Fabric Evolution Research Articles

Taming, controlling and metabolizing flows: Water and the urbanization process of Barcelona and Madrid (1850–2012)

This paper aims to provide a historical reading of the urbanization of the water cycle in Madrid and Barcelona. Starting from an urban political ecology view, the urbanization of the water cycle is understood as the mobilization of water resources to keep pace with and sustain urban growth. This process could not be understood without inquiring into the evolution of the urban fabric in both cities. At the same time an understanding of the power choreographies over the water cycle needs to be brought to the fore. In Barcelona, disputes between the municipality and private capital over the water monopoly deeply shaped the trajectory of the urban supply from the mid-19th century until the early 20th century. Later on, under private monopoly, the search for water resources beyond the urban limits required the development of infrastructures such as dams and channels to keep pace with the intense urbanization of the postwar period. On the other hand, the fully public nature of the supplier in Madrid may help to explain the impressive magnitude of the waterworks, which belittle the urbanization of water of the Catalan city, while showing that ‘modernity’ in the form of ample supplies of good-quality water arrived in Madrid almost 100 years before Barcelona. Recent environmental and economic crises bring to light the hidden, complex and fragile entanglements that permit the flowing of water into the urban and suburban fabrics.

Shear zones in clay-rich fault gouge: A laboratory study of fabric development and evolution

Clay-rich fault rocks have long been recognized to host distinctive fabric elements, and fault rock fabric is increasingly thought to play a fundamental role in fault mechanical behaviour in the brittle regime. Although the geometries of fabric elements in fault gouges have been described for almost a century, the genesis and evolution of these elements during shear, and their links to bulk mechanical properties, remain poorly understood. We characterize the development and evolution of fabric elements with increasing shear in a variety of clay-rich experimental gouges over shear strains of <1 to >20 and at normal stresses of 2–150 MPa in the double-direct shear configuration. In addition to SEM observations of experiment products at a variety of shear strains, we quantified clay fabric intensity and the degree of grain size reduction using X-ray Texture Goniometry (XTG) and particle size distribution (PSD) measurements. We also measured P- and S-wave velocities during shear to further probe the evolution of shear fabric and gouge properties. We find that clay fabric elements develop in a systematic manner regardless of the gouge material. Riedel shears in the R1 orientation and boundary-parallel shears are the dominant fabric elements. Riedel shears nucleate at layer margins and propagate into the layer shortly after reaching yield stress. Clay particles rotate into the P-orientation shortly after Riedels propagate through the layer. The Riedel shears are through-going, but are >10× thinner than similar zones observed in coarser granular materials. Our results suggest that the weakness of clay-rich fault gouge may be less a function of anisotropic crystal structure, as has been suggested previously, and more a consequence of very thin shear surfaces permitting deformation in clay-rich materials with minimal dilation or cataclasis. The very thin shear surfaces are a function of the fine grain size of the materials and possibly polymodal PSD's.

The evolution of crystal fabric in ice sheets and its link to climate history

AbstractThe evolution of preferred crystal-orientation fabrics is strongly sensitive to the initial fabric and texture. A perturbation in climate can induce variations in fabric and texture in the firn. Feedbacks between fabric evolution and ice deformation can enhance these variations through time and depth in an ice sheet. We model the evolution of fabric under vertical uniaxial-compression and pure-shear regimes typical of ice divides. Using an analytic anisotropic flow law applied to an aggregate of distinct ice crystals, the model evolves the fabric and includes parameterizations of crystal growth, polygonization and migration recrystallization. Stress and temperature history drive the fabric evolution. Using this model, we explore the evolution of a subtle variation in near-surface fabric using both constant applied stress and a stress–temperature history based on data from Taylor Dome, East Antarctica. Our model suggests that a subtle variation in fabric caused by climate perturbations will be preserved through time and depth in an ice sheet. The stress history and polygonization rate primarily control the magnitude of the preserved climate signal. These results offer the possibility of extracting information about past climate directly from ice fabrics.

