Dynamic properties of calcareous and siliceous sands under isotropic and anisotropic stress conditions

The strain-dependent dynamic properties of sand are generally described by their relative density and mean effective stress, while the contribution of other factors, like soil origin, mineralogy, grain morphology, and initial stress anisotropy, have not been fully recognized. This paper presents the results of an experimental study on the shear modulus and damping ratio of calcareous and siliceous sands of different origins and their identical grain size distribution and stress-density states. Resonant column and cyclic triaxial tests were conducted on reconstituted samples of these two sands obtained from coastal areas. The significance of the initial effective confining pressure and stress anisotropy on the dynamic properties of the sands is evaluated and compared. It is demonstrated that the small-strain shear modulus of the calcareous sand is more affected by an increase in mean effective confining pressure than the siliceous sand. However, the effect of the initial shear on the secant shear modulus of the sands is unique. Based on the test data, a rigorous correction factor is proposed to account for the influence of the initial stress anisotropy on the small-strain shear modulus of the sands. A comparison between the strain-dependent dynamic properties of the calcareous and siliceous sands reveals that the calcareous sand has a higher secant shear modulus, lower damping ratio, and higher linear and volumetric threshold strain. Since the stress-density states and grain size distribution of the two sands were identical in the experiments, the discrepancy in the dynamic properties can be attributed to other factors, including sand origin, grain angularity, mineralogy, and formation processes, which are not commonly taken into account in the current practice.     

复杂应力路径下饱和砂土静态液化失稳预测

饱和砂土应力–应变关系具有各向异性和状态相关特性,因此其静态液化的触发与应力路径密切相关。现有文献已提出多种静态液化判别准则,但其准确性往往仅在三轴应力路径下被进行校验。对于涉及主应力方向旋转和不同中主应力比的复杂应力路径,现有判别准则是否可以准确预测静态液化的触发有待进一步验证。为此,基于状态相关各向异性砂土本构模型,结合前人的空心圆柱扭剪单元体试验,比较了二阶功、弹塑性刚度矩阵对称部分和失稳模量3种准则对复杂应力路径下砂土静态液化失稳预测的效果。发现包括:基于弹塑性刚度矩阵对称部分的失稳触发表达式不依赖于加载路径,具有更好的通用性,其预测的失稳点早于或与实际失稳点吻合;失稳模量理论可预测实际液化失稳的位置,但判定表达式因加载条件不同而变化。获得了复杂应力路径下的失稳线,分析了静态液化触发前砂土可发挥峰值摩擦角受中主应力、主应力方向等因素的影响。     

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.     

One-dimensional dynamic compressive behavior of dry calcareous sand at high strain rates

Calcareous sand has distinct characteristics in comparison with silica sand, such as dynamic behavior at high strain rates (HSRs). This is closely related to pile driving, aircraft wheel loading and mining activities. To understand the response of calcareous sand at HSRs, a series of dynamic tests is performed using the split Hopkinson pressure bar (SHPB) with steel sleeve, including 6 validation tests of bar-against-bar and 16 comparative tests relevant to the relative density and strain rate of calcareous and silica sands. The apparent dynamic stiffness of calcareous sand is approximately 10% of that for silica sand due to different particle shapes and mineral compositions. The axial stress-strain response of silica sand is mainly governed by the deformation of individual grain and soil skeleton, and particle crushing. However, porous calcareous sand shows yielding and strain-hardening responses that are always followed by particle crushing. As the applied loading increases, the particle crushing of calcareous sand develops from local instability to whole breakage. Calcareous sand has lower viscous flow effects compared with silica sand at HSRs.     

