Static liquefaction behavior of saturated fiber-reinforced sand in undrained ring-shear tests

The problem of static liquefaction of sand is nowadays a classical soil mechanics subject. Using a ring-shear apparatus, we explore the possibility of fiber reinforcement as a new method to improve the liquefaction resistance of sand. In order to understand the effect of the fiber content and sand density on the static liquefaction behavior of fiber-reinforced sand, a series of undrained ring-shear tests were carried out on saturated samples with different fiber content and sand density, and the test results and mechanisms of fiber reinforcement were then analyzed. The results indicate that the undrained shear behavior of fiber-reinforced loose samples is not greatly influenced by the presence of fiber, but for medium dense and dense samples, the presence of fiber clearly affects their undrained behavior. Untreated specimens showed a continuous decrease in shear resistance after failure, while the specimens treated with fiber showed fluctuations even after shear failure, and these fluctuations become stronger with increasing fiber content. The peak shear strength increases with the fiber content, especially in dense specimens. After shearing, all the fiber-reinforced and untreated dense samples maintained structural stability, while the unreinforced loose samples showed a completely collapse of structure. The presence of fibers may thus limit or even prevent the occurrence of lateral spreading that is often observed in unreinforced sand.     

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。     

Effects of small and large shear histories on multiple liquefaction properties of sand with initial static shear

It has been reported that soils belonging to slope grounds show different types of liquefaction behavior than those belonging to horizontal grounds. Some research has also revealed that liquefaction histories can significantly affect the shear behavior of sandy soils. However, the combined effects of the slope angle and the magnitude of past shear histories on the liquefaction properties of soils have not been studied comprehensively. Based on this background, several multiple liquefaction tests with initial static shear were conducted on Toyoura sand. In each of these tests, a single specimen was sheared several times up to small or large double amplitude shear strain under a constant volume condition using a specially designed stacked-ring shear apparatus. The behavior of the Toyoura sand observed in these tests was discussed considering various perspectives, such as the increase in relative density, the induced anisotropy, the change in liquefaction resistance, and the shear strain accumulation. The findings of this study established that shear histories of smaller magnitude had relatively less influence on densification and induced anisotropy than those of larger magnitude. Moreover, shear histories of smaller magnitude also resulted in the relatively higher liquefaction resistance of sand specimens against the next cyclic shear, while the opposite trend was observed in the case of specimens subjected to shear histories of larger magnitude. Finally, shear strain accumulated less easily in tests with small shear histories than in those with large shear histories.     

固有和诱发各向异性对击实粘性土强度和变形特性的影响

应用内外室压力不等的空心园柱扭剪仪，对击实粘性土进行了固有和诱发各向异性的试验研究。结果表明，击实粘性土具有明显的固有各向异性，它对土的应力应变关系、强度和变形特性都有不可忽视的影响；不同主应力方向的破坏强度最大差别可达３０％。试样的初始含水量和应力水平会影响固有各向异性程度。诱发各向异性可由卸载和主应力方向旋转产生，它对排水剪强度影响不大，但对应力应变关系和变形特性有较大影响。     

Stress–dilatancy relationships of sand in the simulation of volumetric behavior during cyclic torsional shear loadings

In order to describe the volumetric behavior of soil subjected to shearing, a relationship that deals with the ratio of plastic strain increments to stress ratio (i.e. a stress–dilatancy relationship) is required in addition to the stress–shear strain relationship. In view of the above, stress–dilatancy relationships during cyclic torsional shear loadings were experimentally investigated in the current study. Based on the experimental results, a bilinear non-unique stress–dilatancy model was proposed for stress controlled drained cyclic torsional shear loading. The stress–dilatancy relationships during virgin loading and subsequent cyclic loading were modeled separately by considering the effects of stress history (over-consolidation or normal consolidation). Then the volume change of Toyoura sand specimens subjected to cyclic torsional shear loading was simulated by combining the simulation of stress–shear strain relationship with the proposed stress–dilatancy relationships. It was observed from the comparison of the experiment results with the simulation of volumetric strain that, after combining with accurate modeling of stress–shear strain relationship, the proposed stress–dilatancy relationship can reasonably simulate the volumetric behavior of sand subjected to various drained cyclic torsional shear loadings.     

