考虑各向异性的软黏土扰动状态本构模型

以大量试验结果为依据,运用物理意义明确、获取简便快捷、经济实用的土电阻率结构指标进行软黏土扰动变量的表征和测定。以三轴试验与电阻率测试结果为基础,进行了扰动函数的构建。在软黏土扰动状态描述中,引入旋转硬化因子,用以考虑应力诱发各向异性,进而构建了考虑各向异性的软黏土扰动状态概念本构模型。模型从一定程度上克服了传统扰动变量获取方法的弊端,克服了传统借用数学演绎手段建立扰动函数的不足,提高了模型应用的适用性。在模型构建中,融入初始应力各向异性的影响,从而使模型能更好地反映软土实际工程情况。三轴排水剪切试验验证结果表明,该模型可较好反映软黏土受荷下的结构变化情况和受力特性。     

Theoretical solution for drained cylindrical cavity expansion in clays with fabric anisotropy and structure

Acta Geotechnica - This paper presents a novel, exact, semi-analytical solution for the quasi-static drained expansion of a cylindrical cavity in soft soils with fabric anisotropy and structure....     

Experimental investigation of shear strength of sands with inherent fabric anisotropy

Loading direction-dependent strength of sand has been traditionally characterized in the principal stress space as a direct extension of the Mohr–Coulomb criterion. A recent study found that it is more appropriate to define anisotropic strength of sand on failure/shear planes, but this proposition has only been demonstrated with discrete element method (DEM) simulations. The present study experimentally investigates anisotropic shear strength of sands in this new framework. Three granular materials with distinct grain characteristics ranging from smooth and rounded particles to flaky and angular particles are tested with the bedding plane inclination angle ψ _b varying over the full range of 0°–180°. The main objective is to study how the peak friction angle ? _p of sand is affected by the ψ _b angle and how the ψ _b–? _p relationship evolves with the change of characteristics of constituent sand particles. We find that the general trend of ψ _b–? _p curves for real sands resembles what was predicted by DEM in a previous study, whereas rich anisotropic strength behavior is revealed by the laboratory data. The effects of normal stress and initial density, as well as shear dilation behavior at different shear directions, are also studied.     

Undrained expansion of a cylindrical cavity in clays with fabric anisotropy: theoretical solution

This paper presents a novel, exact, semi-analytical solution for the quasi-static undrained expansion of a cylindrical cavity in soft soils with fabric anisotropy. This is the first theoretical solution of the undrained expansion of a cylindrical cavity under plane strain conditions for soft soils with anisotropic behaviour of plastic nature. The solution is rigorously developed in detail, introducing a new stress invariant to deal with the soil fabric. The semi-analytical solution requires numerical evaluation of a system of six first-order ordinary differential equations. The results agree with finite element analyses and show the influence of anisotropic plastic behaviour. The effective stresses at critical state are constant, and they may be analytically related to the undrained shear strength. The initial vertical cross-anisotropy caused by soil deposition changes towards a radial cross-anisotropy after cavity expansion. The analysis of the stress paths shows that proper modelling of anisotropic plastic behaviour involves modelling not only the initial fabric anisotropy but also its evolution with plastic straining.     

Numerical simulation of fabric anisotropy and strain localization of sand under simple shear

This paper studies the effects of initial fabric anisotropy of dry sand in simple shear deformation. The effects of anisotropy are taken into consideration through the modification of the mobilized friction in the Mohr–Coulomb‐type yield surface as a function of a fabric parameter. In addition, the constitutive model uses a gradient term that directly incorporates the effects of material length scale. The constitutive formulation is implemented into ABAQUS finite element code and used to simulate shearing of the dry sand under various conditions of simple shear. The numerical simulations show that while the shear stress response is affected by fabric anisotropy, its effects on strain localization in simple shear are minimal. This is in contrast to other devices such as the biaxial shear. The strain localization in simple shear is controlled more by the imposed boundary conditions. The use of material length scale is shown to remove the effects of strain localization in the shearing response. Copyright © 2009 John Wiley & Sons, Ltd.     

