DEEP REPOSITORY R&D FOR RADIOACTIVE WASTE | ROCK TUNNELS
Excavation-Induced Damaged Zone As part of the Meuse/Haute-Marne Underground Research Laboratory (MHM-URL) programme at Bure, experimental studies were performed to characterise the response of the COx claystone to different drift excavation methods. The experiments showed that the fracture patterns and the extension of the fractured zone around the drifts depend essentially on the drift orientation (Delay et al. 2007; Armand et al. 2013). Figure 10 shows the conceptual model proposed
by Armand et al. (2014) for the induced fractures network observed around a drift when the excavation is aligned with the major principal horizontal stress. It was observed that despite the nearly isotropic in-situ stress state perpendicular to the axis of the tunnel, the extent of the fractured zone is markedly larger in the horizontal direction compared to the vertical, indicating the important role played by the inherent anisotropy of the rock. The modelling of the development of localised
fractures during excavation would require the use of advanced numerical techniques intended to provide objectivity to the obtained solution. As previously noted, the huge computational resources required for large 3D geometries makes such application currently impracticable. The fractured zone will thus be represented as the zone of development of shear plastic strains predicted by the hardening/softening model. In Figure 11, contour values of the equivalent plastic
strain are presented for t = 100 years. The dashed line delimits the plastic zone. The result describes the state at the end of the operational phase. Although no regularisation technique was considered,
the obtained extension and shape of the plastic zone appear to be comparable with the fractured zones observed in the MHM-URL and schematised in the conceptual model proposed by Armand et al. (2014). This plastic zone will serve as the basis for the modelling of the EDZ and the size of the zone of permeability variations around excavations. In Figure 12, values of water permeability obtained
from the simulation along the radial profile indicated in Figure 10 are presented. The in-situ experimental measurements reported by Armand et al. (2014) for CGS drift are shown, indicating also the extent of the area where extensional fractures were observed. Since the diameter of CGS drift is smaller than the one considered in these simulations, distance to excavation wall has been normalised with respect to the radius. It can be observed that the simulation satisfactorily
reproduces the hydraulic conductivity increment within the EDZ. It can be noted that the extent of the area where extensional fractures were observed around the experimental drift is comparable to the extent of the softening zone derived from the simulation. It is in this zone where the most significant variations of permeability are obtained. These comparisons are not intended to validate the
capability of model to represent the processes occurring during the formation of fractures. The purpose is to
Mean swelling pressure [MPa]
time = 100 years
time = 150 years
time = 250years
time = 500 years
time = 1000 years 01 2
time = 10000 years 3 4
Top, figure 12: Hydraulic conductivity measured within EDZ (experimental results by Armand et al. 2014)
Above, figure 13: Core mean swelling pressure evolution February 2025 | 29
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Core
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