S. Chanchole

Micro-scale experimental investigation of the swelling anisotropy of the Callovo-Oxfordian argillaceous rock

Abstract An experimental study of the swelling anisotropy of the Callovo-Oxfordian argillaceous rock under hydration is presented. The investigation, which combines environmental scanning electron microscopy (ESEM) and digital image correlation techniques, has been carried out at the micrometric scale of the composite microstructure of the rock. Specimens were hydrated in the ESEM over a wide range of relative humidity and observations conducted on two planes: plane 1 parallel to the bedding plane, and plane 2 perpendicular to it. The observations reveal that the local swelling (which can be quantified at a local gauge length of about 5 μm) is strongly anisotropic in both planes. The global swelling, measured over areas of about 500 μm in width, is also clearly anisotropic in plane 2 (with major swelling direction normal to the bedding plane), but not in plane 1. The global isotropy in plane 1 arises from the uniform distribution of the orientation of anisotropic local strains, while the anisotropic swelling in plane 2 is due to a preferred local orientation.

Experimental device for chemical osmosis measurement on natural clay-rock samples maintained at in situ conditions: implications for formation pressure interpretations.

In order to characterize the so-called coupled processes occurring in compacted clay rocks, the coupling coefficients must be identified. For this purpose, an original device which allows such measurement for undisturbed (natural) samples in their in situ conditions was developed. The present experimental device minimizes the fluid leaks improving the accuracy of the coupling parameter determination. Three chemical osmotic tests were performed on a cylindrical sample of Callovo-Oxfordian argilite. Room temperature variations during the chemical osmosis experiments required the implementation of temperature effects in the numerical model used for the interpretations. These variations offered the opportunity of an alternative method to estimate the compressibility of the fluid in the circuit connected to a measurement chamber located in the center of the sample. An osmotic efficiency of almost 0.2 for a concentration of 0.094 mol L(-1) is obtained for the Callovo-Oxfordian argilite. This value would explain only some part (approximately 0.10-0.15 MPa) of the overpressures (0.5-0.6 MPa) relative to the surrounding reservoirs measured in this formation. Others processes, such as thermo-osmosis, hydrodynamic boundary condition changes due to climate variations or creep behavior of the shale, could explain the remainder of the overpressures.

Study of the Anisotropic Properties of Argillite Under Moisture and Mechanical Loads

Due to various factors, such as sedimentation, layered morphology of clay minerals, in situ stress, etc., argillite rocks often exhibit anisotropic behavior. In order to study the anisotropic properties of the Callovo-Oxfordian (COx) argillite of the Meuse–Haute-Marne site in France considered as a possible host rock for high-level radioactive nuclear waste repository, a series of tests including uniaxial compression and dehydration and hydration at different constant applied stress levels are carried out. In this study, a specific setup combining moisture and mechanical loading with optical observation is used and it allows to continuously capture surface images from which the full-field strains are determined by using Digital Image Correlation techniques. The results show evidence of the mechanical and hydric anisotropy of the material. The anisotropy parameters are identified, assuming the studied argillite as transversely isotropic. The shrinkage and swelling depend on the applied stress and the angle with respect to the vertical direction of the mechanical load and the stratification plane, and this dependence is quantified. The non-linearity and the hysteresis observed during dehydration and hydration cycles are discussed.

Microscale insight into the influence of humidity on the mechanical behavior of mudstones

The mechanical behavior of mudstones strongly depends on humidity. In this paper, we present some microstructural insights into this phenomenon gained from a microscale investigation using a novel experimental method. The experimental method consists of combined hydric and mechanical loading tests in environmental scanning electron microscopy, as well as full-field strain measurement by digital image correlation techniques. The sample is subjected to a stepwise wetting (21%, 80%, and 99% relative humidity); for each equilibrium moisture state, a uniaxial compression test is performed. The microscale observation reveals that humidity-induced changes in the mechanical behavior of mudstones are controlled by the deformation and microcracking upon wetting. With increasing relative humidity, expansion of pores causes the clay matrix to be softer. In addition, because of the reduction in shear modulus and the lessening of capillary effect, shear bands are prone to appear at a high humidity state. The microcracking upon wetting, which results in predamage of the material, also affects the mechanical behavior. Finally, the sample with more moisture exhibits a more ductile behavior that involves more pronounced microcracking at failure.

The Mechanisms of Deformation and Damage of Mudstones: A Micro-scale Study Combining ESEM and DIC

Combining environmental scanning electron microscopy (ESEM) and digital image correlation techniques, the mechanical behaviour of mudstones is studied at the scale of their composite microstructure (that is, grains of carbonate and quartz embedded in a clay matrix). A specially designed apparatus is developed to allow in-situ uniaxial compression tests on samples with controlled humidity states in the ESEM chamber. As the mechanical behavior of mudstones is sensitive to water content, two tests on samples with contrasting water contents (3.8 and 7.4 %) are performed to identify the unified mechanisms of deformation and damage. We illustrate heterogeneous local strain fields that well correlate with the microstructure of mudstones. Three types of deformation bands involving different mechanisms have been classified: orthogonal (compaction of macro-pores and closure of pre-existing cracks), parallel (micro-cracking) and inclined (shear deformation) to the uniaxial compression direction. These deformation modes are activated at different stress levels, and they strongly interact: for instance, a high-strained shear band may result in tensile micro-cracks at its tip. We also illustrate damage phenomena, particularly at the inclusion-matrix interface, which is found to be a hazardous position for nucleation of micro-cracks.

Micro-Scale Experimental Investigation of Deformation and Damage of Argillaceous Rocks under Hydric and Mechanical Loads

Argillaceous rocks are possible host rocks for underground nuclear waste repositories. Such rocks actually exhibit multi-scales heterogeneities, among which the scale of composite microstructure (clay matrix + mineral inclusions) is particularly interesting: the complex matrix-inclusion interaction under hydric and mechanical loadings is a potential source of damage. One important interest of this work is in the insitu investigations at such a micro-scale, based on the combination of high resolution imaging by environmental scanning electron microscope and digital image correlation techniques. The results show complex localization patterns of deformation as well as some damage phenomena, both under purely hydric and combined mechanic/hydric loadings. Based on these observations, the deformation mechanisms can be identified, and the validation of standard poromechanics to describe the hydromechanical behavior of such rocks is discussed.

Conception d'un essai triaxial à succion contrôlée : mesure des déformations

Experimental study of mechanical behaviour of clayey materials under hygrometric condition is usually performed either on unloaded samples or by means of classical oedometer tests used in soil mechanics. Such methods are not well adapted to hard deep clayey rocks with little deformability, porosity and permeability. Moreover, stress and strain tensors having a significant effect on hygro-mechanical behaviour and properties cannot be measured and investigated appropriately by classical tests. This is why a specific triaxial test was designed in which the sample is surrounded by a fiber glass tissue allowing air circulation and then by silicon on which confining pressure is applied. Thus, equilibrium between air and sample was reduced. Stress and strain tensors were also measured in time on the sample subjected to a mechanical loading and to a controlled suction. After presentation of the test, preliminary results are given.