Large-scale failure prediction of clay rock from small-scale damage mechanisms of the rock medium using multiscale modelling

Clay rocks are multiphase porous media whose complex structure is characterised by heterogeneity and possible anisotropy on a wide range of scales. The mesoscopic scale plays a particular role in deformation mechanisms under mechanical loading by cracking. The behaviour of rocks at mesoscale is characterised by the material and morphological (shape and size) properties of its components and their interactions. The accurate reproduction and influence of these mesoscale characteristics on the material behaviour and damage at large scale remain a complex issue. This question becomes crucial when investigating the underground stability during excavation works such as tunnels. In this numerical multi-scale study, the mesostructure characteristics are embedded in a Representative Elementary Area (REA) in a 2D configuration. A double-scale numerical framework, with finite element resolution at both scales (FE2) and computational homogenisation, is considered. The influence of the mesostructural characteristics of a heterogeneous rock and the effect of different inter-granular properties on their macroscopic behaviour, are examined. Additionally, a predictive strategy which is based on the connection between the failure modes of the REA and the failure mechanisms of the macroscale structure is also presented. This study investigates the effect of the mesocracking on the shear banding in a rock specimen during laboratory biaxial shear test and the development of the Excavation Damaged Zone (EDZ) around tunnels. The objective of this work is to explain the failure mechanisms observed up to the engineering scale of underground structures through the morphological and material small-scale characteristics of the REA.</p

Large-scale failure prediction of clay rock from small-scale damage mechanisms of the rock medium using multiscale modelling

Clay rocks are multiphase porous media whose complex structure is characterised by heterogeneity and possible anisotropy on a wide range of scales. The mesoscopic scale plays a particular role in deformation mechanisms under mechanical loading by cracking. The behaviour of rocks at mesoscale is characterised by the material and morphological (shape and size) properties of its components and their interactions. The accurate reproduction and influence of these mesoscale characteristics on the material behaviour and damage at large scale remain a complex issue. This question becomes crucial when investigating the underground stability during excavation works such as tunnels. In this numerical multi-scale study, the mesostructure characteristics are embedded in a Representative Elementary Area (REA) in a 2D configuration. A double-scale numerical framework, with finite element resolution at both scales (FE2) and computational homogenisation, is considered. The influence of the mesostructural characteristics of a heterogeneous rock and the effect of different inter-granular properties on their macroscopic behaviour, are examined. Additionally, a predictive strategy which is based on the connection between the failure modes of the REA and the failure mechanisms of the macroscale structure is also presented. This study investigates the effect of the mesocracking on the shear banding in a rock specimen during laboratory biaxial shear test and the development of the Excavation Damaged Zone (EDZ) around tunnels. The objective of this work is to explain the failure mechanisms observed up to the engineering scale of underground structures through the morphological and material small-scale characteristics of the REA.</p

Etude de la localisation de la déformation à l'aide de modèles second gradient

Cette étude explore le comportement des matériaux en régime de post-localisation, elle traite de certains aspects qui doivent être pris en compte dès lors que l'on cherche à modéliser le phénomène de localisation. On ne peut plus se restreindre à trouver une solution à un problème donné, on doit aussi se poser la question de l'existence éventuelle de plusieurs solutions au même problème. En effet, déterminer la solution la plus critique au problème de l'ingénieur peut nécessiter d'explorer différentes solutions. Puis des résultats avec localisation sont présentés pour différents régimes de comportement dans les géomatériaux, régime fragile ou ductile, montrant des similitudes avec les observations expérimentales de laboratoire

The Alestle - Vol. 57 No. 30 - 01/11/2005

Vol. 57 No. 3

MECHANICAL BEHAVIOUR AND RUPTURE IN CLAYEY ROCKS STUDIED BY X-RAY MICRO-TOMOGRAPHY AND 3D-DIGITAL IMAGE CORRELATION

International audienceThe mechanical behaviour and the rupture of clayey rocks have been experimentally studied by performing in situ triaxial tests on a synchrotron beamline i.e. performing X-ray micro tomography scans under mechanical loading. The 3D images obtained at different steps of the test were then analysed by 3D-Digital Image Correlation method in order to measure incremental strain fields. The results allow to clearly detect the onset of shear strain localization and to characterize its development in a 3D complex pattern

A biaxial apparatus for the study of heterogeneous and intermittent strains in granular materials

We present an experimental apparatus specifically designed to investigate the precursors of failure in granular materials. A sample of granular material is placed between a latex membrane and a glass plate. A confining effective pressure is applied by applying vacuum to the sample. Displacement-controlled compression is applied in the vertical direction, while the specimen deforms in plane strain. A Diffusing Wave Spectroscopy visualization setup gives access to the measurement of deformations near the glass plate. After describing the different parts of this experimental setup, we present a demonstration experiment where extremely small (of order $10^{-5}$) heterogeneous strains are measured during the loading process

Experimental characterisation of (localised) Deformation Phenomena in Granular Geomaterials from Sample Down to Inter-and Intra-grain Scales

AbstractThis paper outlines some recent advances in the full-field experimental characterisation of the mechanics of granular geomaterials (in particular, sands) using a range of methods that provide characterisation at different scales, from the sample-scale down to the inter- and intra-grain scale. The techniques used are “full-field” approaches involving in-situ x-ray micro-tomography, 3D-volumetric digital image analysis/correlation and grain ID-tracking, in-situ 3D x-ray diffraction and in-situ, spatially-resolved neutron diffraction. These methods provide new data on the mechanics of sand at different scales, including continuum measures of strain, porosity, and fabric plus discrete measures of particle kinematics and force transmission. The results of such measurements might be used to advance higher-order continuum theories, and provide the necessary input parameters, or to calibrate discrete grain-scale simulations of sand behaviour to explore loading paths that are inaccessible in the laboratory

Caractérisation expérimentale de la déformation et de la rupture localisée dans une argilite

Nous présentons une étude expérimentale de caractérisation de la rupture par localisation de la déformation dans une roche argileuse. Des essais triaxiaux ont été effectués sur une ligne de lumière de l'ESRF pour obtenir des reconstructions microtomographiques en volume à différents stades de l'essai. Une mesure du champ de la déformation a été faite en associant aux images obtenues une méthode de corrélation d'images en volume. Ces résultats permettent d'identifier très clairement le seuil d'initiation de la localisation, et le mode de déformation dans les bandes