A note on influence of stress anisotropy on the Poisson's ratio of dry sand

In this study, extender and bender element tests were conducted investigating the small-strain Poisson's ratio of variable sands, with a focus on the effect of stress anisotropy in order to quantify the sensitivity of Poisson's ratio to the applied deviatoric stress. Four different uniform sands were tested, including a biogenic sand, a crushed rock and two natural sands, covering a wide range of particle shapes. From these sands, eleven samples were prepared in the laboratory and were tested under variable stress paths,maintaining a constant mean effective pressure while increasing the deviatoric compressive load. Under the application of these given stress paths, the data analysis indicated that the sensitivity of Poisson's ratio to the stress ratio was more pronounced for sands with irregularly shaped particles in comparison to sands with fairly rounded and spherical grains. For sands with very irregularly shaped particles, the increase of Poisson's ratio from the isotropic to the anisotropic stress state reached 50%, while this increase for natural sands with fairly rounded particles was in the order of 20%.     

土体的各向异性及近似模拟

通过真三轴仪试验 ,分别从 3个主应力方向加荷 ,研究产生的应变分量变化 ,从而较清楚地揭示了土体中显著的由应力引起的各向异性。在此基础上提出了土体应力应变柔度矩阵所具有的性质 ,即不同应力方向上加荷所引起的应变分量之间关系的规律 ,这些规律是检验土体本构模型合理性和适用性的重要标尺。依据这些性质 ,提出了一种以邓肯模型为基础修正的各向异性本构模型 ,对于不同的应力方向 ,采用不同的弹性模量和泊松比 ,模型参数与邓肯模型一致 ,由常规三轴试验确定 ,它是一个经验的、但实用性强的各向异性本构模型。     

主应力方向循环变化对饱和松砂不排水动力特性的影响

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主应力轴循环变化的多种模式竖向和扭转双向耦合循环剪切及普通循环扭剪试验。针对福建标准砂, 在均等固结条件下着重研究了振动过程中主应力方向的不同变化模式对饱和松砂不排水循环特性的影响。试验研究结果表明: 振动过程中主应力方向的变化方式对饱和砂土不排水动强度具有显著的影响, 在所采用的五种类型循环剪切应力路径中, 主应力方向连续旋转条件下的动强度最低。进一步的研究发现: 对于初始均等固结条件,分别采用初始平均有效固结压力和循环破坏次数归一化的循环孔隙水压力比与循环次数比之间的关系与振动过程中主应力方向变化方式无关, 这种归一化循环孔隙水压力比随广义剪应变的变化规律及累积广义剪应变与循环次数比之间关系均与振动过程中主应力方向变化方式无关。     

钙质砂中群桩模型试验研究

 针对取自南沙群岛永暑礁的钙质砂,利用室内模型试验研究钙质砂中群桩的承载和变形性能及其影响因素,并进行石英砂中的对比试验。试验结果表明,钙质砂由于颗粒破碎作用,导致其群桩承载和变形性能与石英砂有着显著差异。相对密度对钙质砂中群桩承载力影响较大,闭口群桩的承载力比开口群桩高17%～20%,但与石英砂相比很小,仅为石英砂的56%～71%。钙质砂中桩身轴力衰减速率缓慢,桩侧阻力值仅为石英砂的21%～30%,但具有深度效应,而且钙质砂中桩侧阻力对相对密度的变化没有石英砂敏感,受相对密度影响较小。钙质砂的承载力群桩效应系数在Dr = 46%,75%时均小于1,且随着相对密度的增加而增加,与石英砂有着本质的区别。该研究成果可为钙质砂桩基工程设计提供有益的参考。     

砂土在小应变下考虑应力路径影响的本构模型的试验研究

以标准砂为试样,利用GDS数字控制应力路径三轴仪进行了K0初始应力条件下8条特殊应力路径试验。在小应变区域内,通过刻画在应力空间的等应变应力线的变化规律,推导了可以考虑任意应力路径和应力路径方向变化的杨氏模量计算方法,并对泊松比进行了研究,建立了砂土在小应变区域内可以考虑复杂应力条件的本构模型。模型可以反映小应变时砂土的高模量、非线性以及模量急剧变化等主要特征;也可以模拟一般模型所不能考虑的应力路径方向变化引起的模量变化。     

Effect of Directional Stress History on Anisotropy of Initial Stiffness of Cohesive Soils Measured by Bender Element Tests

