Deep argillaceous rocks are reducing environments. When exposed to air, reduced minerals of these rocks react with oxygen, modifying the surrounding chemical conditions. Thus, oxidation is an issue in studies about the confining properties of such rocks in the framework of geological disposal projects for radioactive waste. Previous studies in several underground research laboratories (URLs) in argillaceous rocks have shown that oxidation reactions mainly occurred in the excavation-induced fracture network surrounding the drifts. In the Callovian-Oxfordian argillaceous rock, at 2490 m in drifts from the Meuse/Haute-Marne URL, oxidized features were systematically looked for in 115 borehole cores. The concerned drifts were of various ages, from a few days to 6.5 years. After 5 months, oxidized features were encountered in numerous excavation-induced extensional fractures. In excavation-induced shear fractures, oxidized features were observed in a few borehole cores after 2 years, and they became frequent after 6 years. In all cases, the oxidized features observed were found on the fracture walls or were connected to them, and were less than 1.8 m from the drift walls. These observations about the oxidation front and its evolution over time provide insights regarding the properties of excavationinduced fractures with respect to oxygen transfer.
The investigation of the induced fractures network around seals in drifts or shafts, and in particular its evolution, is a key issue for the performance assessment of an underground waste repository. Within this framework, a specific experiment was designed and implemented in the Meuse/Haute-Marne Underground Research Laboratory (URL). This experiment, called CDZ (Compression of the Damaged Zone), is dedicated to studying the effect of mechanical compression within the induced fractures zone of the Callovo-Oxfordian claystone (COx). An unequalled level of knowledge in the 3D structure of the fractures network has been attained. A multidisciplinary approach was applied to observe not only the initial state of the induced fracture zone but also its evolution during a loading cycle. The investigations show that the fracture network which composed the Excavation Damaged Zone (EDZ) was initially interconnected and open for flow and then partially closed progressively following the increasing mechanical stress applied on the drift wall. Moreover, the evolution of the EDZ after unloading indicates a self-sealing process.
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