Hollow Cylinder Simulation Experiments of Galleries in Boom Clay Formation

Abstract

The rights are held by Springer-Verlag Wien.The final publication is available at http://link.springer.com. Abstract: 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.

Coupled modeling of Excavation Damaged Zone in Boom clay: Strain localization in rock and distribution of contact pressure on the gallery’s lining

“…9b) which can justify our numerical prediction (see Figs. 7d, and 18), the similar pattern has been observed in the small-scale laboratory experiments as well [43].…”

Laboratory and In Situ Simulation Tests of the Excavation Damaged Zone Around Galleries in Opalinus Clay

“…This is actually the case, but the ovalisation is occurring in reverse directions in the two clays… related to the fact that the damaged zone is developing in reverse directions too. It seems to have an eye-shape in Boom Clay with a major axis in the bedding planes direction (Labiouse et al 2013) whereas it extends in the direction normal to bedding in Opalinus Clay and is limited to the width of the hole. This rather unexpected observation is most probably related to different modes of failure in the two clays: shear failure prevails along conjugated planes in the plastic Boom Clay, while bedding plane splitting and buckling occur in the indurated Opalinus Clay.…”

“…While explanations about the testing device and procedure are provided in a companion paper (Labiouse et al 2013), this paper focuses on the results on Opalinus Clay hollow cylindrical samples cored parallel and perpendicular to bedding. Next, an in situ experiment designed at the Mont Terri URL by the Swiss National Cooperative for the Disposal of Radioactive Waste NAGRA is described and the observed EDZ is commented.…”

“…The testing device consists of a specially-designed triaxial cell with an aluminium body allowing for X-Ray Computed Tomography scans of samples through the cell walls at different stages during testing (Labiouse et al 2013). Four separate hydraulic circuits and pressure-volume controllers enable an independent control of the confining and drain pressures acting at the outer and inner boundaries of hollow cylindrical specimens.…”

“…As they are actually pretty close to the in situ conditions at Mol URL in the Boom Clay formation, the initial testing conditions on Boom Clay samples (Labiouse et al 2013) were kept for the experiments on Opalinus Clay, i.e. total confining pressures of 4.5 MPa and pore water pressure of 2.2 MPa.…”