Hollow cylinder simulation experiments of galleries in Boom Clay formation

In the context of nuclear waste disposal in clay formations, laboratory experiments were performed to study at reduced scale the Excavation Damaged Zone (EDZ) induced by the construction of galleries in the Boom Clay formation. For this purpose, thick-walled hollow cylindrical samples were subjected (after recovery of in-situ stress conditions) to a decrease of the inner confining pressure aiming at mimicking a gallery excavation. X-Ray computed Tomography (XRCT) scans of the specimens were carried out through the testing cell before and after the mechanical unloading and allowed to quantify the displacements undergone by the clay as a result of the mechanical unloading. The deformation of the hollow cylinders and the inferred extent of the damaged zone around the central hole are found to depend on the orientation of the specimen with respect to the bedding planes and show a great similarity with in-situ observations around galleries and boreholes at Mol URL in the Boom Clay formation. In the experiments performed on samples cored parallel to the bedding, the damaged zone is not symmetrical with respect to the hole axis and extends more in the direction parallel to the bedding. So it is for the radial convergence of the hole walls which is larger in the direction parallel to bedding than in the perpendicular one. On the contrary, a test on a sample cored perpendicularly to the bedding did not show any ovalisation of the central hole after the mechanical unloading. These observations confirm the significance of the pre-existing planes of weakness (bedding planes) in Boom Clay and the need for a correct consideration of the related mechanical anisotropy.