Jean-Dominique Barnichon

Characterizing the mechanical behaviour of the Tournemire argillite

Abstract A laboratory programme of uniaxial, triaxial, cyclic and Brazilian tests was conducted to investigate the anisotropic mechanical behaviour of the Tournemire argillite, with different axial loading orientations with respect to the bedding planes (i.e. loading orientation angle, θ=0°, 30°, 45°, 60° and 90°). The experimental results show that both strength and deformation of the argillite are direction-dependent. Failure occurs in a brittle manner with a sudden collapse of the material strength. The failure mode exhibits localization along distinct failure planes and also depends on the loading orientation. This paper summarizes the experimental results and describes constitutive relationships that were developed in order to simulate the stress–strain behaviour of the Tournemire argillite. A microstructure tensor approach is adopted in order to take into account the anisotropic behaviour of the argillite. The identification procedure for material function and parameters is outlined, and the model is applied to simulate the set of triaxial tests performed at different levels of confining pressure and orientation of the bedding planes. It is demonstrated that the model adequately reproduces the anisotropy, the pre-peak stress–strain response and the onset of material collapse in those tests.

Swelling of Highly Compacted Bentonite-Sand Mixtures Used as Sealing Materials in Radioactive Waste Disposal

In the radioactive waste disposal concepts, highly pre-compacted bentonite-sand mixtures are considered as appropriate sealing materials to fill the galleries. Compacted elements of bentonite-sand mixtures are placed adjacent to each others, forming a plug that should limit the transfer of radionuclide. The bentonite has proven to have the capability of retaining radionuclides and the capability of swelling when getting in contact with water and then closing all the voids in the system especially the radial technological void being the gap between the bentonite-sand elements and the host rock. Indeed, when the bentonite-sand elements is emplaced in the gallery, they are first in an unsaturated state and they will start being saturated by the host-rock pore water. They will then start swelling, forming a gel that will close the gap. In this work, such hydration process is experimentally analysed on a small scale. The swelling is analysed by time-lapse photography, its kinetic is also studied by image processing. We can see that the swelling of this material is fast and large, and we can distinguish different states of the material during swelling, starting from the dry centre to the outer boundary. On the other hand, the presence and behaviour of sand grains in the gel formation is evidenced by images as well. The sand grains are found to be covered by bentonite particles and being carried with the bentonite swelling.

Infiltration Column for Studying the Lateral Swell Behavior of Expansive Clay

Infiltration column is usually used to investigate the hydro-mechanical processes in unsaturated expansive clay. In this test, the moisture transfer is often monitored along the column during water infiltration using suction or water content probes. Nevertheless, the lateral swelling pressure developed is rarely considered. This paper describes an infiltration column for studying the lateral swell behavior of expansive clay. The column consists of a rigid cell ensuring the constant-volume condition and a hydraulic system enabling the water intake of the hosted sample. It is equipped with three types of sensors: pressure sensors, force transducers, and displacement transducers to, respectively, monitor the radial and axial swelling pressure of the sample at different positions and to check whether any axial displacement is taking place. A detailed description of the different parts of the cell is first presented. Second, analysis on the results of a test on a compacted bentonite/sand mixture allows the pertinence of such a device to be evaluated.