A numerical investigation and geological discussion of the relationship between folding,kinking and faulting

The layer-parallel compression of a regular bilaminate consisting of layers with materials described by an incompressible power-law elastic model is considered. The average mechanical properties of this idealised multilayer are then represented by those of an equivalent anisotropic continuum with internal resistance to bending. Changes in material properties that accompany uniform finite shortening are accounted for. Interpretation of the internal instability analysis for such a continuum, introduced in the companion paper involves the use of a spectrum which at a given level of strain, scans all directions within the continuum for relative susceptibility to a heterogeneous simple shearing instability.Estimates of nonlinear material properties from reported experiments on the behaviour of various rocks in the time-independent deformation regime, and geometric parameters such as the volume fraction of each material and the number of confined layers are considered. The shapes of the resulting spectra may be used to predict natural conditions that will favour the initiation of repetitive buckle folds or more localized disturbances such as kink-bands and faults. Results suggest that for typical properties of sedimentary multilayers, kinking is strongly favoured over repetitive buckling where the weaker material occupies only a very small volume fraction of the multilayer. The effect of significant imperfections leading to slippage between layers is discussed.Finally, a simple classification of structure genesis is proposed in which the mechanical relationships between apparently diverse structures is illustrated.     

The signature of shear-induced anisotropy in granular media

This paper presents a micro-mechanical study on the characteristics of shear-induced anisotropy in granular media. Based on three-dimensional Discrete Element Method (DEM) simulations, the distinct features associated with the evolution of internal granular structure and different anisotropy sources during drained/undrained shearing of granular samples are carefully examined. The study finds that static liquefaction occurs when the geometrical anisotropy in a sample dominates the mechanical anisotropy in the overall shear strength, and the weak force network features an exceptionally high proportion of sliding contacts and develops certain degree of anisotropy. Phase transformation corresponds to a transitional, unstable state associated with a dramatic change in both coordination number and the proportion of sliding contacts in all contacts. The critical state in a granular material is always associated with a highly anisotropic fabric structure wherein both the critical void ratio and critical fabric anisotropy are uniquely related to the mean effective stress. The relations provide a more comprehensive definition for the critical state in granular media with proper reference to the critical fabric anisotropy.     

Micromechanical inspection of incremental behaviour of crushable soils

In granular soils grain crushing reduces dilatancy and stress obliquity enhances crushability. These are well-supported specimen-scale experimental observations. In principle, those observations should reflect some peculiar micromechanism associated with crushing, but which is it? To answer that question the nature of crushing-induced particle-scale interactions is here investigated using an efficient DEM model of crushable soil. Microstructural measures such as the mechanical coordination number and fabric are examined while performing systematic stress probing on the triaxial plane. Numerical techniques such as parallel and the newly introduced sequential probing enable clear separation of the micromechanical mechanisms associated with crushing. Particle crushing is shown to reduce fabric anisotropy during incremental loading and to slow fabric change during continuous shearing. On the other hand, increased fabric anisotropy does take more particles closer to breakage. Shear-enhanced breakage appears then to be a natural consequence of shear-enhanced fabric anisotropy. The particle crushing model employed here makes crushing dependent only on particle and contact properties, without any pre-established influence of particle connectivity. That influence does not emerge, and it is shown how particle connectivity, per se, is not a good indicator of crushing likelihood.

     

Impact of mechanical anisotropy and power-law rheology on single layer folding

The infinitesimal and finite stages of folding in nonlinear viscous material with a layer-parallel anisotropy were investigated using numerical and analytical methods. Anisotropy was found to have a first-order effect on growth rate and wavelength selection, and these effects are already important for anisotropy values (normal viscosity/shear viscosity) < 10. The effect of anisotropy must therefore be considered when deducing viscosity contrasts from wavelength to thickness ratios of natural folds. Growth rates of single layer folds were found to increase and subsequently decrease during progressive deformation. This is due to interference between the single layer folds and chevron folds that form in the matrix as a result of instability caused by the anisotropic material behaviour. The wavelength of the chevron folds in the matrix is determined by the wavelength of the folded single layer, which can explain the high wavelength to thickness ratios that are sometimes found in multilayer sequences. Numerical models including anisotropic material properties allow the behaviour of multilayer sequences to be investigated without the need for resolution on the scale of individual layers. This is particularly important for large-scale models of layered lithosphere.     