上海浅部黏土渗透系数及其各向异性

为研究上海浅部黏土的渗透系数及其各向异性,沿水平及竖直方向对上海浅部主要黏土层进行了不同压力下的固结试验,联合使用时间平方根法和时间对数法获取了试样的渗透系数,探讨了渗透系数与孔隙比及渗流方向之间的关系,并通过电镜扫描,从微观角度分析了水平和竖直方向渗透系数存在差异的原因。研究结果表明：上海浅部主要黏土层的渗透系数均随着孔隙比的增加而增大。对于单组试验来说,渗透系数k与孔隙比e在e lgk坐标系中呈现很好的线性关系,且渗透变化指数Ck大致呈现Ck = 0.5e0的规律;但对于整体试验而言,渗透系数与孔隙比在e lgk坐标系中大致呈现出曲线关系。通过扫描电镜观察,揭示了沉积形成的絮状微观结构是竖直和水平渗透系数差异较小的主要原因。     

Elastic properties, fabric and seismic anisotropy of amphibolites and their contribution to the lower crust reflectivity

In this paper the elastic properties of amphibolites from the Serie dei Laghi and the Ivrea zone (Southern Alps, N-Italy) were investigated as a function of their mineralogical composition, microfabric and density.Three orthogonal cores were cut parallel and normal to foliation and lineation; from those, bulk and grain density were measured and the interconnected porosity was calculated. Bulk density varies from 2.75 to 3.07 g/cm³ and calculated porosity ranges from 0.02 to 0.88%.The same cores were also used to measure seismic velocity of ultrasonic waves at room temperature and at increasing confining pressure up to 300 MPa. At high pressure the matrix properties are separated from the crack-induced properties. P-wave velocity varies with respect to the direction of propagation: the slowest direction is always normal to foliation and the fastest parallel to the mineral lineation. The mineral lineation is typically defined by the elongation of amphibole crystals, in which the fastest Vp direction is parallel to the c axis, that is also the elongation axis. The Vp ranges between 6.76 and 7.54 km/s in the direction parallel to lineation and between 6.32 and 7.06 km/s in the direction normal to foliation. This defines a Vp anisotropy of up to 14%, whose shape varies from orthorhombic to axially symmetric (either prolate or flattened). It was observed that both Vp and Vp anisotropy increase with the amount of amphibole and decrease with the amount of plagioclase. Moreover, the c axis distribution of amphiboles is responsible of the Vp anisotropy intensity and shape, in agreement with observations from previous studies. The seismic properties calculated with the approach of Mainprice (1990), using the fabric data, the elastic constants and the modal composition, gave results in good agreement with the measurements.Exposed rocks in the Ivrea and Serie dei Laghi zones show that amphibolites are interlayered with metapelites on a scale from 1 to 100 m. Because of the very large acoustic impedance contrast (20.34 ± 1.75 for amphibolites, 17.16 ± 0.4 for metapelites), they represent a very reflective portion in the middle-lower crust.     

A note on thermomechanical anisotropy of clays

Deep clays exhibit a pronounced strain anisotropy both during mechanical loading as well as during heating and cooling at constant stress in drained isotropic conditions. During mechanical loading vertical strain is larger than the horizontal one. During heating the vertical strain is larger than the horizontal one within the elastic range; the opposite is observed in the elasto-plastic range. The above described response can be interpreted adopting a consistent rotational, kinematic hardening thermo-elasto-plastic constitutive law.     

On hardening anisotropy of K0-consolidated clays

The combined isotropic-kinematic hardening rule for clays is presented using the concept of a multisurface model discussed previously in Reference 1. The mechanical behaviour of clays after one-dimensional consolidation is next simulated for the axisymmetric stress stress state. A general three-dimensional formulation of constitutive relations is also presented.     