Hydrostatic and Shear Behavior of Frictionless Granular Assemblies under Different Deformation Conditions

Stress- and structure-anisotropy (bulk) responses to various deformation modes are studied for dense packings of linearly elastic, frictionless, polydisperse spheres in the (periodic) triaxial box element test configuration. The major goal is to formulate a guideline for the procedure of how to calibrate a theoretical model with discrete particle simulations of selected element tests and then to predict another element test with the calibrated model (parameters). Only the simplest possible particulate model material is chosen as the basic reference example for all future studies that aim at the quantitative modeling of more realistic frictional, cohesive powders. Seemingly unrealistic materials are used to exclude effects that are due to contact non-linearity, friction, and/or non-sphericity. This allows us to unravel the peculiar interplay of stress, strain, and microstructure, i.e. fabric. Different elementary modes of deformation are isotropic, deviatoric (volume-conserving), and their superposition, e.g. a uniaxial compression test. Other ring-shear or stress-controlled (e.g. isobaric) element tests are referred to, but are not studied here. The deformation modes used in this study are especially suited for the bi- and triaxial box element test set-up and provide the foundations for understanding and predicting powder flow in many other experimental devices. The qualitative phenomenology presented here is expected to be valid, even clearer and magnified, in the presence of non-linear contact models, friction, non-spherical particles and, possibly, even for strong attractive/ adhesive forces. The scalar (volumetric, isotropic) bulk properties, the coordination number and the hydrostatic pressure scale qualitatively differently with isotropic strain. Otherwise, they behave in a very similar fashion irrespective of the deformation path applied. The deviatoric stress response (i.e. stressanisotropy), besides its proportionality to the deviatoric strain, is cross-coupled to the isotropic mode of deformation via the structural anisotropy; likewise, the evolution of pressure is coupled via the structural anisotropy to the deviatoric strain, leading to dilatancy/compactancy. Isotropic/uniaxial over-compression or pure shear respectively slightly increase or reduce the jamming volume fraction below which the packing loses mechanical stability. This observation suggests a necessary generalization of the concept of the jamming volume fraction from a single value to a “wide range” of values as a consequence of the deformation history of the granular material, as “stored/memorized” in the structural anisotropy. The constitutive model with incremental evolution equations for stress and structural anisotropy takes this into account. Its material parameters are extracted from discrete element method (DEM) simulations of isotropic and deviatoric (pure shear) modes as volume fraction dependent quantities. Based on this calibration, the theory is able to predict qualitatively (and to some extent also quantitatively) both the stress and fabric evolution in another test, namely the uniaxial, mixed mode during compression. This work is in the spirit of the PARDEM project funded by the European Union

Waveform modeling of shear wave splitting from anisotropic models in Iceland

No quantitative model of seismic anisotropy beneath Iceland has yet explained the observed shear wave splitting results in Iceland. In this study we explore the structure of mantle seismic anisotropy associated with plume ridge interaction beneath Iceland by modeling synthetic waveforms using a pseudo‐spectral method. First, predicted SKS waveforms are shown to produce shear wave splitting results for two anisotropic layers that are in generally agreement with analytic solutions. Next, simple models in which anisotropy are imposed at different depths and geographic regions around Iceland are used to examine different hypothesized effects of plume‐ridge interaction. The model that is designed to represent crystallographic fabric due to mantle deformation associated with lithosphere spreading in western Iceland and along‐axis, channeled asthenospheric flow in eastern Iceland can predict the distinct shear wave splitting observations on the east and west sides of the island. Last, we analyze geodynamic models that simulate 3‐D mantle flow and evolution of mineral fabric. The predicted fast directions of shear wave splitting from the models with realistic spreading rate (10 km/Myr) and broad range of plume viscosities are consistent with the observations in central and eastern Iceland. However, because these models are symmetric about the ridge, none of them can predict the observed shear wave splitting results in western Iceland.