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.     

A hypoplastic model for granular soils incorporating anisotropic critical state theory

It is well known that soil is inherently anisotropic and its mechanical behavior is significantly influenced by its fabric anisotropy. Hypoplasticity is increasingly being accepted in the constitutive modeling for soils, in which many salient features, such as nonlinear stress-strain relations, dilatancy, and critical state failure, can be described by a single tensorial equation. However, within the framework of hypoplasticity, modeling fabric anisotropy remains challenging, as the fabric and its evolution are often vaguely assumed without a sound basis. This paper presents a hypoplastic constitutive model for granular soils based on the newly developed anisotropic critical state theory, in which the conditions of fabric anisotropy are concurrently satisfied along with the traditional conditions at the critical state. A deviatoric fabric tensor is introduced into the Gudehus-Bauer hypoplastic model, and a scalar-valued anisotropic state variable signifying the interplay between the fabric and the stress state is used to characterize its impact on the dilatancy and strength of the soils. In addition, fabric evolution during shearing can explicitly be addressed. Modifications have also been undertaken to improve the performance of the undrained response of the model. The anisotropic hypoplastic model can simulate experimental tests for sand under various combinations of principle stress direction, intermediate principal stress (or mode of shearing), soil densities, and confining pressures, and the associated drastic effect of different principal stress orientations in reference to the material axes of anisotropy can be well captured.     

Structural softening of the lithosphere

Structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant. Structural softening is fundamentally different than material softening, where the decrease in stress is generated by a change in rheological properties with progressive deformation, such as grain size reduction resulting from large shearing strain. We study structural softening generated by folding of the crust-mantle boundary, which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere. For ductile rheologies, the stress decrease represents a decrease of the effective lithospheric viscosity, which is proportional to the ratio of stress to lithospheric shortening strain rate. We present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate. The decrease in effective viscosity can be up to 10-fold.     

碎屑砂岩三轴压缩下强度和变形特性试验研究

基于碎屑岩组织结构疏松、含水率较高、物理力学性能较差、呈孔隙式胶结接触等特点,对碎屑砂岩首先开展了物理特性试验分析,认为其微、细观结构复杂、内部破坏严重,矿物成分为石英、长石、绢云母等,化学成分以SiO2为主,属微透水、小孔隙率砂岩,且渗水化学侵蚀并不显著。其次,开展了静水压力、单轴压缩和三轴压缩试验,研究了碎屑砂岩的强度和变形破坏特性。最后,初步探索了物理特性与强度变形特性的关系。结果表明,静水压力为2.6 MPa时,岩样内部微缺陷压密完成;单轴压缩曲线呈明显6阶段特征,峰值应力达0.98 MPa,属脆-延性破坏;三轴压缩条件下,岩样呈压缩为主的延性扩容破坏,轴向压缩和环向体积扩容达6%和4%;曲线无明显破坏荷载,呈现非线性、塑性硬化、存在屈服平台和体积由压缩向扩容过渡等特性。且体积扩容破损应力与屈服应力基本相同,扩容转折点随围压增加而增大,围压可增强岩样抵抗变形破坏的能力。试验结果旨在为岩石工程稳定分析及本构模型构建提供可靠的依据。     

流变物质对软土流变参数影响的蠕变试验

流变性是软土的一种长期变形特性,对软土地基的沉降产生重要影响。现有研究表明,软土中高黏性的流变物质是产生其流变特性的主要物质因素,当流变物质分布具有一定均匀性时其流变特性与流变物质的分布形态无关,仅与流变物质本身性质和含量比例相关。通过蠕变试验研究了流变物质对软土流变参数的影响,视强亲水性的膨润土微小颗粒为具有高黏性的流变物质,均匀地把其掺入到人工土试样中进行一维压缩蠕变试验,引入基于邓肯非线性弹性的流变元件模型对蠕变试验曲线进行拟合分析,根据流变物质含量确定流变模型参数的变化规律,建立了模型等效黏滞系数、等效弹性模量和邓肯非线性弹性模量与膨润土（流变物质）含量的关系。模型拟合分析发现,基于邓肯非线性弹性的软土流变元件模型与试验结果具有良好一致性,模型参数与流变物质含量之间关系为双曲线方程。     

