Description of work
The set-up and the conditions of the laboratory experiments proposed in this sub-workpackage will be adjusted, if necessary, in strong collaboration between the involved partners during the course of the project. Partner 4 will work on Callovo-Oxfordian Argilitte and partners 8, 9, 10 on Boom Clay and Opalinus Clay.
The tests to be performed by different partners are interactive and complementary. The guideline is to perform a series of tests under different controlled conditions following different solicitation paths that cover the temperature, stresses, pore pressure/saturation and time variation independently. Globally, the following tests are proposed to reach the objectives.
- Triaxial shear tests upon heating/cooling : UJF
- Triaxial shear tests upon heating/cooling at different saturation states: EPFL
- Drying/wetting tests (desaturation/resaturation process) at different temperatures: EPFL
- Triaxial shear tests under different constant temperatures : ENPC, GRS
- Triaxial shearing tests with different loading rates under different constant temperatures : Viscoplastic behaviour under high temperature : ULg
- Permeability tests (permeameter) with different artificial fractures upon heating and/or cooling under different chemical conditions: SCK.CEN
- Permeability tests (isostatic cell) with different artificial internal holes under increasing isotropic pressure and temperatures conditions : SCK.CEN
The maximum temperature considered in the triaxial tests upon heating is about 80°C due to technical limitations. Since the consortium thinks that the temperature increase rate (investigated in WP4) is a more penalising criteria than the maximum temperature, no triaxial tests on previously heated samples at higher temperature are foreseen. This point will be debated at the first end-users workshop.
Below is detailed description of the tests to be performed by the different partners.
Triaxial shear tests upon heating/cooling :
UJF will perform triaxial tests to study the impact of heating/cooling on the process of strain localization. More specifically, the effects of a thermal loading on the damaged clay and the damage possibly induced by the thermal loading itself will be studied. A total number of 8 tests (4 on Opalinus Clay and 4 on Boom Clay) will be performed in the high pressure, stress-path triaxial cell at UJF.
Mechanical and thermal loading programs will be defined similar to what occurs around a gallery during the different repository stages: a) an excavation-like stress path (with development of fractures); b) heating , and c) cooling phase. Some improvements to the testing device and procedures developed in the previous European project SELFRAC will be necessary in order to include the possibility to apply a thermal loading to the specimen.
Permeability measurements will be performed at different test phases and should allow for a direct measurement of the effects of the temperature on the extent of the zone of localized damage (which is in principle correlated to the flow properties).
Triaxial shear tests upon heating/cooling at different saturation states :
EPFL will run heating-cooling tests in triaxial conditions at different initial stress and saturation conditions. The goal is to quantify the effect of the thermal cycle on the damage for various degrees of saturation.
Drying/wetting tests at different temperatures:
EPFL will run drying/wetting tests under oedometer conditions at different temperatures to study the thermal influence on the desaturation/resaturation process. The samples will then be submitted to mechanical loads. The results of complementary experiments in progress on Boom Clay at CIMNE (PhD thesis not charged to the project) will be available for the TIMODAZ project. Additional tests will be also carried out to characterise the microstructure of the studied clays at different states. Special attention will be given to the thermal impact on the microstructure.
Triaxial shear tests under different constant temperatures :
ENPC and GRS will realise tests investigating the coupling between damage and water & gas transfer properties of clays at various temperature. Hollow cylinder tests are especially adapted to study this issue when the fracture is passing through the lateral walls of the sample, which is mostly the case in triaxial test. The tests proposed are therefore complementary to the tests described in the previous section. The principle of the test is to develop a temperature controlled hollow cylinder apparatus to be sheared in triaxial conditions. The hollow cylinder sample is designed to allow for running radial permeability tests during shearing (or at various levels of constant shear stress). Hence, the thickness of the hollow cylinder sample would be smaller than existing devices to allow for satisfactory permeability measurements across the cylinder. It is believed that induced fractures would cross the sample thickness more easily than in standard triaxial samples and would allow satisfactory investigation of coupled damage permeability phenomena. The temperature controlled system planned is similar to that used in previous coupled thermo-mechanical studies. ENPC will work on Boom Clay and Opalinus Clays and GRS on Callovo-Oxfordian Argilitte.
Triaxial shearing tests with different loading rates under different constant temperatures :
A number (a minimum of 10) of triaxial tests will be performed, mainly on Boom clay, to further investigate the viscoplastic or viscoelastoplastic behaviour of clay. Tests will be performed at different temperature (20 to 100 °C) and loading rates (around 10-3 to 10-5 MPa/s). For temperature lower then 60°C, sonic velocities will be measured. This work will be performed by ULG and will focus on Boom Clay as viscosity effects are more important than for the indurate clays.
Permeability tests (permeameter) :
The SCK CEN will perform several permeability tests in a permeameter cell (constant volume) on Boom Clay and Opalinus Clay, to evaluate the combined effect of temperature and geochemistry of the pore water on the sealing capacity of the clays. To this end, permeameter cells will be designed to perform permeability tests at increased temperatures or at temperature cycli. The clay samples will be introduced with artificial fractures and the follow-up of permeability will give indications on the time evolution of the sealing process. Moreover, the design of these permeameter cells will allow the use of µCT in order to follow the fracture aperture and density changes within the object during the permeability test. The different chemical conditions that will be considered are: Chemically undisturbed environment, Oxidised environment and Alkaline environment
Permeability tests (isostatic cell)
One permeability test in an isostatic cell of each formation to be studied will be performed on clay samples damaged with an internal hole. Submitting these samples to an increasing confining pressure allows to test the combined effect of temperature, geochemistry and mechanical stress. The initial hole diameter will depend on the clay used and the knowledge obtained from the SELFRAC project. Once again, the results of sealing will be evaluated by means of µCT.
Some of the permeability tests mentioned above will be chosen to perform a radionuclide migration test after the permeability test, in order to evaluate any possible relict of preferential migration along the (sealed) fracture.