Simon Salager

Compression tests on a sandy silt at different suction and temperature levels.

This paper presents a unified thermo-mechanical experimental study on a remoulded unsaturated sandy silt and brings a contribution to the understanding of the fundamental mechanics of unsaturated soils in non- isothermal conditions. The experimental program was carried out at four temperatures and four suction levels using two thermo-hydro-mechanical (THM) cells, one isotropic and the other oedometric. The effect of suction and temperature on the compressibility and on the apparent preconsolidation pressure of the soil is addressed. Finally, an analytical expression of the evolution of the apparent preconsolidation pressure with respect to temperature and suction is proposed.

Water Retention Behaviour Explored by X-Ray CT Analysis

This study aims to experimentally characterise the link between partial water saturation and suction in a sand sample at the micro scale. The paper presents the first results of an experimental study in which high resolution (7.5μm/px) X-ray tomography has been performed on a small (10×10mm) cylindrical sample of Hostun HN31 sand at several different levels of imposed suction. A specialised cell allowing X-ray scanning as well as fine control of suction (imposed both by negative water pressure as well as by positive air pressure) has been developed for this study and is described herein. The 3D images resulting from X-ray tomography are treated in order to define each voxel in the image as either air, water or grain. From these “trinarised” 3D images, local and global values of porosity and degree of saturation are then measured. This method enables the study of water retention behaviour of sand at the grain scale, all the while allowing characterisation of water retention in the sample as a whole.

Application of X-ray Tomography to the Characterisation of Grain-Scale Mechanisms in Sand

X-ray micro-tomography allows 3D imaging at sufficiently high spatial resolution to distinguish all the individual sand grains in a small sample (10mm diameter), as well as the distribution of air and/or water at this scale. Since this imaging technique is completely non-destructive, an imaged sample can be made to evolve by controlling some relevant variable (e.g., imposed deformation, suction), and can subsequently be re-imaged. This allows processes to be followed in 4 dimensions (3D + relevant variable). This paper shows the application of this technique and philosophy to the study of two different phenomena: localised deformation resulting from imposed triaxial compression, and the water retention behaviour of sand. The experimental techniques and setups for these two studies are detailed, and the fundamental steps of image treatment are outlined. Some key results are given to demonstrate the power of this “full-field” characterisation approach, such as rotations and displacements for each of the 50,000 grains of a sample in which a shear band occurs as well as the evolution of local measurements of porosity and degree of saturation in a sand where suction is being varied.

Testing the Thermo-Hydro-Mechanical Behaviour of a Shale

Keywords: Shales ; THM behaviour ; exerimental facilities ; Opalinus Clay Reference EPFL-CONF-174656 Record created on 2012-02-02, modified on 2016-08-09

Anisotropic Mechanical Response of a Shale

The purpose of the study is to characterize the main features of the anisotropic mechanical behaviour of a shale (Opalinus Clay), and to identify an anisotropic constitutive framework adapted to the response of the material under mechanical perturbations. Such a constitutive model is required for the simulation of triaxial tests which were done on OC samples. Firstly, the large quantity of available laboratory tests which characterize the mechanical response of OC has been analyzed. The test series provide clear evidence for anisotropic mechanical behaviour of Opalinus Clay. It appears that the stress history and the micro-structure of the material induce a typical response of the material which is more overconsolidated (higher rigidity, less ductile behaviour…) for loading direction parallel to the bedding plane than perpendicular to it. Concerning numerical investigations, the capabilities of the model of Hujeux have been assessed in reproducing the anisotropic features of behaviour of OC. The constitutive model uses the theory of multi-mechanisms plasticity. Two fundamental observations account for the adequacy of the chosen constitutive framework: the elastic moduli depend on the direction of shearing, and induced anisotropy will affect the way the material hardens or softens depending on the angle of shearing.

Design of a complete characterization of bentonite behaviour under high pressure and high temperature

Repositories in deep clay geological formations are considered one of the most promising solutions for a sustainable management of High Level radioactive Waste (HLW). The Swiss HLW disposal concept consists in horizontal tunnels excavated at high depth in strongly over consolidated clay (Opalinus Clay) where Granular Bentonite is chosen for enclosing steel canisters containing the waste. This paper presents the theoretical approach and the research activities aiming at investigating the behaviour of this material. An elasto-plastic constitutive model taking into account coupled processes of stress, capillary pressure, and temperature is used. In this framework, from an experimental point of view, an exhaustive characterization is necessary in order to calibrate required parameters and to validate the model. Laboratory tests designed for this purpose are described. Particular attention is paid in investigating the swelling behaviour, which is among the features that distinguish materials potentially usable in nuclear waste disposals. First results confirm indeed that the chosen bentonite shows a natural and remarkable swelling attitude.

Analysis of the swelling pressure development in opalinus clay – experimental and modelling aspects

In the context of nuclear waste geological storage, deep argillaceous formations are likely to be subjected to complex mechanical, hydraulic, and thermal loads. In particular, the argillaceous material can be firstly dried, and then re-wetted. During the latter process, the material experiences swelling and can develop swelling pressure if swelling deformations are constrained. In this contribution, the results of swelling pressure tests on shale performed in totally constrained conditions (isochoric tests) are presented. A constitutive model (ACMEG-S) is used to predict the value of the swelling pressure in such conditions. The model is made of two parts. The mechanical part addresses the stress-strain behaviour of the material, as a result of effective stress variation. An elasto-plastic approach is employed, and Bishops unsaturated effective stress, which is a function of the degree of saturation, the suction and the externally applied stress, is used as the mechanical stress. The water retention part of the model defines the relation between the degree of saturation and the suction within the material. The results put into light some factors that control the swelling pressure value, in particular the degree of saturation and the plastic behaviour of the material.