固有和诱发各向异性对击实粘性土强度和变形特性的影响

应用内外室压力不等的空心园柱扭剪仪，对击实粘性土进行了固有和诱发各向异性的试验研究。结果表明，击实粘性土具有明显的固有各向异性，它对土的应力应变关系、强度和变形特性都有不可忽视的影响；不同主应力方向的破坏强度最大差别可达３０％。试样的初始含水量和应力水平会影响固有各向异性程度。诱发各向异性可由卸载和主应力方向旋转产生，它对排水剪强度影响不大，但对应力应变关系和变形特性有较大影响。     

Influence of Fabric Orientation to the Permeability and Dynamic Characteristics of Hydraulic-Filled Sand

In this research, several different kinds of specimen's preparation methods which include the hydraulic transportation method (HY), multiple sieving pluviation method (MSP)and wet-tamped method (WT)were used to compare their initial fabric. Moreover, the anisotropic permeability test was performed to those three kinds of specimens. Then, a series of cyclic simple shear tests was conducted on the specimens of HY and WT to investigate the dynamic deformation properties of the sand. The influence of fabric orientation for each kind of specimens to both permeability and dynamic deformation properties were discussed based on the testing results. It is observed that the difference in fabric orientation of the testing soil given by various sample preparation methods results in the difference of permeability and liquefaction resistance. On the other hand, the strength as well as deformation properties obtained by different shearing apparatus appear to have different results. Nevertheless, from the viewpoint of fabric orientation with respect to flowing direction and shearing force direction, an identical conclusion can be obtained. These phenomena demonstrate that the anisotropic characteristics of the soil have significant effect on the permeability, shearing strength and deformation behavior.     

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.     

主应力方向变化对松砂不排水动强度特性的影响

针对相对密度为Dr=30%的福建标准砂,利用新研制的土工静力-动力液压三轴-扭转多功能剪切仪进行了扭转和竖向循环耦合剪切试验和循环扭剪试验。通过对比三向非均等固结条件下的两类试验结果,着重研究了初始主应力方向变化和振动过程中主应力方向连续变化对松砂的动强度特性的影响。     

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。     

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。     

土的基本特性及本构关系与强度理论

土的基本特性及本构关系与强度理论是土力学及岩土工程学科的重要理论基础之一。本 文针对饱和黏土、砂土及堆石料等粗粒土,总结了这三类土在基本力学特性及本构强度理论方 面的研究现状和发展趋势。饱和黏土部分主要包括压缩特性、剪切特性—临界状态及剪胀/剪缩 、结构性及其破坏、中主应力影响、各向异性及主应力偏转效应、不排水抗剪强度、流变特性 、微观力学解析模型等;砂土部分包括临界状态概念与剪胀性、砂土各向异性、应变局部化等 ;堆石料等粗粒土部分则以颗粒破碎对堆石料等粗粒土的力学性质的影响为主线,重点介绍了 颗粒破碎的度量方法、颗粒破碎对剪胀性、临界状态线的影响以及相应的本构模型、宏观-微观 的力学分析方法。     

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.     

定轴剪切条件下砂土强度与非共轴性试验研究

针对现有定轴剪切试验大多忽略中主应力系数b值变化这一问题,基于不同力系之间的映射关系,严格推导了空心圆柱扭剪仪4个外荷载的加载参数。采用重塑福建标准砂试样,分别在2组围压下开展主应力方向固定的剪切试验,研究主应力方向角α不同时,砂土变形、强度、非共轴性等力学特性的差异。结果表明:砂土的强度、应力-应变关系等力学特性均具有显著的应力方向依赖性;随着应力方向从0°到90°变化,应力路径表现为2个变化阶段,峰值强度先减小后增大,在α为67.5°附近取极小值;对比大主应变增量方向与大主应力方向之间的关系,发现当偏应力比较小时,试样存在明显的非共轴现象,随着偏应力比的增大,应变增量方向逐渐趋近于应力方向。     

Cyclic properties of dense to very dense silica sand

This paper presents results of a series of cyclic laboratory test performed on dense to very dense silica sand. Triaxial and direct simple shear tests were performed on reconstituted sand. The test program aimed to determine the cyclic shear strength and soil deformation properties for foundation design of offshore wind turbine structures on the Dogger Bank site, offshore United Kingdom, but is also applicable for foundation design of other offshore structures and other locations. Two sand batches were used, one with clean silica sand and one with 20% silt content. Normally consolidated specimens from both batches were reconstituted to a target relative density approximately equal to 100% and 80%. On the clean sand with a target relative density of 80%, tests were also performed on specimens with an overconsolidation ratio of OCR = 4. Various contour diagrams, presenting cyclic shear strength and average and cyclic shear strains for different number of cycles, are presented. These diagrams can be used in the front-end engineering design (FEED) stage of commercial design projects on similar soil conditions, to define soil behavior when limited cyclic data is available, or in the planning stage of a cyclic laboratory test program. Effects of precycling and fines content on the cyclic behavior of triaxial and DSS specimens are discussed.     