Electrical anisotropy of clays - its geological significance

Electrical (resistivity) anisotropy is found to vary directly and closely as thermaland hydrodynamic anisotropy in clays. This effect is strongest in clays deposited where hydrodynamic forces are strong, i.e., in shallow water. This is the environment of deposition for arenites, however. Therefore, anisotropy in clays may indicate the presence of oil or gas reservoirs. The behavior of the parameter depends, in the main, upon data obtained by the long lateral sonde. —M. E. Burgunker     

Elastic anisotropy of regularly jointed media

SummaryElastic Anisotropy of Regularly Jointed Media A continuum model of a rock mass containing a major set of regularly spaced parallel joints is adopted for length scales large compared with joint spacing. The intact rock is isotropic and elastic; reversible linear relations between the joint tractions and relative displacements (ignoring slip) are assumed, leading to an overall transversely anisotropic elastic medium. The degree of anisotropy depends on the ratios of the gross joint moduli to the elastic moduli of the rock, and is examined for plane strain and plane stress conditions in planes containing the normal to the joint set. Various ratio limits are considered, showing that some ranges are just regular perturbations of isotropic or anisotropic theory, while others give rise to lower order approximations which can involve boundary layer effects. The latter ranges corresponds to the relatively low joint moduli which make the overall response strongly anisotropic.

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ZusammenfassungElastische Anisotropie eines regelmäßig geklüfteten Mediums Das Kontinuumsmodell eines Felsmassives, welches eine größere Anzahl von regelmäßig angeordneten parallelen Fugen besitzt, ist so ausgelegt, daß die Längenabmessungen groß im Vergleich zu der Fugenstärke werden.Der ungestörte Fels verhält sich isotrop und elastisch. Es wird eine umkehrbare lineare Beziehung zwischen den Zugspannungen in der Fuge und den relativen Verschiebungen (bei Außerachtlassung von Gleiten) angenommen, was zu einem in der Gesamtheit quer durchgehend anisotrop elastischem Modell führt. Der Grad der Anisotropie hängt vom Verhältnis des überschlägigen Fugenmoduls zum Elastizitätsmodul des Felses ab und wird unter Zugrundelegung von ebenen Dehnungs- und Spannungsverhältnissen für Flächen normal auf die Fugen überprüft. Verschiedene Grenzwerte werden in Betracht gezogen, es wird nämlich gezeigt, daß einige Bereiche nicht mit Isotropie oder Anisotropie in Einklang zu bringen sind, in anderen wiederum ist es möglich, mit Näherungen niedrigerer Ordnung zu arbeiten, was die Berücksichtigung von Randeffekten gestattet. Letztere Bereiche gehören zu einem relativ niedrigen Fugenmodul, was das ganze Modell stark anisotrop werden läßt.

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RésuméAnisotropie élastique dans un milieu à joints réguliers Un modèle continu d'une masse rocheuse contenant un ensemble majeur de joints parallèles équidistants est adopté pour le cas où les dimensions longitudinales sont grandes en comparison des intervalles entre les joints. Le rocher intact est isotropique et élastique, et l'on suppose des relations linéaires réversibles entre les tractions sur les joints et les déplacements relatifs (avant le glissement), menant à un milieu transversalement isotropique et élastique dans son ensemble. Le degré d'isotropie dépend du rapport entre les modules de l'ensemble des joints et les modules élastiques du rocher; ce rapport est examiné pour le cas de déformations planes et de tensions planes dans des plans contenant la normale à l'ensemble des joints. On considère plusieurs limites de ces rapports montrant que certaines gammes ne sont que des perturbations régulières de la théorie isotropique ou anisotropique, tandis que d'autres donnent des approximations d'ordre inférieur qui impliquent des effets de couches frontières. Les dernières gammes correspondent à des modules de joints relativement petits, qui tendent la réaction de l'ensemble très anisotropique.