Effects of nonlinear rheology, temperature and anisotropy on the relationship between age and depth at ice divides

Abstract. Ice flow in divide areas is strongly anisotropic. The evolution of ice fabric, from the onset of divide flow towards steady state with a fully developed fabric, has been shown to profoundly affect both the stratigraphy and surface topography of ice divides. Here, we investigate the effects of ice flow on the age-versus-depth relationship at ice divides by using a full Stokes thermomechanical model with a non-linear anisotropic constitutive relation between stress and strain rates. We compare our results with analytical approximations commonly employed in age–depth predictions, such as the Dansgaard and Lliboutry approximations. We show that these approximations systematically underestimate the age of ice at fully developed divides by as much as one order of magnitude. We also show that divides with fully developed fabric are ideal locations for ice-core extraction because ice under them can be up to one order of magnitude older than ice at the same depth at the flanks. In addition, these divides have a distinctive morphological structure that allows them to be clearly identified from satellite imagery or ground-penetrating radar data.

Measurements and numerical simulation of fabric evolution along the Talos Dome ice core, Antarctica

We present measurements of fabrics and microstructures made along the Talos Dome ice core, a core drilled in East Antarctica in the framework of the TALDICE project. Fabric and average grain size data are analyzed regarding changes in climatic conditions. In particular, the fabric strength increases sharply going downward from Holocene to Wisconsin ice. Following (Durand et al., 2007), this change is associated with a positive feedback between variations in ice viscosity, due to variations in dust content, and the impact of a shear stress component, increasing with depth. A ViscoPlastic Self-Consistent modeling approach is used to simulate the fabric evolution for a “perfect dome” configuration. The discrepancies between the measured and the simulated fabrics highlight the depth ranges where shear strongly affects the fabric strengthening. Finally, the grain size and fabric analyses show the occurrence of dynamic recrystallization mechanisms (continuous and discontinuous) along the core.

Strain fabric evolution within and near deformed igneous sheets: The Sudbury Igneous Complex, Canada

Deformation structures differing in style and orientation within a given terrane are often attributed to distinct tectono-metamorphic events. However, mechanical heterogeneities may locally cause strain perturbations that can have a profound effect on the geometry of such structures. Here we document highly variable orientations of planar structures, metre- to kilometre-scale folds and shortening directions inferred from brittle fault analysis within the synclinal and layered Sudbury Igneous Complex (SIC) and its Huronian host rocks. NW–SE shortening during Palaeoproterozoic deformation led to the formation of planar structures in the host rocks that are parallel to the NE–SW striking contact of the southern SIC. During deformation, local contact-parallel shortening became more important than regional NW–SE shortening and generated contact-orthogonal planar mineral fabrics and folds. Local contact-parallel shortening is attributed to the shape change of the southern SIC from a convex outward to a concave inward curvi-planar geometry. Contact-parallel shortening accounts for the formation of a previously unidentified kilometre-scale buckle fold in the SIC and respective axial–planar mineral fabrics in Huronian host rocks. This buckle fold shares similar structural characteristics with known higher-order buckle folds of the eastern SIC. We suggest that non-cylindrical buckling and associated mineral fabric development accommodated the shape change of the SIC. This resulted in mutually perpendicular fabric orientations, which are compatible with overall NW–SE shortening. The original shape of the SIC seems to have had a profound influence particularly on the geometry of metamorphic foliations. Our structural analysis supports earlier structural studies advocating that highly discordant planar strain fabrics can be generated by local strain perturbations near igneous sheets under uniform regional shortening.

Three dimensional fabric evolution of sheared sand

Granular particles undergo translation and rolling when they are sheared. This paper presents a three-dimensional (3D) experimental assessment of fabric evolution of sheared sand at the particle level. F-75 Ottawa sand specimen was tested under an axisymmetric triaxial loading condition. It measured 9.5 mm in diameter and 20 mm in height. The quantitative evaluation was conducted by analyzing 3D high-resolution x-ray synchrotron micro-tomography images of the specimen at eight axial strain levels. The analyses included visualization of particle translation and rotation, and quantification of fabric orientation as shearing continued. Representative individual particles were successfully tracked and visualized to assess the mode of interaction between them. This paper discusses fabric evolution and compares the evolution of particles within and outside the shear band as shearing continues. Changes in particle orientation distributions are presented using fabric histograms and fabric tensor.

