Micromechanisms of inelastic deformation in sandstones: An insight using x-ray micro-tomography

This study investigates the grain-scale mechanisms that lead to failure by strain localisation in specimens of Fontainebleau sandstone with different degrees of cementation. While the effects of inter-particle bonding on the mechanical behaviour of granular geomaterials, including soft rocks, have been largely studied, the physical micro-scale mechanisms governing the material deformation are still poorly understood. In this study, laboratory techniques have been developed to allow a non-invasive investigation of the internal deformation of sandstones during triaxial compression to failure. The material investigated was Fontainebleau sandstone, a quartzite formation from the Paris basin (France), which can be found as very hard, tightly cemented sandstone or more porous and less cemented material. Specimens with porosities of 6 and 21% were investigated. Triaxial compression tests at confining pressures of 2 and 7 MPa were conducted on dry cylindrical specimens of 11 mm diameter and 22 mm height. Three-dimensional (3D) images of the full specimens were obtained by carrying out x-ray micro-tomography scans at key points throughout the test. The high-resolution 3D tomographic images have a voxel size of 8.5 μm (0.033d50), allowing clear identification of the grains. This analysis suggests that dilatancy of the material, which depends on the degree of bonding between grains, plays a fundamental role in the failure mode of granular media. Insights into bonding rupture mechanisms and grain damage by inter- and intra-granular cracking are presented

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

An experimental study of micro-scale deformation in a soft sandstone

This study investigates the grain scale mechanisms that lead to failure by strain localisation in a slightly cemented sandstone. While the effects of interparticle bonding on the mechanical behaviour of granular geomaterials, including soft rocks, have been largely studied, the physical micro-scale mechanisms governing the material deformation are still poorly understood. In this study, laboratory techniques have been developed to allow a non-invasive investigation of the internal deformation of a soft sandstone during triaxial compression to failure. The material studied is Fontainebleau sandstone, a quartzite formation from the Paris Basin, with a porosity of 21 %. Fontainebleau sandstone is a uniformly graded material with a mean grain size of 260 m. Triaxial compression tests at confining pressures of 2 MPa and 7 MPa were conducted on dry cylindrical specimens of 11 mm diameter by 22 mm height. Three-dimensional images of the full specimen were obtained by carrying out x-ray micro-tomography scans at key points throughout the test. The high-resolution tomographic images have a voxel size of 8.5 m (0.033d50) allowing a clear identification of the grains. A preliminary analysis has suggested that the failure mechanisms can be different for the samples tested at different confining pressures, which has a direct implication on the permeability of the material

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

Microstructure evolution on sandstones with different degrees of cementation

This study investigates the grain scale mechanisms that lead to failure by strain localisation in specimens of Fontainebleau sandstone with different degrees of cementation. While the effects of inter-particle bonding on the mechanical behaviour of granular geomaterials, including soft rocks, have been largely studied, the physical micro-scale mechanisms governing the material deformation are still poorly understood. In this study, laboratory techniques have been developed to allow a non-invasive investigation of the internal deformation of sandstones during triaxial compression to failure. The material investigated was Fontainebleau sandstone, a quartzite formation from the Paris Basin (France) which can be found as very hard, tightly cemented sandstone or more permeable and less cemented material. Specimens with porosities 6% and 21% were investigated. Triaxial compression tests at confining pressures of 2MPa and 7MPa were conducted on dry cylindrical specimens of 11mm diameter by 22mm height. Three-dimensional images of the full specimen were obtained by carrying out x-ray micro-tomography scans at key points throughout the test. The high-resolution tomographic images have a voxel size of 8.5um (0.033d50) allowing a clear identification of the grains. The analysis suggests that dilatancy of the material which depends on the degree of bonding between grains, plays a fundamental role on the failure mode of the granular media

A microstructural finite element analysis of cement damage on Fontainebleau Sandstone

This paper presents a numerical simulation that uses tomographic data to reproduce the grain-scale mechanisms taking place during deformation of Fontainebleau sandstone. Previous investigation using x-ray tomographic images acquired during triaxial compression has highlighted the role of bonding rupture mechanisms in the failure mode of the material. The model the deformation of the sandstone, images of the internal topology were used to generate an image-based finite element mesh and the grain-scale phenomena such as, the opening and propagation of the cracks associated with the debonding of the cemented grains, were reproduced using a simple constitutive model

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

Vol. 57 No. 3