A new device for investigating the hydro‐mechanical properties of rock joints

A new experimental system composed of two devices is presented for studying the hydraulic conductivity of rock joints and other interfaces in relation with their mechanical loading. This system allows for investigations on the anisotropy of the hydraulic conductivity, as well as for its heterogeneity. The first device controls the injection of a fluid, regulated in pressure or discharge near the centre of the sample of rock joint, which produces a quasi‐radial flow within the joint. The output flow is measured in real time using a sectorized peripheral membrane for acquisition of data about the directionality or anisotropy of the flow. The second device allows for the measurement of the evolution of the morphology and of the void map of the rock joints during their mechanical loading (compression or shearing). It is a laser beam moved by an (x, y) frame. This frame is installed within the shear box at several stages of the loading, after temporarily moving one rock wall away from the other one, but without unmounting them from the shear box. After the presentation of the experimental system, and of its performances, we give some examples of test results of replicas of rock joints, loaded in compression and shearing. First, these results show that the change in global transmissivity of the samples is strongly correlated with the normal relative displacement of the rock walls. Several statistical analyses of the change in the void map, taking into account the damage of the asperities of the joint, are presented and compared with the results of the directional output flows. Today further studies involving a FEM as well as a finite volume modelling of the flow within the joints, based on the measured void map are in progress for a more precise comparison to the experimental results. Copyright © 2003 John Wiley & Sons, Ltd.     

Modeling the behaviour of alluvial and blasted quarried rockfill materials

Two types of modeled rockfill materials were collected from Renuka dam site, Himachal Pradesh, India and Salma dam site, Afghanistan. The rockfill material collected from Renuka dam site is rounded to sub-rounded in shape and the rockfill material collected from Salma dam site is angular to sub-angular in shape. The prototype gradation rockfill material consists maximum particle size larger than 1,000 mm. Therefore, for carrying out laboratory testing and modeling the bahaviour, the prototype rockfill material is scaled down to the maximum particle size (d_max) of 25, 50 and 80 mm for both projects material using parallel gradation technique. Triaxial compression and Index properties tests were conducted on both project rockfill materials and are presented. From the triaxial behaviour, it is observed that the stress–strain behaviour is non-linear, inelastic and stress dependent for both the materials. The material compresses during the initial shearing and shows dilation effect with further shearing. It is observed that the ?-value for alluvial rockfill material increases with increase in d_max and reverse trend is observed for blasted quarried rockfill material which shows the importance of the type of material. The stress–strain-volume change behaviour of both projects modeled rockfill material was predicted by using hierarchical single surface (HISS) model based on elasto plasticity and compared with the laboratory test results. From the comparison, it is observed that both results match closely. It is, therefore, suggested that the behaviour of both types of rockfill materials can be characterized successfully using HISS model.     

基于大三轴试验的粗粒土剪胀性研究

采用大三轴剪切仪对3种不同相对密度的双江口心墙坝覆盖层料进行固结排水剪切试验,重点研究粗粒土的剪胀特性,分析粗粒土的剪胀性与围压、相对密度之间的关系,并检验修正剑桥模型的剪胀方程、Rowe剪胀方程对粗粒土的适用性。结果表明,粗粒土在低围压下表现出明显的剪胀趋势,随着围压的增加,逐渐由剪胀向剪缩过渡;随着密度的增大,粗粒土的剪胀性明显增强;相变应力比是粗粒土剪胀强弱的一个重要影响因素,其随密度的增加而增加,随围压的增大则呈线性减小趋势;粗粒土的剪胀性与相对密度、围压关系密切,根据试验数据得出最大剪胀率 与相对密度 、围压 之间的关系式;修正剑桥模型的剪胀方程不能反映粗粒土的剪胀性;Rowe剪胀方程在一定程度上能反映粗粒土的剪胀性,但高围压下在压缩阶段低估了其压缩性,而在剪胀阶段则高估了其剪胀性。     

