Laboratory and Numerical Investigation of the Hydro-mechanical Behaviour of the Cobourg Limestone

The excavation of galleries and shafts of a deep geological repository (DGR) can induce damage in the surrounding rock. The excavation damage zone (EDZ) has higher permeability and reduced strength compared to the undisturbed rock and those factors must be considered in the safety assessment of the DGR. Ontario Power Generation is currently proposing a DGR for the management of its low and intermediate level nuclear wastes in a sedimentary rock formation of the Michigan Basin, known as the Cobourg limestone. The authors are conducting experimental and theoretical research in order to confirm the hydromechanical behaviour of the host rock, both in the undamaged and damaged state. The experimental program consisted of triaxial tests with a controlled loading rate up to and beyond failure. During the tests, the permeability and the seismic wave velocity were measured for six specimens cored parallel and perpendicular to the bedding plane. The experimental results show that the Cobourg limestone is an anisotropic material with respect to the mechanical behaviour. However, the measured permeability of the intact Cobourg limestone does not show significant difference for the six specimens. The permeability measured post failure is 2~3 orders of magnitude higher than that of the intact rock. The evolution of the compressional and shear-wave velocities and shear wave splitting as a function of the axial stress confirms the anisotropic behaviour of the Cobourg limestone. An anisotropic elasto-plastic model based on the microstructure tensor approach was developed to simulate the tests. The preliminary simulations successfully re-produced the experimental results.