Speaker: Coffi Gbewade
Date: Thursday 13th of October 2022, 1:15 pm.
Abstract:
The Callovo-Oxfordian (COx) claystone in Meuse/Haute-Marne (France) is considered as a potential host rock in the French concept of high-level radioactive waste disposal at great depth. The excavation of galleries or micro tunnels in this host rock located at a depth of 500 meters in Bure (in the Meuse department), creates a fractured zone around them called the Excavation Damaged Zone (EDZ). The heat generated by waste could affect the favorable properties for containment of this host rock, especially its transport properties. The overpressure generated by the difference between thermal expansion coefficient of pore water and the solid rock skeleton could induce fractures opening or propagation in the near field (i.e., in the EDZ), altering the permeability. Then, laboratory experiments need to be performed to study the effects of increased temperature on the mechanical behavior of the COx claystone. These effects could be related to the evolution of the effective stress due to the temperature increase and to the modification of the physical properties of rock-forming minerals. Deviatoric (uniaxial and triaxial) compression tests are carried out in laboratory on cylindrical samples of the COx Claystone with a diameter of 20 mm and a heigh of 40 mm. Tests are performed under different temperatures from 20°C to 100°C to study the impact of temperature increase on the mechanical parameters of the host rock. The samples are cored in two directions, parallel and perpendicular to the bedding plane. The tests are carried out in a conventional triaxial cell equipped with two heating collars connected to a heat generator to apply the temperature imposed on the system. Triaxial tests are performed at low confining pressures (0 and 4 MPa) representative of the near field conditions (i.e, in the EDZ). Confining pressure is generated by a syringe pump while the axial stress is applied using a hydraulic press. Axial and lateral deformations are continuously measured during the mechanical loading using strain gauges. The deviatoric compression tests are performed under controlled displacement in order to characterize the post-peak behavior. Unloading-reloading cycles are performed at different stress levels in order to estimate the evolution of elastic properties. Uniaxial tests performed at temperatures ranging from 20 to 100°C, for both directions (parallel and perpendicular to the bedding) showed a significant decrease in the peak resistance due to the temperature increase. This decrease in the peak resistance is almost non-existent on triaxial tests (4 MPa confining pressure) in the parallel orientation, thus showing the impact of confining pressure. Nevertheless, an increase in the peak resistance is observed for a temperature of 150 °C for both cases. Further tests will be necessary by adjusting the thermal loading rate and the time between the thermal and mechanical loading, in order to see their impact on the mechanical properties of the COx claystone.