This paper describes the initial stiffness of reconstituted kaolinite clay in both vertical and horizontal planes under three different stress histories. The initial shear stiffness was obtained from bi-directional bender element tests during isotropic and K₀ stress loading and unloading. An empirical correlation was established based on the initial stiffness of normally consolidated soils. Unlike the unique relationship of the initial vertical stiffness of normally consolidated clays, the initial stiffness in the horizontal plane is dependent on the stress ratio and previous stress history; thus, three different relationships of the initial horizontal stiffness were obtained for the three loading programs. The effect of the stress history on the initial horizontal stiffness can be considered properly by defining the degree of overconsolidation in terms of the horizontal effective stress. The change in the initial stiffness has a directional dependency on the stress history in the direction of the particle motion and wave propagation in the bender element tests.     

Liquefaction behavior of fiber-reinforced calcareous sands in unidirectional and multidirectional simple shear tests

A study on the liquefaction resistance of calcareous sands reinforced with polypropylene fibers was reported. Stress-controlled cyclic simple shear tests were conducted on specimens prepared at a relative density of 50%, with and without fiber reinforcements. The liquefaction behavior was investigated by considering the effects of fiber contents ranging from 0% to 1%, fiber lengths varying from 3 mm to 12 mm and loading patterns. The results indicated that increasing fiber content and fiber length resulted in a decrease in the deformation, a reduction in pore pressure accumulation rate, and improved the liquefaction resistance of calcareous sands. Additionally, the risk of soil liquefaction could be significantly reduced when the fiber content was greater than 0.8%. The multidirectional loading had a considerable effect in reducing the liquefaction resistance compared to unidirectional loading. Further, the stiffness degradation of calcareous sands decreased with increasing fiber content and fiber length. The pore pressure generated in the cyclic tests was analyzed and was found to be affected by fiber content. A pore pressure prediction model was proposed to obtain the pore pressure characteristics of fiber-reinforced calcareous sands under various fiber content conditions.     

基于应力响应包络的土体典型本构模型比较

应力响应包络是本构模型切线刚度的一种几何表达,也是研究本构模型定性特征的有效工具。针对亚弹性、弹塑性及亚塑性等 3 种类型的土体本构模型,分别以邓肯模型、剑桥模型和 Gudehus-Bauer 模型为例,讨论了相应的应力响应包络形态,并比较了 3 类模型各自的特点。并且,通过分析不同模型的应力响应包络及其在主应力空间中的变化规律,说明了亚弹性模型用于模拟土体的加、卸载存在着本质缺陷;弹塑性模型在屈服面附近的中性变载和卸载响应存在一定的问题;亚塑性模型描述的土体切线模量随应变增量方向连续变化的特征比较合理,但简单亚塑性模型难以准确模拟土体不排水剪切应力路径。     

大型三维土与结构接触面试验机的研制与应用

已有的土与结构接触面试验研究集中在二维加载条件下的接触面力学特性,而实际工程中接触面的受载条件大多为三维的,即接触面的剪应力方向在剪切过程中是不断变化的。针对粗粒土与较粗糙结构面的大型三维接触面的试验手段目前较为缺乏,自主设计研制了一台用于土与结构接触面特性试测的大型三维加载试验机。该试验机具有高加载能力,提供大试样截面,能够进行较大粒径土体与较粗糙接触面在较大应力变化范围内的单调和循环加载试验;采用液压伺服控制系统,法向可提供应力、常位移、常刚度等边界条件;切向两个方向均可按应力或位移控制,能够实现接触面上单向往返、十字、圆形、椭圆等不同的任意设定的加载路径。试验表明,该试验机能够很好地再现粗粒土与结构接触面在上述的复杂加载条件下的力学响应,为接触面的三维力学特性的研究提供了基础。     

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.     