考虑干湿循环路径的石灰改性红黏土路用性能试验研究

红黏土是一种对环境湿、热变化敏感的高塑性黏土,用作路基填料时,为改善其路用性能,并延长路基使用寿命,工程中通常掺入一定剂量的石灰对其进行改性。在地下水、降雨入渗和蒸发等自然营力作用下,运营期间路基土的含水率反复变动会对其工程特性产生一定影响。采用两种与实际工况接近的典型干湿路径,开展红黏土及其石灰改性土的强度和变形特性对比试验研究。试验结果表明：均匀干湿循环作用下,石灰改性土的黏聚力减小,内摩擦角小幅度增大,抗剪强度略高于红黏土,改性效果不明显;定向干湿循环作用下,石灰改性土的收缩裂缝较少,强度和变形参数均显著提高,路用性能得到明显改善。     

基于突变理论的地下工程洞室围岩失稳判据研究

地下工程围岩体系是高度非线性的复杂大系统,其稳定判据至今没有一个统一的认识。突变理论注重研究系统状态发生突变时外界的控制条件,主要阐述非线性系统如何从连续渐变状态走向系统性质的突变。应用突变理论对围岩体失稳的突变过程进行探索,旨在从理论上弄清其突变条件。能量突变判据和熵突变判据从物理学的观点考虑了围岩系统失稳的可能性,位移模突变判据、洞周屈服区面积突变判据和广义黏塑性剪应变突变判据则主要是根据数值计算中直观判别条件（关键点位移、洞周屈服区面积和广义黏塑性剪应变）考虑围岩失稳的可能性。将以上5种判据应用于某地下洞室断面,对围岩系统的稳定性进行分析,根据系统突变条件从不同的方面判别其发生失稳的可能性,可为工程的设计施工提供一定参考。     

各向异性对软土力学特性影响的离散元模拟

软土预压工程中,初始和诱发各向异性对软土力学性质的影响十分显著,而现有研究缺乏对初始和诱发各向异性的统一研究方法。采用离散单元法,以颗粒长宽比作为定量评价指标,构建真实形态的颗粒模型,生成5组不同沉积角的初始各向异性试样,并进行竖直和水平两方向加载的双轴模拟实验,研究了初始各向异性和诱发各向异性对软土力学特性影响;在细观层面,以颗粒为对象研究了颗粒接触形式和转动角度的变化规律,以接触为对象研究了配位数和接触法向各向异性的发展趋势,在此基础上探究抗剪强度指标与各向异性关系。结果表明:初始和诱发各向异性共同影响试样力学性质,当加载方向和软土沉积方向垂直时,土体有最大的峰值强度。颗粒接触形式中面面接触的比例随加载的进行逐渐增大,并影响着试样初始模量和抗剪强度,配位数和接触法向各向异性受颗粒接触形式的影响有不同的演化规律,并在加载后期趋于稳定;同时,初始各向异性试样相较各向同性试样有更大的黏聚力,诱发各向异性主要影响试样内摩擦角,进而影响试样抗剪强度。     

Modelling the pre-failure instabilities of sand

A simple incremental model describing the pre-failure behaviour of granular soils is presented. The model describes both the dry/fully drained and undrained response. It takes into account an initial anisotropy of soil and an initial state defined as either contractive or dilative. A physically sound definition of loading/unloading is assumed, which differs from elasto-plastic approaches. The model is based on extensive empirical data and gives predictions conformable with experimental results. It also describes pre-failure instabilities of granular soils, both dry/fully drained and undrained. The Hill’s criterion was used to examine stability. It was shown that this condition can be formulated either in terms of the effective stresses or by the total stresses. In the extreme cases of either dry/fully drained or undrained conditions, these alternative formulations are equivalent. This is not so in the case of partial drainage of pore water and associated volumetric deformations as well as pore pressure changes. The model describes the pre-failure instabilities well, and additionally allows for analytical derivation of the instability line. It was shown that the second order work, appearing in the Hill’s condition, is equivalent to the entropy source.     