Kinematic analysis using AMS data from a deformed granitoid

This study highlights the usefulness of anisotropy of magnetic susceptibility data from a deformed granitoid in deciphering its kinematic evolution vis-à-vis shear zone. Data are presented from the Chakradharpur Granitoid (CKPG) that lies to the north of the northerly dipping, ENE–WSW striking Singhbhum Shear Zone (SSZ; eastern India). Whilst the foliation recorded in the field in some parts of the granitoid is parallel to the SSZ, the magnetic foliation is N54°E/90° (mean orientation). It is suggested that the magnetic fabric provides a window into an evolutionary stage prior to the final shearing/thrusting event, the evidence of which is preserved on the mesoscopic scale. It is envisaged that during the initial stages of deformation there was simple shear along the evolving SSZ that resulted in sinistral strike-slip movement; the vorticity axis at this stage was steeply plunging and sense of rotation was anticlockwise. Space was generated in a direction ∼N25°E (perpendicular to maximum-Instantaneous Stretching Axis) into which CKPG emplaced synchronously with regional deformation and evolving SSZ. With continued deformation, there was thrusting along the SSZ. The vorticity axis flipped to a sub-horizontal orientation, thus leading to the development of down-dip stretching lineations and sheath folds within the SSZ. However, at the same time, the vorticity axis responsible for fabric evolution within the syntectonically crystallizing/cooling CKPG was steeply plunging with clockwise rotation. The magnetic foliation (mean orientation N54°E/90°) developed during the final stage of syntectonic crystallization. However, deformation in the region and thrusting along the SSZ continued even after the CKPG had fully crystallized and solidified, which led to the development of the ENE–WSW striking mesoscopic foliation that is parallel with the SSZ. We propose that the angle between the magnetic foliation and the SSZ/foliation recorded in the field, enables to decipher the kinematic vorticity number of flow responsible for fabric evolution of the CKPG. It is concluded that transpression was an important mechanism, and during regional deformation, whilst the SSZ developed structures by dominantly simple shear, the CKPG underwent dominantly pure shear.

Fabric evolution and accessible geometrical states in granular materials

We analyze the geometrical states of granular materials by means of a fabric tensor involving the coordination number and fabric anisotropy as the lowest-order descriptors of the contact network. In particular, we show that the fabric states in this representation are constrained by steric exclusions and the condition of mechanical equilibrium required in the quasi-static limit. A simple model, supported by numerical data, allows us to characterize the range of accessible fabric states and the joint evolution of fabric parameters. The critical state in this framework appears as a jammed state in the sense of a saturation of contact gain and loss along the principal strain-rate directions.

Induced fabric under cyclic and rotational loads in a strain space multiple mechanism model for granular materials

SUMMARYThe strain space multiple mechanism model idealizes the behavior of granular materials on the basis of a multitude of virtual simple shear mechanisms oriented in arbitrary directions. Within this modeling framework, the virtual simple shear stress is defined as a quantity dependent on the contact distribution function as well as the normal and tangential components of interparticle contact forces, which evolve independently during the loading process. In other terms, the virtual simple shear stress is an intermediate quantity in the upscaling process from the microscopic level (characterized by contact distribution and interparticle contact forces) to the macroscopic stress. The stress space fabric produces macroscopic stress through the tensorial average. Thus, the stress space fabric characterizes the fundamental and higher modes of anisotropy induced in granular materials. Herein, the induced fabric is associated with monotonic and cyclic loadings, loading with the rotation of the principal stress, and general loading. Upon loading with the rotation of the principal stress axis, some of the virtual simple shear mechanisms undergo loading whereas others undergo unloading. This process of fabric evolution is the primary cause of noncoaxiality between the axes of principal stresses and strains. Although cyclic behavior and behavior under the rotation of the principal stress axis seem to originate from two distinct mechanisms, the strain space multiple mechanism model demonstrates that these behaviors are closely related through the hysteretic damping factor. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Micromechanical study of fabric evolution in quasi-static deformation of granular materials