考虑主应力轴方向的砂土各向异性强度准则与滑动面研究

主应力加载方向对土体强度产生影响的根本原因是土体存在各向异性。对于横观各向同性砂土而言,沿不同平面的抗剪强度随该平面与沉积面夹角增大而增大。认为砂土固有各向异性强度与该平面的各向异性参数密切相关,给出了各向异性砂的峰值强度表达式。在SMP准则中,各个潜在滑动面上的剪正应力比相同,各向异性砂土的抗剪强度和滑动面位置由强度最低的潜在滑动面所决定。综合考虑主应力轴、滑动面以及沉积面之间的位置关系,得到了砂土的各向异性强度准则。采用福建标准砂进行了一系列定轴剪切试验,系统地观测了定轴剪切试验中试样滑动面的特征。已有试验数据和理论结果的对比表明,各向异性强度准则可以较好地预测各向异性砂土的强度与滑动面位置。     

Study of Strain Localization and Microstructural Changes in Partially Saturated Sand During Triaxial Tests Using Microfocus X-Ray CT

It is well known that strain localization and microstructural changes are important issues in the onset of failure problems. In particular, unsaturated soil exhibits more brittle failure due to the collapse of the water meniscus, caused by shearing or the infiltration of water, than saturated soil. The aim of this paper is to observe the strain localization behavior and the microstructural changes in partially saturated soil during the deformation process using microfocus X-ray CT. The microfocus X-ray CT system employed in this study has a very high spatial resolution of 5 μm, which is enough to visualize the sand particles and the other particles individually. In addition, X-ray CT scans can be performed under triaxial conditions. The strain localization of fully saturated, partially saturated, and air-dried sand specimens during triaxial compression tests is observed and discussed. The microstructure of unsaturated soil, consisting of soil particles, pore water, and pore air, is successfully observed in partial CT scans. Through a comparison of the microstructures in the shear bands and in the initial state, the microstructural changes are discussed.     

各向异性砂土剪切带角度的理论分析

针对平面应变条件下各向异性砂土剪切带角度的试验规律,采用传统的3种理论和分叉理论进行对比分析。将平面应变条件下剪切带角度的试验结果按照传统3种理论整理发现,尽管传统3种理论可以估算同种砂剪切带角度的极小、中间和极大值,但无法解释其各向异性规律。砂土在平面应变条件下破坏时会产生明显的剪切带,当剪切带方向和砂土沉积面方向接近时,会较早诱发剪切带的产生,使材料强度降低,造成了平面应变条件下各向异性强度规律明显不同于常规三轴条件下的试验规律,采用分叉理论结合各向异性模型则可以有效解释这个规律。随砂土沉积面角度的变化,模型可以从细观角度解释常规三轴条件下剪切带角度的单调变化的试验规律,结合分叉理论可以描述平面应变条件下其先减小然后增大的规律。通过几种理论对比分析表明,模型结合分叉理论不但能够描述多种应力状态下的平面应变和常规三轴应力条件下剪切带角度表现的不同规律,而且能够从细观角度解释其各向异性成因。     

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。     

Yielding Characteristic and Non-Coaxiality of Toyoura Sand on p′-Constant Shear Stress Plane

In order to investigate the deformation characteristics under a wide selection of stress history, ten series of stress probing tests on p′-constant shear stress plane on dense Toyoura sand are conducted by using hollow cylinder apparatus. This paper presents yielding behavior and non-coaxiality of Toyoura sand obtained from the tests. The stress probing tests consist of ten series of shearing tests each of which starts from systematically chosen individual initial stress point. Each initial stress point is subjected to load-unload stress history. The yielding characteristic is interpreted by means of a concept of multiple yield surfaces model which has three yield surfaces representing linearly elastic behavior limit, commencement point of rapid development in plastic strain and elasto-plastic range. As a result, an experimental evidence of the isotropic hardening, i.e., isotropic expansion of yield surface, induced by anisotropic loadings is obtained. This is what was tacitly assumed in the application of plastic flow rules. However, misalignment of directions between principal stress and principal plastic strain increment is observed. Moreover, plastic strain increment direction is found to be influenced by the given stress increment direction. These facts suggest the non-coaxiality between stress and plastic strain increment due to shear stress increment, even during monotonic loading with isotropic hardening. The plastic strain increment direction diverges from the normal direction to a surface which is a circle with its center nearly at the origin on p′-constant plane and which passes through the current stress point.