英砂砾的粒径分布演化规律和颗粒破碎特性,基于分形模型和粒径分布实测数据研究破碎过程中粒径分布的分形行为。研究发现:颗粒破碎增长导致粒状土趋向分形分布的过程与颗粒破碎量密切相关,并可以通过增大的分形维数来描述。尽管石英砂砾的初始分布和粒径有所不同,分形维数大于2.2的粒径分布实测数据均展示了较为严格的自相似性,因而该数值可作为分形分布的分形维数下限值。研究还发现:相同破碎状态下Hardin相对破碎率小于Einav相对破碎率,但二者对应力和体应变的响应规律是一致的。颗粒破碎发展至粒径分布成为分形分布时,体应变与相对破碎率的比值将保持恒定,并且受初始分布的均匀性和颗粒大小的影响很小。这一特点可用于分形分布的识别,并意味着试验中如果粒径分布是分形的,则无须为了粒径分析而终止试验,只需测量到体应变就可估计相对破碎率。     

On the Relationship between Particle Breakage and the Critical State of Sands

Ring shear and shear box tests were used to investigate the relationship between volume change and particle breakage during the shearing of two sands. One sand was a carbonate sand which was sheared under a high confining stress to examine whether, in the region of compressive shearing behaviour due to particle breakage, the breakage would ever cease and the soil reach a stable grading. The other sand tested was a quartz sand that was sheared at low confining stresses, to investigate whether a dilatant sand would also be subject to particle breakage. In both cases breakage was found to continue to very large strains, with no evidence of a stable grading being reached within the range of strains used. While the breakage was very small for the quartz sand it was large for the carbonate, emphasising that any definition of a critical state by means of conventional triaxial or shear box testing would be approximate only, because of the limited strains that they allow.     

Mechanical property and deformation behavior of geogrid reinforced calcareous sand

The strength and deformation properties of maritime geotechnical structures made primarily of calcareous sand are critical for project safety, and geogrid reinforcement is a promising new approach. A series of consolidated drained triaxial experiments were conducted to evaluate the mechanical property and deformation behaviors of geogrid reinforced calcareous sand (GRCS), taking into consideration the impacts of the geogrid layer, relative density, particle size, and confining pressure. In comparison to the unreinforced calcareous sand, the strength of the GRCS is greatly enhanced, and the deviatoric stress-strain curves are altered from slightly softening to hardening, as well as the suppressed shearing dilatancy. The geogrid, relative compactness, particle size, and confining pressure are all intimately related to the volume changes and shearing dilatancy of reinforced specimens, but particle crushing is mostly impacted by the confining pressure. The interactions of geogrid ribs and calcareous sand particles are summarized as two types of constraint and friction using scanning electron microscope tests to establish a simplified calculation method of horizontal and vertical equivalent additional stresses that could provide a reference for revealing the mechanical mechanism of GRCS.     

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.     

The Effect of Fines on Critical State and Liquefaction Resistance Characteristics of Non-Plastic Silty Sands

Monotonic and cyclic triaxial tests were carried out on sand-silt mixtures for the investigation of the effect of fines content on their critical state and liquefaction resistance characteristics. Both the undrained and the drained monotonic tests produce a unique critical state line for each tested mixture, which moves downwards with increasing fines content up to a threshold value of 35% and then upwards. At a given void ratio and mean effective stress, the liquefaction resistance ratio decreases with increasing fines content up to the same threshold value of 35%, and increases thereafter with further increasing fines content. However, at a given intergranular void ratio, defined as the ratio of the volume of fines plus voids to that of sand particles, liquefaction resistance ratio increases with increasing fines content up to the threshold value. The threshold fines content value, which is an important parameter in determining the transition from the sand dominated to the silt dominated behaviour of sand-silt mixtures, is related to their particle packing. An expression is proposed for the estimation of the threshold fines content as a function of the mean diameter ratio, d₅₀/D₅₀, and the void ratio. The results, presented herein, also show that for each tested mixture the liquefaction resistance ratio is related to the state parameter and that this relation is influenced by the effective stress level and fines content. The results on the sand-silt mixtures are supported by similar results on natural silty sands.