     

Hypoplastic and viscohypoplastic models for soft clays with strength anisotropy

This paper presents a new viscohypoplastic model for soft clays accounting for their typical features—strength anisotropy and rate dependency. The model is based on the hypoplastic model for clays enhanced by the anisotropic shape of the asymptotic state boundary surface. It has been shown that if the surface is skewed, the model predicts different ultimate strength in compression and in extension. Additional enhancement makes the tensor L bilinear in the strain rate, which more realistically predicts the stress paths of K₀ consolidated samples. The new model has been evaluated by simulating laboratory experiments on soft marine clays (Singapore and Bangkok clays). The model can be easily calibrated using only undrained triaxial and odometer tests. The model is subsequently enhanced by the rate effects. The resulting viscohypoplastic model has been evaluated using experiments of remolded kaolin clay and St. Herblain clay. It is shown that the enhanced model can predict important features of soil viscous behavior, such as rate dependency of strength and preconsolidation pressure, relaxation, and creep.     

Effects of geologic time on the initial stiffness of clays

The paper addresses the effect of geologic aging on the initial stiffness of clays through the analysis of in situ shear stiffness data obtained using a seismic cone penetrometer at three sites in Boston blue clay (BBC), a 12,000–14,000-year old post-glacial marine clay deposit. The effects of this aging period are estimated using as reference for the unaged behavior the results of over 90 triaxial shear phases conducted on specimens of resedimented Boston blue clay (RBBC), a soil prepared in the laboratory from natural BBC, and tested after 1 day of laboratory aging.At the same OCR and effective stress level, the natural clay displays an initial stiffness, which on the average, is approximately 50% greater than that of the laboratory resedimented clay. Based on the estimated age of the natural deposit, this corresponds to an increase of the stiffness per log cycle of time of 7%. This value of the aging coefficient, N_G, is at the low end of values presented in the literature for clays, which are however all based on shorter aging durations (days to weeks).     

Numerically observed shear bands in soft sensitive clays

The numerical challenges that arise in modelling shear bands in soft sensitive (SS) clays have not yet been fully resolved. Convincing and well-accepted solutions have yet to be found. This paper presents some novel information related to the shear band phenomenon in SS clays. In this study, the hypothesis is that the generation and dissipation of excess pore pressure from shear bands could regularise the strain softening and result in a mesh independent shear band thickness. The generation and dissipation of excess pressure is modelled by a coupled consolidation process. The simulation aims at modelling two counteracting mechanisms in the SS clay. First, the shear band narrows because of strain softening. Second, the internal pore water pressure drainage reduces the rate of strain softening. This counteracting mechanism provides an inherent regularisation technique for SS clays. This study presents some numerical results involving these two counteracting mechanisms. This study also shows that an inherent internal parameter applicable for SS clays can be defined by the ratio between soil permeability and the applied strain rate. In the case of SS clays, the range of this parameter varies from 0 to 0.0002 mm.     

软土初始应力各向异性弹塑性模型

自然土体处于初始应力状态,其强度和应力-应变关系都呈现出各向异性,而以往广泛使用的剑桥模型是建立在重塑土试验结果的基础上的,因此,计算实际问题时有一定缺陷.在总结了一些在修正剑桥模型基础上进行扩展而得到的各向异性模型,尤其是S-CLAY1模型.然后,假定了初始屈服面的倾角为K0线,这样使S-CLAY1的计算更加简单.此外,还编制了相关程序,进行了比较计算.结果表明,该模型简单合理,参数正确,可以在实际工程中应用.     

Modeling of elastic anisotropy due to one-dimensional plastic consolidation of clays

Experiments indicate that in one-dimensionally consolidated natural clays the elastic anisotropy is much stronger than the plastic strain anisotropy. Moreover, the elastic anisotropy appears to be dependent on the pre-consolidation strain. Coupled elasto-plastic constitutive law is shown to be able to simulate these anisotropy effects of natural clay deposits. In this law the elastic potential is not only a function of stress, but additionly of the plastic strain. The plastic strain comprises the geological process of pre-consolidation idealized as an one-dimensional plastic straining as well as a mechanically induced strain due to engineering activity. Calibration of the model and simulation of some stress paths are presented and related to the classical experimental results by Mitchell (1972).     

Modelling undrained behaviour of sand with fines and fabric anisotropy

Acta Geotechnica - Fabric anisotropy and fines content (fc) in sands modify significantly their mechanical behaviour, particularly as related to static liquefaction under undrained conditions. The...