ROCK TUNNELS | DEEP REPOSITORY R&D FOR RADIOACTIVE WASTE
Interface elements
C3b C3a C2a
Right, figure 6: Model geometry and mesh: details around the sealing core and output points position
C1a
Concrete lining
C2b C1b
Compress. lining
Regarding the sealing materials (core, plug,
backfill), the initial stress state is given by the self- weight of the components. Initial saturation degrees were assumed for the expansive core (≈ 0.35), the concrete plugs (≈ 0.80), and the backfill (≈ 0.70), respectively. All the phases described in Figure. 3 were simulated.
Table 1 summarises the numerical features related to the processes involved in these phases. It is assumed that the excavation, lining construction, lining partial removal, and seal construction phases are almost instantaneous in comparison to the other phases. For this reason, a duration of 1 day is adopted for each of them. Post- closure phase is simulated up to maximum of 10,000 years. In Figure. 6, ‘interface elements’ are located between
the concrete lining and the seal components (core, plugs, and backfill). They are 8-node hexahedral linear elements and transmit the normal and shear stress from one medium to other. They are provided by a Mohr–Coulomb based constitutive model, which limits the maximum shear stress according to the applied normal stress. The stress–strain response is quasi- rigid-perfectly plastic. Only frictional shear strength is considered and dilatancy is neglected.
A value of interface friction angle of 10° was
adopted for both the core-lining interface and the backfill- lining interface, which can be associated with the residual strength of the clay that constitutes these components. For the plug-lining interface, a typical value for concrete–concrete contact of 30° was initially adopted. The effect of the plug-lining interface strength on seal performance was subsequently studied by varying.
Constitutive Model for the Host Rock The modelling of the failure process and EDZ creation, and specially the consequent increase of permeability, requires the selection of an adequate constitutive model. It must be capable of simulating the most important features of COx claystone behaviour, such as strength anisotropy, brittle failure, strain softening, time- dependent deformation, and permeability variations. The model adopted was specifically developed for
the COx claystone, as described in Mánica et al. (2017). It is outlined here. It is important to note that the model was developed
in a local form (i.e., the history variable employed in the hardening/softening law is the one computed at the current integration point).
Table 1: Modelling phases No. 1 2 3 4 5 6
Phase name Excavation
Lining construction Operational phase
Lining partial removal Seal construction Post-closure phase
Duration 1 day 1 day
100 years 1 day 1 day
10,000 years
Numerical stages
Radial stress and water pressure are reduced at the excavation contour from initial values to zero Lining elements are generated
Relative humidity conditions are prescribed at the opening contour Specific lining elements are removed
Core, plugs, and backfill elements are generated No changes
24 | February 2025
4m
2m
3m
1m
Plug
Core
z = 10m
z = 5m
x = 0m
x = 4m
R = 5m
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