辽西地区黄土的强度与本构特性

采用应变控制式三轴仪对取自辽宁西部地区阜新-朝阳段黄土的力学性质进行了试验,获得了黄土的应力-应变关系和抗剪强度的变化规律;采用人工制备不同含水率试样的方法,测试并分析了含水率对黄土本构和强度特征的影响。结果表明：(1)试样的应力-应变关系在不排水剪条件下呈强软化型,表现为脆性破坏;在固结不排水剪条件下随初始含水率的增加由弱软化型向硬化型转变;而在排水剪条件下低含水率时呈软化型,随含水率增加,呈弱硬化型和硬化型。(2)饱水后试样的抗剪强度有很显著的降低:c_UU为13.7～363.3 kPa时,φ_UU为18.6°~40.2°;而c_UU为0.2～29.6 kPa时,φ_CU则为4°～23.4°。(3)土的有效抗剪强度指标均随含水率增加呈非线性减小的关系,粘聚力与含水率呈半对数函数关系,内摩擦角则与含水率呈幂函数关系。给出了黄土本构的数学模型和黄土抗剪强度指标的取值方法。     

Effects of Previous Deviatoric Strain Histories on the Undrained Behaviour of Hostun RF Loose Sand

The effects of previous deviatoric strain histories on the undrained behaviour of very loose and saturated Hostun RF clean sand are investigated in this paper. From an initial isotropic stress state in the triaxial plane, strain histories are generated by isotropic consolidation followed by standard drained triaxial preshear or presheared cycle, either in compression or in extension, up to a desired value of axial strain or mobilized stress ratio. Deviatoric strain histories are achieved by having nearly the same void ratio at the beginning of the undrained shearing for all tested samples. Subsequent undrained behaviour in triaxial compression and extension is analyzed in detail. Previous deviatoric strain histories can progressively transform the compressive and unstable behaviour of loose sand into a dilative and stable behaviour of dense-like sand, while being loose. Experiments show a common response induced by recent strain histories in terms of effective stress paths, independently of the axial strain attained during the drained presheared cycle, a unique initial gradient of the effective stress paths, a progressive appearance of dilatancy, an evolution the undrained behaviour and a systematic partial static liquefaction associated with softer behaviour when sheared in the opposite direction of the initial presheared direction. This paper offers a comprehensive understanding of the mechanisms of a specialized induced anisotropy created by simple linear stress paths in the classical triaxial plane.     

Strength anisotropy of granular material consisting of perfectly round particles

The strength anisotropy of granular materials deposited under gravity has mostly been attributed to elongated particles' tendency to align long axes along the bedding plane direction. However, recent experiments on near‐spherical glass beads, for which preferred particle alignment is inapplicable, have exhibited surprisingly strong strength anisotropy. This study tests the hypothesis that certain amount of fabric anisotropy caused by the anisotropic stress during deposition under gravity can be locked in a circular‐particle deposit. Such locked‐in fabric anisotropy can withstand isotropic consolidation and leads to significant strength anisotropy. 2D discrete element method simulations of direct shear tests on circular‐particle deposits are conducted in this study, allowing for the monitoring of both stress and fabric. Simulations on both monodispersed and polydispersed circular‐particle samples generated under downward gravitational acceleration exhibit clear anisotropy in shear strength, thereby proving the hypothesis. When using contact normal‐based and void‐based fabric tensors to quantify fabric anisotropy in the material, we find that the intensity of anisotropy is discernible but low prior to shearing and is dependent on the consolidation process and the dispersity of the sample. The fact that samples with very low anisotropy intensity measurements still exhibit fairly strong strength anisotropy suggests that current typical contact normal‐based and void‐based second‐order fabric tensor formulations may not be very effective in reflecting the anisotropic peak shear strength of granular materials. Copyright © 2017 John Wiley & Sons, Ltd.