In micromechanical studies of granular materials, relations are investigated between macro-level, continuum characteristics and micro-level, (inter) particle characteristics. For quasi-static deformation of granular materials, the fabric tensor is an important micromechanical characteristic that describes the orientational distribution of contacts between particles. The evolution of the fabric tensor during quasi-static deformation has been investigated, using two-dimensional Discrete Element Method simulations of constant-pressure tests of a dense and a loose isotropic initial assembly. Invariants of the fabric tensor are the coordination number, i.e. the average number of contacts per particle, and the fabric anisotropy, i.e. the difference between the principal values of the fabric tensor. At large strains, both coordination number and fabric anisotropy become constant. This constant value is the same for the dense and the loose initial assembly, i.e. a critical fabric state is attained. The evolution towards the critical state value for coordination number is much faster than for fabric anisotropy. The three micromechanical mechanisms through which the fabric tensor can change are contact creation, contact disruption and contact reorientation. Contact disruption is the dominant mechanism (except at the steady state) that is strongly anisotropic. The rate of contact disruption decreases rapidly with increasing imposed strain. Contact creation is approximately an isotropic mechanism. The rate of contact creation is fairly constant over the range of imposed strains. Fabric change due to contact reorientation is small, except at the steady state. An evolution relation is proposed for the changes in the fabric tensor as a function of the fabric tensor and the strain increment tensor. With its coefficients obtained by fitting with the data for the fabric tensor from the Discrete Element Method simulations, good agreement is observed between results from these simulations and from the proposed fabric-evolution relation.

Does AMS data from micaceous quartzite provide information about shape of the strain ellipsoid?

Anisotropy of magnetic susceptibility (AMS) in micaceous quartzites with mean susceptibility (K m) >50 × 10−6 SI units is known to be on account of the orientation distribution of the para/ferromagnetic minerals (e.g. micas, magnetite), which comprise the minor phase in the rocks. However, the strain in such deformed micaceous quartzites is dominantly accommodated by the quartz grains, which are the major phase in them. The objective of this paper is to explore the extent to which AMS data from micaceous quartzites provide information about the shape of the strain ellipsoid. AMS analysis of 3 quartzite blocks is performed, and the shape of the AMS ellipsoid is recorded to be oblate. From AMS data, the three principal planes of the AMS ellipsoid are identified in each block and thin sections are prepared along them. Quartz grain shape (aspect ratio, R q), intensity of quartz and mica shape preferred orientation (κq and κmi, respectively) and 2D strain (E) recorded by quartz are measured in each section. R q, κq, κmi and E are all noted to be minimum in the section parallel to the magnetic foliation plane as compared to the other two sections. This indicates that the quartz grains have oblate shapes in 3D and accommodated flattening strain, which is similar to the shape of the AMS ellipsoid. The role of mica in causing Zener drag and pinning of quartz grain boundaries is discussed. It is concluded that during progressive deformation, migration of pinned grain boundaries is inhibited. This causes enhanced recrystallization at the grain boundaries adjacent to the pinned ones, thus guiding the shape modification of quartz grains. A strong correlation is demonstrated between κq and κmi as well as κmi and E. It is inferred that fabric evolution of quartz was controlled by mica. Hence, the shape of the AMS ellipsoid, which is on account of mica, provides information about shape of the strain ellipsoid.