飞机移动荷载下层状路基应力路径特性研究

为研究飞机移动荷载作用下跑道地基受力特性及土体单元经历的应力路径,采用积分变换和传递矩阵法推导了层状地基各应力分量的半解析解,将道路面层-基层-地基视作多层跑道结构体系,将飞机荷载考虑为作用于跑道上的多个移动轮载的叠加,分析了不同移动轮组荷载组合形式下地基内部土单元的偏应力大小、主应力方向的变化规律,揭示了偏应力路径的基本特性;进一步探讨了跑道不同功能区土单元应力路径的差异性,发现不同功能区域土单元应力路径曲线有显著差异,呈现为全心形-上半心形及下半心形的形态,并开展动态空心扭剪试验观察了三种应力路径下土体累计变形的差异性,从而揭示了跑道差异沉降产生的内在机理。     

基于功能系统分析技术的葵岗隧道围岩施工方案选择研究

 基于价值工程中的功能系统分析技术,结合专家知识和群决策(DSS)技术,对高速公路隧道施工方案的优化选择方法进行研究,建立一个针对不同围岩类别的施工方案优化理论模型。然后结合一个具体案例(葵岗隧道施工优化)来阐述该模型的应用。实践证明,这种模型很适合大型隧道工程的施工方案优化选择决策。     

主应力方向对饱和松砂不排水单调剪切特性影响的试验研究

利用大连理工大学新开发的“土工静力–动力液压–三轴扭转多功能剪切仪”,针对相对密度为30%的福建标准砂,在不排水条件下控制试验过程中主应力方向、中主应力系数、平均主应力保持不变,进行了单调剪切试验。以此着重探讨了主应力方向对有效内摩擦角、有效应力路径及应力-应变关系的影响。研究表明,主应力方向对在不排水条件下饱和松砂的单调剪切特性具有显著影响。     

土的统一硬化模型及其发展

土的统一硬化模型是以修正剑桥模型为理论基础,以采用统一硬化参数使其简明反映土的剪胀性、以采用变换应力方法使其合理反映土的三维特性为标志性特征,可针对不同类型土的特性,通过对统一硬化参数作适当调整,来建立与其相应的模型。统一硬化模型被发展为黏土和砂土的统一硬化模型、统一硬化超固结土模型、统一硬化K0超固结土模型、考虑砂土软化的模型、考虑土的材料各向异性的模型等十余种模型。这些模型能分别描述土的剪缩、剪胀、硬化、软化、超固结性、初始应力各向异性、材料各向异性、结构性、蠕变压缩和渐进状态等特性以及大应力条件下颗粒破碎和循环加载条件下土的变形特性等。     

Effect of Curing Stress and Period on the Mechanical Properties of Cement-Mixed Sand

Cement mixing is one of the popular ground improvement technologies in geotechnical engineering practice. In order to effectively and confidently design cement-mixed soil structures for specific purposes, its stress-strain behavior needs to be well understood. Though there have been many studies on cement-mixed soils using different types of soils, their behaviors have not been generalized yet. As is the case with concrete materials, the hydration of cement in cement-mixed soil continues with time, thereby improving the strength and deformation characteristics of cement-mixed soil over time. In the field, the cementation bonds are formed under stress in case of in-situ soil. However, in the usual testing techniques, cementation bonds under stress has not been a point of consideration in most of the previous studies. This has led to an underestimation of the stress-strain behavior of cement-mixed soil. On the other hand, soils are subjected to confining stress during loading which has also some effect on the strength and deformation characteristics of soil which has not been considered yet in the case of cement-mixed sand. This study investigates the effect of curing stress and period on the strength and deformation characteristics of cement-mixed sand. The effect of confining stress in the triaxial test is also investigated in another series of specimens. A series of consolidated drained (CD) triaxial compression (TC) tests were done along with the small strain cyclic loading and bender element tests during monotonic loading to determine the small strain Young's modulus (E_v) and shear modulus (G_vh) respectively. The effect of the curing period is significant in the peak strength, stiffness, E_v, G_vh and also in the post peak regime. The curing stress also has a significant effect on the peak strength, E_v and G_vh. The confining stress has an effect on the peak strength, stiffness and in the post peak regime. However, the effect is small compared to clean sand.