Numerical investigation of damage and crack porosity closure effects on the soil-structure interaction around a deep drift at the Meuse/Haute-Marne URL

Abstract : Since 2000, the French National Radioactive Waste Management Agency (Andra) has been constructing an Underground Research Laboratory (URL) at Bure to perform experiments in order to obtain the in situ data necessary to demonstrate the feasibility of a geological repository in the Callovo-Oxfordian claystone (COx) formation. The excavation of galleries at the main level of the laboratory showed a significant fracturing induced by the excavation. One can distinguish a connected fractured zone (with extensional and shear fractures) and a zone of discrete fracturing. The extent of the fractured zones depends on the orientation of the drift with respect to the initial stress state (Armand et al. 2014). The short and long terms hydromechanical behavior is closely related to the behavior of the induced fractured zone as indicated by the in situ strain measurements. Similarly, the nature and extent of these fractured zones directly act on the magnitude of loading transferred to the concrete supporting elements. Hence it is necessary to account for the damage induced by the excavation in order to understand the long-term behavior of the couple COx – structural supports. Indeed, the loading-unloading of the plate test carried out at the wall of the GET gallery (parallel to the major horizontal stress) shows a change in the stiffness of the fractured zone as a function of the load (Vu et al., 2015). The increase of the rigidity can be explained by the mechanical closure of the induced fractures under compressive loading. This phenomenon is similar to those observed in the mechanics of rock joints or the re-closure of initial gap observed at the beginning of triaxial tests on soft rocks or soils. The evolution of the stiffness of the fractured zone, i.e., stiffness degradation due to excavation and partial stiffness recovery due to fracture closure affect the loads transferred to structural elements. The main goal of the present paper is to provide insights on the role of this stiffness change and its effect for a more accurate design of different repository structures.