Late Cretaceous extensional denudation along a marble detachment fault zone in the Kırşehir massif near Kaman, central Turkey

In the Central Anatolian Crystalline Complex (CACC), 100 km scale metamorphic domains were exhumed in a context of north-south plate convergence during late Cretaceous to Cenozoic times. The timing, kinematics and mechanisms of exhumation have been the focus of previous studies in the southern Niğde Massif. In this study, we investigate the unexplored northern area regarding the tectonic features preserved on the edges of the Kırşehir Massif, based on detailed field-mapping in the Kaman area where high-grade metasediments, non-metamorphic ophiolites and monzonitic plutons are locally exposed together. Close to the contact with the ophiolites, west-dipping foliated marble-rich rocks display mylonites and discrete protomylonites with normal shear senses indicating a general top-to-the W–NW direction. Both of these structures have been brittlely overprinted into cataclastic corridors parallel to the main foliation. The mylonite series and superimposed brittle structures together define the Kaman fault zone. The study of the evolution of calcite deformation fabrics along an EW section supported by Electron Back Scattered Diffraction measurements (EBSD) on representative fabrics indicates that the Kaman fault zone represents an extensional detachment. In Ömerhacılı, in the vicinity of the Baranadağ quartz-monzonite, the metamorphic sequence shows static annealing of the calcite mylonitic fabrics. This evidence suggests that intrusion took place at shallow depth (∼10 km) into an already exhuming metamorphic sequence. As a consequence for the Kaman area, buried metasediments have been rapidly exhumed between 84 and 74 Ma (∼1 km/Ma) where exhumation along a detachment zone, displaying a top-to-the W–NW shear motion, took place in the mid to upper crust prior to magmatic intrusion in the late Campanian. As the intrusion cut through the detachment fault, the main shearing deformation ceased. Brittle tectonics coupled with erosion likely took over during the final unroofing stages at a slower rate (<0.2 km/Ma), until the pertinent rocks reached the Earth’s surface in the late Paleocene.

Sand fabric evolution effects on drain design for liquefaction mitigation

This paper revisits the seminal work of Seed and Booker (1977) [21] on the design of infinitely permeable drains for liquefaction mitigation. It is shown that their basic mathematical assumption for the rate of earthquake-induced excess pore pressure generation overlooks sand fabric evolution effects during cyclic loading and eventually leads to underestimation of the drain effectiveness. This is because such effects cause peak excess pore pressures to be attained at the early stages of partially drained shaking, followed by a gradual attenuation even if shaking continues undiminished, a response feature not predicted by the original formulation. In addition, special emphasis is given to the analytical relation describing the excess pore pressure build-up until liquefaction in undrained tests. This relation was considered unique in the original work, for reasons of simplicity, thus neglecting sand fabric evolution effects that may differentiate it for various sands, densities and loading conditions. Hence, a revised analytical formulation is proposed, which takes into account both above effects of sand fabric evolution. The paper provides a quantitative assessment of their influence on drain effectiveness and establishes a new set of charts for drain design. Experimental measurements from shaking table tests, as well as robust numerical simulations are shown, which underline the necessity for the revised solution and design charts.

A full Stokes ice flow model for the vicinity of Dome Fuji, Antarctica, with induced anisotropy and fabric evolution

Abstract. A three-dimensional, thermo-mechanically coupled ice flow model with induced anisotropy has been applied to a ~200 × 200 km domain around the Dome Fuji drill site, Antarctica. The model ("Elmer/Ice") is based on the open-source multi-physics package Elmer (http://www.csc.fi/elmer/) and solves the full Stokes equations. Flow-induced anisotropy in ice is accounted for by an implementation of the Continuum-mechanical, Anisotropic Flow model, based on an anisotropic Flow Enhancement factor ("CAFFE model"). Steady-state simulations for present-day climate conditions are conducted. The main findings are: (i) the flow regime at Dome Fuji is a complex superposition of vertical compression, horizontal extension and bed-parallel shear; (ii) for an assumed geothermal heat flux of 60 mW m−2 the basal temperature at Dome Fuji reaches the pressure melting point and the basal melting rate is ~0.35 mm a−1; (iii) in agreement with observational data, the fabric shows a strong single maximum at Dome Fuji, and the age of the ice is decreased compared to an isotropic scenario; (iv) as a consequence of spatially variable basal melting conditions, the basal age tends to be smaller where the ice is thicker and larger where the ice is thinner. The latter result is of great relevance for the consideration of a future drill site in the area.

An insight into the water retention properties of compacted clayey soils

Experimental data from different testing methodologies on different compacted clayey soils, with dominant bimodal pore size distribution, are presented and analysed, to provide a comprehensive picture of the evolution of the aggregated fabric along hydraulic and mechanical paths. Fabric changes are analysed both from the porous network viewpoint, by means of careful mercury intrusion porosimetry investigation, and from the soil skeleton viewpoint, by quantifying swelling and shrinkage of the aggregates in an environmental scanning electron microscopy study. The consequences of the aggregated fabric evolution on the water retention properties of compacted soils are analysed and discussed. A new model for water retention domain is proposed, which introduces a dependence of the intra-aggregate pore volume on water content. The model succeeds in tracking correctly the evolution of the hydraulic state of the different soils investigated along generalised hydromechanical paths. The proposed approach brings to light coupling between intra-aggregate and inter-aggregate pores in the retention properties of compacted clayey soils. Dependence of the air entry and the air occlusion values on swelling and shrinking of aggregates, besides void ratio, is introduced and discussed.

THM analysis of a large‐scale heating test incorporating material fabric changes

SUMMARYEngineered barriers are basic elements in the design of repositories for the isolation of high‐level radioactive waste. This paper presents the thermo‐hydro‐mechanical (THM) analysis of a clay barrier subjected to heating and hydration. The study focuses on an ongoing large‐scale heating test, at almost full scale, which is being carried out at the CIEMAT laboratory under well‐controlled boundary conditions. The test is intensely instrumented and it has provided the opportunity to study in detail the evolution of the main THM variables over a long period of time. Comprehensive laboratory tests carried out in the context of the FEBEX and NF‐PRO projects have allowed the identification of the model parameters to describe the THM behaviour of the compacted expansive clay. A conventional THM approach that assumes the swelling clay as a single porosity medium has been initially adopted to analyse the evolution of the test. The model was able to predict correctly the global THM behaviour of the clay barrier in the short term (i.e. for times shorter than three years), but some model limitations were detected concerning the prediction of the long‐term hydration rate. An additional analysis of the test has been carried out using a double structure model to describe the actual behaviour of expansive clays. The double structure model explicitly considers the two dominant pore levels that actually exist in the FEBEX bentonite and it is able to account for the evolution of the material fabric. The simulation of the experiment using this enhanced model provides a more satisfactory reproduction of the long‐term experimental results. It also contributes to a better understanding of the observed test behaviour and it provides a physically based explanation for the very slow hydration of the barrier. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Plastic anisotropy and fabric evolution in sheared and recrystallized quartz single crystals

[1] The effect of a lattice preferred orientation on the flow strength of quartz aggregates dynamically recrystallized from single crystals of synthetic quartz was investigated using general shear experiments in a Griggs apparatus. Experiments were conducted at shear strains (γ) up to 5 at a temperature of 900°C, confining pressure of 1.5 GPa, and shear strain rate of 10−5 s−1. Three starting orientations of crystal were used, to activate three slip systems: basal 〈a〉, prism [c], and prism 〈a〉, although slip-induced rotation of the crystal axes in the first two orientations led to the activation of additional slip systems. For crystals with higher water contents, basal 〈a〉 and prism 〈a〉 orientations are relatively weak and prism [c] orientations are stronger. All three initial crystal orientations undergo dynamic recrystallization with increasing shear strain, although the strain required for 100% recrystallization varies: γ ≈ 2 for prism [c] slip, γ ≈ 3.8 for basal 〈a〉 slip, and γ ≈ 5 for prism 〈a〉 slip. For all three starting orientations, distinct domains of recrystallized grains develop with c axes parallel to Y of the strain ellipsoid (Ymax), replacing recrystallized grains of other orientations; the Ymax domains increase in size with increasing strain. In addition, strain markers show that strain is highly localized within the Ymax domains, indicating geometrical softening of up to an order of magnitude in effective viscosity.